NITEO STATES PARTMENT OF 5MMERCE

JBLICATION

NAT'L INST. OF STAND & TECH

AlllDb 3bD22D

NBS SPECIAL PUBLICATION 260"39

U.S.

PARTMENT

OF

COMMERCE

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Standard Reference Materials:

THE EDDY CURRENT DECAY METHOD

FOR RESISTIVITY CHARACTERIZATION

OF HIGH PURITY METALS
NATIONAL BUREAU OF STANDARDS

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fiona! Bureau of Standards ace

Standard Reference Materials:

The Eddy Current Decay Methi

For Resistivity Characterizatic

Of High Purity Metals

A. F. Clark, V. A. Deason, J, G. Hust, and R. L. Powell

Institute for Basic Standards

(/, S National Bureau of Standards

Boulder, Colorado 80302

U.S. DEPARTMENT OF COMMERCE, Peter G. Peterson, Secretary

NATIONAL BUREAU OF STANDARDS, Lawrence M. Kushner, Acting Director

Issued May 1972
Library of Congress Catalog Card Number: 72-600 100

National Bureau of Standards Special Publication 260-39

Nat. Bur. Stand. (U.S.), Spec. Publ. 260-39, 53 pages (May 1972)

CODEN: XNBSAV

Issued May 1972

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The Eddy Current Decay Method for Resistivity

Characterization of High-Purity Metals. (This

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vii
CONTENTS

PAGE

1. Introduction 2

2. General Description and Application 4

2.1 Eddy Current Decay Time 5

2.2 Resistivity - Decay Time Relations 7 2.3 Residual Resistivity as a Measure of Purity.. 10

2.4 Residual Resistivity as a Measure of Homogeneity 12

3. Specimen Preparation 13

4. Experimental Apparatus 14

4.1 Electronic Equipment

4.2 Specimen Probe Assembly 21

5. Typical Experiment 29

6. Precision and Accuracy 33

7. References 36

Appendix A: Useful Formulae 38

Appendix B: Tables of Typical Numerical

Calculations 41

Appendix C: Assembly Instructions for Series

I Probes 42

List of Figures

PAGE

Figure No.

1. Correction Factor for Length to Diameter

Ratio 8

2. Experimental Schematic and Pulse Shapes , 15

3. 3 A Power Supply Schematic 18

4. 110 mA Power Supply Schematic; Including

Current Shut-Off, Time Delay, and Attenuator

Circuits 19

5. Eddy Current Probe System 22

6. Primaries and Secondaries for Probe System... 24

7. Storage Dewar Fitting for Probe System 25

vii i
8. Plug Assemblies for Probe System 27

9. