NBSIR 76-847

ELECTROCALORIC REFRIGERATION FOR SUPERCONDUCTORS

Ray Radebaugh and J.D. Siegwarth

Cryogenics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

and

W.N. Lawless

Research and Development Laboratories

Corning Glass Works

Corning, New York 14830

and

A.J. Morrow

Development Laboratory

Corning Glass Works

Raleigh, North Carolina 27604

February 1977

Sponsored by

Advanced Research Projects Agency

ARPA Order No. 2535

Arlington, VA 22209

Covers May 1, 1973 to June 30, 1975

NBSIR 76-847

ELECTROCALORIC REFRIGERATION FOR SUPERCONDUCTORS

Ray Radebaugh and J.D. Siegwarth

Cryogenics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

and

W.N. Lawless

Research and Development Laboratories

Corning Glass Works

Corning, New York 14830

and

A.J. Morrow

Development Laboratory

Corning Glass Works

Raleigh, North Carolina 27604

February 1977

Sponsored by

Advanced Research Projects Agency

ARPA Order No. 2535

Arlington, VA 22209

Covers May 1, 1973 to June 30, 1975

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Betsy Ancker-Johnson . Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

CONTENTS

Page

1. INTRODUCTION 1 1.1. Purpose of Work 1 1.2. Description of Refrigerator 1 1.3. History and Organization of Project 2 2. HEAT SWITCHES 6 2.1. Types of Heat Switches and Minimum Requirements 6 2.2. Mechanical Heat Switches 7 2.2.1. Multiple leaf switch 7 2.2.2. Helium gap switch 15

2.3. Magnetothermal Heat Switches 16

2.3.1. Survey of various metals 16

2.3.2. Beryllium 20

2.3.3. Magnesium 26

2.3.4. Thermal conductance of joints 29

2.3.5. Magnet requirements 3 5 3. REFRIGERATION MATERIALS 3 6 3.1. Theoretical 36

3.1.1. Thermodynamics 36

3.1.2. Criteria for materials selection 38

3.2. Previous Work 39

3.3. SrTiO^ Glass-Ceramic Research Samples 4 0 3.3.1. Dielectric properties 40

3.3.2. Relation to capacitance thermometer manufacturing . . 42

3.4. SrTiO^ Glass-Ceramic Multilayer Samples 42

3.4.1. Fabrication method 42

3.4.2. Fabrication problems 51

3.5. Alternate Materials Investigated 57

3.5.1. Glass-ceramics 57

3.5.2. Ceramics 57 » 3.6. Experimental Methods 66

3.6.1. Dielectric properties 66

3.6.2. Thermodynamic properties 7 0 3.6.3. Thermal conductivity 80

3.7. Experimental Results 82"

3.7.1. SrTi0

3 glass-ceramics 82

3.7.2. KTa0

3 glass-ceramics 108

3.7.3. Single crystals . 108

3.7.4. Ceramics 113

iii
3.8. Discussion of Experimental Results 143

3.8.1. Dielectric Properties 143

3.8.2. Thermal Properties 150

3.8.3. Recommendations for further work 175

4. CONCLUSIONS i76

5. REFERENCES i78

iv
DISCLAIMER

Certain commercial materials are identified in this report in order to specify adequately

the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the material identified is necessarily the best available for the purpose.

The views and conclusions contained in this document are those of the authors and should

not be interpreted as necessarily representing the official policies, either expressed or implied, of the Advanced Research Projects Agency, the U. S. Government, Corning Glass Works

or its subsidiaries.

v
PROJECT SUMMARY

The purpose of this work was to investigate a technique for the refrigeration of superconductors at 4 K which has the potential of better reliability and lower cost than present

4 K refrigerators. The poor reliability in present 4 K refrigerators is a major handicap in

the technological use of superconductors. In this project a solid-state type of refrigeration

was investigated for use as the final stage of refrigeration to span the range between 4 and

15 K. This proposed new technique uses the electrocaloric effect in certain dielectric

materials and is the electrical analog of cooling by adiabatic demagnetization. In essence,

a high electric field is used to change the entropy of a paraelectric material. The advantage over the magnetic case is that electric fields in thin layers are easier to apply than

magnetic fields.

A continuous refrigerator operating with a load at 4 K and a heat sink at 15 K is accom- plished by separating the refrigeration material from the load and heat sink by heat switches.

The original problems in the project were to develop appropriate heat switches for use on a prototype, 1-watt refrigerator and to develop the refrigeration material based on SrTiO^

glass-ceramic technology. Preliminary studies had indicated that certain SrTiO^ glassceramics would be the most promising refrigeration material in this temperature range. The

program was well along before it was found that SrTiO^ glass-ceramics would not be suitable

as the refrigeration material; this conclusion then led to the investigation of other potential materials.

The heat-switch problem was attacked by investigating both mechanical and magnetothermal

heat switches. The mechanical switch, which has moving parts, used forced contact between

multiple leaves of gold-plated copper. Conductances on the order of 5 W/K could be achieved

with about 10 such contacts and with on-to-off heat transfer ratios on the order of 1000.

Such a ratio is significantly higher than the required value of about 30.

A literature review showed that both single crystal gallium and tungsten would be useful

materials for the lower (between refrigerator and 4 K load) heat switch. Such materials show

large changes in the thermal conductivity at low temperatures when a transverse magnetic

field is applied. Switch ratios on the order of 100 are possible. The magnetothermal conductivity of single crystal beryllium was measured in this program and the results show that

this material can be used for both the upper and lower heat switches with a switch ratio of

about 100 in both cases. A compilation of literature data on the thermal conductance of joints (solder, grease,

adhesive, pressure) was made and revealed a lack of data between 4 K and room temperature on

solder joints. Such joints have higher conductances at room temperature than any other type

and would be used in the electrocaloric refrigerator. Measurements were made on the thermal

conductance of indium solder joints between 2 K and 120 K. The conductance per unit area

2 reached a peak of 100 W/cm K at 20 K, which is sufficiently high to eliminate most problems

with joint conductances.

In any dielectric material the entropy change or refrigeration power brought about by a change in electric field is proportional to the change of polarization with respect to temvi
perature. When no remanent polarization occurs in the material, the polarization can be

derived from the dielectric constant, an easily measured quantity. Therefore, early in the

program the change zz dielectric ccr.star.t with respect tc temperature was a. pritary guar.city

for estimating the refrigeration power of a material. The SrTiC^ glass-ceramic showed a peak

in the dielectric constant at about 30 K with a large positive slope below that. The positive slope implied cooling would occur when the electric field was increased.

Many manufacturing problems had to be overcome in making satisfactory multilayer SrTiO

glass—ceramics with large temperature derivatives of the dielectric constant and large electric

field breakdown strengths and over half the experimental work on the program was devoted to

these problems. Sample porosity was the major unsolvable problem. However, it was then

discovered that the inherent electrocaloric effect in SrTiO^ multilayer glass-ceramics at 4 K

not only was very small and dominated by hysteretic effects, but that it probably had the

wrong sign. Definite electrocaloric cooling was seen in the 10-40 K temperature range when

the field was decreased, but the temperature changes were only about 0.03 K. Direct measurements of dc polarization were made on the samples and the results were consistent with the

electrocaloric measurements but inconsistent with the dielectric constant behavior. The dc

polarization showed only a negative slope with temperature, and the polarization became

essentially independent cf temperature below about 10 K. The dielectric constant, on the

ether hand , had a teak at about 3C K with a large positive slope below that-. Extensive dc polarization and dielectric constant measurements were made on the SrTiO^

glass-ceramics as well as many other materials to try to understand the fundamental discrepancy between the two types of dielectric measurements. It was found that many dielectric

materials show a broad peak in the dielectric constant at low temperatures. In the past such

peaks have usually been taken as an indication of ferro- or antif erroelectric ordering. The

model developed here to explain the peak in the dielectric constant is based on the electret

behavior of impurity-vacancy dipoles in the material. Impurity levels in the parts per

million range car. give rise tc such effects. Application of an electric field produces a remanent polarization which remains after the field is removed. It is a result of the electret behavior and not that of a ferroelectric. The polarization, and not the dielectric

constant, is then the proper thermodynamic quantity for predicting the electrocaloric effects.

Many other materials were investigated by polarization, electrocaloric, and specific

heat measurements to find a suitable material for electrocaloric refrigeration. These other

materials included glass-ceramics, ceramics, and single crystals. Most of the investigations

were on SrTiO^ and KTaO^ based materials. Other materials investigated were of the Pb^Jtt^C^

type ceramics, the polar PZT ceramics, and TIBr single crystal. The largest electrocaloric

effects were seen in SrTiC^ ceramics, followed closely by KTaO^ single crystal. In SrTiO^

ceramics electrocaloric temperature changes on the order of 0.5 K were observed at about 10 K. The effect decreased significantly at 4 K. Jcr various practical reasons the electric fields

applied to these samples were usually limited tc relatively modest values . Without any

existing theoretical model for the thermodynamics of the materials, it was uncertain how much

larger the electrocaloric effects would be if higher fields were applied.

vii
A theoretical model, based on the lattice dynamics of the materials, was then developed

to explain the observed cooling effects. Dielectrically active materials are classified as either displacive type or order-disorder type. The materials studied in this program were of

the displacive type since they are usually more active at low temperatures than the orderdisorder materials. The theoretical model developed is valid for the displacive type materials. In this model only the entropy associated with transverse optic phonons can be altered

with an applied electric field. The calculated temperature changes for SrTiO^ , KTaO^, and

TlBr agree reasonably well with the observed values. This model shows that at 4 K the maxi- mum possible entropy changes in the displacive type materials are at least an order of magnitude too small to make a practical refrigerator.

The report is concluded by stressing the need to concentrate on a search for an appropriate order-disorder type of dielectric because the possible entropy changes are orders of

magnitude higher than the displacive type materials. Unfortunately order-disorder materials

order at rather high temperatures, thus removing the available entropy for cooling. It is

suggested that for any future work thermodynamic studies be made on lithium thallium tartrate

and NH^H^PO^, which may be order-disorder materials with enough entropy at 4 K for useful

refrigeration. However, at this time the basic physics of the dielectric materials must be

better understood before a worthwhile development program could begin.

viii
ABSTRACT

A solid state type of refrigeration, which utilizes the electrocaloric effect in certain

dielectric materials, has been investigated. Such a refrigerator would operate with a load

at 4 K and reject heat to a reservoir at 15 K. Heat switches for such a refrigerator were

studied. One type was a multiple leaf contact switch. The other type was a magnetothermal

switch utilizing single crystal beryllium. Based upon earlier preliminary work, the refrigeration material was to be a SrTiO^ glass-ceramic. It was found here that such a material

has no useful electrocaloric effect at 4 K. Many other materials were studied but none were

found with sufficiently high reversible electrocaloric effects for a practical refrigerator.

The largest effects were seen in SrTiO^ ceramics, followed by KTaO^ single crystal. Temperature reductions of about 0.5 K at 10 K were observed during depolarization. A theoretical

model, based on the electret behavior of impurity-vacancy dipoles was developed to explain

the observed dielectric behavior in the materials investigated. Another theoretical model,

based on the lattice dynamics of displacive dielectrics, was used to explain the observed

entropy and temperature changes seen in such materials. The model points out that displacive type materials have too low entropies at 4 K for practical refrigeration. An investigation of certain order-disorder dielectrics is suggested.

Key words: Beryllium; ceramics; cryogenics; dielectric-constant; electrets; electrocaloric

effect; entropy; f erroelectrics; glass-ceramics; heat switches; magnetothermal conductivity;

polarization; potassium tantalate; refrigeration; specific heat; strontium titanate.

ix

1 . INTRODUCTION

1.1. Purpose of Work

One of the largest applications of cryogenics is in the area of superconductivity.

These applications range from the low-power level devices employing the Josephson effect up

to the large scale superconducting magnets and rotating machinery. These superconducting

devices require temperatures below 15 K, and in most cases temperatures of about 4 K. In certain limited cases, infrared detection also requires 4 K temperatures or lower to gain

sensitivity.

The unreliability of present 4 K refrigerators is probably the main reason for the

relatively slow growth of applied superconductivity in commercial markets. A mean-timebetween-failure (MTBF) of about 3000 hours is a typical figure for such machines. Specialized mechanics are required to work on such machines and their presence every few thousand

hours serves as a reminder that 4 K is not easily achieved. Rotating compressors and ex- panders now offer some hope for a MTBF of about 10,000 hours for large 4 K refrigerators.

The Stirling cycle and Gif ford-McMahon cycle refrigerators can also achieve a MTBF of 10,000

hours even on a small size machine of about 1 watt. Unfortunately the regenerative heat

exchangers used on such machines become ineffective below about 10 K. What is needed then to

reach 4 K with such a machine is a very reliable last stage to span the gap between about

15 K and 4 K. In addition, this last stage should operate close to Carnot efficiency to maintain a good overall efficiency. A survey of cryogenic refrigerators"'' shows that 4 K refrigerators operate at the same fraction of Carnot efficiency as do 10-30 K refrigerators

of the same power rating.

2 Preliminary studies showed that a refrigerator utilizing the electrocaloric effect in

certain glass-based materials would make a promising last stage to span the gap between 15 K

and 4 K. Since it is inherently a solid state type of device, the reliability of the electrocaloric refrigerator should far exceed that of the 15 K upper stage. Thus 4 K can be

reached with the reliability of a 15 K regenerative type refrigerator.

The purpose of this project was to study the feasibility of an electrocaloric refrigerator which would absorb roughly 1 watt at 4 K and reject the heat to a reservoir at about 15

K. Originally the refrigeration material was to have been a glass-ceramic of SrTiO^. That

2 choice was based on preliminary mea surements and calculations of the thermodynamic properties of research samples of SrTiO^ glass-ceramics. Subsequent measurements on multilayer

samples manufactured from this material showed relatively small cooling effects and the

program was altered to study the potential of using other materials with large electrocaloric

effects.

1.2. Description of Refrigerator

The electrocaloric refrigerator is analogous to a magnetic refrigerator, which is often

referred to as adiabatic demagnetization. 3 The difference is that in the electrocaloric

refrigerator the entropy of the working material is changed by an electric field rather than

a magnetic field. The first experiments with the electrocaloric effect were on Rochelle salt

by Kobeko and Kurtschatov4 in 1930. Since then several measurements of electrocaloric cool-
ing effects in various materials have been measured, but until now no attempt has been made

to make a refrigerator utilizing the electrocaloric effect. Quasicontinuous refrigeration is

achieved by taking the refrigerating material through a closed loop on an entropy versus temperature diagram. Such a path is shown in figure 1. The entropy of the material is shown

for four different values of electric field. If we start at the temperature in a field of

E^ and then change the field to E^, the material cools adiabatically to the temperature T^.

When going from the field to E^ in an isothermal process, heat in the amount = T^AS is

absorbed. The change in field from E^ to E^ causes the sample to heat adiabatically to T^.

An amount of heat = T

2 AS is then rejected to a thermal reservoir at T^ during the field

change from E^ to E^. The ratio of heat rejected at T^ to the heat absorbed at T^ is then

simply the Carnot result S^/Qj = T2^T

1 ^or t^ie -"-^ ea^ cYc ^- e discussed here. If now two re- frigerators are operated simultaneously such that at any time one or the other is in that

2 part of the cycle where T = T , then continuous refrigeration at T, is achieved . Prelimi2 nary studies showed that cycle times on the order of 1 second would be optimum for a re- frigerator of SrTi0

3 glass-ceramic operating between 4 and 15 K.

2 A schematic diagram of an electrocaloric refrigerator capable of continuous refrigeration is shown in figure 2. The working material is separated from the reservoir at 15 K and

the load at 4 K by heat switches. The working material has closely spaced metal fins in it, as in a capacitor, to provide heat transfer and a means of obtaining a high electric field

with a reasonable voltage. The two halves can be made in the form of two half cylinders (see

Fig. 2b) to conserve space. A rather unique refrigeration cycle was proposed by van Geuns^ which has applications to the electrocaloric refrigerator. His idea is to use high pressure helium gas as a regenerative material to change the temperature of a paramagnetic or paraelectric material between 15

and 4 K. In that case the adiabatic lines in figure 1 are replaced by constant field lines. In the real cycle, however, there must be a small adiabatic segment at the end of the cooling

or heating step to account for the finite AT required between the helium gas and the refrigeration material. With this regenerative cycle the heat load due to the lattice heat capacity is nearly eliminated from each cycle, but the initial cool down from 15 K is then done in

very small steps until equilibrium is established in the helium gas. The helium gas must be

at a high pressure (above the critical pressure) to provide a high heat capacity. The high

pressure can pose certain mechanical problems in displacing the gas with respect to the

refrigeration material. Whether this type of mechanical device would be any more reliable

than a mechanical 4 K refrigerator can only be determined by experiment. We have not pursued

this type of cycle in this project. Instead the heat switch arrangement has been pursued

since it can be made with no moving parts.

1.3. History and Organization of Project

Glass-ceramics of SrTiO^ developed by Lawless show large changes in dielectric constan

with respect to temperature. Because of that behavior, such a material has been useful for

6 7 low temperature capacitance thermometers. ' Corning Glass Works and the Cryogenics Divisio

of NBS collaborated on extending measurements of the capacitance thermometer to temperatures

2
TEMPERATURE

Figure 1. The entropy vs. temperature curves for a material useful

for electrocaloric refrigeration. The ideal refrigeration

cycle, shown by the arrows, is traced out by changing the

electric field from E to E3 to E4 to E2 and back to Ej_

.

3
Figure 2a. Vertical cross section of the model refrigerator: (A)

Mechanical thermal valve (left-hand valve is closed,

right-hand valve is open) ; (B) Temperature sensors to

monitor operation; (C) Metal post running lengthwise

through the element and acting both as low voltage lead

and heat conductor (see also Fig. 2b); (D) Dielectric

element with interspersed metal electrodes (see also

Fig. 2b) ; (E) Magnetic thermal valve which thermally

connects or isolates the element from the load depending

on the magnetic field.

4
Figure 2b

.

Horizontal and partial vertical cross sections of the model

refrigeration showing the arrangement of the metal post and

electrodes with the dielectric material.
below 2.4 K . In addition, the material has been proposed for use as a low temperature

9 bolometer

.

2 10

Lawless then proposed ' that the SrTiO^ glass-ceramics be used as an electrocaloric

refrigerator. This proposal was based on several factors. First, thermodynamic calculations

from the dielectric constant behavior showed large cooling effects would occur in the 4-15 K

temperature range. Second, large breakdown strengths had been achieved in similar materials.

Third, it is possible to fabricate glass-ceramic materials into almost any size and shape.

In addition to the refrigeration materials, the construction of an actual refrigerator

required the development of heat switches and a general experience in cryogenics in order

to put together an efficient refrigerator. Thus, a collaboration between Corning Glass

Works and the Cryogenics Division of the National Bureau of Standards evolved as. a sound

approach to the development of the electrocaloric refrigerator. A two year project came about with ONR and ARPA funding for two men for two years. The Cryogenics Division subcontracted half of those funds to Corning Glass Works for manufacturing the refrigeration

samples. In addition, Corning Glass Works and the Cryogenics Division each funded one additional man for the two years. Thus the project became a four man effort for two years.

The Cryogenics Division was responsible for the heat switches, Corning Glass Works was responsible for manufacturing the refrigeration material, and Dr. Lawless of Corning Glass

Works became a guest worker at the Cryogenics Division to make the properties measurements

on the refrigeration samples and to oversee the development of the glass-ceramic processes

and compositions. Subsequent problems with the refrigeration materials led the Cryogenics

Division to change its effort in the last six months from heat switches to studies of

refrigeration materials.

2. HEAT SWITCHES

2.1. Types of Heat Switches and Minimum Requirements

Two heat switches are required for the electrocaloric refrigerator, one to connect the

cooling element with the 15 K reservoir, and the other to connect the cooling element with

the 4 K load. The ratio of heat conducted in the "on" state of the switch to that conducted

in the "off" case will be called the switch ratio and should be as high as possible. The

switch ratio should probably be at least 30 to maintain a high refrigerator efficiency. In the "off" condition the switch must span the temperature difference between 15 K and 4 K. In the "on" case the upper heat switch will be at about 15 K whereas the lower heat switch

will be at 4 K. The temperature drop across these switches in the "on" case should be as small as possible. As a first approximation we take this AT to be 1 K for both switches.

For the refrigerator size considered in this project, the lower switch must then have an

"on" conductance of about 1 W/K and the upper switch about 5 W/K. To achieve a switch

ratio of 30 means that the conductance ratio must be about 11 x 30 = 330 because the AT in

the "off" case is 11 times that of the "on" case. These switches must be able to operate

at roughly one cycle per second.

6
There are several ways in which the switch conductance can be varied with some external

parameter. These parameters could be such things as force, magnetic field, and electric

field. We know of no other parameters which could reversibly change the conductance of a heat switch. An electric field controlled heat switch is especially attractive since

electric fields are easy to establish and are already being used for the cooling element.

Though the thermal conductivity of a material like SrTiO can be changed with an electric

11 field, the effect is too small to make a useful heat switch.

Superconducting heat switches are commonly used for adiabatic demagnetization but

their use is restricted to temperatures below about IK. A large difference in the normal

state and superconducting state thermal conductivity occurs only for temperatures much

below the transition temperature. The operation of the switch requires a magnetic field to

drive the material into the normal state. The mechanical switches commonly used in calorim12 etry will operate in the 4-15 K temperature range but their conductances are usually on

the order of only a few milliwatts per kelvin instead of watts per kelvin.

Previously developed heat switches have been designed for lower temperatures, lower

power levels, or longer cycle times than that required for this electrocaloric refrigerator.

Hence, a program was necessary to develop the proper heat switches. Several types were investigated and compared.

2.2. Mechanical Heat Switches

2.2.1. Multiple leaf switch

Most pressed contact switches have conductances, k, of only a few mW/K at 4 K. Generally, the forces, F, applied have been only a few kg because opening the switch causes heating that increases with increasing forces. The conductance of a switch has been found

to be dependent on Fn where n is usually less than 1. The conductance is generally indepen13 14 dent of the macroscopic surface area of the contacts. ' 14

Berman and Mate have measured the conductance of solid gold contacts at 4 K that were closed at room temperature and found a value of 0.2 W/K at 4 K for a 43 kg closing

force. This suggests that a mechanical contact type switch might achieve the desired

conductance. The conductance of the gold switch might be expected to be qualitatively

14 similar to copper, so k is reduced by a factor of 4 when the switch is closed at 4 K and

3/4

is proportional to F . This being the case, several hundred kilograms closing force

would be required to achieve the desired conductance. High forces are undesirable because

of the structural parts that must extend to room temperature. In addition, it is possible

that the surfaces can be mechanically damaged causing a reduction in the conductance of the

contact. It is conceivable that a high switch conductance could be achieved if the heat

could flow through several independent contacts instead of one contact. The geometry could

be arranged so that a single applied force could close all of the contacts.

Switch design : A switch designed to achieve high conductance with a low closing force

is shown in figure 3. The contacts are stacked in parallel so that the single clamp mechanism closes several. When the clamp is opened, the spring of the leaves can move them

apart. The stiffness and alignment of the leaves will determine how much residual closing

force remains between contact surfaces.

7

In figure 3 the leaves A, A are either copper with free ends gold plated or silver.

They are soldered into slots in copper base blocks B, B with pure indium. Carbon thermometers are mounted either on copper tabs indium soldered to top and bottom outer leaves C, C, or on the copper blocks as shown by D, D, since only two carbon thermometers are used at a time. A heater E is attached to the lower copper block to establish the desired heat flux.

A germanium thermometer F is used to calibrate the carbon thermometers in place. A second

heater (not shown) is attached to the upper block and used to heat the whole switch assembly

during calibration. The upper base block B is thermally anchored to the flat circular heat

exchanger G. Liquid helium is admitted through needle valve H and the vapour is removed

through the pump line and valve I. This line is jacketed to a point above the bath so the

enthalpy of the gas as well as the latent heat can be used to cool the switch when measurements are being made above 4 K. The switch is clamped shut by the two brass jaws J, J. The force is applied over an area of 3.2 mm times the leaf width, about 9 mm. The closing force is applied by an air

piston K. The forces are transmitted through the wire L and a stainless steel tube M. The

jaws are moved by two pairs of bell cranks N, N which are coupled to the pull wire L through

an equalizing device P. The wire is thermally anchored with copper braid at Q at 4 K and vacuum jacketed so a sliding O-ring seal R can be placed at room temperature. The switch

assembly is placed in a vacuum container S sealed by a Woods metal joint near the top. The vacuum can is immersed in liquid helium in a set of glass dewars T. There are a number of questions that must be answered about the switch shown in figure

3. For example: Can a switch of this geometry be fabricated out of high conductivity

materials? Can plated contacts be used or must they be solid gold? How high is the con- ductance when the switch is closed at low temperature? What is the effect of cycling the

switch while cold? How large is the on-to-off ratio? Is there a surface cleaning problem?

Can such a switch be adapted to a small dielectric cooling stage? These questions are resolved to varying degrees by measurements on four modifications of the switch.

Switch 1 This switch had 13 leaves made of copper with a residual resistance ratio (RRR) of

500. This ratio is the ratio of the electrical resistances at 300 K and 4 K. The

leaves were approximately 0.33 mm thick, 8-1/2 mm wide, 25 mm long and arranged to give

12 parallel contacts. The contact ends were plated with about 2 . 5 x 10

3 mm thickness

of commercial gold plating.

Switch 2 This switch had 3 leaves, hence two contact surfaces. The leaves were high

purity copper with an RRR of 1900, and were 0.75 mm thick, about 10 mm wide and 18 mm

long. The contact ends were plated with pure gold to a thickness of about 8 x 10 3 mm

The leaves were mechanically polished before plating.

Switch 3 This switch was the same as switch 2 in every way except that the leaves and

contacts were silver with an RRR of 1400. Silver contacts were tried because measurements at 77 K showed the contact conductance of silver higher than gold.

15 The contacts were mechanically polished.

9
Switch 4 This switch used identical leaves to those of switch 2. The base blocks were

larger, made of high purity copper and there were 11 leaves, hence 10 contacts.

All the leaf materials were rolled from 6.3 mm or 9.5 mm rod and annealed. The switch

2 material was rolled to size, etched with HNO and oxygen annealed at 1000°C in air at 5 x

-4

10 torr for 24 hr. The high purxty copper was annealed once during rolling, and annealed

afterward at 750°C in a vacuum for 1 hr. The silver was rolled to size and annealed at

550°C in hydrogen for 1-1/2 hr. The residual resistance ratios were measured after all

rolling and annealing was completed. All the leaves were soldered into the base blocks with

pure indium. High temperature solders were not used to avoid damaging the contact surfaces

or diffusing impurities into the leaf material.

Since the proposed electrocaloric refrigerator is to reject heat at about 15 K, most of

the conductance measurements were made at that temperature. Temperature measurements were made either on the base blocks, to get the conductance of the whole switch, or at the leaves,

so the results could be corrected to the contact conductance. The latter results can be

14 compared directly to the pure gold contact results. Because of the geometry of the leaf

switches, the temperature drop between contacts cannot be directly measured.

For the temperature measurements on switch 1, a calibrated carbon thermometer was used

to measure the temperature on one side of the switch. The carbon thermometer on the other

side of the switch was calibrated against this each run. The switch was closed during

calibration and heat was supplied by a heater on the heat exchanger, figure 3, to bring the

switch assembly to a constant temperature. Under these conditions the heat flow in the

switch should be zero. For the remainder of the switches, both carbon thermometers were calibrated by a germanium thermometer. The calibrations were done at three temperatures

between 4 and 15 K, and the results were used to find A, B, and P of the equation^

-P Log R = A + BT

where R is the thermometer resistance at the temperature T. The conductance measurements were made by adjusting the needle valve, H, figure 3, to fix the heat exchanger temperature at the desired value while heat is applied to the opposite side of the switch. The heater power, temperatures and closing force were recorded

when the temperatures reached equilibrium. The closing force was determined from the cylinder air pressure and area of the piston.

The switch contacts were cleaned with an abrasive copper-cleaning powdered soap, rinsed

with distilled water, then rinsed with freon liquid and dried with freon gas. Switch 4 was also cleaned with freon in an ultrasonic cleaner. There was a slight increase in contact

conductance. Switch 2 was cleaned only with freon liquid at first. The contact conductance

was about 1/3 the conductance after cleaning with the soap. There was some variation in

conductance from run to run and occasionally it was necessary to reclean the contacts. The

higher conductance results are presented in the figures below.

10
Results : So that the switch conductances can be more readily compared to the measure14

ments of Berman and Mate for solid gold, the conductances are given in figure 4 as W/K per

contact. Most of the measurements presented were done around 15 K since this was the temperature at which the switch was to be used. Data are shown for switch 3 at 4 K, however.

The switches were cycled from 1 to 150 times while cold. Since there was no change observed

in conductance after cycling the switch, the data are presented without specifying the

number of cycles.

The conductance of switch 1 is shown by point 1 after it was cooled from room temperature to 15.2 K with a 54 kg closing force. The contact conductance is about 1/4 that of

solid gold contacts. After opening and closing the switch at 15 K, the switch conductance

per contact is shown by curve 1A. The measured conductance is from base block to base

block.

The conductivity of the leaf material of switch 1 was measured and found to be 160 W/cmK

at 12 K. This is high for an RRR of 500 when compared to other conductivity measurements on

17 high purity copper. This measurement indicated that most of the measured aT was in the

contact. Cobalt or silver impurities are used in the commercial gold plating for hardening.

Since this impurity could possibly affect the contact conductance, pure gold was used to

2 plate the remaining switches. The indium joint conductance was estimated to be 10 W/K cm from the conductivity measurements.

The contact conductance of pure gold plated contacts is shown by curve 2 of figure 4 for switch 2. The switch was closed with a 54 kg force before cooling. At 15 K the force was increased resulting in some increase in conductance.

The contact conductance of the switch, after cycling at 15 K, is shown by curve 2A.

The conductance is reduced by slightly more than two by opening and closing the switch cold.

There is a thermometer on both sides of the indium solder joint for this measurement, a carbon thermometer on the leaf and the germanium thermometer on the base block. If the AT

is corrected for the leaf conductance and the conductance of the 0.9 cm square base block,

then the conductance of the indium joint can be determined. This gave an indium joint

conductance of 00 to 20 W/K depending on whether 7 or 14 W/cmK was used for the OFHC copper

conductivity

.

The total switch conductance per contact is shown in curve 2B. The thermometers were mounted on the blocks. A correction has been applied to exclude the AT due to the base

blocks since they can be made of a much higher conductivity material than the OFHC copper

used here. There is only one set of low- temperature conductivity data^ for OFHC copper and

for that the state of work hardening is unspecified. The error bars on the data are for a

7 W/cmK minimum conductivity and a 14 W/cmK maximum conductivity for OFHC copper at 15 K. The section of curve 2B showing little force dependence was measured while reducing the

air cylinder pressure. This apparently different force dependence is due in large part to drag in the air cylinder and linkage to the switch. Applying the force with a spring scale

brought the pressure applied and pressure released curves closer together.

The conductance of the silver contacts of switch 3 after opening and closing at 15 K is

shown by curve 3A. The point 3 is the conductance of the switch after cooling closed to

11
12
15 K. This conductance is about the same as for gold plated copper. The conductance of the

silver at 4 K is shown by curve 3B. The conductance per contact of switch 4 after cooling closed was essentially the same

as for switches 2 and 3. The closed cold contact conductance, however, was only about 3/4

of the conductance observed in the earlier switches. This could perhaps result from the

stiffness of the leaves or a movement of one set with respect to the other. If, for some reason, one or both of the leaves on which the thermometers were mounted made poorer contact

than the next, the measured AT would be between the AT of the contacts and the AT of the

whole switch.

In figure 5, the conductances of the switch contacts of switches 1, 2, and 3 (switches

cooled from ambient while closed) are compared to the solid gold contacts of Berman and

Mate. Switches 2 and 3 were cooled at a higher closing force so the conductance was cor3/4 rected to a 43 kg closing force assuming the conductance varies as F . The conductances

agree well with the results of Ref. 14. The temperature dependence of the conductance of

the silver contacts, after cycling cold, was found to be proportional to T in agreement with

Ref. 14. On-to-off conductance ratios were measured for switches 1, 2 and 4 for closing forces

around 90 kg. For switch 1, this ratio was about 2000. For switch 2, this ratio varied for

different runs from 900 to 3000, probably depending on the alignment of the leaves. This

ratio was actually measured for an open AT of about 10 K and no radiation corrections have

been applied. Switch 4 had an on-to-off ratio of about 45. The low ratio is due to the

impracticability of aligning a large number of relatively stiff leaves so there are no appreciable contact forces remaining when the switch is open. The longer and thinner leaves

of the first switch were sufficiently flexible so that the open conductance was low in spite

of the even larger number of leaves.

Discussion : A maximum conductance of 1.2 W/K at 15 K with a 90 kg closing force and a

10

3 on-to-off ratio was achieved with the switches reported above. Although 5 W/K at 15 K was desired, the obtained value is sufficient for testing a refrigerator. The conductances

of both pure gold plated copper and silver contacts are comparable to the solid gold mea14

surements when the contacts are closed before cooling. The conductance after reclosing

the switch cold is reduced by a factor of 2 or 3. The pure gold plated contacts used for switch 2 were definitely superior to the com- mercial plating, which contained a dilute impurity. This difference may be caused by the

presence of the impurity, but it should be noted that the leaves of switch 2 were mechanically polished before plating, and the plating was thicker, either of which may have caused

the improvement

.

The power, n, of the relation k a F

n for the contact conductances was found to be about

0.66 for switch 2 and 0.6 for switch 3 from log-log plots of k as a function of F. These

slopes do not extrapolate to k = 0 at F = 0 on a linear plot. No corrections have been made

to the indicated forces for friction in the switch mechanism.

The above measurements show that a high conductance switch (on the order of 1 W/K) can

be made by plating and that the fabrication can be done using pure indium solder. The

13
.01

1 10 50

TEMPERATURE OF HEAT SINK, K

Figure 5. T dependence of the conductance of one contact. Curve 4, data of

Berman and Mate extrapolated to 17 K. Closing forces are 43.0 kg

unless otherwise specific

.

Point 1 - Switch 1 closed at ambient

Point 2 - Switch 2 closed at ambient

Point 2A - Switch 2 closed at 15 K

Point 3 - Switch 3 closed at ambient

Curve 3A - Switch 3 closed at 15 K with a 54 kg force

Curve 4 - Berman and Mate

14
conductance is not degraded by cold cycling the switches 150 times. On-to-off ratios of

greater than 10^ are possible if the leaves are properly aligned (switches 2 and 3) or sufficiently flexible (switch 1) . Cleaning and maintaining that cleanliness appears to be

no problem. No heating was observed due to opening or closing the switch, which is not

surprising due to the heat fluxes present.

The conductance of a switch of the above design can probably be increased by optimizing

the leaf dimensions and the number of leaves. Probably, a switch with even higher conductance is possible using a more elaborate disengaging mechanism for the leaves.

2.2.2 Helium gap switch . In order to transfer heat across contacts in a vacuum (previous section) large forces

are required. A second mechanical switch considered uses a low pressure of helium gas

around the contacts to provide good heat transfer without using high contact forces. The

thermal conductivity of helium gas is independent of pressure until the pressure becomes so low that the mean free path of the molecules becomes comparable to the gap spacing. For a pressure of 1 torr the mean free path of helium gas is 1.1 ym at 4 K and 2.1 ym at 15 K. Therefore, for a gap of 5 um or more the thermal conductivity of the gas will be the classical bulk value. The thermal conductance across the helium gap is simply

K = kA/£,

where k is the thermal conductivity of the helium, A is the cross sectional area of the gap, and £ is the gap spacing. The conductance can then be varied by changing the gap spacing.

For the "on" case the two copper surfaces are brought into light contact. For reasonably

smooth surfaces, the average gap spacing may be about 5 ym. The "off" condition is achieved

by separating the two plates by a gap of about 5 mm. The conductance then decreases by a factor of 1000. For a AT of 11 K in the off case, the switch ratio would be £ -,/ll £ off on

for k independent of temperature. Actually k varies from 0.084 mW/cmK at 4 K to 0.22 mW/cmk

18 at 15 K, so the switch ratio becomes £ __/7.5 £ = 133 for the upper switch and

or r on

£ ,/19-4 £ =52 for the lower switch as £ changes from 5 ym to 5 mm. The biggest uncerof f on tainty is the "on" conductance since the spacing depends on the quality of the surfaces. In

2 order to transfer 5 watts at 15 K with a AT of 1 K, a surface area of 11.4 cm is required

with a gap of 5 ym. An apparatus to test the heat conductance across a helium gap was designed and built.

However, a shift in emphasis from heat switches to refrigeration materials prevented a test

of the helium gap heat switch.

An actual working heat switch would have a cycle time of about 1 second so that the

heat absorbed from one surface during the first half second can actually be stored in a movable plate with a high heat capacity. During the next one half second the plate is moved

to make contact with the upper reservoir and transfers its stored heat to it. Thus a simple

solenoid could move the plate from one position to the other. A 200 g movable plate of lead

could be used for an upper switch but a lower switch would require as much as 8 kg of lead.

Thus such a helium gap heat switch would only be practical for the upper switch.

15
2.3. Magnetothermal Heat Switches

2.3.1. Survey of various metals . The thermal conductivity of a metal is composed of the lattice and the electronic term,

i.e., k = k^ + k^. The electronic contribution can be reduced considerably in many metals

by the application of a transverse magnetic field. The amount of reduction is proportional

19 to the electrical magnetoresistive effect. A reduced Kohler diagram of the transverse

magnetoresistivity of most metals is very helpful in selecting those metals with high magnetoresistive effects. A high magnetoresistive effect also reduces the eddy current heating. However, additional conditions must be met to obtain a large change in thermal con- ductivity with a magnetic field. The zero field thermal conductivity, which should be high,

is proportional to the residual resistivity ratio (RRR = P 2 95/p o^ ' w^ere p o^ s usua^y

measured at 4.2 K. The lattice thermal conductivity, which should be low, is given theoret20 ically by the expression

3.1 x 10 k

00 2 k

l = 2 2 T (W/Cm K)

' GN

o %

where k is the constant thermal conductivity at high temperatures, e.g., room temperature,

G is a numerical factor which is about 70, is the number of free electrons per atom, and

6^ is the Debye temperature at 0 K. From this expression it is obvious that a high 9^ will

give a low k^. The important properties of a metal which would make a good heat switch are summarized as follows:

(1) high magnetoresistive effect

(2) high residual resistance ratio (RRR > 1000)

(3) high Debye temperature, 6 > 300 K. o

Even though some materials, such as bismuth, have extremely high magnetoresistive

effects, they are inappropriate as heat switches since they do not satisfy conditions (2) and (3) above. Table 1 lists the various metals which are known to satisfy all three con- ditions above and therefore would be potential heat switch materials.

Table I. A summary of metals which have the potential for making good magnetothermal

heat switches in the 4-15 K range. The metals are listed in order of decreasing magnetoresistive effect.

highest RRR 9 sufficient literature

Metal ^ o to date (K) data on k(T,H) exists

Ga 2 x 10 324 yes

Be 3 x 10

3 1160 no

Mg 10

6

- 10

7 342 no W 3 x 10

4 405 yes

3 Fe 3 x 10 464 no

3 Ru > 10 600 no

16
Of the six possible candidate materials only Be and possibly Ru have high enough 6 q to be

potentially useful as the upper switch where the "on" temperature is about 15 K. The other

materials would be useful only for the lower switch where the "on" temperature is 4 K. Until now, only Ga and W had been measured in enough detail to predict the heat switch

performance. In fact Ga has already been used as a heat switch in the 1-4 K temperature

21 range. Our first efforts were then to make a switch of Ga. Gallium ; Figure 6 shows the behavior of the thermal conductivity of gallium as a function of temperature for various transverse magnetic fields. The H = 0 curves are from

22 21 the work of Boughton and Yaqub, and the H ^ 0 curves are from Engels, et al. In the

"off" state of the switch (H j- 0) the temperature will be 15 K on one side of the switch and

4 K on the other side. The heat flow must be evaluated by integration over the k(T) curve.

2 For k^T the average conductivity is the value of k at about 11 K. For the lower switch,

the switch ratio is then approximately (1/11) k^/k^^ and for the upper switch the ratio is

(1/11) k^/k^. Table 2 lists these switch ratios for gallium as well as for several other

metals

.

Table 2. Measured or estimated thermal conductivity at three different temperatures

and the switch ratio for several heat switch metals. Units for k are W/cm K.

H = 0 H = 14 k Oe Switch Ratios

Metal k (4 K) k (15 K) k (11 K) lower upper

Gallium 200 5.0 0.20 91 2. 3 Beryllium 23 73 0.040 52 166

Magnesium 60 28 1.6 38 18

Tungsten 800 115 0.45 162 23

Iron 50 70 y 0.5 9 13

Ruthenium 50 80 0.1 - 0.3 15 - 45 24 - 73

Two gallium magnetothermal heat switches were made by growing a single crystal of

23

99.9999% Ga in a teflon mold like that described by Yaqub and Cochran. Each crystal had

relatively large diameter discs grown on each end to reduce the thermal resistance at the

bond between gallium and copper. Both crystals broke upon cooldown. The first was subject

to slight tension from the stainless steel support system, whereas the second was subject to compression from the teflon support system. Because of the brittleness and low melting

point (29.9°C) of gallium, and because of the encouraging results obtained with beryllium,

further work on gallium was abandoned.

Tungsten : The magnetothermal conductivity appears to be quite well characterized from

24 25

the measurements by de Haas and de Nobel and by Long. Figure 7 shows the thermal conductivity behavior and table 2 gives the expected switch ratios. The results show that tungsten would make a satisfactory lower switch.

17
1 2 4 6 10 20 40 60 100

Figure 6. The thermal conductivity of single crystal gallium in transverse

magnetic fields^l. The heat flow is parallel to the a axis and

the field is parallel to the c axis. The thermal conductivity in zero field is somewhat size dependent because of the long electron

mean free path^2 #
1 2 4 6 10 20 40 60 100

T,K

Figure 7. The measured thermal conductivity of single crystal tungsten in

transverse magnetic fields 2 ^* 25 . A size dependence occurs at

low temperatures in zero field for the higher purity sample.

19
Other metals : The rest of the metals listed in table 1 have not been measured in

enough detail to permit an evaluation of them in regard to heat switches. Magnetothermal

conductivity measurements were then made on two of these metals (beryllium and magnesium) to permit an evaluation of them. The estimates in table 2 show that it is not fruitful to study iron and ruthenium.

2.3.2. Beryllium . Until now the only measurements of the thermal conductivity of beryllium in a magnetic

26 27 field were those of Griineisen. ' His sample had a resistance ratio, P295/p o' °^ 9^8. He found that the maximum field effect occurred with the heat flow along the hexagonal c axis

and the magnetic field along an a axis such as {1120>. Unfortunately his measurements were

made at only 23.5 K and 81 K. Our measurements on the magnetothermal conductivity of beryllium have shown this material to be superior to all other metals for heat switches in the 4-

15 K temperature range. For that reason a fairly extensive study of its thermal and electrical conductivity was made.

Samples . Two different single crystal beryllium samples were measured. Most of the

measurements were made on a very high purity sample, which we will denote as sample 1. This

sample was loaned to us by Dr. R. J. Soulen of the Heat Division of the National Bureau of

Standards. He in turn received the sample from Dr. W. Reed of Bell Telephone Laboratories.

That sample was one of several that Dr. Reed cut out from a much larger single crystal grown

by Nuclear Metals, Inc. The size of the crystal we received was 3 mm x 3 mm x 25 mm. The

orientation of the crystal was determined from back scattered x-ray Laue photographs. The

hexagonal c axis was along the long axis of the crystal to within one degree. The resistivity ratio, p /p , was found to be 1340.

z. y b u Sample 2 was spark cut from a single crystal disc piece given to us by Dr. S. K. Sinha

of Iowa State University. This sample was 2.3 mm x 3.7 mm x 16 mm with the c axis within

two degrees of the long axis of the crystal. The resistivity ratio, P 1Q r/P n > was found to

295 0 be 79. After the sample was spark cut, it was etched in a solution with composition: 26.5

ml cone. H

2 S0^, 450 ml cone. H3 P04

, 53 g Cr0

3

, held at a temperature of about 70°C. The ends

of this etched sample were then tinned with 99.99% pure indium using an ultrasonic soldering

iron and zinc chloride flux.

Experimental techniques . The thermal conductivities of the two beryllium samples were measured by using the standard technique of observing the temperature drop across a portion

of the sample while a steady heat current flowed through the sample. Carbon thermometers of

1/8 watt size and 220 U normal resistance with a flat ground on one side were used as temperature sensors. Two thermometers were varnished to the sample to measure the AT across it

and one thermometer was mounted on the reservoir close to the sample. The reservoir thermometer then allowed measurements of the thermal resistance at the boundary between the

sample and the reservoir. A constant current source and a digital voltmeter were used to read the carbon thermometers. A germanium thermometer, calibrated by the manufacturer, was

also mounted on the reservoir to calibrate the three carbon thermometers. A capacitance

thermometer, also mounted on the reservoir, served to calibrate the other thermometers in a magnetic field. Between 100 K and 300 K thermocouples of type KP versus Au-0.07% Fe were

20
used for measurements of both T and AT. Published thermocouple tables were used to convert voltages to temperatures.

An aluminum foil radiation shield surrounded the sample and was attached to the reservoir. The reservoir, made entirely of copper, was a 10 cm

3 pot suspended inside a vacuum

can by a 6 mm diameter stainless steel pumping tube. Liquid nitrogen or liquid helium

outside the vacuum can could be let into the reservoir through a stainless steel capillary

tube with a valve in the liquid bath. With such an arrangement the reservoir could be

cooled down to the bath temperature within a few minutes by opening the valve in the liquid

bath while the pumping tube on the reservoir is vented to the atmosphere. Measurements

below the bath temperature were made by closing the needle valve and pumping on the reservoir. Measurements above the bath temperature were made by first closing the needle valve

and applying current to a reservoir heater to boil off the liquid in the reservoir. When

the temperature rose to the desired point the needle valve was opened just enough to provide

a cooling effect to nearly balance the heat input to the reservoir. The carbon thermometers

were calibrated during that procedure. Next the power to the reservoir heater was turned

off as the same power was applied to the sample heater for measurement of the thermal conductivity. This procedure gave rather rapid equilibrium times and usually yielded very

stable temperatures.

The reservoir and vacuum can were made small in diameter so as to fit in the tails of

nested glass dewars. These tails, in turn, were fit between the poles of an iron core electromagnet which could attain a field of 955 kA/m (12 kOe)

.

The high purity sample 1 was thermally anchored to the reservoir with a low temperature

varnish. The heater was attached to the other end of the sample with varnish also. The

thermal boundary resistance of the varnish joint was much higher than the thermal resistance

of the sample in zero field. Thus accurate measurements in zero field were difficult to make because of the necessity for low power levels. That sample was to be used later for

superconducting fixed point measurements and contamination with solder was not desirable.

Sample 2 however was tinned with pure indium on each end as described previously. One end

of this sample was then indium soldered to the reservoir and the heater mounting bracket

soldered to the other end. The indium solder provided much better thermal contact for this

sample.

The electrical conductivities of both beryllium samples were measured potentiometrically

.

Current and voltage leads were pressure contacts to the sample. Pointed screws were used

for the voltage taps. Currents up to 2A were used when the sample conductivity was high.

The same thermometers used for the thermal conductivity measurements were also used to determine the sample temperature during the electrical conductivity measurements.

Experimental Results . The temperature dependence of the electrical resistivity of

sample 1 is shown in figure 8. These data are for the current along the hexagonal c axis

26 29 30

and the magnetic field along the a axis. Previous measurements ' ' in zero field on less pure samples are also shown for comparison. The total resistivity p

fc is conventionally

expressed as

P t = P i + P r (2.D

21
10

St.

Figure 8

id

5

1

1

|

I I I I

| Beryllium

1 1—I

| I I I

l

H=955kA/m (12kOe)

637kA/m (8kOe)

/

^

Reich et al

318kA/m (4kOe)

B

0^

159kA/m (2kOe)

80k A/m (IkOe)

O This Work

R.W. Powell

A Gruneisen and Adenstedt

10 20 50 100 200 500 1000

The electrical resistivity of high purity single crystal beryllium

as a function of temperature in various transverse magnetic fields

Previous work26,29,30 on i ess pUr e samples is shown for comparison

22
where is the intrinsic resistivity due to scattering of electrons by phonons and is

the residual resistivity due to impurity scattering of the electrons. For sample 1 we have

-3 -9 4

p = 3.35 x 10 ufi'cm and p. =1.0 * 10 T ufi'cm. This expression for p. is identical

r 1 301 with that found by Reich, et al. on a less pure sample. For our sample we find p 0

_-. = 3.69 viQ'cm and p__ c , „ = 4.48 yfi*cm. These data are in good agreement with the value

p =3.58 u^*cm found by Grtineisen and Adenstedt.

2/3 • Id K

Figure 9 shows the thermal conductivity of beryllium sample 1 as a function of temperature for several magnetic fields. The heat flow was along the hexagonal c axis [OOOl]

and the transverse magnetic field was along one of the a axes, e.g. [ll20], of the crystal.

26

Such an orientation has been shown to give the maximum field effect on the thermal con- ductivity. Figure 10 shows how the thermal conductivity varies with the angle of the magnetic field within the basal plane. As expected, a 60° symmetry exists. Such a plot was used to find the exact orientation of the a axis before taking the data shown in figure 9. 29

The data taken near room temperature blend very well with the data of Powell above room

26

temperature on his purest sample in the annealed state. The data of Grtineisen and Adenstedt

on a less pure sample is shown for comparison. Their data is for H = 0 and H = 955 kA/m (12

kOe) and is consistent with the present data considering the differences in purity.

For H = 0 and T < 150 K the thermal conductivity of the high purity sample is dominated

by electron conduction. For that range the data are fit very well by the theoretically

expected form

2 -1

W = W. + W = ctT + (BT) , (2.2) e i o

where W

g is the total electronic thermal resistivity, is the ideal resistivity due to phonon scattering, and W

q

is the resistivity due to impurity scattering. The data of figure

9 give a = 1.04 x 10 ^ cmW ^"K ^ and B = 5.7 W cm ^. This was the first beryllium sample

measured that was pure enough to allow a determination of the ideal thermal resistivity.

The total thermal conductivity, k, has a contribution from the electrons, k , and from

the lattice, k , such that k = k + k . The electronic thermal conductivity is so easily

g eg

reduced by the application of a transverse magnetic field that the lattice thermal conductivity can contribute a sizeable fraction to the overall conductivity in sufficiently high

fields. The application of a magnetic field then allows a separation of the electronic and

lattice components of conduction. Several methods exist for separating the two components

from measurements in magnetic fields. In this work we use the method of Grtineisen and

26 Adenstedt in which they write

k(H) = k (H) + k = L To (H) + k , (2.3)

e g e g

where L is the electronic Lorenz number and a (H) is the electrical conductivity in a mage netic field. In this expression Legand k are assumed independent of magnetic field. A plot of k(H) versus Tcr(H) for different field strengths yields a rather straight line for

H > 150 kA/m (2k0e) . Thus L and k are determined simultaneously. This technique was used

e g

23
E

U

^-Extrapolated Latti

Conductivity

Gruneisen and Adenstedt

(RRR = 948)

O This Work

(RRR = 1340)

10 20 50 100 200

T,K

Figure 9

.

The thermal conductivity of high purity single crystal beryllium

as a function of temperature for various transverse magnetic fields

The work of Gruneisen and Adenstedt^^ i s shown for comparison.

24
E

u

E

16

15

i r

b — A x i

30 50 70 90 110 130 150 170 190

<f> , deg

Figure 10. The thermal conductivity of single crystal beryllium at 2.1 K as

a transverse magnetic field of 796 kA/m (10 kOe) is rotated about

the basal plane. The angle 0 is an arbitrary angle.

25
at several temperatures to obtain the lattice conductivity shown in figure 9. For T < 30 K

for the lattice conductivity can be expressed as

k

g

= YT

2

, (2.4)

-4 3 31 -4 3 where y = 1-6 x 1° W/cm K . Powell, et al. estimated y = 2.0 x 10 W/cm K from

thermal conductivity measurements of beryllium alloys. The value of y is lower than that

of any other metal and is to be expected because of the high Debye temperature of beryllium.

For temperatures below about 20 K the thermal conductivity of this high purity beryllium sample can be changed by over three orders of magnitude with a magnetic field of 955

kA/m (12 kOe) . This is roughly equivalent to going from the thermal conductivity of high

purity copper to that of stainless steel. Table 2 lists the heat switch ratios which could

be obtained with such a sample. As this table shows, beryllium is the only metal which can be used as the upper switch. It can be used as the lower switch also, but for that switch

tungsten would be just as good.

The relative change of electrical resistivity at 4 K of beryllium sample 1 is shown as

32 n

a Kohler diagram in figure 11. This figure shows that Ap/p <* H , where n = 2.00 ± 0.02.

o This field dependence agrees with the theoretical value n = 2 for a compensated metal with

no open orbits. Also shown in figure 11 is the relative change in electronic thermal

2 resistivity. This change is nearly proportional to H also, except that the coefficient of

2 H is smaller than for Ap/p^. The difference, however, is about within the experimental

uncertainty of AW^/W^O). The biggest uncertainty is in W

e

(0), the electronic thermal

resistivity in zero field. The rather small value of W

e

(0) at low temperatures along with

the large cross section of the sample made it difficult to measure w

e

(0) accurately. The

uncertainty in W

g

(0) for T < 70 K is about ± 25%. At higher temperatures and in magnetic

fields the thermal resistivity increases considerably and the uncertainty eventually drops

to about ± 5%.

2 The fact that AW

e/We

(0) is proportional to (H/p Q

) points out the importance of using

samples with a low p Q

, i.e., high resistance ratio. A lower p Q not only means that a higher ratio between k(H = 0) and k can be attained but also that a lower field can be

g

used to make k(H) approach k , the lattice thermal conductivity. Such behavior is evident

g

in figure 12, which shows the magneto-thermal conductivity of sample 2, a less pure beryllium single crystal (RRR = 79) . The thermal conductivity for H = 0 is considerably

lower than for sample 1, but also the thermal conductivity for H = 955 kA/m (12 kOe) is

considerably higher for this sample than that for sample 1. Beryllium crystals with resistance ratios as high as 3000 were made several years ago

for a government contract, but these samples (our sample #1 is one of those) are now essen- tially all gone. About $5 , 000-$15, 000 would be required to establish facilities for growing more of these crystals.

2.3.3. Magnesium

The magnesium sample measured in this work was kindly grown for us by R. Reifenberger

of the University of Arizona. It was a rather small crystal with dimensions 0.5 mm * 1.0 *

5.0 mm. The long axis was along the [ll20] direction and the rectangular sides were {0001}

26
O &p/pQ

a AWe /We (o)

i i i i i i i i i i i iii 12 5 10 20 50

H/yo 0

, 10

13 A/fim

Electrical and thermal KohsLer diagram for high purity single

crystal beryllium at 4 K. The relative change in electrical

resistivity, Ap/p , and of the electronic thermal resistivity,

AW /W (0) , are shown as a function of magnetic field divided

e e by the residual electrical resistivity.

27
1 5 10 50 100 300

TEMPERATURE, K

Figure 12. The thermal conductivity of low purity single crystal beryllium as

a function of temperature for various transverse magnetic fields.

The lattice conductivity derived from the measurements on high

purity beryllium is shown for comparison.

28
and {1010} type crystal faces. The resistance ratio of similar crystals were greater than

6 33

10 , which makes these crystals among the purest substances known to man. These crystals were grown from the vapor phase by a vacuum sublimation technique.

33 Copper was sputtered

onto the ends of the sample to provide a base for indium solder.

The crystal was indium soldered to the copper reservoir of the thermal conductivity

apparatus described previously. The holder for the heater was indium soldered to the other

end. Small copper spots had been sputtered on the side of one of the {0001} faces but it was not possible to tin those spots. Consequently, a low temperature varnish was used to mount the two carbon thermometers on the {0001} face. These thermometers were mounted by

first varnishing them to a piece of 0.05 mm thick copper foil. The copper foil had a narrow tab (*V 0.4 mm) which was wound around the sample and varnished in place. The two thermometers were placed 2.1 mm apart on the sample. Because of the small size of the

sample the measurement inaccuracy was at least 15%. In zero magnetic field where the

thermal conductivity is extremely high the inaccuracy is even greater.

Figure 13 shows how the thermal conductivity of magnesium at 4 K varies with the angle

of the magnetic field. In this case the heat flow is along the [ll20] axis (a axis) and

the field is rotated in the plane perpendicular to this axis . The minimum thermal conductivity occurs when the angle of the magnetic field is halfway between the c and b axes. A somewhat larger transverse magnetic field effect may occur for heat flow along the hexagonal

26

c axis as in the case in beryllium. The data in figure 14 were taken with the magnetic

field aligned so as to give the minimum thermal conductivity. The zero field thermal

conductivity below about 10 K was so high that it could hardly be measured in this small

sample. The data above 30 K agree to within experimental error with previously published

data for H = 0. Until now there has been no measurement of the thermal conductivity of

magnesium in a magnetic field. The thermal conductivity shown in figure 14 for a given

magnetic field is not as low as those for beryllium, gallium, or tungsten. However, even

at 955 kA/m (12 kOe) there is no indication that the thermal conductivity is approaching

the lattice conductivity. Thus it may be as much as an order of magnitude below the highest

field curve in figure 14. For heat switches made of very high purity magnesium, the switch ratios would be

approximately those given in table 2. As is evident from this table, magnesium would not

make as good a heat switch as beryllium or tungsten.

2.3.4. Thermal conductance of joints

Any low temperature apparatus is composed of many parts. In the electrocaloric refrigerator there will be a 15 K reservoir, refrigeration element, heat switches, and load.

Each of these parts must be joined together in some fashion that will allow large amounts

of thermal power to be transferred between these parts. An improperly designed joint may

have a thermal resistance which could be larger than most other resistances in the system.

The electrocaloric refrigerator has a cyclic operation mode so that the mass of material

connected to the cyclic element must be kept small enough to prevent its heat capacity from

thermally overloading the system. This is accomplished by using a small amount of high

purity copper for the transfer of heat. The size of a magnetothermal heat switch also

2 9
E

o

60 80 100 120 140 160 180 200 220 \

b-axis 0 deg

b-axis

Figure 13. The thermal conductivity of single crystal magnesium at 4.3 K as

a transverse field of 955 kA/m (12 kOe) is rotated through the

arbitrary angle 0.

30
Magnesium

1 5 10 50 100

T, K

Figure 14. The thermal conductivity of single crystal magnesium as a function

of temperature for various transverse magnetic fields.

31
needs to be kept small for the additional reason that the magnet size should be small. The

small volumes of copper and of the magnetothermal switch material mean that the surface area of the joints will necessarily be quite small and therefore may have high joint resistances . Figure 15 is a compilation of existing data on the thermal conductance of solder

joints, grease joints, and pressure joints. In the case of the pressure joints the thermal

conductance, in units of W/K, is for the whole joint, independent of the surface area. For

2 the grease and solder joints the thermal conductance is for 1 cm . The curves shown in figure 15 are taken from the following references

:

Solder joints (T < IK): ref. 34,35

Solder joints (1 K < T <4 K) : ref. 36,37

Solder joints (room temperature) : ref. 38

Grease and adhesive joints: ref. 34,39-42

Pressure contacts: ref. 12,14,41-43.

It is rather amazing that there are no data in the literature on the thermal conductance of solder joints in the region between 4 K and room temperature. But rough interpolations (shown by the dashed lines in figure 15) , based somewhat on values of the bulk

thermal conductivity of certain solders, suggest that solder joints are superior to all

other joints in the region above 4 K. Because of the importance of and uncertainty of the thermal conductance in solder

joints, measurements were made on such joints in this work. Indium solder was chosen since

it has a higher thermal conductivity than alloy solders. Indium of 99.99% purity was used

for these tests. Two joints were measured. The first was between beryllium sample 2 and

an ETP copper surface. The second joint was between an annealed 99.999+% pure copper

sample and the same ETP copper surface. Each joint was made by first tinning the two

surfaces with generous amounts of indium. The parts were than pushed and rubbed together

as heat was applied to the joint. The excess indium was trimmed off with a razor blade.

The resultant joint thickness was on the order of 25 ym. Figure 16 shows the thermal conductance of the two indium solder joints. These con- ductances were measured during the same time the thermal conductivity of each sample was

being measured. It was necessary to extrapolate the temperature gradient in the sample to the surface. This correction was less than 10% for temperatures below 30 K with the annealed

99.999+% pure copper sample. For the beryllium sample, however, this correction amounted

to as much as 90% for temperatures around 10 K. Thus only the high temperature data for

beryllium is fairly reliable. With that in mind, the two sets of data (beryllium and

copper) are in good agreement with each other. This agreement shows that even though it is

difficult to wet beryllium with the indium, successful solder joints can still be made to

it. The thermal resistance of the joint is more than an order of magnitude larger than

that expected across a 25 ym thick pure indium sample. The increased resistance, which is

typical in solder joints, is probably due to imperfections both in the solder layer and in

the actual bond.

32
Figure 15. The thermal conductance of grease, adhesive, solder, and pressure

joints as a function of temperature. The conductance is per cm^

of contact for the grease, adhesive, and solder joints. However,

the curves for the pressure joints give the total conductance of

the joint, independent of the area, but dependent on the total force

applied. The dashed lines are our estimates of the conductance in areas where no data exist.

33
evj

E

500

200

100

5 0

20

10

- Normal

1

1 I I I I I I I I I I I I

o Cu — In —Cu

Be — In —Cu

Superconducting

I 1

I 111 L

10 20 50 100 200

T, K

Figure 16. Thermal conductance as a function of temperature for two indium

soldered joints. The normal state is produced by applying a magnetic field.

34
The indium joint conductance in the 4-15 K temperature range is considerably higher

than estimated in figure 15. In order to transfer 5 W of power with a AT much less than 1 K

. . 2 would require a joint area on the order of 0.1-0.5 cm .

2.3.5. Magnet requirements

The magnetothermal switches require a transverse magnetic field on the order of 1200

kA/m (15 kOe) to operate them. This is low enough that iron can be used for most of the

magnetic flux path. A gap in the iron on the order of 1 cm in length would be large enough

to fit around, but not touch, the heat switch material. The magnet must be able to operate

at 15 K, since it will be thermally anchored to the upper reservoir. Even if it were attached to the load, it would have to start operation at 15 K. Other requirements for the

magnet are that the heat load to the 15 K reservoir be small compared with 5 W and that the

rise time of the magnetic field be much less than 1 s. The magnet wire could be made superconducting if Nb

3 Sn were used. It has a transition

temperature of about 18 K. The only heat load would be from heat conduction down the leads

but this could be made much less than 5 W. It should be possible to apply or take off the

field in a Nb^Sn magnet in less than one second in light of the present superconducting

magnet technology. However, tests were not made of the possible cycle times in a Nb

3 Sn

magnet.

For resistive windings the power dissipation in an air core solenoid can be expressed

44 as

w = fc]

2

A (Gj ' (2.5)

where p is the resistivity in ohm-cm, r is the inside radius of the magnet coil in cm, A is

the fraction of coil volume occupied by conductor, H is the field in Oe, G is a geometry

factor typically in the range 0.15 to 0.2, and W is the power in watts. For commercial

purity copper at 15 K, for r = 0.5 cm, and for H = 1200 kA/m (15 kOe) , the power dissipation

is about 200 watts. The use of an iron core with a 1 cm gap can reduce the power to about 2 watts. High purity copper wire could reduce the power dissipated by another order of magnitude. A commercial iron core transformer was modified to make a C-type magnet capable of

generating the required fields. The copper wire in the coil was 0.5 mm diameter. The rise

and fall times for generating the field were on the order of 0.2 s. The magnets for the magnetothermal heat switches could be made of copper windings and

it would be certain to work. The power dissipated in that case would be higher than desired

unless high purity copper were used. The use of Nb Sn would probably be better but there is

the uncertainty of how fast the field can be changed.

35
3. REFRIGERATION MATERIALS

3.1. Theoretical

3.1.1. Thermodynamic s The general TdS equation for a dielectric material is

TdS = C„dT + T(3P/3T)„dE, (3.1)

where C is the specific heat in the electric field E and P is the polarization of the

sample. The refrigeration absorbed during the isothermal step from E^ to E^ in figure 1 is

given by

QL

= TlAS = T

1 (3P/3T) dE. E

(3.2)

The temperature change during the adiabatic step from E^ to E^ is best determined by equating the entropy changes along a fictious path from E^ to at the temperature T^ and then

from T to T^ along the field line E^. These entropy changes are given by

dT = (3P/3T) dE.

2 (3.3)

The specific heat in an electric field is given by

45

= C + T ,

I Ut2 o

E^2

3 P

dE, (3.4)

where CQis the specific heat in zero field. Equations (3.2) to (3.4) point out the fact

that the thermodynamic behavior of a dielectric material can be determined entirely from

CQ

(T) and P(T,E). This fact can be visualized by considering the entropy in figure 1 as a surface in (T, E) space. The entropy at a point (T^, E^) is found by integrating (3.1) from

(0, 0) where S=0. One may choose any integration path, but the path considered at the

moment follows the E=0 plane from T=0 to T=T^ and then along the T plane from E=0 to E=E

1

- Each of the two segments of the path use only one of the terms in eq. (3.1) because first

the field is held constant and then the temperature is held constant. The total entropy is

then given as

S(T

1 ,E1

) =

1 c„

^dT +

T

0

3P

3Tj

dE

T1# E

36
The dielectric constant e of a dielectric is given as e tion can be derived from the dielectric constant by the expression

(3P/3E) T

so that the polarizaP(T,E) = e K' (T , E

' ) dE 1 + P(T,0) , (3.5)

where eQis the dielectric constant of free space and K

1 is the relative dielectric constant. The last term accounts for any remanent polarization in the material. A material

with any remanent polarization is normally referred to as a pyroelectric. If the direction

of polarization can be reversed, then it is called a ferroelectric. Dielectric absorption,

or the storage of charges, can also make a contribution to any measured remanent polarization. In addition, at low temperatures there may be some very long dipole relaxation times

which can given rise to a measured remanent polarization. These last two contributions were

not considered at the beginning of this program. Much has been learned about the charge

storage and relaxation mechanisms during this project, but there still remains much to be

learned. The presence of a remanent polarization usually implies irrversibility in P(T,E).

Therefore, P(T,E) would no longer be a state function from which other thermodynamic properties could be derived. Irreversibility in P(T,E) would also give rise to hysteretic

heating.

The temperature-entropy diagram of a dielectric material can also be determined en- tirely from specific heat measurements without recourse to polarization measurements. This can be done by choosing the integration path for eq. (3.1) to be first along the T=0 plane

In that case the total

x " i entropy is given as

from E=0 to E=E^ and then along the E^ plane from T=0 to T=T1<

S(T

1 ,E1

) =

• E

l r 3p'

3T

dE + E,T=0

T

l CE. dT,

which must be the same as that derived from any other path. The third law of thermodynamics

requires S=0 at T=0, so the first term for S(T, ,E,) must be zero, i.e., (3P/3T)„ = 0 at T=0. 11 & The entropy in any electric field is then simply given as

S(T,E) = T c

rrr dT'

0

(3.6)

To use this type of expression for the entropy requires measurements of the specific heat in

electric fields down to temperatures much less than T^ in figure 1. In that case, the

uncertainty in the entropy due to the extrapolation of the specific heat down to 0 K is

usually negligible. Equation 3.6 includes the lattice entropy, so that field-induced entropy

changes small compared with the lattice entropy would be difficult to detect from eq. (3.6).

Because of that reason and because specific measurements are time-consuming, polarization

measurements were used more often in this work to find AS upon a field change via eq. (3.2).

The uncertainty in AS determined from 3P/3T is unaffected by the size of the lattice entropy

37
Large entropy changes upon a change in the field will occur at temperatures not too far

removed from a transition. The entropy curves shown in figure 1 are indicative of a zero field transition near T^. Such a transition in a dielectric could be associated with a change from a ferroelectric or antiferroelectric state to the paraelectric state. The

minimum entropy change required at T^ can be roughly estimated. In going from T^ to T^ in

figure 1, the dielectric material must be able to cool itself as well as metallic heat

conductors and any inactive dielectric. If we assume that the metallic parts and the inactive dielectric occupy about 10% of the total volume of the refrigeration material, then the

3 entropy change in the active dxelectric would be on the order of 1 mJ/K cm in cooling down

the other materials from 15 K to 4 K. The lattice entropy at 15 K of the active dielectric

would also be about 1 mJ/K cm^. Therefore, the entropy available at T needs to be somewhat

3 larger than 2 mJ/K cm if any isothermal refrigeration is to be performed. In terms of the

-2

gas constant we require > 10 R. If the cool-down from 15 K to 4 K is to be done in a regenerative mode or in several stages instead of one, then the entropy at 4 K can be smaller

-2 3 than 10 R. Since the lattice specific heat of the dielectric material increases as T , the

upper limit of refrigeration cannot be much above 15 K.

3.1.2. Criteria for materials selection

The previous section established the thermodynamic groundwork upon which criteria were established for selecting materials to try for electrocaloric cooling effects. It should be

mentioned at this point that much was learned during the course of this project in regard to

the criteria for selection of materials. For instance the original set of criteria was not

adequate and the material (SrTiO^ glass-ceramics) originally chosen turned out to be

unsatisfactory

.

Originally it was thought that the P(T,0) term in eq. (3.5) would be zero or negligibly

small for a paraelectric or antiferroelectric material. In that case the dielectric con- stant could be used to find P(T,E) and the related thermodynamic properties. From eq. (3.1)

it is evident that large refrigeration effects require a large 3K'/9T. The original set of

criteria then consisted of the following points: (1) a large zero field 9K'/3T in the 4-15

K temperature range, (2) the term 9K'/9T must remain sufficiently high up to the maximum

field to be used, (3) a high breakdown strength to provide wide limits of integration in eq. (3.2), (4) negligible hysteresis in K' (T,E) , and (5) the phonon-dipole relaxation time be

much less than 1 s. Experimental results before the program began on research samples and

on small plant manufactured samples of SrTiO^ glass-ceramics indicated that all these con- ditions could be met. However, only the plant-manufactured multiple layer samples could

give the high breakdown strengths. Considerable effort was then put into making large

2 (about 5 cm area) multilayer samples with process variable that maximized both 9K'/9T

at zero field and the 4 K breakdown strength.

After it was found that cooling effects did not exist at 4 K in the SrTiO^ glass ceramics, measurements of the DC polarization were made on these samples. These measurements

showed a relatively large P(T,0) term in the total polarization. The resulting temperature

dependence of this total polarization turned out to be considerably different from that

derived from the dielectric constant at ac frequencies greater than 100 Hz. The dc

38
polarization showed zero temperature dependence at 4 K in the SrTiO^ glass-ceramics, consistent with the electrocaloric measurements. A negative slope in P vs T did occur in the 10-

15 K temperature range and some cooling effects were observed there.

Several variations in the SrTiO^ glass-ceramics were then tried to try to understand

the origin of the remanent polarization. These variations included changing the crystallization temperature, which changes the SrTiO^ crystallite size and changing the composition of

the glass phase. It was felt that these variations would determine if the remanent polarization came from the glass phase or the SrTiO^.

We also decided to measure the DC polarization and the electrocaloric effect in several

other candidate materials. The hope was to find a material with a large enough 8P/3T at

4 K to make an effective refrigerator. The study of the other materials would also allow

us to obtain a more general understanding of the behavior of dielectrics in an electric

field. In general, the alternate materials chosen had high dielectric constants which

showed a temperature dependence near 4 K. Several of the materials chosen were reported in

the literature to have a transition to either the ferroelectric or antiferroelectric state

in the vicinity of the 4-15 K temperature range. Various solid solutions in SrTiO^ and

KTaO^ were tried in the hopes that such solutions may lower the temperature at which the dc

polarization still shows a temperature dependence.

3.2. Previous Work

4 The first measurements of the electrocaloric effect were done in 1930, only four

46

years after the first measurements of the magnetocaloric effect. The magnetocaloric

effect was utilized for adiabatic demagnetization below 1 K within a few years after the

first measurements. However, further work on electrocaloric effect was not done until some twenty years later, probably because so little was known at that time of the dielectric

behavior of materials. After the ferroelectric state was better understood, several more

:ic 48

47

measurements of the electrocaloric effect were made on such materials. The electrocaloric

measurements on these ferroelectric materials were all done near room temperature. Granicher

first suggested the use of SrTiC>

3 for electrocaloric cooling at low temperatures. His

suggestion was based on his dielectric constant measurements of SrTiO^. This material has

a large negative temperature derivative around 20 K even though it does not become ferro49 electric. At 3 K and below the dielectric constant of SrTiC>

3 is independent of temperature. That behavior is consistent with Barrett's theory50 and means that no transition to the ferroelectric state occurs. Hegenbarth51 was the first to make electrocaloric effect measurements on SrTiC^,

which in that case were ceramic samples. With an electric field of 8 kV/cm, he saw adiabatic

depolarization cooling of 6 mK at 78 K and 60 mK cooling at 17.5 K. Somewhat larger effects

52 53 were seen in later measurements ' on single crystal SrTiC>

3

, but these cooling effects

disappeared below about 4-5 K. The temperature changes observed in the single crystal

SrTiO were in reasonable agreement with values calculated from the specific heat and the

52 54

observed dielectric constant versus temperature and electric field. ' The uncertainty

in the calculation was dominated by that of the specific heat, since no accurate specific

heat data existed in the liquid helium temperature range.

39
Electrocaloric cooling at still lower temperatures was first demonstrated in OH doped

KC1 by three independent investigations. 55 57 Cooling to 0.36 K from a starting temperature

of 1.3 K was reported.

57 Various theoretical studies^ 5 ' 58

and experimental investigations59 '^ 0

on other doped alkali halides followed shortly. Cooling to as low as 0.05 K was demonstrated

60 61

in CN doped RbCl. Electrocaloric refrigeration using KC1:0H has been used for thermostating crystals below 1 K while they were being radiated with short light pulses. Patents

have been issued^ 2 ' 63

for electrocaloric refrigeration with doped alkali halides as the

refrigerant.

Of all the materials studied previously, none showed large enough electrocaloric effects

to be useful as a refrigerator in the 4-15 K temperature range. The doped alkali halides

appear to be quite powerful for temperatures below 1 K. However, because of the very low

20 21 3 ordering temperature and the limit of about 10 -10 impurities per cm , these materials

have only very small cooling effects in the 4-15 K range. The studies on SrTiO^ (single

crystal and ceramic) indicate that material is better suited for cooling at higher temperatures than the doped alkali halides. Unfortunately it tends to order at too high a temperature so that almost no cooling is available at 4 K. Development of a practical 4 K electrocaloric refrigerator was then dependent upon the discovery of a more suitable material.

3.3. SrTiO^ Glass-Ceramic Research Samples

3.3.1. Dielectric properties

A new class of dielectric materials were discovered recently by one of the authors

6 (WNL) . The dielectric properties of these materials in the 1-20 K temperature range appeared

ideal for an electrocaloric refrigerator operating in this temperature range. A patent"*"0 was

issued for an electrocaloric refrigerator utilizing such materials. These new dielectric

materials are the SrTiO^ glass-ceramics. The useful glass-ceramics are select compositions

in the SrO-Ti0

2 -Nb

20,.-Al202-Si02 system, where the Al

20^-Si02 component is the glass phase.

The initial work on these materials was done with bulk material ground down into thin sheets.

The field of the glass chemistry of crystallized glasses has grown steadily following the

64 work of Stookey at the Corning Research Laboratory, and the reader is referred to the

literature^' 55 for a detailed account of the preparation of these materials. In brief,

crystals of SrTiO^ about 5-10 ym in size are grown in situ at about 50 volume percent in an alumino-silicate glass matrix at temperatures near and above 900°C

The SrTi0

3 glass-ceramics exhibit large changes in the dielectric constant with respect

to temperature below 20 K, as shown in figure 17. For some research samples 3K'/3T was as high as 25 K . The positive value of 3K'/3T at low temperatures is in marked contrast to

49 54 66-69

that found in single crystal SrTiO^/ ' ' which has only a negative derivative. The

dielectric constant in figure 17 follows a Curie-Weiss law from room temperature down to approximately the temperature of the peak at 33 K. The loss tangent (tan 6) has a maximum at

20 K. The lattice parameter data from the two diffraction lines shown in figure 17 were

identical above 77 K but showed a splitting at and below 50. This structural transformation

was tentatively identified as cubic-*tetragonal, but the data were too limited to determine

the c/a ratio. Measurements of the loss tangent as a function of frequency indicated the

dipole-lattice relaxation times were much less than 1 second.

40
400

3.904

3.900

3.896

50 100 150 200 250

TEMPERATURE, K

Figure 17. Dielectric data (K' and tan 6) and lattice parameter data (a)

measured on a SrTiU3 glass ceramic from 2 to 250 K. The CurieWeiss behavior of K' extends to, and above, normal temperature,

41
A serious problem with the bulk samples was that the breakdown strength was extremely

low (a few hundred V/cm) due to minute cracks in the ground glass. Thus the hysteresis and

field dependence of 9K'/3T could not be measured in these samples.

3.3.2. Relation to capacitance thermometer manufacturing

A similar SrTiO glass-ceramic material has been developed previously for a low tempera6 ture capacitance thermometer. This material has a value of 3K'/<)T which is nearly independent of temperature over a wide range, but the value is much less than 25 K \ The thermometer-manufacturing process is similar to the process used to fabricate multilayer samples

to be discussed in the next section. With this process breakdown strengths in excess of

300 kV.cm have been achieved.

Hysteresis measurements, shown in figure 18, were made on the thermometer type samples

at 4.2 K by numerically integrating K' (E) data measured at 100 Hz. As the curve indicates,

very little hysteresis is present except for the region below about 40 kV/cm. From such

results the polarization appears to be essentially reversible as required for the thermodynamic

analysis.

2 An electric field reduces 3K'/9T rapidly as shown in figure 19, although calculations

based on the results in figure 19 show that the refrigeration power is sufficiently high for

practical use, provided that OK'/3T) i s on the order of 25 K ^

.

E—

0

The problem which had to be investigated in this project was the use of the research

material with high (9K'/3T) in the multiple layer process to provide the high breakdown

E—

U

strength. Such a step would also be a test of the prototype manufacturing process. The

material made in this process was then expected to meet all the criteria discussed in section

3.1.2.

3.4. SrTi0

3 Glass-Ceramic Multilayer Samples

3.4.1. Fabrication method

The term "glass-ceramic multilayer sample" refers to a capacitor fabricated using the

Corning Glass K II process. Essentially it consists of alternate layers of glass-ceramic

and precious metal electrodes sealed under heat and pressure to form a monolithic unit. The ease of forming using this process far exceeds conventional ceramic methods and was, therefore, a good candidate process for use in the fabrication of the proposed refrigerator

elements.

The initial phases of the fabrication process were performed at Sullivan Park Research

and Development Laboratory, Corning, New York. The constituent oxides and/or carbonates were mixed by J. E. Sage in a mason jar in 1000 g batches, then melted by the Melting Technology

Department in platinum-crucibles, cocktail-mixed, and quenched between two water cooled

rollers to produce a tinted ribbon glass which was slightly reduced due to the high melt

temperature. The ribbon glass was subsequently oxidized by heating to approximately 700°C

in air for 3-4 hours which is well below the nucleation temperature. The oxidized glass was

a lighter shade of yellow to greenish-brown. This concluded the portion of work which was performed at the laboratory in Corning, New York. The ribbon glass was next shipped to the

Capacitor Product Development laboratory at the Corning Glass Works Plant in Raleigh, North

Carolina.

42
0 100 200 300

E. kV/cm

Figure 18. Polarization vs. electric field at 4,2 K for SrTi03 glass-ceramic

from integration of dielectric constant data.

43
0 50 100 150

FIELD STRENGTH, kV/cm

Figure 19. Empirical dependence of 9K'/3T on field strength for three glassceramics, 2 < T < 6 K. The solid curve through the points is a plot of the g(E) function shown. Samples cerammed at three

different temperatures were measured.

44
The work in Raleigh was performed by A. J. Morrow, R. P. Ruddock, and W. D. Claborn.

The first step was to grind the ribbon glass in water in a 1.33 gallon carburundum ball mill

with Al

2°3 cylindrical media. A grind of 44 hours at 60 rpm reduced the glass to a particle

size of 2.75 ym. It was dried overnight, mixed with an aqueous binder system, and ball

milled for 4 more hours. The resulting slurry was de-aired for 16 hours and cast into a thin

film which, when fired, was approximately 28 ym thick.

In order to produce capacitors, a wet stacking method was used in which the electrodes

are screened onto the dielectric film. The screen is shifted after each application to effectively produce several capacitors in parallel. The electrode metals used are listed in

table 3.

TABLE 3

Manufacturer 1 s Number

Electrode Metal Compositions3

Weight Percent

Pt Au Pd Vehicle

A-2664 4.5 42 13.5 40

A-2860 49 12 — 39

9545b48 12 — 40

A-2721 — 45 15 40

All inks manufacturered and composition information supplied by

Engelhard Industries, 1 W. Central Avenue, E. Newark, New Jersey.

A different (and less satisfactory) vehicle was used for this ink.

A-2664, Pd-Pt-Au, was used as a standard metal throughout most of the work. Normally, 8 weight percent glass frit was added to the paste using a 3 roll mill. The geometry of the

samples for this project was limited in large part by the screening operation. Since an 11.5

x 11.5 cm area is the maximum practical area which can be screened, the maximum sample which

could be made would be approximately 10 x 10 cm. This size is impractical, however, since

only one sample is produced per stack. Therefore a 1.9 x 1.9 cm sample was agreed upon which

would yield 25 samples per stack and yet give a thermally efficient volume. In the original

design (Fig. 20) a so-called "floating electrode" was inserted in the center of each effective capacitor and a generous "pullback" was allowed. It was thought that it would be possible to apply larger voltages with these modifications. In a later design, the "floating

electrode" was removed, the pullback was reduced to increase the amount of dielectric exposed

to the electric field, and the dielectric thickness was increased. The thicker dielectric

layers were made by pressing 11.5 cm squares of green film together at a pressure of 30 psi.

This procedure allowed a uniform thickness of film to be used and also made it highly

45
FIGURE 20

GLASS -CERAMIC MULTILAYER SAMPLE DIMENSIONS*

Front Cross-Section Side Cross-Section

19.43

14.13 >

2.65 1

19.30

12.54

1.56

•3.38

Floating

Electrodes

ORIGINAL CONFIGURATION

19.05

17.40

-0.83

-1.78

19.05

15.40

1.02

Floati ng Electrodes

Configuration for

Increased Efficiency.

-19.05

17.53

0.76

No Floating

Electrodes

BOTH CONFIGURATIONS :

Glass Thickness: 0.028 mm/pieces

Electrode Thickness: 3.81 urn

ORIGINAL CONFIGURATION :

2 15 effective layers of 0.056 mm, 2.06 cm (15 floating layers)

NEW CONFIGURATION:

Floating: 15 eff. of 0.056 mm, 2.87 cm (15 floating layers)

.002 Type: 15 eff. of 0.056 mm, 3.05 cm

.003 Type: 10 eff. of 0.084 mm, Same area as above

.004 Type: 6 eff. of 0.112 mm, Same area as above

.006 Type: 5 eff. of 0.168 mm, Same area as above

* All measurements in millimeters.

46
unlikely that a pinhole in the film could extend from one electrode to another, reducing the

probability of shorts and increasing the resistance to breakdown under voltage.

At the end of the stacking operation an 11.5 x 11.5 cm stack containing 25 samples was fabricated. The next step effected the removal, through oxidation, of the organic binder,

and the fusing of the stack into a dense glass plate. The first portion of this operation,

"burn-out", was a 20-hour cycle in which the temperature of the stack was very gradually

raised to 350°C, then to 600°C in a drop-bottom furnace. The stack, sandwiched between

layers of mica (to prevent sticking) and stainless steel plates, was then removed from the

burn-out furnace at 600°C and transferred quickly to the press-ceram furnace at 725°C. A

pressure of 100 psi was applied using an air cylinder. The furnace was maintained at 725°C

for one hour, in the standard cycle, then raised at 135°C/hour to 915°C where it was held for

2 hours. The entire assembly, pressure plates, mica, and stack, was then removed and placed

in an oven to cool.

A diamond saw was used to cut the samples apart. They were checked for shorts by dipping

the exposed electrodes in Du Pont 8225 silver and air drying. This procedure made it possible

to determine whether shorts were being formed in the press-ceram or firing operations

.

Due to temperature limitations of the press-ceram furnace, the final firing step was performed in an ordinary box furnace. Variables were rate, soak time, and soak temperature.

The normal procedure was to sandwich the samples between polished plates of fused silica

protected by 2.5 x 2.5 cmx 25 urn pieces of platinum foil to prevent reaction. A weight of

400 g/sample was applied to prevent curling. After firing, the ends of the samples were

ground on 600 grit SiC paper to ensure good electrode exposure. Dupont 8225 was applied to

the electrodes and fired at 620°C for 10 minutes in a drop-bottom furnace.

The ac capacitance and dissipation factor were measured on an automatic bridge at 1 KHz

and 1 VAC. On some samples, measurements were also made at 77 K by immersion in liquid

nitrogen. A teflon fixture allowing the simultaneous immersion of 10 samples was constructed

for this purpose.

Both to optimize properties and to eliminate processing problems a large number of

variations were made in the process during the project. Table 4 is a summary of parameters

varied. The experimentation breaks down chronologically into several distinct matrices

primarily involving a particular glass composition.

The first work done used the 899 FEP glass and was exploratory in nature. After elimination of a host of problems, including batching and film casting problems and the major

problem of substrate adherence during firing, manufacture of the first FEP matrix was carried

out. There were 64 matrix elements altogether including 2 electrode metal, 4 press-ceram, 2 firing rates, and 4 firing temperature variations. Feeling that several of these variables

had been acceptably eliminated, the second matrix, using 899 FEL glass, was carried out using

2 electrode metals and 6 firing temperature variations while holding other factors constant.

This conclusion turned out to be somewhat premature; therefore, a second FEP matrix was fired

in which 3 firing rates, 2 electrode metals, 2 press-ceram cycles, 6 firing temperatures, and

3 soak times were varied. In addition, three configurations were made: 28 um layers with

floating electrodes, and samples without floating electrodes having dielectric thicknesses of

56 and 84 um.

47
Table 4 Glass-Ceramic Multilayer Experimentation Summary

Initial Matrix: Sticking Problem Solved, 9 Samples Tested.

Glass

Firing Temperature:

Press-Ceram:

Firing Rate:

Metals

Floating Electrodes

Firing Substrates:

899 FEP

1075-1200°C

Seal A (Standard)

150°C/Hr. (2 Hr. Soak)

A2664

Zr0

2

, Mica, BN, A1

2 0

3

, Si0

2

, Graphite,

Platinum Foil.

FEP-1 : 132 Samples Tested.

Glass

Firing Temperature:

Press-Ceram:

Firing Rates:

Metals

Floating Electrodes

899 FEP

1100, 1120, 1140, 1160°C

Seals A, B, C, D (Variation in Soak Time

at 725°C and 915°C)

100, 150°C/Hr. (2 Hr. Soak)

A2664, 9545

FEL-1 Porosity Problem First Identified, 78 Samples.

Glass

Firing Temperature:

Press-Ceram:

Firing Rate:

Metals

Floating Electrodes

FEP-2 : 209 Samples

Glass

Dielectric Thickness:

Firing Temperature:

Firing Rate:

Firing Soak Times:

Metals

Electrode Fritting:

Press-Ceram:

899 FEL

1100, 1120, 1145, 1160, 1180, 1215°C

C

150°C/Hr. (2 Hr. Soak)

A2664, 9545, A2860 (Detective)

899 FEP

.001" Layers Floating Electrodes

.002" Layers

.003" Layers

New Artwork (Better Efficiency)

1080-1200°C

30, 60, 90°C/Hr.

2, 8, 20 Hours

A2664, A2860, A2721

0, 8 (Normal), 12% by Weight

Normal (A) , High Pressure, Extended Pressure

FHJ-1 : Processing Matrix, Approximately 400 Samples

Glasses

Firing Temperature:

Firing Rates:

Samples

Burnout:

Metals

Binder

Press-Ceram:

899 FHJ (Primarily), 899 FEL (Control),

899 FEP

1100-1200°C

30, 60, 90°C/Hr. (10 Hr. Soak)

Multilayer (.002"), Bulk Glass, Unelectroded Samples, Small Samples

Normal, Extra Long

A2664, A2721, A2860

Aqueous, Toluene (FEP), Methacel (FEP)

Capacitance Thermometer (DXO) Variations

(involves application of pressure during

cycle and soak times)

AVC Suggested Variations (rates and temperature changes).

48
Table 4 (Continued)

FEL-2 : 75 Samples Tested.

Glass

:

Firing Temperature:

Samples

Metal:

899 FEL

1080-1200 in 10° Increments 60°C/Hr,

(10 Hr. Soak)

.003", .004", .006" Layers

3/4", 1-3/8", 2" Square Samples

A2664

FEP-FHN : Approximately 100 Samples.

Glasses: 899 FEP and 899 FHN (Niobium-Free)

Firing Temperature: 1100, 1115, 1130, 1145, 1160

Samples: .003" Dielectric, A2664 Metal

60°C/Hr. (10 Hr. Soak)

Press-Ceram: Low Temperature (825, 850°C Max. Temp.)

(FEP)

FHS-FID : Boroaluminosilicates , 20 Samples Tested.

Glasses: 899 FHS , 899 FID (Niobium-Free)

Firing Temperature: No Fire, 1100°C

Samples: .003" Dielectric, A2664 Metal.

60°C/Hr. (10 Hr. Soak)

49
A great deal of experimentation was performed in an attempt to solve specific problems

(e.g., excessive porosity). The primary matrix involved in this work used the 899 FHJ glass

(FEL batched using carbonate-free starting batch powders) . Nearly every variable in the

process was varied in this matrix which sought a solution to the porosity problem. This work

will be detailed in a later section.

When it became clear that no process variation was likely to eliminate porosity altogether, a second FEL matrix was fabricated wherein most variables were fixed. A standard

processing procedure was determined at this time: a 60°C/hour rate combined with a 10 hour

soak at temperature using Pd-Pt-Au (A2664) electrode metal. The major variables were firing

temperature and geometry. Samples having dielectric thicknesses of 0.11 mm and 0.17 mm and

external dimensions of 3.5 cm and 5.1 cm square were made for the first time. The larger

samples were never fired but were press-ceramed showing that samples this size could be made.

After the discovery of the irreversible heating effect in multilayer samples, an attempt

was made to reduce or eliminate the effect through glass composition. The first matrix along

this line involved 899 FEP and a niobia-free version of FEP: 899 FHN. Samples were made

using the standard process referred to above.

In order to help pin down the cause of the heating effect, a series of samples were also

made in which the press-ceram peak temperature was varied from 825°C to the normal 915°C.

Some of these samples were fired at low temperatures, for example 1000°C, after press-ceram.

Later, a second matrix was performed using 899 FHS and 899 FID which were boroaluminosilicate compositions with and without niobia, respectively.

A final phase of the fabrication method involved work in Raleigh to bond multilayer

samples together with an eye to the finished refrigeration unit. High purity foils of silver,

tungsten, and copper in 25 \im and 254 ym thicknesses were used along with several bonding

agents: epoxy (Epibond 100-A) , Corning glass code 750 AGB devitrifying glaze, and DuPont

8225 glass gritted organic silver paste. Bonding samples were fabricated from previously

tested multilayer samples. A stainless steel fixture was devised to hold the sandwiched

multilayers together during processing. All foils were used with the epoxy, which was a one component, thermally activated bonding agent. The epoxy was heated to 175°C and spread on the chips which were then held together until hard using the fixture. Producing a thin

uniform layer was the only difficulty encountered. Due to the temperature involved, only

silver foil was tested with the DuPont 8225 and the Corning 750 AGB. Using the same con- figuration, the application of these materials was made easier by the ability to work at room

temperature. The bonded layers were fired in a drop-bottom furnace. The DuPont 8225 was fired at 620°C for 10 minutes; 750 AGB was fired at 350°C and 560°C each for 30 minutes. The

750 AGB glass, supplied in frit form was converted to a slurry using Reusche Oil, an organic

vehicle. Results for all the above techniques were tested by cycling the bonded units between

liquid nitrogen and warm water. The only failures were found in the DuPont 8225 samples

which was therefore not recommended.

It is obvious that a large number of samples were fabricated during this project. The

process involved is a sensitive one and yields often fell below 50% for a given stack. In addition, most of the variations, both in configuration and process, had never been attempted

50
previously. However, among the dozens of problems encountered, and for the most part overcome, two stand out as having the greatest significance, which are considered in detail

below.

3.4.2. Fabrication problems

Two critical problems were encountered in the fabrication of the cooling samples, 1) substrate adherence during the final firing operation, and 2) porosity. The former was aggravated by the fact that the initially recommended firing temperatures were high and

caused considerable softening in the samples. After experimenting with a variety of materials, including mica, BN powder, Si0

2

, Al

2 °3 Powder and substrate, ZxO^, SiC, graphite, and

solid Pt, the idea of using the Pt foil was suggested. Although some sticking was still

encountered, the foil was flexible enough to be peeled off. The solution to this problem

allowed the fabrication of high quality samples, which previously had not been possible.

The problem of porosity was considerably more difficult. Especially when fired at relatively high temperatures (1120°C or above) , pores in the samples increased drastically in

size so that at 1160-1200°C they were easily visible to the naked eye. On the electrode

edges of the samples which had been ground, the pores looked like pits in the surface, but

scanning electron microscope work (Figs. 21-24) showed clearly that their origin was internal.

An intensive effort was made to determine the cause of this porosity in both Raleigh and

Sullivan Park. Diverse theories to account for the formation of the pores led to several

widely varying investigations which break down into roughly three areas, 1) electrode variations, 2) composition changes, and 3) process variations.

As the pores tended to form along the metal electrodes , it was suspected that the electrodes were the cause of the problem. Dr. Andrew Herczog (Sullivan Park) suggested a possible

mechanism whereby the platinum in the electrode could be causing porosity through the formation of PtTiO^ at intermediate temperatures. To test this theory, Pd-Au (A2721) and Pt-Au

(A2860) electroded samples were made. No differences in porosity was observed. In a definitive experiment, samples without electrodes were made which showed the same porosity as electroded samples and shifted emphasis to other possible causes. Dr. Brent Wedding (Sullivan

Park) also did DTA and TGA analyses of electrode pastes and observed no gassing at firing

temperatures

.

Dr. I. Lachman (Sullivan Park) successfully fabricated honeycombed structures of FHN by

an entirely different fabrication process than the one used in Raleigh. However, measurements on these samples at helium temperatures proved disappointing.

Carbonates were used in the original glass melts for electrocaloric refrigeration

glasses (899 FEP, 899 FEL) but not for the capacitance thermometer glass (899 DXO) , a similar

glass which had been used for multilayer samples previously. It appeared from SEM photos

that there was less porosity in the DXO samples than in FEP or FEL samples. A batch was prepared using no carbonates (899 FHJ) having the same nominal composition as FEL to test the

possibility that carbon or carbon-dioxide could be remaining in the glass after melting and

resolidifying the starting materials. This glass was also used for an extensive processing

matrix in Raleigh.

Among the parameters varied, which are listed in table 4, many were part of the pressceram cycle which had not been extensively investigated previously. The incentive for this

31
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research was the observation that DXO produced lower porosity samples than did FHJ. Aside

from a slight difference in composition, the major difference in the processing of this glass

involved the application of pressure and soak times in the press-ceram cycle. Samples of FHJ

produced using the so-called "DXO Cycle" did in fact produce lower porosity samples than

those produced using the so-called "ECR Cycle" described previously. However, these samples

exhibited no capacitance; the electrodes were apparently discontinuous. A search to reproduce the low porosity effect without loss of capacitance was performed by exhaustively

varying all differences between the two cycles. The result was that all variations produced

higher porosity chips with capacitance.

Also involved in the processing matrix was a change in binder system. FEP glass was used and a slurry was developed using a toluene base. The resulting samples showed increased

porosity compared to the normal aqueous batch.

In a third approach, 152 pm sheets of glass 1.27 cm square were fabricated from solid

glass at Sullivan Park by John Sage. Platinum electrodes were sputtered onto both sides of

the squares in the typical pullback configuration. In Raleigh, each square was sandwiched

between two layers of cover glass and 3 samples were press-ceramed at approximately normal

pressure using the standard press-ceram furnace. The resulting samples, which were electrically satisfactory after press-ceram, lost capacitance during the final firing. Apparently the thickness of the platinum was not sufficient to maintain continuity at elevated temperatures. By the time this experiment had been completed, the irreversible heating effect

was discovered by Dr. Lawless in Boulder. It was therefore not repeated using thicker

electrodes.

Other variations in the process were suggested by work done in Sullivan Park by R. J. Colegrove using the Automatic Viscosity Control (AVC) furnace. The rate of increase in

temperature in this device is controlled by a feedback loop to the viscosity of a piece of

sample glass in the furnace. A minimum limit of viscosity can be maintained using this

system. It was discovered that significant crystallization occurred at 960°C, above the

final press-ceram temperature. Therefore, a new press-ceram cycle was devised having 960°C

as its upper limit. No improvement was observed in Raleigh samples using this cycle.

Several samples fired under viscosity control appeared less porous than average and were

measured, however.

Chips without electrodes were made from FHJ and showed the same porosity as FEL. This

eliminated the possibility that the carbonates were causing the problems. Despite the fact

that blisters formed on the surface of samples during firing were shown to be carbon dioxide

by Dr. Hans Holland at Sullivan Park using mass spectragraphic analysis, no evidence of

carbon could be found in the unfired chips.

It is evident from the above that a considerable amount of effort was devoted to solving

the fabrication problems associated with making these larger samples, an effort which involved manpower and resources at Corning Glass works in excess of what the contract called

for. In the final analysis, undeniable progress was made in learning how to make these sam- ples, which involved temperature ranges, composition fields, and plate sizes previously not

attempted

.

56
3.5. Alternate Materials Investigated

3.5.1. Glass-ceramics

Aside from the variations in the strontium titanate aluminosilicate and boroaluminosilicate systems discussed above, one completely new glass ceramic system was investigated:

the potassium tantalate-aluminate system. Work on this system was suggested by results

obtained using a potassium tantalate single crystal and it was hoped that by formulating a glass ceramic, multilayer samples could be fabricated.

The initial work on this system was performed at Sullivan Park by John Sage. Using the same basic processes as were used for the original research samples, compositions in the.

systems KTa0

3 -Al2

C> 3

, KTa0

3 -Al

2

C> 3 -6Si0

2

, and KTa0

3 -ZrC>2 -Si02 were melted in Corning and. the

low-temperature properties measured in Boulder. The most promising ' appeared to be* the

KTaC>

3 -Al2 03 system; therefore, a 10 pound batch of ribbon glass was made by John Sage, with

the assistance of Dr. A. A. Erickson, and shipped to Raleigh for processing into multilayer

samples.

In Raleigh, an attempt was made to process the new glass (899 FIL) in the same manner

as previous glasses. However, it was found that no sealing was taking place in the pressceram operation. Higher and lower sealing temperatures were tried, buc the result in every

case was a plate which showed no glassy properties and which cracked very badly when pressure

was removed.

A possible explanation of this phenomenon based upon experience with similar glasses

was proposed by Dr. R. D. McBrayer, Raleigh. He suggested that a reaction may be occurring

in the aqueous batching operation producing a cement-like ceramic from the glass. In order

to test this theory a non- aqueous toluene-based batch was prepared (the same as was used

with 899 FEP to test the effect on porosity) . The results after press-ceram were essen- tially the same. No explanation for this effect has as yet been advanced and no multilayer

samples could be produced for testing.

3.5.2. Ceramics

After the irreversible heating effect was observed in glass-ceramic samples, it was decided to eliminate the glass matrix and make pure ceramic samples. At first, only the

strontium titanate and potassium tantalate systems were attempted. Later, however, solid

solutions of other oxides in the above systems and new compositions in the lead-pyroniobate

family were made.

The following general procedure was used in the preparation of all ceramic compositions:

(1) The composition was calculated in weight percent from the chemical formula.

(2) Using an "00" size porcelain ball mill with porcelain media, the reagent grade

powders were ground for 6 hours at a speed of 73 rpm. Loading was 350 g powder to 350 ml of liquid media (methanol, water, or acetone).

(3) The mixture was dried and ground with a mortar and pestle, then put into Zr0

2 saggers and calcined.

(4) The calcined material was reground in the mortar and pestle.

At this stage, a decision was made whether to use a cast film approach to make samples

or to press pellets. This depended on the compatibility of the ceramic chemically to the

57
aqueous batch and the volatility of the ceramic expected during firing (e.g., KTaO^ had to be covered in its own powder to minimize K^O loss and therefore was unsuitable for multilayer samples)

.

The process used for multilayer samples without electrodes (often called K-Squares) was

as follows:

(1) The powder above was ground in a "00" size ball mill with an aqueous binder for 6 hours.

(2) Film was cast from the resulting slurry to produce fired film of approximately

33 pm thickness.

(3) Twenty 11.4 cm square pieces of film were laminated using pressure and heat into a solid plate.

(4) The plate was cut into 2.5 samples 1.9 cm square using a razor blade.

(5) Samples were placed on a ZrO^ pallet and prefired at 450°C to remove the binder

then fired in a box furnace. For the strontium titanates, Pt foil was used to prevent reaction between the ceramic and the pallet.

(6) Electrodes were applied using DuPont 8225 silver to the top and bottom surfaces of

the K-Square. A small margin was left around the edges to decrease the possibility

of voltage breakdown. The silver was fired at 700-800°C in a continuous furnace.

(7) Room temperature capacitance and dissipation factor were measured and dielectric

constant calculated.

The process used for pressed pellet samples was as follows:

(1) A 5% by weight addition of Carbowax, dissolved in acetone, was made to the powder.

(2) Pellets, .95 cm in diameter, were pressed from 3 g of material.

(3) The pellets were fired in a box furnace on ZrC>

2 pallets. A special atmosphere was produced when necessary by burying the pellet in its own powder (e.g., KTaO^)

.

(4) The fired pellets were sliced using a diamond saw into rectangular samples approximately 254 ym thick by cutting in a direction parallel to their axis.

(5) The samples were silvered, silver fired, and measured as described above.

X-ray diffraction analysis was performed at Sullivan Park by Dr. Hans Holland on all

successfully fired samples, and determined that the desired phase had been produced in all cases

.

The ceramics produced fall into three general types : strontium titanate and solid

solutions, potassium tantalate and solid solutions, and lead pyroniobate and related com- pounds. The specific processing information for each of these compositions is compiled in

table 5. The SrTiO^ compositions were all fabricated into K-Squares. This appeared to be a reasonable course since no volatile components were included (with the possible exception of

V^Oj.) and considerable experience was available on the batching of this material. It was discovered early that the samples curled during firing due to reaction with Zr0

2 and therefore Pt foil was used (as with the glass-ceramic samples)

.

The KTaO^ compositions were more difficult. A calcination temperature which would

react the K

2 C03 and Ta

2

C> 3 without causing K

2 0 loss had to be determined. It was also clear

58
COMMENTS

Reacted

with

ZrO

pallet.

2

Only

1450°C

firing

was

densely

sintered

.

Curled

during

firing

.

Analyzed

as

solid

solution

.

FIRING CONDITION

4-> +J

0) C O

r-l O rH

rH rH

IXI rH (TJ Pi -H Pi O

CN fci CN O O

iH -U U

ta Pi n

+> C CU O rHrH

rH (TJ •H pi O

fa CN O

-P u

Cu N

-p C 0) O rH

rH rH (TJ •H Ol 0

fa CN o

4J U

CU N

-P C CO O -HrH

rH (TJ •H Pi 0

fa CN O

-P U

Cu N Pt

Foil

On

Zr02

Pallet

Pt

Foil

On

Zr02

Pallet

FIRING TEMPERATURES U —

0 U o E

inm cn

rH »»

U U U X X E

CN CN CN u u u

0 0 0 o o o

in o in In *T

rH rH rH

1450°C

(2

Hr) O ~

0 H O E

in

*3" CN H

CJ 0 H O E m

Tf CN rH «—

u -> 0 u O E

inT (N <H •-»

TEST CONFIGURATION

K-Squares K-Squares K-Squares K-Squares K-Squares K-Squares

CALCINE TEMPERATURE

Pre-Calcined Pre-Calcined

1200°C

12

Hrs

1200°C

12

Hrs

1200°C

12

Hrs

12

00°C

12

Hrs

GRINDING MEDIUM

|

Pre-Ground Pre-Ground o

CN B oCN E oCN E o

CN X

COMPONENTS

Wt.

%

TAM

505

Commercial

Grade TAM

8

54

CP.

Grade rH vo in

i m

m i O CN

CJ O

IH *H

CO Eh

CN

Cn Cr> CN • • r4 lf>

1 ro | en | in O (NO

CJ O CN M -H X)

CO Eh 2

in o>

\o Ch . rH in 03 rH

1 CN 1 m | in O CN o

CJ O CN U -H X)

tO Eh 2

f~ o

. n . VO • CO in in rH

1 CN 1 m i m

O CN O

CJ O CN U -H XI

CO H 2

INTENDED COMPOSITION m

o

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to

n

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to o

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CO

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LOT

NUMBER

ST-1 ST-2 ST-3

STN

(10)

-1

STN

(20)

-1

STN

(30)

-1

59
COMMENTS

Both

firings

too

low.

Over

calcined.

Over

calcined.

1350°C

firing

optimum.

FIRING CONDITION

-p C <D O rH

rH rH rg •h en 0

fci CM o

P u

Ph N

i

i

I

p

C 0) O rH

rH

rH rd •HO

Ch ro o

P u

Ph CSJ

i

|

Pt

Foil

On

Zr02

Pallet

-p c ai O rH

rH

rH Id •H Pn 0

Cn CM o

•P u

Pn tsj

FIRING TEMPERATURES

1450°C

(2

Hr)

1500°C

(4

Hr)

i

i

U —

0 u o K

inro rH

rH »»

i

i

i

u -» o u o a

in

ro rH

rH »-

1350°C

(2

Hr)

1450°C

(2

Hr)

TEST CONFIGURATION

K-Squares

i

i

i

K-Squares

i

i

K-Squares K-Squares

CALCINE TEMPERATURE

1200°C

12

Hrs

Repeated Twice o w

o u O EC

(N (N

rH rH

1000°C

6

Hrs

1200°C

12

Hrs.

1000°C 6

Hrs

1100°C 6

Hrs

GRINDING MEDIUM Methanol AcetoneMethanol Acetone

Methanol Methanol

COMPONENTS

Wt.

%

in cn

. . to H

1 ro 1 ro i in O (NO

CJ O (N M -H X)

CO E-i Z

>*

in ro . rH

vo fN .

1 ro

ro I I o oj in U o o

U -H (N

CO Eh >

in ro • rH

Id (N

1 ro u- ro I I o (N in U o o

M -H CN

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00

• rrH . in

VO CTl

1 (N 00 ro 1 1 o cn in

C_> o o

U -H CN

CO Eh >

00

rH • in

VO <T\ '

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that K-Squares were unsuitable due to the "soft" calcine and necessity of firing the samples

in an atmosphere. Considerable experimentation in firing procedure eventually produced

dense KTaO^ ceramic samples.

The PbO-Nb

2

0,_ system also had the disadvantage of a volatile component, PbO. However,

this was not as severe a problem as with KTaO^. Two niobates: Pb

2 Nb

2 07 and Cd^rNbO^ were produced in K-Squares although significant loss of PbO in the former and probably Cr 2°3 an<^ CdO in the latter caused considerable staining of the ZrO^ pallets upon which they were

fired. The remainder of the niobates were made using pressed pellets, often producing

somewhat porous samples (perhaps due to low temperature calcination) . Some compositions

would not fire to a dense ceramic at the conditions tried and had to be abandoned. PbSO,

4 was used in all Pb compositions due to its high temperature of decomposition. It was thought

that this would allow better reaction during calcine with the very refractory Nb

20^.

3.6. Experimental Methods

3.6.1 Dielectric properties

Dielectric constant and derivative ; The quantity OK'/3T) was the main property of

the multilayer samples to be tested. Such measurements had to be made on hundreds of these

samples to test the effects of various manufacturing procedures. A cryostat insert was made

whereby a fifteen sample teflon holder was mounted on the end of the stainless steel tube.

The holder was immersed in a bath of liquid helium which could be pumped down to about

1.5 K. A germanium thermometer was used to measure the temperature of the helium bath,

which was controlled with a manostat. Each of the fifteen samples were connected in turn to

a commercial capacitance bridge operating at 1 kHz. The capacitance of each sample was measured at 4.2 K and at a few lower temperatures. The data were computer analyzed to find

K' and 3K'/8T for each sample.

For measurements of the dielectric constant at temperatures above 4 K, the specific

heat apparatus was used. In that apparatus (described in section 3.6.2) only two samples

could be run at the same time.

Breakdown strength at 4 K : The breakdown strengths of the glass ceramic capacitors

were measured at 4 K in liquid helium. The dewars used were unsilvered so any external

breakdown could be observed. The breakdown voltage was applied to the capacitor and monitored

using the simple circuit shown in figure 25. The leakage current appears on the ammeters

placed in the ground return leads. Currents of about 1 yA could be detected. A 1.5 megohm

current limiting resistor was placed in the high voltage lead.

DC polarization ; As shown in section 3.1.1. on thermodynamics, the polarization P(T,E)

is used to determine the refrigeration power. When a remanent polarization is present,

P(T,E) can no longer be determined from the dielectric constant but must be measured directly

with a dc technique.

The circuit in figure 26 was built to measure P(T) directly. A similar technique has

recently been used by Gesi.

7 ^" This circuit can be used to measure both P(T)1 and P(E)] . E T A voltage, V, is applied across a series combination of the unknown capacitor C and a known

5 and much larger capacitor, C . Both capacitors must have very low leakage, RC > 10 sec

66
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68
for this system to work. When the ratio of C-^/C <K 1 the voltage across C^, i.e., V^, is

very nearly the applied voltage V. The voltage, V on C is measured with a vibrating reed

s s

5 electrometer. The input impedance, R^n

f is so high that R

^n

c

s >> 10 sec. The output of

the electrometer is measured with a digital voltmeter. The precision of the measurement is

about 0.01% for the larger capacitors measured.

For the capacitors connected in series with no leakage,

q = q = C V = C.V.

.

1 s s s 11

For parallel plate geometry,

D = q/s,

where D is the electric displacement and s is the plate surface area. Since P = D - £ Q

E, then

p = q-,/Si - e E = C V /s- for e/e >> 1. (3.7)

1 1 o s s 1 o

The quantity P is the total induced polarization and will be called P

fc

. If any ferroelectric ordering takes place or an electret is formed when the field is

applied, then a charge will remain on V when V is removed. Then

s

P = [c V /s,l

r s s 1 ^

where P^ is the remanent polarization. An AC measurement of polarization will presumably

measure P^ - P . t r A liquid helium cryostat was used for these measurements. The samples were mounted in

a vacuum can on a heat exchanger through which liquid helium could be valved. After a desired temperature was reached, the valve was closed. A carbon resistor sensor coupled to

a heater by an electronic feedback circuit kept the temperature constant. The cryostat was

the same as used for the multi-leaf switch measurements.

Most of the P^CT) measurements, where p

t

( T ) is the total polarization observed when a field is applied, were made by applying the field to the sample around 77 K then cooling it to 4 K and recording P at several temperatures between. The temperature was brought to equilibrium at each point. Then the field was removed and P^, the remanent field, was measured by warming the sample back to about 77 K. Hysteresis loops were measured by re- cording V

s while switching through E values at a constant T and calculating P from eq.

(3.7) .The dielectric constant at low frequencies could be measured with the same system by

measuring hysteresis loops as a function of T. The high frequency measurements were done by

connecting a commercial capacitance bridge to the leads in place of the polarization system.

Most all of the materials on which measurements of P and P were desired had a suft r ficiently high R such that the decay time was negligible.

69
3.6.2. Thermodynamic properties

Measurement theory : From eq. (3.1) the thermodynamic state function can be written as

(3P/3T)

E = -CE

6 E/T, (3.8)

where g = (9T/9E) , the electrocaloric coefficient. The function (9P/9T)_ is then determined

E S E from measurements of C and g . E E Since the field-dependent quantities C and g are involved, the experimental arrangeE E ment has to be compatible with handling large voltages. For this reason, the sample with

voltage hookup leads should not be subjected to motion. Also, the use of helium exchange

gas is proscribed for electrical discharge reasons. Consequently, a rigid system was adopted where the sample is secured to the reservoir by a thermal link in a high-vacuum

environment. This situation is illustrated in the inset of figure 27. Consider first the specific heat case. The heat flow is described by the equation

Q = <2_(dT/dt) + ^(T - T ), (3.9) E O

where Q is the power dissipated in the sample, <2 is the heat capacity of the sample (in- E eluding addenda) , ^ is the thermal conductance of the thermal link, T

q

is the resevoir

temperature, and T(t) is the sample temperature. Equation (3.9) is simply a power-balance

equation, and it is assumed that steady-state conditions are involved. This is a reasonable

assumption at the low temperatures of interest because thermal dif fusivities are generally

large.A specific-heat datum by the pulse method consists of two parts, shown in figure 28: A heat energy pulse of duration At, followed by a temperature decay through the thermal link

back to the reservoir temperature T . It is found experimentally that during the (short)

heat pulse, the sample temperature rises linearly with temperature,

T(t) = 6 + (6 - T )t/At, (3.10) o

and the corresponding solution to eq. (3.9) is

e = 2£t_ E (6 - T ) (1 + At/2x) (3.11)

where

t = e/Jr . (3.12)

hi

The factor (1 + At/2x) in eq. (3.11) accounts for the heat energy flowing into the reser- voir through the thermal link during the pulse duration At. At the end of the pulse, Q = 0 and the solution to eq. (3.9) is

T - T = (9 - T )e"t/T

. (3.13) o o

70
10'

ioCARBON-RESISTOR THERMAL LINKS

Link^ /

O 10 n - 1/4 w _ 10 ft - 1/2 W

A 2.7 ft - 1/2 W O 10 ft - 1 w

RESERVOIR

at T

Thermal Link

SAMPLE

at T(t)

10 15

T, K

Figure 27 Time constant characteristics for various resistor thermai links

between 2 - 20 K. Glass samples varied 3-5 gm. A hypothetical

metal link is shown, and the inset illustrates the experimental

arrangement

.

71
72
That is, the sample temperature decays exponentially to the reservoir temperature with the

time constant x, eq. (3.12).

Therefore, measuring Q, At, T and determining 6 and x from a T - t plot, the heat

o capacity of £- can be determined from eq. (3.11). Finally, the specific heat of interest,

E C (T) , is obtained from <3 by making the addenda correction,

E E

C_(T) = (C_(T) - £m.C.(T))/m, (3.14) E E j j

where m is the mass of the dielectric sample, and the m.C.(T) are mass times specific heat

3 3 of the addenda.

Considering next the electrocaloric coefficient, 3„> the exp«rimental situation is to

E make a field change AE about E and measure a temperature change, AT. A special consideration here is the phonon-polarization relaxation time. That is, following a polarization

change the phonon spectrum will come into equilibrium with the polarization exponentially

with a characteristic time called the phonon-polarization relaxation time. For the type of

2 72 dielectric materials of interest, this relaxation time is faster than 10 ys. ' This fact

insures that adiabatic conditions are fulfilled in the experiment [i.e., 3 = (9T/3E) J

.

E b Therefore, for the time scales involved experimentally the dielectric material exposed

to the field instantaneously changes in temperature from T^ -> T. , whereas (6 - T ) is the

temperature change of the entire sample. The energy balance condition is

T. - T = (9 - T )[mC (T) + Tm.C . (T) ]/mC_(T) , (3.15)

l o o E 3 3 E

and it is important to recognize that m in eqs. (3.14) and (3.15) refers to the mass of the

dielectric exposed to the field. Those portions of the dielectric not exposed to the field

are included in the addenda.

Following the temperature change due to AE, eq. (3.9) with Q - 0 applies, so the sample

temperature decays back to T

q with the time constant x, as in the specific heat case, eq. (3.13). The electrocaloric coefficient is then given by

(6 - T )[mC (T) + lm ,C (T) ] 3 = (T. - T )/AE = -e-—— J-3 " (3.16) E i o mC

E (T)AE

The eqs. (3.14) and (3.16) describe the means for measuring C

E and 3E

» respectively,

from which the state function (9P/3T) , eq. (3.8), can in principle be determined. Note

E that the zero-field specific heat has to be known for the dielectric so that the addendum

correction can be made for the dielectric material not exposed to the field. Also the

field-dependent specific heat C (T) has to be known to determine the electrocaloric coeffiE cient, eq. (3. 16)

.

We will now describe a scheme for considerably expediting these measurements. Suppose

that a sample has been fixtured and wired for field-dependent measurements, and suppose that

the zero-field specific heat has been measured. These measurements will yield C

q

(T) from

eq. (3.11) and x (T) from fitting the T - t data. Consequently, the thermal link is calibrated,

73
Jf (T) = 6 (T)/T(T) , (3.17)

and it is important in what follows to recognize that m#\T) is field-independent.

Without altering the sample fixturing, the field-dependent properties are measured.

Specifically, a field change AE about E leads to temperature change (G - T ) followed by an exp(-t/x) decay. If the resulting T-t data are analyzed for x, then

<2E

(T) = t^HT) (3.18)

where *y(T) comes from the calibration, eq. (3.17). Substituting in eq. (3.14),

C

E

(T) = [V(T) - £m.C. (T)]/m, (3.19)

and in eq. (3 . 16)

,

(6 - T ) t (T)

6 E

(T) = AELx^T) -V-C.(T)J " (3 ' 20)

Finally, combining eqs. (3.8), (3.19), and (3.20),

(3P/9T) = - ( 0 - T )x>*(T)/mT . (3.21) E o

The measurement scheme then is to measure first the zero-field specific heat, thus

calibrating the link, eq. (3.17). Then, from a single AE-AT event, the field-dependent

specific heat, eq. (3.17), the electrocaloric coefficient, eq. (3.20), and the state function, eq. (3.21), can all be determined at those T- and E- values.

The result eq. (3.21) is particularly attractive because it means that the state function, which is of primary interest, can be measured directly rather than by synthesizing 8 and c data. The cancellation of the addenda corrections in arriving at eq. (3.21) is E fortunate experimentally, because these corrections, though small, are difficult to determine to high accuracy.

The application of the method for measuring zero-field specific heat will be emphasized,

because measurements of the field-dependent properties are a straightforward extension of

the zero-field methods.

Cryostat insert : The calorimeter is a two-can system where the inner can is a temperature-controlled isothermal shield, and the vacuum spaces of the two cans are connected.

Hookup leads (0.015-cm manganin) run from an interchange can at room temperature down the

pumping line and are heat sunk to a copper collar bolted onto the flange of the outer can. The highvoltage leads are encased in Teflon sleeving and are heat sunk to a sapphire rod

mounted in the collar with varnish.

The lid of the inner can is suspended from the flange with copper straps, chosen to give 77->-4 K cooldown times of about 12 nr. A second heat-sinking station and interchange is

mounted on the underside of this lid and a sapphire rod is provided for heat-sinking the

voltage leads. In this fashion, all leads to the samples are tempered to the reservoir

74
temperature, T . The inner can bolts onto this lid (both copper) , and a heater wire is

spiralled on the outside of this can. A temperature controller powers this heater using the

signal from a silicon diode thermometer mounted on the lid. Finally, the sample and thermal

link are mounted on a copper fixture which bolts onto the bottom of the heat-sinking station

and interchange.

Thermal links : The time constant x in eq. (3.12) depends on the thermal conductance of the link,^, and the heat capacity of the sample, & . It is not practical to have t smaller than about 30 seconds nor larger than about 600 seconds. Also, as will be seen

below, a portion of the thermal link is an addendum, so its mass should be kept small.

3 2 Now, C^T and for a metal link,

aT. so that a metal link would have TaT (approximately) in the temperature range of interest. This would lead to an unacceptably small

(large) x at 2 K (20 K) associated with an acceptable x at 20 K (2 K) . Different metal

links could of course be used spanning overlapping temperature ranges.

73 2 It has been reported that radio resistors have (approximately) at low temperatures.

Some available Allen-Bradley resistors were studied for possible thermal links: 10 ft, 1W; 10 ft, 1/2W; 10 ft, 1/4W; and 2.7 ft, 1.2W. The phenolic resin was machined off these resistors,

the leads trimmed to 2 mm, and they were attached to glass or glass-ceramic samples in the manner described below. The sample weights chosen were 3-4 gm. The x-data were measured as

a function of temperature by heat-pulsing the samples through attached heaters and measuring

the exponential time decay on a chart recorder. The results of these measurements are shown

in figure 27. Also shown in figure 27 is a hypothetical metal link for which x = 40 seconds

and 2 K. These data reveal that x is approximately proportional to T, as expected, and that

the 10 ft, 1W and 2.7 ft, 1/2W resistors have favorable ranges of x-values. The 2.7 ft, 1/2W

resistor was selected because of its smaller mass.

In the above section, the technique for determining C , 3 , and (3P/3T) from a single

E E E AE-AT datum hinges on determining v^(T) for the thermal link accurately from the zero-field

specific heat measurements, eq. (3.17). The JKt) data so determined from two different

specific heat runs are shown in figure 29. Thermometry : The small 220 ft, 1/8W Allen-Bradley resistors were chosen for sample

thermometers because of their 2-20 K sensitivity. These were calibrated against a germanium

resistance thermometer mounted on the reservoir. From 2 to 20 K, the R-T could be fitted

to

logR = A + BT~

P

, (3.22)

where typically the residual standard deviation was about 10 mK. Typical parameter values

are A = 2.37, B = 3.47, P = 0.78.

The thermometry procedure adopted was as follows : At the equilibrium temperature

before a heat pulse or field change, T = T , and the resistor thermometer on the sample and

a standard thermometer on the reservoir were measured. These data collected during the run

were curve-fitted according to eq. (3.22), but only the B and P coefficients were retained.

Then, at each equilibration point, the coefficient A was repeatedly determined from the

reservoir temperature T and the B and P coefficients. This procedure makes use of eq.

75
76
BLANK

77
(3.22) in such a way that the temperature changes AT reflect the accuracy of the overall fit

while insuring that at equilibrium the sample thermometer matches exactly the standard

thermometer

.

Sample fixturing : A schematic of the fixtured sample is shown in figure 30 (voltage

leads are not shown) . The sample consists of two plates (labelled J) varnished together

with a heater wire (G,H) sandwiched between. A thin layer of fritted silver (DuPont 8225)

is fired (680°) on a portion of one edge of each plate (C) . The manganin heater wire is

next mounted on the face of one of the samples with varnish, and this step requires con- siderable care because of the relatively large specific heat of the varnish (^ 30 times

larger than that of copper) and its dependence on curing conditions.

The fired-on silver section (C) is indium-soldered to the thermal link (B) and the

solder joint (D) is trimmed with a knife. The machined-down resistor thermometer (F) is

varnished to the sample, and short portions (about 1 cm) of the hookup leads (E) are tempered

to the sample with varnish. Finally, the other end of the thermal link is indium-soldered

to the reservoir (A)

.

The addenda weights are determined by cumulative weighings. The weight of the manganin

heater wire is determined by measuring the room-temperature resistance and using the calibration, 5.7 x 10 ^ g/fi. Typically the heat capacity of the addenda amounted to about 4% of

that of the sample above 10 K and about 20% at 2 K. The components attached to both the sample and reservoir involve two types of corrections: A temperature gradient is established across, say, the thermal link, and it is easy

to show that one-half the mass of the link is "on" the sample as an addendum. The second

consideration involves the power flow in the manganin hookup leads (I) . Note that if these

74 leads were copper, they would constitute the thermal link. Neighbor has shown that to a very good approximation, one-half the power generated in these leads flows to the sample.

This means that one-half the resistance of these hookup leads has to be added to the heater

resistance to obtain the total effective heater resistance at room- temperature

.

Instrumentation : For the specific heat case, the heat dissipated in the sample is

Q = i

2 RH

(T)At (3.23)

where i is the d.c. current, Rjj(T) is the heater current, and At is the pulse duration. The

current supply was a 9-volt battery bank and the current was measured by measuring the

voltage drop across a 1 kfi standard resistor. A switching panel was assembled where the

pre-set current could be applied to the sample heater by depressing a relay switch which

also triggered a digital clock. Releasing the switch stopped both the current flow and the

clock.

The exciting current to the resistor thermometer on the sample was battery-supplied and

measured by measuring the voltage drop across a 1 kfi standard. The current was adjusted

during the run to maintain the thermometer voltage at ^ 20 mV, and this signal level produced negligible self-heating. All but 1 mV of this voltage signal was "bucked" potentiometrically, and the residual voltage was displayed on the 1 mV scale of a strip-chart

78
79
recorder. For a specific heat datum, the heater current was interrupted when the chart pen

deflection reached 0.5-0.8 mV, and this AV/V corresponded to AT/T values ^ 2-4%.

Electric fields were produced by applying a voltage to the samples from a decade high

voltage dc power supply. The power supply could provide up to 3000 volts. Step changes in

field were produced by changing the decade setting on the power supply.

Accuracy of the method : The major sources of error in the method are: (1) The addenda

corrections, ^ ± 0.5%, including some imponderables such as solder joints; (2) The heater

resistance, R^(300), ^ ± 1%, due to uncertainties in correcting for hook-up leads; (3) The

uncertainty in determining t = 0 from the chart trace which translates into an uncertainty

in computing the intercept 6, ^ ± 2%; and (4) The curve-fitting uncertainties both in the

exp(-t/x) fits and the thermometer handling according to eq. (3.22). These uncertainties

reflected in x and 6 are believed to be ho greater than ^ ± 2%. The uncertainty in t = 0 does not affect x, because this uncertainty is absorbed in the

amplitude in the curve-fitting.

The repeatability of the method is an approximate measure of precision. At the same reservoir temperature, a sample was pulsed ten times using different heater currents, pulse

times, and chart speeds. The resulting zero-field specific-heat values agreed to ± 2%. This repeatability is about the value expected from the t = 0 uncertainty.

The inaccuracy of the method was expected to be ^ ± 5%. An attempt was made to verify

this by measuring the zero-field specific heat of Si0_. A sample of Corning fused silica,

-3 Code 7940, was measured and these data are shown in figure 31 plotted as CT vs T. Also

75 76 shown in figure 31 are literature data for I. R. Vitreosil and vitreous silica. The

position of the peak in CT

3 agrees well with the literature data, - 10 K but the height of

the peak is = 16% higher than the literature data. At 3 and 20 K, the measured and literature data agree to 1.5% and 6.5%, respectively.

-3

The difference in the heights of the CT peaks in figure 31 appears to be much larger

than the experimental error in the method. This difference is believed due to the different

sources of Si0

2

. That is, the temperature of the peak in the CT

3 curve is related to the

frequency at which the phonon density of states reaches its first maximum, and the height of

77 the peak correlates with the number of phonon modes in the density of states peak. Lowfrequency defect modes related to the open structure of SiO are believed responsible for

Z

3 this departure of the specific heats of amorphous materials from a T law. Consequently,

the height of the CT

3 peak can be expected to depend on the pre-history of the Si0

2 samples.

3.6.3 Thermal conductivity

Knowledge of the thermal conductivity of the glass-ceramic was needed for eventual

prototype design, so a method for measuring thermal conductivity in the range 2-20 K was established in parallel with the above specific heat-electrocaloric coefficient. The two

methods involve much of the same equipment and thermometry handling. The conventional

techniques of using two carbon thermometers on the sample with a heater attached to one end

of the sample was used in the initial runs. The carbon thermometers were calibrated against

a germanium resistance thermometer mounted on the reservoir. From the first set of measurements it was determined that the thermal resistance of the heater-sample joint and the

80
6

2

1 1 1 1 1 1 1 1

|

Corning Fused

Si0

2 (Code 7940) —

o/ / V >?v

I.R. Vitreosil P / Vitreous Silica \

(White & Birch) / / (Flubacher et al.& Simon)\

1 1 1 1

2 5 10

TEMPERATURE , K

Figure 31. Comparison of specific heat data measured on a sample of Si02 with

literature data. The difference between the measured and literature

data at the maximum in CT~3 is too large to be attributed to experimental uncertainty but is believed due to differences in the number

of low frequency phonon modes in the different samples of SiC>2.

81
sample-reservoir joint were negligible. Subsequent measurements were then done with the

thermometers permanently mounted on the heater and reservoir. That procedure greatly

simplified the mounting procedures.

3.7 Experimental Results

3.7.1. SrTi0

3 glass-ceramics

Maximizing (9K'/9T)

0and the breakdown strength: The quantity (9K'/9T)q at 4 K was measured for over 200 samples in the FEP composition series. The series consisted of a preliminary matrix and two main matrices. About 64 different combinations of fabrication

variables were used in these matrices. The most important variable was the ceram temperature and so the plot in figure 32 is of (9K'/9T)g vs the ceram temperature. The figure

compares (9K'/9T)

0 for the FEP matrices with that of the bulk research samples. All the

data for the matrices lie within the cross-hatched region.

Two features are immediately evident from figure 32. First, the high values of (9K'/9T) as seen in the research samples could not be duplicated in the FEP matrices. Second, for

low ceram temperatures the FEP values are higher than that of the research samples, but the FEP values drop abruptly for ceram temperatures above 1100°C. As discussed in section 3.4

it is just that temperature at which pitting in the samples began to occur. A plot of

dlnK'/clT increases smoothly through the pitting regime which indicates the sudden drop of

9K'/9T is probably not an intrinsic material behavior. Most likely the pits cause the rapid

decrease in K' and 9K'/9T. The maximum value of K' at 4 K is about 250 and also occurs just

before the onset of pitting. There appears to be only a slight correlation between 9K'/9T

and the other process variables. Slower heating rates and longer soak times have the most

influence in increasing 9K'/9T. The use of seal A with paste A2664 gives a slightly higher

3K'/9T.

The first measurements of voltage breakdown strengths on 0.025 mm thick samples showed

three types of behavior: (i) conducting at all voltages, (ii) conducting above a certain

voltage, and (iii) a classical breakdown with a sudden appearance of large current flow at the breakdown voltage. Arcing was often seen in this last case. It is difficult to define

a breakdown strength for the first two cases, but it was arbitrarily taken as that field

strength where 10 pA of current was drawn. If each element in the refrigerator drew that

much current, then the heating effect would be comparable to the refrigeration power desired.

As defined, the breakdown strength ranged from less than 10 kV/cm for the first case to as high as 600 kV/cm for the third case. A study of the breakdown data for this first matrix suggested that flaws or pin holes

clear through the 0.025 mm dielectric layer could account for the conducting behavior seen

in many of the samples. It was then suggested that two or more layers of dielectric between

the electrodes be used. With two or more layers the chances of having two pinholes line up

to penetrate straight through the dielectric becomes essentially negligible. The second FEP matrix was then made with many samples containing two and three layers of the basic 0.025 mm

thick dielectric and with no floating electrodes between the layers. For the cases where

pitting did not occur, the breakdown strengths of these samples averaged about 550 kV/cm,

82
50

Figure 32. The temperature derivative at 4 K of the dielectric constant for

the FEP multilayer samples as a function of the ceram temperature.

Shown for comparison is the results on a bulk research sample.

83
whereas that of the samples with floating electrodes between the layers was about 160 kV/cm.

The other process variables appeared to have little effect on the breakdown strengths. The

2 550 kV/cm is well above the 300 kV/cm strength used in the original design calculations.

It was found that some correlation existed between the loss factor, tan 6, in the

dielectric constant and the breakdown strength. Large values of tan 6, i.e., greater than

about 0.01, are associated with low breakdown strengths although very small tan <5 values do

not necessarily mean large breakdown strengths. Measurements of tan 6 were then useful in

eliminating most of the samples which had low breakdown strengths.

A second series of samples with a different composition (FEL series) was made to test

the effect of composition on the dielectric properties. The only difference between the FEP

and FEL compositions is that FEP has a SiC^/A^O ratio of 4 while FEL has a ratio of 3. The FEL series also consisted of two matrices with essentially the same process variables as used with the FEP series. The results of measurements on (3K'/3T) at 4 K are shown in

o figure 33 as a function of the ceram temperature. The behavior is somewhat similar to the FEP series in that the maximum (3K'/3T) o values are about 3.5 K

^" and that a rapid decrease

in 3K'/9T occurs above a certain ceram temperature where pitting occurs. The onset of

pitting for the second FEL matrix occurred at about 1160°C compared with 1100-1110°C for the FEP series and the first matrix of FEL. Extensive tests on the composition of the two FEL

matrices showed no differences between the two, although there was a difference in color.

No satisfactory explanation could be found for the difference in behavior. In any case, the

maximum (3K'/3T)

o values still did not exceed 3.5 K which is about an order of magnitude

below that of the bulk research samples. The largest values of K' at 4 K are about 250,

which are the same as for the FEP series.

The results of breakdown measurements on the FEL series gave similar results as for the FEP series except that there were far fewer samples which were conducting at low voltages.

The average breakdown strength for the better samples was about 650 kV/cm, which is slightly

higher than the FEP series.

As discussed in section 3.4 the FEP and FEL series contain 2 mole % SrNb-O doping. To

2 o test the effect of this niobium doping, the FHN series was made which is just FEP without

the niobium oxide. The resulting K' and (3K'/3T)

o values are decreased by about a factor of

three and two, respectively. Consequently, niobium has a rather strong effect on (3K'/9T) . One approach to solve the pitting problem was to inhibit strontium feldspar crystallization. This was accomplished by adding B

2°3" Single layer research samples were made from

this material and are known as the SrTiO^ boroaluminosilicates. Two compositions were

tested, FHS, which has 1% niobate and FID, which is niobate free. The ceram temperatures

were mainly 1050 and 1100°C. For the FHS composition with 1100°C. ceram temperatures the K' and (3K'/8T)

o values were about 550 and 10 K , respectively. The dielectric constant reached

a peak of about 600 at a temperature of 50 K. The niobate free FID samples had K' and

(3K'/9T) values of about 300 and 3 K , respectively. Since these were only single layer

o research samples the breakdown strengths would be very low and were not even measured.

Further attempts to increase (3K'/3T) were halted when electrocaloric and polarization

o measurements showed that (3K'/3T) was not the relevant parameter as far as predicting

o cooling effects at 4 K.

84
1100 1200 1300

Tceram

The temperature derivative at 4 K of the dielectric constant for

the FEL multilayer samples as a function of the ceram temperature.

Shown for comparison is the results on a bulk research sample.

85
Electrocaloric measurements ; Considerable time was spent trying to solve the pitting

problem. Finally it was decided to make electrocaloric measurements on the best FEL samples

cerammed at 1100°C under optimum conditions. Two such samples were mounted in the apparatus

for measurement of specific heat and electrocaloric coefficient. These samples are designated

as ECR3 and ECR6.

The first observation was that the initial voltage application at 4 K caused heating

rather than the expected cooling. Moreover, this heating persisted for very long times.

Data are shown in figure 34 for two samples where at t = 0 the field was switched 0 * 239

kV/cm. These data were obtained by analyzing T(t) data knowing thermal conductance data

for the thermal links. For times of about 40 min. , the heating has fast and slow com- ponents with time constants of about 2 min. and 20 min. , respectively. Considerable energy

3 is dissipated in this process (about 0.1 - 0.2 J/cm from figure 34).

It was found experimentally that once these initial mechanisms stabilized at some field

value, the field could then be reduced, cycled, etc. without re-triggering these mechanisms. , The next observation was that at 4 K, where (3K'/3T) was observed to be the largest,

heating was seen on both positive and negative field changes AE about E. The samples were

first stabilized as mentioned above, and data on this heating are shown in figure 35. The

actual heat input was calculated using measured values of the specific heat. The results

are shown as dashed lines in figure 35 corresponding to AQ = 1.90 and 6.37 uJ/cm^. Here AQ

was assumed temperature independent.

Electrocaloric measurements were also made on the same two stabilized samples between

20-40 K, and these AT-data are shown in figure 36. In the range 20-40 K, reversible electrocaloric heating (AE > 0) and cooling (AE < 0) are observed tor the two samples. The dashed

curves in figure 36 were drawn to represent exactly reversible electrocaloric temperature

changes.

The deviation of the position AT points between 20-25 K from the dashed curve in figure

36 probably result from the hysteretic-heating effects (note that the field changes in

figure 36 are considerably larger than the figure 35 field changes) . That is, hysteretic

heating would raise both the positive and negative AT points. Due to the uncertainty of the

AT's, no attempt was made to correct the data for hysteresis effects.

The ceram temperature has a large effect on the resultant SrTiO^ crystallite size. The

possible effects of crystallite size on the electrocaloric properties were tested by measuring FEL samples cerammed at 915°C and 1160°C. The two samples measured previously were cerammed at 1100°C. The crystallite size changes from a few angstroms for 915°C samples to about 0.7 ym for 1160°C samples.

In figure 37 are shown temperature decays at 4.0 K following the first and second

voltage pulses for FEL 1160 and FEL 915. The long time-constant self-heating appears absent

in both samples because the t's match the link time-constants. Also, the 0-»-E and E-*0 x's are essentially the same. There is hysteretic heating present in the uncrystallized FEL 915

sample, and both samples in figure 37 show evidence of depolarization cooling superimposed

on the more dominant hysteretic heating.

86
TIME, s

Figure 34. The time dependence of self heating in two FEL glass-ceramic after

an electric field is first applied to the samples at 4 K. Both

samples had ceram temperature of 1100°C.

87
100

5G

E

I—' <

CD

<

10 —

5 -

a r

\

HYSTERETIC HEATING

Open Symbols 59.6—71.6 kV/cm

^ Closed Symbols 71.6—59.6 kV/cm

\

\/-AQ=6.37/*J/cm3

\ "\

\o \D

\ \ECR6

\- v

\° \

ECR3 \ \

\

°

AQ=1.9/*J/cm 3-v"\

\

\

\

\

\L

8

TEMPERATURE. K

ure 35 The temperature rise in two previously polarized FEL samples during

a change in the electric field. The open symbols are for an increase

in field and the closed symbols are for a decrease in field.

88
E

40

30

20

10

0

-10 -

-20

-30

-40

i r FEL 1100

Polarization Heating

0—120kV/cm

o

A

ECR3

ECR6

Depolarization Cooling

120—0 kV/cm

_ 6

/

A

\

\

1

20 30

TEMPERATURE, K

40

Figure 36. The temperature change in two previously polarized FEL samples during

a change in the electric field. This data is for the 20 - 40 K

temperature range where cooling is seen for a decrease in field.

89
5000 -

1000

E

TIME, s Figure 37. Temperature decays at 4 K following the first and second voltage

pulses for FEL 1160 and FEL 915. The open symbols are for the

first pulse, 0—72 kV/cm and the full symbols are for the second

pulse, 72 kV/cm—0.

90
In figure 38 are shown data on the effects of polarity reversal at 4.0 K for the two

samples. For example, on switching CH- + 72 kV/cm, FEL 1160 heated 2.2 K; after decay back

to 4.0 K, the field change + 72 -> 0 resulted in a temperature rise AT = 0.8 K; the subsequent

CH- - 72 switch yielded AT = 3.5 K, etc. The residual polarization +P remaining in the sample after a +E -> 0 switch is reversed

to -P on the subsequent 0 -* -E switch and results in the large AT = 3.5 K in FEL 1100. The

±E -» 0 switch involves a smaller amount of heating, AT = 0.8 K for FEL 1160.

Also shown in figure 38 for FEL 1160 are the AT's obtained on subsequent 0 + +E > 0 -» +E,

etc., switches (measured in a second run). The initial 0 -* +E switch gives a larger AT (2.2

K) than subsequent 0 -+ +E switches (AT = 1.2 K) provided polarity is not changed (this

initial AT decays quickly, figure 37)

.

The FEL 915 data in figure 38 track the FEL 1160 data, implying that the heating

mechanisms are qualitatively the same in these end members. In figure 39 are shown the

2 hysteretic and electrocaloric components for FEL 1160 at 4.0 K plotted vs E . These data

show that at 4.0 K the hysteretic component is not only larger than the electrocaloric

component but is apparently increasing more rapidly with applied field. The electrocaloric

component is defined as the reversible part of the temperature change and the hysteretic

component is that which always causes heating. This definition assumes the nysteretic

heating is the same for both increasing and decreasing fields. Because the hysteretic

component is much larger than the electrocaloric component in figure 39, the inaccuracy of

the electrocaloric component is rather large. The first resolvable cooling effects were

seen at about 7 K. By 15 K, electrocaloric heating and cooling effects are well established,

and electrocaloric data are shown in figure 40 on FEL 1160 from 15 to 31 K. Polarization measurements : All the SrTiO^ glass ceramics show qualitatively the same behavior for P^(T) and P

t

(E), where P is the total polarization. The most extensive polarization studies were done on the FEL glass ceramics, however, so the representative curves

shown will be mostly for this material. Measurements of dc polarization for FEL glasses

crystallized at different temperatures are shown in figure 41 for a 3.6 kV/cm field applied

around 77 K. This p

fc (T) data and most subsequent P (T) data are normalized to P (T) = 1 at

4 K so the temperature dependence can be compared easily. The normalized values of P and

P are designated P^ and P , respectively, where P „ = P /P^ (4 K) . There is no maximum in

r tN rN rN r t

P (T) evident above 4 K for any crystallization temperature. The slope of P does increase

with increasing crystallization temperature since p

t

(4) and 9P

tN

/8T increases with increasing crystallization temperature up to 1150°C. The 1160°C glass has a slightly smaller

3P

t/3T because the maximum value of P is smaller even though ^ P

^N

/^ T ^ s larger. The maximum dP^/dT with crystallization temperature cannot be confirmed without more samples measurements. None of the materials show any appreciable change of P with temperature below 10 K, unlike that of K' . All of the materials with any temperature dependence of P displayed a

25 to 30% remanent at 4 K when the field was removed after cooling from 77 K. This remanent

was quasi-stable; a very slow but detectable decay was observed at constant T. This remanent

rapidly decreased as T was increased. The remanent curves approach P^ = 0 assymptotically

at some value of T above 77 K. Figure 42 shows normalized polarization curves measured at 3

91
CO

CO

CD

CO

I

LO

CM

II

i LO

CO

II

CSl CD

LO

CO

CSJ CO

TT

CD

II 1—

1 LU

k:-a

CO CM CO

CD

CM

CD CD

CM

+

CM

o

M-4 rH O

rH

t-4 o W •rl

4-> CJ

r-i 0) O r-l M-l •H

Tj

0)

<!-

4->

4-> cd CO>

rH o

cd

CO CO M

cu a) > r-l U M

•H

4J •H a)

}-i cd 4-1 rHo 14-1 a o

4Ju

cda

CO

CU ^

60 cu

fi &

cd o

rH a

cu 13

rH H

CD •H

M-l CU LO bC

M-4 rH G

O CTi cd

CO hJ a

4-> w

O

CD rH

14-4 T3 cu iw C •H W cd

>l IV 3SIH 3HniVH3dlAI31 iua/A>| '3V

92
FEL 1160 (36)

T=4.0 K

E

2

, (kV/cm)

2

x10"

3

Figure 39. The hysteretic and electrocaloric components of temperature changes

at 4 K as a function of electric field.

93
20

E

0

30

20

10 -

0

-10 -

-20 —

-30

i—i—

r

i—i—

r

FEL 1160

Hysteretic Heating AE= ±72 kV/cm

-| I—I h H 1 h

Polarization Heating, 0—-72 kV/cm

H

1 h

-\ 1 h

Depolarization Cooling, 72—^0 kV/cm

J I I I I I I L

Figure 40

,

20 25 30

TEMPERATURE, K

Electrocaloric and hysteretic effects in FEL 1160 between 15 and 31 K,

94
Temperature, K

Figure 41. Curves of P tN and Pr^ for FEL materials crystallized at various

temperatures

.

Specimen P^(4 K) , ycoul/cm E, kV/cm

FEL 915 0.0058 3.63

FEL 1000 0.046 3.66

FEL 1090 0.116 3.66

FEL 1150 0.141 3.66

FEL 1160 0.118 3.64

95
0 10 20 30 40 50 60 70 80

Temperature, K

Figure 42. Curves of P

tI

r and Pr^ for FEL 1090 and various fields. Curve 1,

E = 3.7 kV/cm, P t (4 K) = 0.11 ycoul/cm2

; Curve 2, E = 7.3 kV/cm,

P t (4 K) = 0.18 ycoul/cm2

; Curve 3 , E = 59 kV/cm, P t (4 K) = 0.89

coul/cm2

; Curve 4, E = 167 kV/cm, P t (4 K) = 1.6 ycoul/cm2 .

96
different field values for FEL 1090°C. The normalized slope decreases in magnitude with

increasing field. The relative magnitudes of compared to P

fc at 4 K for voltages applied

at 77 K decreases from about 33% to about 14% as E is increased. The remanent appears to approach the same percentage around 70 K for all three field values. The remanent was measured once after applying a field of 167 kV/cm to the sample at 4 K. There was only a very slight decrease, one part in nearly 700, in P^ upon warming the samples to 67 K. This

remanent may have resulted from a momentary breakdown of the FEL capacitor during the application of the field. Such occurrences were observed during the breakdown tests. This would

leave a net charge on the standard capacitor when the high voltage lead is shorted. However,

sufficient remanent would be expected to remain on the FEL capacitor for a temperature

dependence to be observed even though it is superimposed on a leakage generated charge.

This question was not pursued any further.

Figure 43 shows normalized P^CT) and P^ (T) curves for three other glasses of approximately the same crystallization temperature. The P curves of FEP 1130 and FEL 1150 curves

are essentially co-linear though P is larger for FEL. Removing the niobate from FEP (FHN

1130) reduces the T dependence of P . The DXO 1199 glass ceramic (used for capacitance

thermometers) is unfairly displayed. It has twice the field applied to it because the plate

spacing was half that of the other capacitors. A rough estimate of the P curve at half the

field using the field dependent P data of figure 42 would bring the high temperature end of

this curve down to the FHN curve. Hysteresis loops for FEL glasses to a field of ± 2.2 kV/cm are shown in figure 44. In figure 45 the hysteresis loop is shown to fields of ± 228 kV/cm. The 0 to +345 kV/cm hys2 teresis loop which has been offset by P^ = 1.0 ucoul/cm in figure 45, shows a very small

opening. This is further reduced if the field is not brought below 60 kV/cm. The maximum

field in the SrTiO is probably less than 30 kV/cm due to the presence of the glass matrix

with a low dielectric constant.

2 The half width of the hysteresis loop in figure 44 is about 0.0034 ucoul/cm . This

2 would be about 0.006 ucoul/cm for a field of 3.7 kV/cm. When 3.7 kV/cm is applied while

2 cooling the sample from 77 to 4 K, the remanent is about 0.035 ucoul/cm , which is about 5 times larger.

For a period of time after a field is applied, the polarization increases with time.

In figure 46, P

t

(°°) - P

t

(t), where t is time and P (») is the estimated P at t = <*>, is

plotted as a function of time after E is raised to 300 kV/cm from 240 kV/cm. The curve can be approximated by two exponentials.

A maximum was observed in polarization measurements made while decreasing T if the

electric field was switched off at each temperature point to measure P . Figure 47 shows

the results of a P measurement of FEL 1150 for E first applied at 4 K and measured as T is

increased to 56 K. The temperature then was decreased and P measured without changing the

applied field. A peak is observed in P measured while increasing T. The 4 K to 56 K section of the curve is a quasiequilibrium curve as P increases slowly with time up to the

peak, then decreases slowly with time above the peak. The peak of P occurs around 25 K

while the peak of K' at 1 kHz occurs at 35 K.

97
Temperature, K

Curves of P „ and P „ for four different glass ceramic capacitors

tN rN

crystallized at similar temperatures.

2 Curve No. Specimen P

r

(4 K) ycoul/cm E kV/cm

1 FEP 1130°C 0.113 3.6

2 FEL 1150°C 0.141 3.7

3 FHN 1130°C 0.094 3.6

4 DXO 1100°C 0.12 7.1

98
Figure 44. A P t (E) hysteresis loop for FEL 1150°C at 5 K for small fields.

The cycle frequency is approximately 10~3 Hz.

99
-300 -200

200 300

E (kV/cm)

-^-2.4

Figure 45. Hysteresis loops of P

t (E) for FEL 1090 at 4 K and high field

strengths. The cycle frequency is about 10~3 Hz. The upper curve

is P t (E) for cycling E between 0 and maximum field. The zero of

the P

t scale is arbitrary.

100
1(T

FEL 1090

TIME, s

Figure 46. Change of P t of EEL 1090 with time after increasing the field to

300 kV/cm from 240 kV/cm. The data are plotted as [? t (~) " PtC*)]/

P t

(oo) where PtC 33 ) is the estimated maximum value of Pt

.

:::
.13,-

10 20 30 40 50

Temperature, K

60 70

Figure 47. Polarization curves for FEL 1150 in a 3.7 kV/cm field showing the

difference in P t (T) depending on whether the sample is heated from

4 K or cooled to 4 K after the field is applied. Curve 3 is

Pt(T) - Pr

(T) . Curves 2 and 3 are nearly identical in shape. The

magnitude of curve 3 is lower by the amount of remanent polarization

obtained when the field is applied at 4 K. Curve 4 shows K'(T) at

1 kHz and 0.36 V/cm.

102
Judging from the results of K' (T) measured for FEL (1090) for four frequencies between

0.01 Hz and 10 kHz, figure 48, the peak in K' (T) for 0.01 Hz could be about 10 K lower than

the 1 kHz peak for FEL 1150 bringing it down to the same temperature as the observed peak in

P

t

(T) warming. The curves of K' (T) of FEL (1090) show obvious dispersion. The loss tangents

measured were small and probably dominated by lead losses. Dispersion exists in other glass

ceramics as shown by the plot of C as a function of f in figure 49 for a DXO glass ceramic

at 3 K.No cooling is possible in the temperature region where P has a positive slope (curve 2 in Fig. 47) since the field must be applied to cause cooling, but the P^fT) curve would then

follow a negative slope curve similar to curve 1. Only adiabatic depolarization cooling is

possible since P

t

(T) always has a negative slope when the material is cooled. Such behavior

is consistent with the electrocaloric measurements.

Specific heat : The zero field specific heat of an unelectroded FEP sample cerammed at 1100°C is shown in figure 50. Specific heat measurements between 3 and 5 K on an unelectroded

FEL sample cerammed at 1100°C gave essentially identical results as the FEP sample. This

figure also shows the specific heat of a glass, fused SiO^, and the specific heat of a single crystal SrTiO . The single crystal SrTiO^ specific heat curve is from unpublished

data by J. H. Colwell of NBS, Washington. The FEP glass-ceramic is approximately 50% SrTiC>

3 and 50% glass phase, though the glass phase is an alumino silicate rather than fused SiO^.

The specific heat of the FEP glass-ceramic appears consistent with what would be expected of

a mixture of SrTiO^ and glass. The data show no indication of an anomaly in the range 3-20

K. A transition from an antiferroelectric state (or ferroelectric state) to a paraelectric

state would give rise to a peak in the specific heat at the transition temperature. Such

peaks are usually spread out somewhat in temperature, but the specific heat of FEP at 20 K

shows no apparent indication of any approaching peak at 33-40 K. The possibility of a sharp

peak at 33-40 K cannot be ruled out until zero field specific heat data are obtained in that

temperature range.

The specific heats of the two FEL samples used in the electrocaloric effect experiments

were measured at 4.4 K as a function of electric field. For an electric field of 220 kV/cm

the specific heat decreased less than about 5% compared with the zero field specific heat.

Thus the change in entropy at 4 K with electric field is also probably less than about 5%.

2 2 From eq. (3.4) it can be seen that a decrease of C with electric field implies 3 P/3T < 0.

2 2 A negative value of 3 P/3T at 4 K is also seen in the polarization measurements.

Thermal conductivity : The results of thermal conductivity measurements on several

different glass-ceramics are shown in figure 51. The results are typical of values seen in

glass or some plastics. Results on fused SiO^ are also shown in figure 51. The amount of

glass phase decreases from FEP to FEO to Code 9658. These results suggest that the ceramic

component contributes significantly to the thermal conductivity above about 7 K. Though the

thermal conductivity values are low in comparison with metals or single crystal dielectrics,

the values are sufficiently high that thermal equilibrium times in thin sections between

electrodes are much less than 1 second.

103
0 10 20 30 40 50 60 70 80 90

Temperature, K

Figure 48. Curves of K'(T) at 4 frequencies between 0.01 Hz and 10 kHz for

FEL 1090. E = 3.6 kV/cm max. at 0.01 Hz and 0.036 kV/cm max. at

the higher frequencies.

104
(jd) aoueinedeQ

105
10 5

TEMPERATURE. K

Figure 50. Specific heat of unelectroded FEP 1100 SrTi03 glass-ceramic sample

compared with fused Si02 and single crystal SrTi03.

106
10

-2

E

o

o

a 10

-3

2x10"

Figure 51.

V

Machinable Glass

Ceramic, Code 9658

SrTi03 Glass Ceramic,

Code FED

SrTi03 Glass Ceramic,

Code FEP

Fused SiO

2

5 10

TEMPERATURE, K

20 30

Thermal conductivity of three different Corning Glass Works glassceramics as compared with fused SiG^. The slopes for T^ and T^ are shown in the graph for visual comparison.

107
3.7.2. KTa0

3 Glass-Ceramics

Large crystallization yields of KTaO^ were realized in the glass-ceramic system K^OTa^O^-Al^O^. The dielectric constants and their temperature derivatives were measured for

three different compositions, 75%, 80%, and 85% KTaO^. The 80% composition had nearly an order of magnitude higher K' and 9K'/3T as the other two compositions. This composition

gave K' and 9K'/3T at 4 K of 731 and 9.4 K , respectively. The dielectric constant reached

a peak of about 800 at a temperature of 19.6 K. The behavior is thus qualitatively the same

as that of the SrTiO^ glass-ceramics. The loss tangent of 0.006 is quite low for this type

of research sample.

Polarization measurements on these samples were not possible due to the low dc resis12 tance (< 10 Q) . Likewise reliable electrocaloric measurements could not be made on such

samples.

3.7.3. Single crystals

The difficulties in growing single crystals would make the proposed refrigerator more difficult but not necessarily impractical. A study of some single crystals was done to better understand the fundamentals of the dielectric behavior. In the case of the SrTiO^

glass-ceramics, measurements of P

t

(T) showed that 3P

t/3T approaches zero well above 4 K. Thus cooling at 4 K would not be expected, in agreement with the actual electrocaloric

measurements. Polarization measurements are much easier to make than electrocaloric measurements, thus many of the subsequent materials were tested only by polarization. A material

with a relatively large 3P

t/3T at 4 K would then be a good candidate for cooling.

KTaO^: A KTaO^ single crystal obtained from Dr. Lou Grabner, NBS Washington, was first tried in the P (T) apparatus and found to be conducting over the temperature range of

interest. All the measurements presented below for a single crystal were made on a single

crystal obtained from Dr. George Samara of Sandia Corporation.

In figure 52, P

t

(T) and P^_ (T) are shown for the KTaO^ single crystal from 77 to 2 K at fields of 0.25, 0.84, and 4.05 kV/cm. The polarizations again are normalized to P = 1.0 at

4 K. The data were taken without warming the crystal above 77 K after the initial cooldown.

Some hysteresis curves were run before the data for curves 2 and 3 were taken, then 2 and 3 were measured using the opposite polarity from 1. The application of a high field before

measuring the remaining two P (T) curves is most probably responsible for the relationship

of the P curves and the fact that P is higher at 77 K. In any case, there is a fairly rN tN ' ?8 large remanent in this crystal also. There would be maxima in ITC, the ionic thermal

current, which is proportional to SP^/ST, in the 13 to 18 K range, unlike the glass ceramics

in which any ITC maximum appears to be below 4 K. The hysteresis was measured at 4 K at about 0.001 Hz. The loop was wide initially then closed down by a factor of 6 after two

cycles. No saturation effects were evident at either 4 or 77 K, figures 53 and 54, and only

a very slight hysteresis appears at 77 K. Hysteresis was measured when the field on the

crystal was cycled between 0 and 3 kV/cm in figure 55. This loop opens more than the similar

loop for the FEL glass ceramic.

The change in temperature of the crystal estimated from P (T) for a field reduced from

15 kV/cm to 0 at 10 K is AT = 0.26 K. This can be compared to the measured value of about

0.3 K, which is discussed below.

108
u

10 20 30 40 50 60 70 80

Temperature, K

Figure 52. Polarization temperature curves for the KTaC>3 single crystal for

various applied fields

.

Curve 1, P t (4 K) = 0.076 ycoul/cm2

, E = 0.25 kV/cm;

Curve 2, P t (4 K) = 0.23 ycoul/cm2

, E = 0.84 kV/cm;

Curve 3, P t (4 K) = 1.27 ycoul/cm2

, E = 4.05 kV/cm.

109

Ill
112
Results of the electrocaloric measurements on the same KTaO^ single crystal are shown

in figure 56 as a function of temperature. The results are separated into the hysteretic

and reversible electrocaloric components. Addenda corrections have not been made for this

data, but such a correction could give intrinsic values about twice that shown. Figure 57 shows the same data as a function of electric field at T = 10 K. These electrocaloric

effects are considerably larger than those seen in the SrTiO^ glass-ceramic and about com- parable to published values for single crystal SrTiO^.

The specific heat of the KTaO single crystal was measured with the results shown in

3 figure 58. A rapid increase in C/T is observed from 2 K to 8 K, but such behavior is quite

79 typical of many solids in this temperature range. Both adiabatic and pulse methods were

used as described in section 3.6.2. The agreement between the two methods serves as a check

on the accuracy. The results are expected to be accurate to about 5%. _ 80 TIBr and T1C1 : The dielectric constants of TIBr, T1C1, and Til were measured by Samara

between 76 and 400 K. All three materials show very similar behavior of the dielectric

constant, which increases with a decrease in temperature. At 76 K and below values of

3K'/3T are the order of -0.02 K

1 compared to about -80 K

1 for SrTiC^. Measurements in

this laboratory on T1C1 between 0.015 and 10 K show that the dielectric constant levels off

and becomes independent of temperature below about 3 K. The dielectric constant of TIBr is

expected to behave the same way in that temperature range. Such behavior is typical for a material which remains paraelectric at all temperatures down to 0 K. The electrocaloric effect in both T1C1 and TIBr were undetectable. This was to be

expected because of the small value of 3K'/3T. The specific heat of each of these materials

was measured between 2 and 20 K. The results for TIBr are shown in figure 59 and are similar

to that of T1C1. The peak in C/T^ is typical of most solids and will be discussed more in

section 3.8.

3.7.4. Ceramics

SrTiO^ and solid solutions: The polarization as a function of temperature was measured

for various SrTiO^ ceramics. Curves of Pt

(T) and P

r

(T) are shown in figure 60. The slope,

9PtN

/3T, of the KTaO^ single crystal is larger over the whole region below 40 K, though

3Pt/3T of SrTiO^ ceramics is larger by virtue of a higher value of P

fc at 4 K. Unfortunately,

3Pt/3T is essentially zero at 4 K and so refrigeration at 4 K is not possible. All the

materials show a remanent though the temperature dependencies vary somewhat. The unique

feature of P^ of the SrTiO^ ceramics is that in three cases, P^ apparently changed sign.

A partial hysteresis loop for a SrTiC>

3 ceramic at 4 K is shown in figure 61. The

material seems to be less prone to breakdown than the KTaO^ single crystal so data could be

taken to higher fields. The first time the fields were applied momentary breakdowns

appeared at 14 and 24 kV/cm as shown. Subsequent cycles showed no further discontinuities.

The hysteresis loops for E between 0 and +30 kV/cm had small areas such as observed in the

glass ceramics.

Electrocaloric measurements were made on sample ST2(1450). The hysteretic heating

component and the reversible electrocaloric component were separated from the results and

shown in figures 62 and 63. The reversible electrocaloric component for this SrTiO^ ceramic

113
500 i

i .1 i |

i

i i i |

i I I I |

I I I I |

I I I I

100

10 —

0

AE=15 kV/cm

KTa03

AE=4.1 kV/cm

Electrocaloric

Component

Hysteretic

Component

ATn

oc 1/T

(Uncorrected

for Addenda)

10 15

TEMPERATURE, K

20 25

Figure 56. Electrocaloric and hysteretic effects in KTa03 single crystal for a field change of 4.1 and 15 kV/cm.

114
Figure 57. Electrocaloric and hysteretic effects in KTaO-^ single crystal at

10 K as a function of electric field change.

115
116
25

5 -

0

2 5

]—i—i—i—i—

|—i—i—

r

i i I i i i i I i i i i i i I i i i i 10 15 20

TEMPERATURE. K

25

Figure 59. Specific heat of TIBr single crystal between 2 and 22 K.

117
0 10 20 30 40 50 60 70 80

Temperature, K

Figure 60. P

t (T) and P

r

(T) curves for various SrTi03 ceramic samples.

Curve E P t (4 K) P t (4 K)/E

No. Specimen (kV/cm) (yCoul/cm2

) (uCoul/kV cm)

1 Commercial grade 0.53 0.21 0.40

2 ST2 (1450) 0.46 0.33 0.71

3 ST2 (1350) 0.39 .064 0.16

4 ST3 (1450) 0.34 .068 0.20

118
8.0

0 5 10 15 20 25 30

Electrical Field (kV/cm)

Figure 61. A partial P^CE) hysteresis loop for SrTiC^ ceramic ST2 (1450) over

the field range 0 to +30 kV/cm. Discontinuities are due to momentary

charge leakage or breakdowns

.

119
1000

_o

-\

I

I I J

ST2(1450)

500 rV-1

\

\- \

\

\

100

50

10

Figure 62.

\

\ •

\ 7.69 kV/cm

\

\

AQh =213/*J-g-L

AQ h

=22.7-^"

\

\

\

\

Open Symbols - AT^

Closed Symbols - ATe

J I l_

10

TEMPERATURE, K

15 18

Electrocaloric and hysteretic effects in SrTi03 ceramic as a function

of temperature for two different values of field change.

120
0 100 200 300 400 500 600 700

E

2

, (kV/cm)

2

Figure 63. Electrocaloric and hysteretic effects in SrTiC^ ceramic as a function

of electric field change for several temperatures.

121
is somewhat larger than that seen in KTaO^ single crystal, as predicted by the polarization

results. Total cooling effects of about 0.1 K were seen at 10 K. Without the hysteretic

component, a cooling effect of about 0.5 K would be expected at 10 K for a field of 23

kV/cm. Because of the hysteretic component, no cooling appears possible at 4 K in this

sample.

The specific heat of the ST2(1450) SrTiO ceramic is shown in figure 64. For the sake

81 82 83 of comparison, the specific heat of single crystal and polycrystal ' SrTiO^ are also

shown in this figure. The reason for the slight upturn in the ceramic curve below 4 K is

not clear, but it could be a result of the specific heat of metallic impurities such as

iron. The impurity level in this ceramic is probably on the order of 1%, which is large

enough to account for the upturn in the specific heat.

Also shown in figure 64 are specific heat curves for vanadium doped SrTiO^ ceramics.

This doping appears to increase the specific heat to some extent, though anomalous effects

begin to dominate below about 4 K. The higher specific heat of the vanadium doped samples

could mean a higher entropy would be available for refrigeration at 4 K. That would be true

only if the additional specific heat is electric field sensitive. Polarization measurements

would be able to indicate if the additional specific heat and entropy are field dependent.

The SrTiO. ceramics containing various amounts of SrV_0. or SrNb-O,. show dc polariza3 z o z b tions that have less T dependence than nominally pure SrTiO^ ceramics. Curves of P

t

<T) are shown in figure 65 for STV(IO) and STV(20) compared to nominally pure SrTiO^ and in figure

66, curves for STN(IO), STN(20) and STN(30) are shown compared to a nominally pure SrTiC>

3 ceramic made at the same T. A couple of points on the remanent curve for a few of the

samples were measured. The STV(IO) and STN(30) specimens have very large remanents. The

polarization measurement was repeated a second time for STV(IO) to measure in more detail. The results are plotted in figure 65 along with the earlier data. The temperature at which the field was applied was higher in the earlier measurement so both P^_ and P^ were reduced at 4 K. The fact that P^ seems to drop below 0 will be discussed later.

Hysteresis curves were then measured for the STV(IO) sample at 4 K, 39.4 K and 70 K, figure 67. The maximum field applied at 4 K is almost 10 times the field that was applied

at 70 K to measure P

fc (T) while cooling. After cooling the sample to 4 K, however, the two remanents are about the same size. This was observed before for some of the glass ceramics.

The magnitude of P

fc (4 K) is dependent on the temperature at which the field is applied.

This was noted but not appreciated in the initial glass ceramic measurements. The first

actual measurement of P

t

(T) when the field was applied at 4 K for the first time was for

this STV(10) sample. The dielectric constant was measured first and found to have a peak at about 45 K about 1.5 times larger than the 4 K value in figure 68. The polarization, P

t

, shows a similar peak, figure 68, when the field is applied at 4 K and P is measured while

warming. The curve is a quasi equilibrium curve since P increases slowly with time at temperatures below the peak and decreases slowly with time at temperatures above the peak.

The curve of P

t

(T) after the field is applied at the high temperature end seems to be Stable

with time. Subtracting P

r

(T) from P^tT) measured while cooling gives a curve the same as

P (T) warming. Measurements of K' (T) for four frequencies between 0.01 Hz and 10 kHz are shown in figure 69 for the STV(10) sample. The applied maximum field was 10 times larger

122
1 5 10 50

TEMPERATURE, K

Figure 64. The specific heat of SrTiC^ ceramic and SrTi03 ceramic doped with

various amounts of V. The behavior of single crystal"-'- and

O O Q O polycrystal 0 ^ °-> SrTiC^ from other measurements are shown for

comparison.

123
Temperature, K

Figure 65. P tN (T) and P rN (T) for two vanadium doped SrTi03 ceramics compared

to a nominally pure SrTi03 ceramic. Curve 1, STV(20) -2 (1350)

,

P = 0.046 Ucoul/cm2

, E = 1.57 kv/cm; Curve 2, STV(IO) -2 (1350) , P = 0.084 ycoul/cm2

, E - 0.36 kv/cm; Curve 2a, STV(10)-2(1350)

,

P = 0.083 ucoul /cm2

, E - 0.36 kV/cm; Curve 3, ST-2(1350), P = 0.064

ucoul /cm2

, E = 0.39 kV/ cm

.

124
10 20 30 40 50 70 80

Temperature, K

Figure 66. PtN

and P rN for SrTi03 containing Srltt^O^ compared to a nominally

pure SrTi03 ceramic.

"rve

e P (4 K) ycoul/cm

2

E(kV/cm)

No . Specimen t

1 STN(30)-1(1450) 0.031 0.37

2 STN(20)-1(1450) 0.028 0.35

3 STN(10)-1(1450) 0.026 0.39 >

4 ST2-(1450) 0.33 0.46
_ .5

CME

u

^ -4 +

o

u

3.

c

» rwm o

N

o

.3

-3.0 -2.0 -1.0

70K

—-.1 —-.2

-.3

~-.4

1.0 2.0 3.0

Electric Field (IcV/cm)

-.5

Figure 67. P t (E) hysteresis curve for STV(IO) at 4, 39.5 and 70 K. The 39.4 K

loop opening is the same as the opening at 4 K. At 70 K the loop

opening was only 1/5 that at lower T.

126
0*ooK«om^-cocN oooooooo ( 2 ujD/|noDrY) *j 'uojjdz u d joj

Figure 68. The P t (T) curves for STV-10 for E = 0.36 kV/cm. Curve 1, field

applied at 81 K then the sample was cooled. Curve 2, field applied

at 4 K then the sample was warmed. The diamond points on curve 2 are from Pt

(T) - P r

(T) for curve 2a of figure 65. Curve 3, K

T measured at 10

-

^ Hz

.

127
0 10 20 30 40 50 60 70 80 90

Temperature, K

Figure 69. Curves of K'(T) at four frequencies between 0.01 Hz and 10 kHz for

STV(IO). E = 0.35 kV/cm max at 0.01 Hz and 0.02 kV/cm max. at the

higher frequencies.

128
for the 0.01 Hz measurement but since P(E) is fairly linear at least at these low fields,

the K' (T) curve should not be shifted significantly by a lower amplitude measurement. There

is definitely a frequency dependence in K' (T)

.

The dielectric constants of a SrTiO^ ceramic, STV(2), STV(IO), and STV(20) are shown in

figure 70 normalized to the 4 K value. The peak occurs at a lower temperature for STV(2)

than for STV(IO) though the peak to 4 K ratio of P is about the same. The peak is flattened

for STV(20). There appeared to be some change in K' when a dc field was applied.

The dielectric constant of an SrTiO^ ceramic prepared at 1350°C was then measured to

see if an observable peak exists in the dielectric constant. The results for P and K ' are

shown in figure 71. A peak exists in K' but the peak value is only about 0.5% higher than

the 4 K value.

The peak to 4 K ratios of K' were largest for the vanadium doped SrTiO^ . The 20%

niobium doped SrTiO^ has a peak 6% higher than the 4 K value as shown in figure 72. The

shape of the curve appears to be slightly different than most of the K(T) data in that

3P/3T seems to decrease in magnitude as 4 K is approached. The STN(5) specimen shows a peak

only about 0.3% higher than the 4 K value and at a much lower T. One SrTiO^ ceramic with 5% antimony doping also had a small peak in K' (T) at about 10

K. The peak was only about 0.2% higher than the 4 K value.

KTaO^ and solid solutions: Polarization of a number of the KTaO^ ceramics were

measured. The Pt

( T ) curves for those measured are shown in figure 73. In only one case were the ceramics superior to the single crystal. Sample KT(1)1 had a larger polarizability

,

P

t/E, by a factor of 3-1/2 than did the single crystal. Also, P was a considerably larger

percent of P . This results in a larger value of 3P/3T at intermediate temperatures but not

much improvement below about 10 K as can be seen in the figure. The behavior of the remanent

was significantly different from that of the single crystal. At about 29 K, P drops with

time at constant temperature. This behavior was still observed at 42 K. This sample broke

down when a field of about 5 kV/cm was applied. The value of P /E at 4 K was not as large

for sample KT-K1300) and the remanent was almost half of P at 4 K. The remanent is only

^ 10% for KT-K1200). The remanent was only measured at the two extremes of T for most for

the remaining samples. There would be a peak in ITC for both the KT(1) and the single

crystal though perhaps the temperature of the peaks differ. The ceramic materials all had

P^ values that were a higher percentage of P than did the single crystal.

The single crystal had a rather poor dielectric strength which was definitely undesirable

for an electrocaloric refrigerator. The breakdown strength of a ceramic such as KT (1)-1

appeared to be even lower than the single crystal, although no effort was made to improve

the ceramic.

The dc polarization of some of a series of KTaO^ ceramics containing 10 and 20% vanadium

in place of tantalum were measured and the results of the P^ and P measurements on three

t r samples of vanadium doped KTaO^ are shown in figure 74 along with the P

fc (T) and P^CT) measurements of a nominally pure KTaO^ ceramic. A small amount of data were taken on three samples.

The data for KTV (10) -1 (1250) was taken on warming. The magnitude of P (4 K) would presumably

be larger and 3Pt/3T would be increased some if P (T) was measured from high T. None have

as large a value of P at 4 K as the nominally pure KTaO^ ceramics or as large a value of

129
0)

b^s co CN

CO « 4-> O C

•H

00 O

d a

•Hd T3

•H

03

-Ul d

o

o

00

« o

iH II v > > ^

C/3 <f O -

II o

m

O ^ H

on cu CM o

•H H H

EHH

E/D S o

S-l £

O •H CJ rH

IW a o-»

co > 4-1 II •H

4J ro u 00

CO 0. O

•H iH O II 03 O H u w 4J a P

cu w

TD \

0) CN N +J •H 3 •H >

rH •H H tO T) 6 CTJ u fi o

O CO CN O

C > >

EH H

o cn

Csl II MH MH o o a

CO CO

CD>

QJ d o

u H 4-J

N » '4UD4SU03 Dijpd|8g(3 paziptujofsj

o

a)IHd

oo

•H

PL4

130
0 2 4 6 8 10 12 14 16

Temperature, K

Figure 71. K' (T) and P t

(T) curves of ST 2(1350) normalized to 1 at 4 K.

There is a very small peak at about 6 K observed in K '

.

131
0 10 20 30 40 50 60 70 80

Temperature, K

Figure 72. P t (T) and K*(T) for STN ceramics. Curve 1, K

f (T) of STN(20)

,

K' (4 K) = 1370. Curve 2 P t (T) of STN(20) , E = 0.35 kV/cm,

Pt (4 K) = 0.0028 ycoul/cm^. Curve 3, K'(T) of STN2

.

K' (4 K) = 1702.

132
0

10 20 30 40 50 60 70 80

Temperature, K

Figure 73. P tN (T) and P

r^(T) curves for various KTa03 specimens.

Curve

No. Specimen E(kV/cm)

P

t (4 K)

(yCoul/cm2

)

P t (4 K)/ E (yCoul/kV'cm)

1 Single crystal 0.25 0.76 0.3

2 Ceramic KT(1)-1 0.38 0.35 0.92

3 Ceramic KT-1(1300) 0.37 0.106 0.29

4 Ceramic KT-1(1200) 1.7 0.072 0.043

5 Ceramic KTV(20) ' 1.4

(1300)

0.041 0.029

133
0)N

75

E

0.3

0.2

i 10 20 o 40 50 60

Temperature, K

n so

Figure 74. P t ^ and PrN

for vanadium doped KTaO-^ ceramics compared to a nominally

pure KTaO^ ceramic, curve #4,

Curve

No.

1

2

3

4

Specimen

KTV(20)-1(1250)

KTV(10)-1(1250)

KTV(20)-1(1300)

KT-1(1300)

P (4 K)y Coul/cm"

.0093

.027

.041

.106

E(kV/cm)

0.42

1.38

1.40

0.37

134
3P/3T. The dielectric constant of KTV (20) -1 (1300) was measured at 1 kHz. The normalized

curve of K' (T) is shown in figure 75 and compared to p tN

- There is a peak in K' at about

12 K. The dielectric constant shows a field dependence.

Polar ceramics: A well known series of polar ceramics is that of the PbZr Ti, 0„ x 1-x 3 (PZT) system. The system is ferroelectric at room temperature for x < 93% but for x > 93%

84

the system is antiferroelectric. The antiferroelectric 95/5 PZT is interesting for this

program since hysteresis effects should be small. Polarization measurements at low temperatures had not been done previously. However, the high ordering temperature would suggest

that 3P/3T is small at 4 K. Measurements of P(T) below 77 K were made for both 65/35 PZT and 95/5 PZT with very

similar results. Both showed negative 3P/3T values, but for the antiferroelectric 95/5 PZT

the slope should be positive. We have no explanation for the observed behavior except to say that 95/5 PZT appears to be a ferroelectric below 77 K. Electrocaloric measurements were made on the 95/5 PZT sample with depolarization resulting in cooling. Such an effect is consistent with the observed negative 3P/3T. Figure 76

shows the results of the electrocaloric measurements made at 12 K and figure 77 shows the

temperature dependence of the electrocaloric effect. This material showed only heating

effects at 4 K. The specific heat is shown in figure 78. The effective Debye temperature

varies from 185 K at low temperatures to 119 K at the peak in C/T

3

. Pb_Nb„0_ type ceramics: The dc polarizations of PCTT (Pb_Cd n ,_Ti n

. TaO.) , PN (Pb_Nb„0_)

,

2 2 1 2 1/ 2 1/ 2d 2 2/

and CCN (Cd CrNbO ) are shown in figure 79. The remanent was measured at only one temperature

2 o on each. Only CCN showed appreciable P

r and that was measured only at 48 K. The remanent

for PN is zero at 4 K to within the limits of the resolution of the measuring system. Both

PCTT and PN exhibit the typical P

fc (T) curve. The magnitude of P

fc (T) asymptotically approaches

the limiting valve at 4 K. It has been suggested that PCTT is an antiferroelectric below

85

the peak in K' (T) at 40 K. A discontinuity appears in the P (T) of CCN somewhere between

86

31 and 42.5 K. A discontinuity appears in the K' (T) measurements also but at a temperature

of about 25 K. Detailed measurements were made on Pb Nb 0 . A peak was noted in K' (T) at 15.4 K for a

87 2 2 7 1 kHz measuring frequency. In figure 80, P (T) and K' (T) at a number of frequencies are plotted as a function of T on a much expanded scale. This work shows a peak at about 17 K but the percent decrease of the 1 kHz value of K' (T) at 2 K from the peak value is almost

87 exactly the same as measured by Hulm, about 2.6%. This peak has been attributed to an antiferroelectric phase transition. If an antiferroelectric transition takes place, then

P

fc (T) should decrease with decreasing temperature. There was no apparent decrease in P^(T)

except in one measurement, curve P, figure 80 which rules out an antiferroelectric transition.

Perhaps the measuring field used destroyed the antiferroelectric ordering. Measurements of

P were made at a field less than 1/3 the field used for the data in figure 80. No peak was observed in P (T) . Also, neither the slope or magnitude of K' (T) were changed by applying a steady electric field during the measurement. Furthermore, a hysteresis loop was measured

at 4 K to E = ± 7 kV/cm, figure 81. p

t

( E ' i s linear with E over this range and there is no

loop opening to within the uncertainty of the instruments. If an antiferro- to ferroelectric

transition is caused by the dc field, there should be hysteresis loops above some value of E.

135
36 \

CO «—

c/> c

o

o

u

OS o

20 30 40

Temperature, K

50

Figure 75. Details of P fcN and Kvr for KTV(20) -1 (1300) . The P t-Pr curve is

calculated from the data for curves 3, figure 73,

field applied reduces K'

.

A steady state

136
Hysteretic Heating

0 100 200 300 400 500

E

2

, (kV/cm)2

Figure 76- Electrocaloric and hysteretic effects in 95/5 PZT at 12 K as a function of electric field change.

137
l

! I I I

\—I—I—i—I\—i—

r

-

:

3 5 10 15

TEMPERATURE, K

Figure 77. Electrocaloric and hysteretic effects in 95/5 PZT as a function

of temperature for a field change of 13.9 kV/cm.

138
Figure 78. Specific heat of PZT 95/5 between 2 and 23 K.

139
0.8

08 0.7

1-1 Q.~ 0.6

©

CO

to©

Q_

toE

0.5

0.4

0.3

0.2

0.1

10 20 30 40 50

Temperature, K

60 70 80

Figure 79. Pt( T ) °f tne pyrochlores Pb2Nb20y and Cd2CrNb06, and the antiferroelectric perovskite Pb2Cd^^2Ti-L^TaO^ . This latter material shows

no evidence of any antiferroelectric ordering over the range measured

Curve

No.

1

2

3

Specimen

Pb

2 Nb 2°7

Cd

2

CrNb0

6 Pb

2 Cd

l/2

Ti

l/2

Ta0

6

P

t (4 K)

2

(UCoul/cm )

0.0066

0.016

0.021

E (kV/cm)

1.47

0.30

1.0

140
0 10 20 30 40 50 60

Temperature, K

Figure 80. Polarization and dielectric constant of PD2ND2O7 on an expanded axis.

The polarization curve is for a 1.47 kV/cm electric field. 'The

K ' (T) curve at 1 kHz and = 0.44 kV/cm was unchanged from the zero steady state field curve when a 1.47 kV/ cm field was applied.

The other K'(T) curves were measured at the same field amplitude

except the 0.01 curve, for which Emax = 1.47 kV/cm.

141
142
This result along with the lack of a maximum in P

fc

( T ) suggests that neither a ferro- nor

antif erroelectric transition takes place around the maximum of K' (T) . In figure 80, K' (T) is shown for frequencies of 10, 1, and 0.1 kHz as well as for a frequency of about 0.01 Hz. The curve of K

1 (T) at 0.01 Hz was determined by measuring hysteresis loops at a series of

temperatures with the dc polarization instrument. The dielectric constant measured using

the dc system gave a slightly lower value for K' at higher T. This curve is raised by 0.8%

to place it slightly above the higher frequency K' values at 52 K. The voltage out of the

electrometer read by the DVM was found to be ^ 1% low for one polarity which agrees with the

correction made. The observed loss tangents are quite small but increase monotonically with

decreasing T by about a factor of 100 when an estimated lead loss is subtracted. The in- crease in tan 6 and the dispersion in the K' (T) curves along with the dc polarization mea- surements suggest that the peak in K' (T) is the result of a dielectric relaxation mechanism

rather than an ordering transition.

The specific heat of the Pb

2Nb.,0^ ceramic is shown in figure 82. Measurements were made for E = 0 and E = 27.3 kV/cm. To within the precision of about 1%, there is no difference in specific heat for the two electric field values. Such behavior argues against

the material being in an antiferroelectric state below 15 K. Also, the lack of an anomaly

in the specific heat seems to suggest no transition to the antiferroelectric state. Thus,

both the polarization measurements and the specific heat measurements are consistent with

each other and indicate the material remains in the paraelectric state down to at least 3 K.

3.8. Discussion of Experimental Results

Several questions have been brought to light by the measurements in this program. In this section we discuss and attempt to explain the following observations, (i) the lack of

cooling in SrTiO^ glass ceramics at 4 K, (ii) the agreement between polarization and electrocaloric measurements, (iii) the peak in the dielectric constant in most dielectrics at low

temperatures, (iv) the reason for the remanent polarization in most dielectrics at low

temperatures, and (v) the specific heat and its relation to electrocaloric refrigeration.

Then finally we make some general conclusions regarding the low temperature behavior of

dielectrics and the potential of electrocaloric refrigeration.

Adiabatic polarization cooling of SrTiO^ glass ceramics was predicted because of large

positive values of 3K'/3T at 4 K. The polarization calculated from K' , assuming no remanent,

would also have this positive slope. However, actual measurements of P on glass ceramics,

ceramics, and single crystals not only showed the presence of hysteresis, but also no sign

of a positive slope in P(T) when cooled in a field. Electrocaloric measurements agreed well

with that predicted from measured P(T) data. What then gives rise to the peak in the dielectric constant and the remanent polarization?

3.8.1. Dielectric properties

66 Earlier measurements of the electrical properties of SrTiO by Weaver have shown

88 hysteresis in the P-E data up to 45 K. Burke and Pressley saw hysteresis in the P-E

curves with stress applied and attributed the hysteresis to surface charge build up. Saifi

and Cross^ have measured apparent antiferroelectric hysteresis loops in some cases. Earlier

89 work on ceramic SrTiO^ showed a peak in K' (T) in one case but not in another.

143
TEMPERATURE, K

Figure 82. Specific heat of PD2ND2O7 ceramic between 3 and 23 K in zero field

and in a field of 27.3 kV/cm. The arrow indicates the temperature

of the observed peak in K' (T)

.

144
Similar results have been obtained for KTaO . A peak in K' (T) was observed originally

90 91 by Hulm, et al. Recent measurements by Demurov and Venevtsev have shown a peak at 10 K. 92 Dielectric measurements in this laboratory on the single crystal of Samara and Morosin

showed a slight peak in K' (T) at 3 K. Hysteresis loops have been observed in P-E curves up

to 56 K. 91

There exists no model of electrical behavior for SrTiO^ or KTaO^ that explains hysteresis

or peaks in K' (T) without assuming a ferroelectric or an antiferroelectric transition. In this section, a model consisting of permanent dipoles, due to impurities, that have an electret state and give rise to dielectric relaxation effects will be proposed and used to explain the behavior of the materials measured here. The model also explains the presence

of apparent ferroelectric hysteresis curves without requiring an ordering transition.

93 The total polarization per unit volume, P

fc

, can be described as the sum of three parts,

P = P + p + p

t e a d

where P is the polarization due to displacement of the electrons, P is the polarization

6 Si due to displacement of the atoms, and P^ is the polarization due to alignment of any permanent dipoles. For many substances, all the temperature dependence of P comes from the

dipole term. Any T dependence in P^ is negligible. For many materials the only T dependence

in P^ would be expected to come from lattice transformations or thermal expansion. The

thermal expansion contribution would be expected to be small compared to the T dependence of

the dipole term. For the soft mode materials such as SrTiO^ and KTaO^, however, P^ can have

a larger T dependence than the dipole term. The P

t

(T) measurements reported here show

3P

t (T)/3T = o for all specimens at 4 K so both P^ and P^ are T independent at 4 K. The

P^CE) data taken at 4 K should only measure the stretch of the electronic and atomic dipoles.

If the dipoles are in harmonic potential wells, the p

t

(E) curves should be linear until

electrical breakdown occurs. The fact that the curves deviate from linearity at high fields

suggests anharmonicity due to a flat bottomed potential well, which would be expected for

soft mode materials. The shape of the P(E) curves in the T independent region tell nothing

about the electrocaloric cooling capabilities of the material.

Measurements of P (T)] for a number of applied fields should have been made to higher

t E fields to determine 9P/3T as a function of E in the temperature region of interest. The

data on the KTaO^ single crystal show only that P is about proportional to E up to E = 4 kV/cm for all temperatures. Some correction may be required for the temperature dependence

of the remanent since it adds a nonreversible part to P

t

- This correction would be expected

to be large for the glass ceramic where most of the T dependence is associated with the

remanent.

Because of the lattice symmetry, there are presumably no permanent dipoles in pure f SrTiO^ and KTaO^ . Since remanent polarizations and hysteresis effects are present in the

materials, permanent dipoles of some sort must exist in these samples.

Usually, a peak in K' (T) is assumed to indicate a transition to ferroelectric or antiferroelectric ordering. If the ordering is antiferroelectric then both p

t

(T) and K' (T)

145
should decrease below the transition temperature. Since P^.( T ) does not decrease with T for

any of the specimens measured here, an antiferroelectric state is immediately ruled out. The observed T dependence of K

1 and do not exclude a ferroelectric phase, but P (T) does

not behave as would be expected. The remanent would be expected to remain nearly constant

71 as T increases then decrease rapidly to zero at the Curie temperature, T , as seen by Gesi

using the same dc technique. The polarization must go to zero around the same temperature

that the peak of K

1 (T) occurs, though there can be some asymtotic behavior at T^ if there is a spread of transition temperatures in the material. Contrary to the expected behavior, the

SrTiO^ and KTaO^ materials showed the most rapid decrease of P (T) with increasing T at the

lowest temperatures. Then P^fT) asymtotically approaches zero at temperatures well above 77

K, though the peaks of K' (T) occur from 6 K to 45 K. In addition, some of the P

r

(T) data reverse sign. Only a formation of domains that proceeds rapidly at low T then slows down at higher T could account for such behavior. If domains form and the domain walls move so easily, then the walls should move easily under fields. The P(E) data, however, show no sign of saturation at the highest fields applied indicating that domain walls do not move easily or do not exist.

An alternative cause of the K' (T) peaks might be dielectric relaxation. A simple model

for the peak in K' (T) is that motion of the dipoles in the material is increasingly inhibited

as the temperature decreases and also they are less able to follow a field at constant T as the frequency increases. A decrease in dielectric constant and an increase in dissipation

94 results. This was first reported by Skanavi and Matveeva for impurity doped SrTiO^ and was later observed by Tien and Cross^"' and Johnson, Cross and Hummel^ for a number of

97 different impurities in SrTiO^. Nomura and Kojima have observed similar effects in some tantalates and niobates containing potassium, bismuth and magnesium. The dielectric relaxation effects in solids have been observed mainly in the perovskites. The peaks in the

dielectric constant are accompanied by a large increase in dissipation. The loss tangents

observed in the specimens measured in this work increased as T decreased. Detailed results were not obtained because the loss tangents were small and apparently dominated by lead

losses. There was some insensitivity to the dissipation factor also, perhaps because of the

poor lead shielding in the cryostat.

The observed remanent polarization is not a necessary result of relaxation effects but

is probably the cause of them. The perovskites are nonpolar prior to a ferroelectric transition. There is another common source of polarization in dielectrics known as the electret

state. There is not a large number of papers in the literature dealing with the electrets

probably because they are commercially important. Some review papers on electrets have been

98-101

published.

Electrets are sometimes specified by the activating process used in their formation.

Those formed by lowering the specimen temperature in a field are called thermoelectrets.

Their formation is thought to result from the trapping of charge or dipoles in a material

after it had been cooled in the presence of an electric field. The dipoles would be trapped

so they cannot freely rotate, or charge may be trapped on the surface, at defects or on grain boundaries. It may be difficult to establish which mechanism or how many of the

mechanisms are causing the remanent polarization. An electret can be formed without raising

146
the temperature of the material though the polarization is not as large. The electret may

have a final charge after the field is removed that gives a field the same direction as (homocharge) or opposite to (heterocharge) the forming electric field. The heterocharge is

due to the induced polarization while the homocharge results from charge leaking from the

electrodes on to the surface of the dielectric. The polarization of an electret can actually

reverse if both charges are formed initially and the heterocharge leaks off more rapidly.

This polarization reversal is common, especially after poling in high fields.

The reviews do not make a distinction between an electret and a ferroelectric. For

this discussion, we will make the distinction that a ferroelectric is the result of a collective interaction that reduces the lattice energy by reducing the symmetry in such a way

that polarization results. The electret, on the other hand, is a mechanical trapping of

dipoles or charge and a polarization results only if a field has been applied. If only one dipole or charge existed in the whole of the material an electret can be formed.

102 Gubkin and Skanavi reported thermoelectric behavior in a number of ceramics, including SrTiO^, at room temperature. The apparent sign reversal on some of the measurements

in this work indicate an electret behavior where both homocharge and heterocharge are present.

The fact that a much higher value of at 4 K results when the specimen is cooled in a field rather than when the field is applied at 4 K suggests the electret state also. If

this was due to refusal of domain walls to move at 4 K under a field, then P would not have

r the high T dependence around 4 K that was generally noted for these specimens.

The model being suggested here is that of a thermoelectret formed from permanent dipoles.

Grain boundary charging is assumed negligible since a similar P

r

(T) was observed even for

single crystal KTaO^. Space and surface charge effects may contribute but such effects are considered negligible because the remanent seems to be repeatable. The dependence on impurities suggest permanent dipoles similar to the impurity-vacancy (I-V) dipole that has been

studied extensively in alkali halides. The I-V dipole in alkali halides consists

of a divalent metal ion accompanied by the metal ion vacancy required to maintain charge

neutrality. Permanent dipoles consisting of impurities and vacancies should be considerably

more complex in the perovskites because the impurity valancy is not a simple 2 to 1 ratio

with the lattice cation and the valancy of both the lattice cation and the impurity are variable. Because of the range of valances available, the electret properties should vary

depending on the temperature, and atmosphere present when the material is manufactured as well as on the impurity included.

Dreyfus"'" 0^ measured polarization current vs. time and found a charging current decaying

as the sum of two exponentials. He interpreted the double exponential as being the sum of

at least two different rate processes, one being the alignment of impurity-nearest neighbor

vacancy dipoles and the other with impurity-next nearest neighbor vacancy dipoles. The

polarization current of FEL (1090) is the time derivative of the polarization P

t

(°°) - P

fc (T) and is shown in figure 46. This polarization current is approximately the sum of two ex- ponentials. In this respect, at least, an I-V dipole model seems compatible with the measured results.

The presence of permanent dipoles with long relaxation times can cause the hysteresis

loops observed for almost all the specimens measured. Since electrets can be formed at low

147
temperatures by field application, there is a source of polarization of long relaxation

time.

The suggested model is one in which I-V type dipoles in SrTiO^ and KTaO^ base materials

are sufficiently free to move and contribute to the ac dielectric constant. As the temperature is reduced, these dipoles freeze out. The dispersion noted arises from the freezing

out process. A low frequency field can move more of the dipoles at any given T. The remanent in STV(IO) is quite large but a fairly large amount of vanadium was added.

The peaks in K' (T) in the glass-ceramics are very similar but the SrTiO^ crystallites are supposed to be pure. The crystals are grown in glass which certainly can be a source of

impurities. If it is assumed that the dipoles in STV(IO), for instance, are a positive and

negative electronic charge one lattice spacing apart then around 10 parts per million impurities, if all form dipoles, are all that is required to give the observed P at 4 K. This impurity level for the precipitated crystals is quite conceivable. The electrical

behavior of the glass ceramics is similar to that of STV(IO), suggesting that the glass

matrix is making no significant contribution to the behavior of the glass ceramics.

Curves of K' (T), P^ (T) , and P „(T) are shown in figure 83 for the simple model just N tN rN discussed. The model assumes that the dielectric constant due to the soft mode effects,

K' . varies as 1/T down to 20 K whereupon it curves over and becomes constant at ^ 5 K. smN

This polarization is normalized to 1 at 0 K and is arbitrarily set to be 0.2 at 80 K. £he,

T_ T ^ o c dielectric constant due to permanent dipoles, K, £N

» is assumed to be proportional to e where T is 14 K and T is zero for the K' ,„ curve shown. This quantity divided by K' ,„ at

o c dN dN

T = 00 is the fraction of dipoles not trapped by the potential kT^. The trapped dipoles can

no longer contribute to the ac dielectric constant, but can contribute to the remanent if

polarized. The ac polarization measured is the K' N(T) curve of figure 83, which is the sum

of K' and K' . This gives a peak in K' at 12.5 K that is 1.066 times the 0 K dielectric dN smN N constant. This peak is changed in temperature and size when T

q

, or K^N

at T = », or the

value of T for which K

' sm departs from 1/T behavior, is changed. A wide range of peak

temperatures and peak to 0 K ratios of K' are possible by varying all three parameters.

The remanent polarization could be proportional to Kj

N

(°°) ~ K

(jN^ T ^ ^ a ^ ^'•P0^ es are aligned by the applied field while cooling the specimen. Since the ac dielectric constant,

K'^, has a strong T dependence, the measured polarization would be expected to be proportional

to K' also. Thus,

P "[k (<») - K (T) ]K' (T) .

This relation gives the P (T) curve shown in figure 83. This assumes a long lifetime for

rN

trapped dipoles and that any dipoles falling into the traps align with the remainder. It is

not obvious what mechanism might produce such dipole alignment but the curve is similar to the remanent polarization observed when heating the specimen.

When a specimen is cooled in an applied field, E, a polarization £ 0

EK '

(

T ) i s observed.

Assuming that the remanent contribution is the same cooling as observed in 0 field while

heating, then

148
1.1 rTemperature, K

Figure 83. Normalized dielectric constant, Kjj, and polarization, P TM and P^Nj

as a function of temperature based on the mathematical model described

in the text

.

149
(T) = e EK 1 to (T) + P (T) . r

The Pj_„ (T) curve in figure 83 is derived from the above equation by assuming a value for e E tN o such that e ONrN EK' (0) + P (0) = 1. No peak is generated in P. (T) regardless of the T depent dence of P (T) and K' (T) because of the relationship between K' ., and P . The curves obtained

r d r from this simple model have temperature dependences for P^, p tN ? and K" similar to those

observed, though the model doesn't specifically fit any particular specimen. For the ceramic

materials containing additives,

sm probably deviates from 1/T at a much higher T, then the

peak in K' (T) and the peak temperatures are increased. The T dependence of P^

N

is probably

less than calculated since all the dipoles would presumably not align in the applied field.

Of the materials measured in any detail, only Pb

2 Nb2

C> 7 differed significantly from the

behavior discussed above. The peak in K' (T) is probably due to dielectric relaxation but

there is no significant remanent at 4 K or hysteresis in the P-E curve. Apparently the

relaxing dipoles are not sufficiently inhibited to form an electret state even at 2 K. 3.8.2. Thermal properties

For temperatures somewhat above 4 K, the term 3P/3T in most materials studied became

reasonably large and negative, expecially for SrTiO^ and KTaO^ (single crystal, ceramic, or glass-ceramic). The largest 3P/3T values, and hence largest entropy changes, occur in the

20-50 K temperature range. The observed adiabatic depolarization cooling in this temperature

range agreed well with that predicted from the measured 3P/3T and specific heat via eq.

(3.3). The actual tmeperature changes were always less than 1 K for the field strengths

used. It was uncertain how much further the temperature could be reduced by using much

larger fields, since 3P/3T was not known at higher fields. If 3P/3T could be measured in sufficiently high fields to make 3P/3T apprach zero, then the maximum entropy change could be

calculated. Since all the materials studied had 3P/3T<0, an applied electric field always

decreases the entropy. The maximum entropy change then would be just the value of the field

dependent entropy of the material, or the dipolar entropy in zero field. When that is

subtracted from the zero field entropy, the remainder is the intrinsic lattice entropy,

which is field independent. As will be discussed later the lattice entropy can be influenced

slightly by an electric field in some cases. In those cases the maximum entropy change can

be larger than the case where the lattice entropy is field independent. In any case the

maximum entropy change can be no larger than the total entropy in zero field. Such a limit

indicates the maximum refrigeration power which can be expected from a material, even though

the material could cool itself down from any temperature.

At this point we are faced with the problem of determining the total zero field entropy

and then separating that into the dipolar and lattice contributions. Equation (3.6) can be

used to determine the total zero field entropy if the specific heat is measured to a sufficiently low temperature.

2 2 For low enough temperatures, 3P/3T = 0 and 3 P/3T = 0, and the dipolar contribution to the specific heat should be negligible. According to eq. (3.4) the specific heat is independent of field in that temperature region. For all the materials studied in this program, 2 2 2 K was a sufficiently low temperature to insure 3P/3T and 3 P/3T were zero. Specific heat

150
results on these samples showed that C/T

3 was beginning to level off in the range 2 - 4 K and

approach the value predicted by elastic constants. For some of the materials, such as SrTiO and KTaO , it would be desirable to have specific heat measurements down to 1 K or

3 3 below to accurately extrapolate C/T to 0 K. However, because C « T , the temperature range

below 1 K contributes only about 2% to the entropy at 4 K. Considerable uncertainties in

3 C/T below 1 - 2 K are then of little consequence.

The only cause for concern in extrapolating the specific heat to 0 K is that an anomaly

in the specific heat may be present in that temperature interval. Only an anomaly which is

influenced by an electric field is of any consequence since any other type would simply

shift all the curves in figure 1 by the same amount. For the case of SrTiO the dielectric

49

constant shows no anomaly down to 0.025 K. The dielectric constant of SrTiO glass8 ceramics goes through a minimum at about 0.1 K and continues to increase as the temperature

is decreased even to 7 mK. "'"^ Similar behavior has been seen in measurements in this laboratory on a KTaO^ single crystal. Because other work in this laboratory has shown that in

SrTiO^ glass-ceramics the minimum in dielectric constant is a function of the ac measuring

field amplitude and frequency, it is probable that a thermodynamic anomaly does not occur

there. That also is consistent with the specific heat measurements on a SrTiO^ glassceramic sample at 4 K as a function of electric field. Since the change in specific heat at

4 K was only 5% or less in a field of 220 kV/cm, the entropy at 4 K is little affected by

such a field. That suggests that if any anomaly did exist, it must be orders of magnitude

lower in temperature than 4 K and thus has no influence on the relative entropy at 4 K and

above. In addition, there is no theoretical reason to expect any anomaly in the range below

1 K. Separation of the total entropy into the lattice and dipolar parts could be done experimentally if a sharp transition occurred. The dipolar part would simply be the transition

entropy and the lattice part would be the background entropy. This separation is commonly

88

done for materials with ferroelectric transitions near room temperature. When the dipolar

entropy is small and there are no sharp transitions, it becomes difficult to separate the

two from experimental specific heat results. Such is the case with all the materials studied

in this program. To help in the separation, we will make use of theoretical models.

Order-disorder materials : Ferroelectric and antiferroelectric transitions are usually

88,108 classified into order-disorder or displacive type transitions. For order-disorder

materials a permanent dipole moment exists even in the non-polar phase which implies the

existence of a double potential well for the ion or group of ions. This kind of dielectric

material is very much analogous to magnetic substances. The elementary magnetic moment of a material is independent of temperature and exists both above and below the transition temperature. In such materials the thermal energy tends to randomize the direction of each

moment so that no net moment exists in the material above the transition temperature . In magnetic materials short range forces of the exchange, super-exchange, or double exchange

type bring about an ordering of the dipoles below the transition temperature. For dielectric

and ferroelectric materials long range forces, such as the Columb interaction, are just as important as the short range forces in bringing about ordering of the moments. Nevertheless

151
the thermodynamic treatment of the magnetic and dielectric cases is identical. Thus depolari

zation cooling with an order-disorder dielectric should be completely analogous to adiabatic

demagnetization of a paramagnetic material. In paramagnetic materials with total angular

momentum of j there are 2j + 1 possible orientations of the magnetic moment and so the

entropy in the disordered state is S = Rln(2j +1). A pseudo-spin model may be used to 109 describe order-disorder dielectrics. For a material with two minimums in the single

particle potential well, spin 1/2 operators are used in the description. Because of the two

equilibrium positions of the particle in the disordered state, the dipolar entropy is S = Rln2 in the disordered state. Sufficiently high electric fields can order the dipole moments

and so maximum entropy changes of the order of Rln2 are possible with order-disorder dielectrics above the transition temperature.

There are a few materials such as potassium dihydrogen phosphate and tri-glycine sulfate

which are typically order-disorder type ferroelectrics. Specific heat measurements do

indeed show entropy changes on the order of Rln2 at the transition from the ferroelectric to the paraelectric state. The transitions, however, occur at much too high a temperature to be useful for refrigeration at 4 K. The lowest transition temperature for a known order108 108 disorder ferroelectric is 96 K for KH AsO and 122 K for KH PO . The theoretical

109

entropy and spontaneous polarization of such materials were derived by Devonshire and

are shown in figure 84 as a function of the reduced temperature. According to that calcula-

-2 tion S/R falls below 10 for T/T < 0.3 and continues to drop exponentially. Thus for

c KH AsO below about 30 K the dipolar entropy is too small to make a useful refrigerator. In

-2

order to meet the requirement of S/R £ 10 at 4 K, it would be necessary to have an orderdisorder material with a transition lower than about 13 K. If the ordered state is ferroelectric, then there still would be the problem of hysteretic heating during field changes.

The material lithium thallium tartrate (LiTIC .H .0 *H„0) , hereafter abbreviated as LTT, 4462 110-113

has shown indications of a ferroelectric transition at about 11 K. A fairly pronounced peak in the dielectric constant occurs at 11 K for semi-transparent electrodes as

112 well as a rapid softening of the lattice. Both the elastic compliance and dielectric

constant are strong functions of electric field in the region around 11 K. The structural

similarity to Rochelle salt indicates it may be an order-disorder ferroelectric. Unfortunately there was not time to measure the specific heat and electrocaloric effects of this

material because of the difficulties in obtaining a sample. Existing measurements "'""'" ~* of the

spontaneous polarization P^ can be used to give estimates of the entropy change near the

transition. This estimate is done via the Clapeyron equation,

AS = P

2 /2e C, (3.24)

s o

where e is the dielectric constant and C is the Curie constant in the Curie-Weiss expression

K' = C/(T - T ) . (3.25) o

3 -7 2 3 If we use C = 1.4 x 10 K and P = 2 x 10 C/cm and a molar volume of about 40 cm /mole,

S -4

the entropy change is only AS/R = 8 x 10 . Such a change would be too small for useful

152
153
refrigeration but further measurements on LTT would be useful.

Displacive materials : The other class of dielectric transitions is the displacive type.

In these materials the potential well, V(X), has only one minimum and is only slightly anharmonic. A permanent dipole moment does not exist in these materials above the transition temperature, but the application of an electric field induces a moment. As the temperature is

lowered in such a dielectric, the long and short range forces on the ion can change in such

2 2 a way that they tend to cancel each other at some termperature . When this occurs, 3 V/3X =

0 at X = 0 and the ion becomes unstable at X = 0. A small displacement of the ion can occur

at and below that temperature and the material becomes a displacive ferroelectric or anti114 ferroelectric. A lattice dynamic description of such materials, as introduced by Cochran,

and Anderson^"'" has proven very useful."'"

0 ^ In this description the frequency, oi, of the optic

phonon mode decreases as the temperature is lowered. At the transition temperature cj becomes

zero at some value of the wave number q. The process is called a condensation of a soft mode.

If the condensation occurs at q = 0, the transition is to the ferroelectric state, whereas if

it occurs at the Brillouin zone boundary, an antiferroelectric transition occurs. An electric

field has a strong effect on the soft mode.

The ABO^ compounds having the perovskite structure usually display the displacive type

transitions. The material PbTiO^ is the most ideal example in the sense that the soft optic

mode is sharp and underdamped in contrast to BaTiO where the soft optic mode is highly over116 damped. Both SrTiO^ and KTaO^ are good examples of displacive or soft optic materials

though the optic mode never quite condenses at q = 0 even for T = 0 K. These materials

could be called incipient ferroelectrics . All of the materials studied in this program are probably of the displacive type.

The lattice dynamic description of the displacive materials provides a simple way of

separating the lattice and dipolar entropies. In such a model the lattice entropy is that

associated with the acoustic modes and the dipolar entropy is that of the optic mode. Recent

neutron scattering experiments using high flux reactors have been valuable for mapping out the

phonon dispersion curves of several of the materials studied in this program. Various thermodynamic properties of the material can then be calculated from these phonon dispersion

curves. For instance, when the phonon dispersion curves are independent of the temperature,

117

the specific heat is given as

where V is the volume, k is Boltzmann's constant, q is the wave number, p refers to summation

over all polarizations, and x is given by

The summation over the polarizations refers to the various transverse and longitudinal modes

in the acoustic and optic branches. The transverse modes are doubly degenerate in both the

acoustic and optic branches. In general, the phonon dispersion curves have not been measured

in enough directions or in enough detail to warrant solving eq. (3.26) exactly. Instead we

(3.26)

x(q,T) = fra)(q)/kT. (3.27)

154
make the assumption that the phonon dispersion curves are the same in all directions and

approximate the Brillouin zone with a sphere. With those approximations we get

E

1 2 x Y dy, (3.28) 3R , . X ,,2

p ' 0 (e - 1)

where y = q/q . For a cubic crystal with a zone boundary of q^ _ along a [lOO] direction,

for the spherical approximation is w - 3/^%.z- (3 - 29)

With these approximations the entropy is given as

3R " o

J 0 eX

- 1

-x 1 2 -ln(l - e ) | y dy. (3.30)

J

If the dispersion curves are functions of temperature, the entropy is still given by eq. (3.30),

but then the specific heat must be calculated from

C = T3S/3T. (3.31)

The materials studied in this program had phonon dispersion curves that were nearly temperature independent below 15 K. Thus, eqs. (3.28) and (3.30) were used in all the calculations

since the region of main interest in this program is that below 15 K. The calculations for

C and S were carried out to 40 K and, in general, the error introduced in C at that temperature by not using the exact technique of eq. (3.31) is probably the order of 10%. That

uncertainty is still smaller than that of the complete phonon dispersion carves. Figure 85 shows the phonon dispersion curves for SrTiO^ used for the thermodynamic

calculations. For low q the curves are based on the neutron scattering work of Yamada and

118 Shirane. ~ The slopes of the acoustic modes at q = 0 are consistent with the elastic

119

constant measurements of Bell and Rupprecht. Ir.e rebye temperature, normalizes, tc cr.e atom per molecule, derived from the elastic constants is 390 K. The double valued part of

the transverse optic (TO) curve at low q and E = 0 indicates the uncertainty of the curve.

" " z The upper curve is based on the neutron work* and the lower curve is based on Raman scatter120 ing work. The difference is outside experimental error and has been attributed to thermal

aging which causes an increase in the energy with time. The lower curve was used in our calculations because it is more consistent with that derived from dielectric constants using

the Lyddane-Sachs-Teller relation,

kvk'~ = (3 - 32)

133
Figure 85. Phonon dispersion curves for SrTi03 used for thermodynamic calculations

The curves are based on neutron and Raman scattering work.

156
where K

1 and K' are the static and high frequency dielectric constants, io„ and u _ refer

O « 3 -i j LO TO

to the longitudinal and transverse optical phonon frequencies. In any case the total entropy

or specific heat would be reduced by less than 1% if the upper curve was used. The dispersion

curves for various values of electric field are estimates based on the q = 0 values from

120

Raman scattering cara. For high values of q the dispersion curves in figure 85 are based on the work of Shirane

and Yamada 2 ^ and of Cowley.

2 The dashed line in figure 85 shows the soft acoustic mode,

?25' a-^on<3 ^e [m] direction which is responsible for the antiferrodistortive structural

transition at 105 K. The curve as drawn is the position of the mode below about 40 K. The

highly anisotropic nature of the T mode means that its contribution to the specific heat

and entropy cannot be calculated by eqs. (3.28) and (3.30) since they assume an isotropic

mode. Its contribution, however, is probably small and so a crude estimate was made. This

was done by using eqs. (3.28) and (3.30) and multiplying the resultant C and S by a constant

factor to give reasonable agreement with experiment. The factor used here turned out to be

lO"

2

. 3 Figure 86 shows the calculated specific heat curves for SrTi0

3

» plotted as C/RT to

rer.cve z.csz zz z'r.e units ar.d the temperature dependence . The lever lashed, curve is the

contribution from the three acoustic modes. The middle dashed curve is the specific heat

after adding on the contribution from the soft T^- mode. Notice the hump it tends to give

near 5-6 K, which is in reasonable agreement with that seen experimentally in SrTiO^ ceramic.

The top dashed line is the specific heat after the addition of the contribution from the

doubly degenerate soft TO mode for E = 0. For E = 12 kV/cm the specific heat contribution

from the TO mode is decreased. The maximum effect occurs between 5 and 10 K but the total

specific heat is lowered only about 2-3%. Such a small change in specific heat with field is

consistent with our observation of less than 5% change at 4.4 K for a field of 220 kV/cm on a 123

SrTiO^ glass- ceramic. It is also consistent with the results of Lombardo, et al., who

found no change within 3% in the specific heat of single crystal SrTiO^ for a field of 1.8

kV/cm. Shown in the same figure are the results of measurements on a ceramic in this labora81 82 83 tory, a single crystal, and a polycrystal. ' The agreement between the theoretical and

experimental curves is very good over most of the temperature range. The difference is

largest below 4 K, which could be a result of impurity contributions and an uncertainty in

the details of the acoustic dispersion curves for hu/k < 20 K. The elastic constant value

is based on measurements above the 105 K transition and may not be correct for the 4 K value.

124

The phonon dispersion curves for KTaO^ have been measured by Shirane, et al. and Axe,

et al.^"

2 ^ The curves shown in figure 87 are based on their work. The slopes of the LA and

126

TA modes are consistent with elastic constant measurements made at 2 K and give a Debye

temperature, normalized to one atom per molecule, of 327 K. A slight flattening of the TA

mode occurs at about q/a = 0.2, which has been attributed to an interaction between the TO

125 W

and TA modes. Figure 88 shows the calculated specific heat of KTaO^ in comparison with

the measured value. The agreement is good except for the region of 10-15 K. The discrepancy

there indicates the TA mode is too low near <3/<3max

= 1- That may be possible since the TA

mode shown in figure 87 is based on neutron measurements only along the [lOO] direction. The TA mode could be higher along other directions which would mean the average value shown in

157
30

20

CO

COCD

10

0

1 l

|

i I l_M

SrTiO.

polycrystai

elastic

constants

1_L J L I l I I I

200

5 10

TEMPERATURE, K

50 100

Figure 86. Specific heat of SrTi03 calculated from the phonon dispersion curves

as compared with experimental measurements. See text for explanation

of the three theoretical curves

.

158
Figure 87. Phonon dispersion curves for KTa03 used for thermodynamic calculations.

The curves are based on neutron scattering work.

159
150i

1 |

I I I

KTa0 3

100

CO

I

COCD

Experimental

Theoretical

50 —

elastic constants

0

I I I I

L 1

i I M

125

- 150

200

250

350

5 10

TEMPERATURE, K

50 100

Figure Specific heat of KTa0 3 calculated from the phonon dispersion curves

as compared with experimental measurements

.

160
figure 87 would have to be raised. The TO mode contribution to the specific heat is much

less than that in SrTiO^. It is only for temperatures above about 30 K where the TO mode

begins to contribute significantly to the total specific heat (see upper dashed line in

figure 88)

.

A material studied in this program which showed no cooling effects was TIBr. For the

sake of comparison we also have calculated the thermodynamic properties of this material.

Figure 89 shows the phonon dispersion curve, which are based on the neutron scattering work

. 127

of Cowley and Okazaki. The TA mode is fairly anisotropic with the (0,0,1) direction

giving the lowest values and the (1,1,1) direction the highest. Thus, instead of considering

the TA mode doubly degenerate as for SrTi0

3 and KTaO^ , these two curves were used in the

thermodynamic calculations. Such a process tends to average the two curves. As drawn, the

acoustic modes are consistent with a Debye temperature, normalized to one atom per molecule,

128

of 100 K. Elastic constant measurements yield a Debye temperature of 131 K, which is

rather high to be consistent with the dispersion curves. The TO mode is quite high and independent of temperature and would not be considered a soft mode. The dielectric constant

obeys a Curie-Weiss law but the temperature dependence is a result of anharmonic lattice

80

effects rather than a softening of the TO mode. The material is paraelectric for all

temperatures and never even approaches a ferroelectric transition as do SrTiO^ and KTaO^.

The calculated specific heat is shown in figure 90 and compared with the experimental

curve. There is excellent agreement between the two curves. This material has a rather

large specific heat which is a result of the fairly soft acoustic modes. As shown by the

upper dashed line, the TO mode contributes little to the total specific heat.

Theoretical calculations for the thermodynamic properties of PZT and P

2 Nb20

7 cannot be

done since there have been no measurements of the phonon dispersion curves. The three specif

i

heat comparisons (SrTiO^, KTaO^, and TIBr) between theory and experiment indicate that the

lattice dynamic model gives a good description of the thermodynamic properties of the displacive type materials. In all three cases, however, the TO mode contributed only a small part

to the total specific heat. Thus the specific heat measurements are not a very sensitive measure of the thermodynamic properties associated with the TO mode. The TO mode contribution could be factors of 2-5 higher before the agreement with experiment is deteriorated.

Nevertheless, because of the good agreement in the total specific heat, we shall proceed with

the assumption that both the acoustic and TO mode contributions to the thermodynamic propertie

are well described by the lattice dynamic model.

With the lattice dynamic model, we now have the means for separating the dipolar and

lattice entropies for the displacive type materials. The soft transverse optic (TO) mode is

in general strongly temperature dependent and is responsible for the polarization of the

material. We then assign the dipolar entropy as that associated with the soft TO mode and

the lattice entropy as that of the acoustic modes. Electric fields have a strong effect on

the static dielectric constant and by the LST relation in eq. (3.32) they also have a strong

effect on the frequency of the soft TO mode. An applied electric field has the effect of

hardening the TO mode, i.e., increasing the frequency of the mode. The effect is greatest

for that part of the Brillouin zone with the lowest frequency of the TO mode. This is the

zone center for ferroelectric type materials and the zone edge for antiferroelectric type

161
CO

3

4=

q/q max

Figure 89. Phonon dispersion curves for TIBr used for thermodynamic calculations

The tranverse acoustic mode, TA, is shown in two principle directions

These curves are based on neutron scattering work.

162
900

800

700

600

500

400

300

200

100

I l I 1 I I I I I

TIBr

0

Experimental

Theoretical

1 J I I I II I

65

70

CD

80

90

- 100

- 125

5 10

TEMPERATURE, K

50 100

e 90. Specific heat of TIBr calculated from the phonon dispersion curves

as compared with experimental measurements

.

163
materials. At the other extreme of the zone, the electric field should have little effect

on the TO mode. The materials studied here are of the ferroelectric type so that an electric field increases the frequency at the zone center, but has little effect on the frequency

at the zone edge. We then expect that as the electric field increases to infinity the soft

TO mode hardens and becomes nearly constant for all wave numbers. This constant value is

just that of the original soft mode at the zone edge. These two extreme positions for the

TO mode then can be used for the dipolar entropies at E = 0 and E = 00 . We have made the

assumption that the electric field has no effect on the acoustic modes, though we show later

that this is not entirely correct.

Figure 91 shows the entropy of the acoustic and optic modes calculated via eq. (3.30)

for SrTiO^. For E = 00 we have taken the value hco/k = 185 K for the TO mode, which tends to level out the hump in that mode. The calculated entropy for that case is just two-thirds

the Einstein entropy of a material with an Einstein temperature of 185 K. The difference

between the E = 0 and E = °° curves gives the maximum entropy change possible, provided the

acoustic mode is unaffected by the field. We recall that it was desirable to have entropy

_2

changes on the order of AS/R « 10 at 4 K for a practical refrigerator. As shown in figure

91, the maximum dipolar entropy change is over three orders of magnitude less than the

desired amount. Because the dipolar entropy is also so much less than the lattice entropy,

temperature changes during adiabatic depolarization would be expected to be small. For

small temperature changes, i.e. , AT/T << 1, we calculate these changes by

= AS AS

T TOS/3T) C'

1 '

where C is the total specific heat of the material in zero electric field.

Figure 92 shows the calculated temperature changes possible in SrTiO^ when depolarizing

from various E values. Shown for comparison are some experimental measurements. The work

53 of Hegenbarth was for a field of 10 kV/cm on a single crystal and that of Kikuchi and

52 Sawaguchi was for 7 kV/cm on a single crystal. The experimental work shown for this

program was for a field of 26 kV/cm on a ceramic sample. The open symbols indicate actual

observed cooling effects, whereas the solid circles indicate the reversible cooling effect

which would occur if the hysteretic heating component is subtracted. According to the

theoretical curves, higher electric fields would not significantly increase AT below 15 K. We note that some of the observed cooling effects are somewhat larger than the theoretical maximum. If this is a real effect, then the electric field must affect the entropy not

only of the optic mode but of the acoustic modes as well. An interaction between the TO and

the TA modes must then exist in order for the electric field to influence the TA mode. A

125

TO-TA mode interaction is known to exist in the case of KTaO^, as discussed earlier in

regard to a flattening of part of the TA mode at low temperatures. It would be very desirable to study this interaction further by such means as observing the field dependence of

the phonon dispersion curves. It is probably a general occurrence that as the TO mode

softens and approaches the TA mode, the TA mode also softens in part of the Brillouin zone because of the interaction between the two modes. The existence of electrostrictive or

164
TEMPERATURE, K

Figure 91. Entropy of the acoustic and optic modes of SrTiC^ calculated from

the phonon dispersion curves

.

165
SrTi0 3

EXPERIMENTAL #

• } This Work

Kikuchi & Sawaguchi — Heqenbarth - THEORETICAL

TEMPERATURE, K

Figure 92. Calculated maximum temperature change due to optic mode during

depolarization of SrTi03. Experimental values are shown for

comparison

.

166
piezoelectric effects in a material is an indication of coupling between optic and acoustic

119 modes. It is known that an electric field does have an effect on the elastic constants,

i.e., the slopes of the acoustic modes at the zone center. A field of 20 kV/cm decreases

the slope by 4% at 125 K, which is the opposite direction for the effect we are looking for.

Presumably the field increases the acoustic phonon frequency farther out from the zone

center. Because the entropy associated with the acoustic mode is so much higher than that

of the TO mode, a rather small shift of the acoustic mode with electric field can make a large correction to calculated temperature changes. In the case of SrTiO^ a shift of the

acoustic mode entropy by 15% would account for the observed high values of AT/T. Even if

the total entropy of the acoustic modes is completely removed by the electric field, the

-4

resulting entropy change of AS/R = 10 is still two orders of magnitude less than the

desirable amount for a practical refrigerator.

Figure 93 shows the calculated optic and acoustic mode entropies for KTaO^ . The soft

optic mode entropy is less than that for SrTiO , whereas the acoustic mode entropy is higher

-2

than that of SrTi0

3

- The entropies at 4 K are much less than the desired value of S/R = 10

The calculated electrocaloric temperature changes for field changes from E = 00 to E = 0 are

sho.-.T in figure 94. Experimental results on a KTaO^ single crystal with a depolarizing

field of 15 kV/cm are shown for comparison. Both the experimental results and the calculated

limit are less than for SrTiO^. As for the case of SrTiO^, the experimental values for AT/T

are higher than the theoretical limit. A 10% reduction of the acoustic mode entropy in an applied field would account for the larger observed AT/T.

The entropies of the acoustic and TO modes for TlBr are shown in figure 95. The TO

mode (shown in figure 89) for TlBr, unlike SrTiO^ and KTaO^, does not soften at the zone

center, i.e., the phonon frequency is nearly independent of wave number. Thus an electric

field should have little effect on the TO mode and its entropy, as is indicated by the

calculated curves in figure 95. The entropy of the acoustic mode is on the order of S/R =

-2

10 , but little interaction would be expected between the TO and acoustic modes because of

the large difference in energy between the two modes. Thus, the electric field should have

little effect on the acoustic mode. Theoretically the temperature changes during adiabatic

depolarization are very small (about 1 mK at 7 K) and probably not observable. As expected,

no temperature changes could be detected experimentally.

Except for PZT, none of the displacive type materials showed a transition to the ferroelectric or antiferroelectric state. Instead they remained incipient ferroelectrics down

to OK. If a displacive transition to the ferroelectric state did occur at a low-temperature

would the dipolar entropy then be high enough for a practical refrigerator? To answer this

question we look at the general behavior of the soft TO mode dispersion curve. For the TO

129 mode we consider a fairly simple but reasonable dispersion curve of the form

2 2 2 2 u (q) = A^ + s q , (3.34)

where Aq

is the phonon frequency at q = 0 and s is a constant. Putting this equation in

temperature units and in units of y = <3/<3max

gives

167
TEMPERATURE, K

Figure 93. Entropy of the acoustic and optic modes of KTaO^ calculated from

the phonon dispersion curves.

168
10

-1

10-

10

-3

KTaO

q } Experimental — Theoretical Limit

10"

5 10

TEMPERATURE, K

50 100

Figure 94. Calculated maximum temperature change due to optic mode during

depolarization of KTaO.

comparison

.

Experimental values are shown for

169
TEMPERATURE, K

Figure 95. Entropy of the acoustic and optic modes of TlBr calculated from

phonon dispersion curves

.

170
nco/k = [(fiA Q

/k)

2 + (cy)2J

1/2

. (3.35)

Figure 96 is a plot of equation (3.35) for various values of A as c is varied to keep fiw/k o at 200 K for y = 1. Condensation of the TO mode would follow the behavior of these curves

as Aq approaches zero. The condensation of the mode, which gives rise to a ferroelectric

transition, can occur at any temperature.

Figure 97 shows the entropy of the TO mode, calculated via equation (3.30), at various

temperatures as a function of ftA^/k. From these curves we note that the dipolar entropy at the transition (A =0) increases with temperature. Whereas S/R is significantly greater

-2 ° -2

than 10 for a transition at 50 K, it is significantly less than 10 for a transition at 5

K. In SrTiO_ a transition does not occur and instead -fiA /k comes down to only 16 K. At 5 K

3 ° -4

the dipolar entropy for this case as shown in figure 97 is about 3 x 10 R. If the transition did occur, the entropy is increased by only a factor of three as seen in figure 97. Thus a transition in SrTiO^ to the ferroelectric state would not significantly increase the

dipolar entropy. The entropy change which can be brought about by an electric field change

from E = 0 to E = 00 is just the entropy at a given value of fiA^/k minus the entropy at hA

Q

/k = 200 K. For -fiA^/k = 200 K the phonon dispersion curve is independent of phonon momentum (see

figure 96) and probably cannot be raised any further by an electric field. The maximum

entropy change an electric field causes on the mode occurs when the model undergoes a transition (fiA^/k = 0) . This maximum entropy change caused by a change in field from E = 0 to E =

°° is the entropy at fiA^/k = 0 minus the entropy at fiA^/k = 200 K. The curve marked displacive

material in figure 98 shows this entropy change as a function of temperature. The significance of the curve is apparent when it is compared with the region of practical refrigeration,

also shown in figure 98. The lower boundary for that region is defined by AT/T

f = 1 below

about 20 K and gradually decreasing to AT/T^ = 1/3 at higher temperatures. In this expression T

f is the final temperature reached after cooling from some higher temperature. The

curve was calculated using the Debye specific heat of a material with Debye temperature of

200 K. This definition of practical refrigeration pertains only to the ability of a material

to cool itself to T^ and says nothing about the refrigeration power once T^ is reached. If

the refrigeration power were also considered, the lower limit of the practical refrigeration

would be at least a factor of two higher, depending on the specific refrigeration requirements. In any case it is clear that the lower boundary for practical refrigeration is in

reality a somewhat fuzzy band with a width of at least a factor of two centered on the sharp

line drawn in figure 98. The example worked out for figure 98 is for a material with a Debye temperature of 200 K. The low temperature part of the AS/R curves for the displacive material and the region of

practical refrigeration will change for different Debye temperatures. However, each of the

two curves will change by the same percentage amount so the relative position of the curves

in figure 98 remains unchanged. It is evident from figure 98 that the possible entropy

change in a displacive material is much too small to be useful for practical refrigeration

at any temperature. This results from the fact that the TO mode is always higher than the TA mode and thus the dipolar entropy is always less than the lattice entropy. This means

171
Figure 96. Plot of equation (3.35) for various values of A and c. These curves

are typical of the dispersion curves for transverse optic modes as they soften and condense.

172
1 I I

I I I

1

1

1 1—I—I I I I I

I

T=100 K

TiAo/k, K

Figure 97. Calculated entropy of a transverse optic mode from the dispersion

curves of figure 96 as a function of hA /k for various temperatures.

173
T. K

Calculatea maximum entropy change for a field change from

E=0 to E=°° in a model displacive material in which the TO

mode has condensed at the zone center. The dispersion

curves used in the calculations are from Figure 96. Shown

for comparison are the maximum entropy change expected in an order-disorder material and the region of practical re- frigeration.

174
that a displacive material cannot cool itself significantly in any temperature range, unless

the TO-TA mode interaction is large. In a material with a large TO-TA mode interaction the

Debye temperature would have to be less than about 80 K (normalized to one atom per molecule)

to provide enough lattice entropy for practical refrigeration. No such a material is known

to exist.

For an order-disorder material with a two position dipole the maximum dipolar entropy

change is simply AS/R = ln2 for all temperatures. The ordering temperature must be below

the desired refrigeration temperature in order for the entropy to be changed by R£n2 . As shown by figure 98 an order-disorder material could provide practical refrigeration below

about 30 K. Adiabatic demagnetization of a paramagnetic salt below 1 K is a classic example

of the practicality of the magnetic order-disorder materials. Because of the many spin

orientations possible in some magnetic substances, e.g., gadolinium, the entropy changes may

be as high as R5,n8. In that case the order-disorder line in figure 98 is moved up into the

region of practical refrigeration even at room temperature. It should be noted however that

the removal of most of the R£n8 entropy at 200-300 K requires extremely high magnetic fields.

In the case of dielectric materials, either order-disorder or displacive, extremely high

electric fields are required to remove most of the dipolar entropy at room temperature,

whereas at lower temperatures more moderate fields would be sufficient.

3.8.3. Recommendations for further work . We consider here one final possibility in the search for materials useful for electrocaloric refrigeration at 4 K. Suppose we use an order-disorder material and by some means

we lower the transition temperature down to about 5 K. There are a couple of methods whereby this can be done. One method uses hydrostatic pressure to lower the transition temperature and the other uses an electric field bias on an antiferroelectric material. Samara

130

has measured the transition temperature as a function of pressure for the ferroelectric

material KH„P0„ and the antiferroelectric material NH„H„P0„. The transition temperatures

2 4 4 2 4 decrease to 0 K at 17 kbar for KH P0„ and 33 kbar for NH„H PO,,. The transition temperatures

2 4 4 2 4 in zero pressure are 122 K for KH„P0„ and 151 K for NH„H P0„. What happens then to the

2 4 4 2 4 transition entropy, or the dipolar entropy, as the transition temperature is lowered? To answer this we use the Clapeyron equation,

AS = Av^-, (3.36) dT

c

where Av is the change in molar volume at the transition from the ferroelectric to the

paraelectric state and dp/dT is the slope of the pressure vs. temperature curve separating

° 89 the two phases. At p = 0 the entropy change is AS/R = 0.37. This number is used with

Samara's

130 value for dp/dT at p = 0 to calculate Av at p = 0. Then if Av is assumed independent of temperature, we get the following expression for the entropy change below 50 K by

using Samara's data for T^ vs p:

AS/R = 6.0 x

10~ 3 T . (3.37)

c

175
-2

At T = 5 K the entropy change is then AS/R = 3 x 10 , which is sufficiently high for a

c -2

refrigerator. For NH^H^PO^ we expect the entropy change at 5 K to be about AS/R = 2 x 10

Because NH^H^PO^ is antiferroelectric, it would not have the problem of hysteresis. However,

the need for a pressure of 33 kbars probably makes such a method impractical.

In the case of an antiferroelectric, such as NH^H^PO^, an electric field also reduces

the transition temperature. For that case the Clapeyron equation is

dE

AS = -P

s —, (3.38)

c

where P

g is the sublattice polarization and E is the electric field along the transition

from the antiferroelectric to the paraelectric state. No data exists for dE/dT , but we

c assume it falls off linearly with temperature below about 50 K, just as for the case of

-2

dp/dT

c> At 5 K we then expect the entropy change to be about AS/R = 2 x 10 . The field

necessary to lower the transition to 5 K may be on the order of 100 kV/cm. The application

of a field to an antiferroelectric in some cases may introduce a ferroelectric state between

the antiferroelectric and paraelectric states. If this occurs in NH

4 H2 PO^ the entropy

change at 5 K would be reduced but the field required to push the transition down to 5 K would also be reduced. A study of NH„H P0„ in an electric field would be very desirable in

4 2 4 establishing its usefulness as a refrigerator material.

For a material to have a transition to a ferroelectric or antiferroelectric state at a very low temperature requires a very small dipole moment. In fact the transition temperature

will be proportional to the square of the distance between ions. At some point the zero point motion of the atoms would tend to make smaller displacements, and hence lower transition temperatures, unstable. Kurtz"*"

3 ^ has shown that 20 K is an approximate cutoff temperature below which a transition to a dielectrically ordered state cannot occur. Naturally a lower transition could occur in dilute materials, but the entropy per unit volume is also

decreased by the dilution.

4. CONCLUSIONS

This study showed that the SrTiO^ and KTaO^ glass-ceramics and doped ceramics have more pronounced peaks in the dielectric constant vs. temperature curves than do the other nominally

pure dielectric materials studied. The positive slope at temperatures below the peak cannot

be used for electrocaloric refrigeration as originally believed. The fundamental thermodynamic quantity, the dc polarization, had only negative values for 3P/3T when the sample was cooled in the applied field. The slope was nearly zero for temperatures below about 5 K in

all samples tested. It was concluded that the peaks in the dielectric constant were a result of the thermal electret behavior of impurity-vacancy dipoles.

Electrocaloric cooling effects could be seen in many of the samples tested for the

temperature range 10-30 K where 3P/9T is large and negative. The largest effects were seen

in SrTi0

3 ceramics rather than glass-ceramics with similar effects seen in a KTaO^ single

crystal. The observed cooling effects were the order of 0.5 K or less for fields of 2 7 kV/cm. Such effects were not large enough for a practical refrigerator, although the fields were not very high.

176
In this study, glass-ceramics, ceramics, and single crystals were investigated to better understand the general principles of dielectric behavior and to search for possible

cooling materials. All of the materials studied for possible cooling materials were of the

soft optic phonon mode (displacive) type dielectrics. Theoretical calculations using the

phonon dispersion curves were made of the thermodynamic quantities and showed good agreement

with the experimental results. From these calculations it was concluded that the entropy in

such materials is one to three orders of magnitude too small for practical refrigeration at

4 K even with infinitely high electric fields. However, discovery of a material with a low

Debye temperature and a large TO-TA mode interaction would warrent further investigation.

Dielectric materials of the order-disorder type have orders of magnitude higher entropy

(AS/R ss 1) above their transition to the paraelectric state than do the displacive type

materials. Unfortunately, an order-disorder dielectric with a low enough transition temperature has not been found. It is recommended that the material lithium thallium tartrate,

which has anomalous dielectric behavior at 11 K, be investigated further. Further low

temperature studies are also needed on NH^H^PO^, which from theoretical estimates could have

enough entropy change remaining at 5 K in a high field to make a useful refrigerator.

Both mechanical and magnetothermal types of heat switches can be used successfully for

a cyclic refrigerator operating between 15 and 4 K. Multiple leaf contact switches are useful for loads up to the order of 5 W/K. The magnetothermal switches where a transverse

magnetic field alters the thermal conductivity of a metal single crystal are particularly

useful because they have no moving parts. Single crystal beryllium can be used for both the

upper and lower switch. It is also possible to use tungsten, but for the lower switch only.

Further engineering work on an electrocaloric refrigerator is not practical at this

time. Instead more basic research on the thermodynamic behavior of dielectrics must be done

to find a suitable refrigeration material or to show that such a material cannot exist

because of certain fundamental reasons.

177
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U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 76-847

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE 5. Publication Date February 1977

6. Performing Organization Code

275.08

7. AUTHOR(S)

Ray Radebaugh, J. D. Siegwarth, W. N. Lawless & A. J. Morrow

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 2750489

11. Contract/Grant No. NAonr-1-75 and

ARPA 2535

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Advanced Research Projects Agency

Arlington, VA 22209

13. Type of Report & Period Covered , Final

May 1973 - June 1975

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less tactual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

A solid state type of refrigeration, which utilizes the electrocaloric effect

in certain dielectric materials, has been investigated. Such a refrigerator would

operate with a load at 4 K and reject heat to a reservoir at 15 K. Heat switches

for such a refrigerator were studied. One type was a multiple leaf contact switch.

The other type was a magnetothermal switch utilizing single crystal beryllium.

Based upon earlier preliminary work, the refrigeration material was to be a SrTiO^

glass-ceramic. It was found here that such a material has no useful electrocaloric

effect at 4 K. Many other materials were studied but none were found with sufficiently high electrocaloric effects for a practical refrigerator, the largest

effects were seen in SrTiO^ ceramics, followed by KTaO^ single crystal. Temperature

reductions of about 0.5 K at 10 K were observed during depolarization. A theoretical

model, based on the electret behavior of impurity-vacancy dipoles,was developed to

explain the observed dielectric behavior in the materials investigated. Another

theoretical model, based on the lattice dynamics of displacive dielectrics, was used

to explain the observed entropy and temperature changes seen in such materials. The

model points out that displacive type materials have too low entropies at 4 K for

practical refrigeration. An investigation of certain order-disorder dielectrics

is suggested.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons) Beryllium; ceramics ; cryogenics ; dielectric-constant; electretsa

electrocaloric effect; entropy; f erroelectrics ; glass-ceramics; heat switches;

magnetothermal conductivity; polarization; potassium tantalate; fefrigeration;

specific heat: strontium titanate.

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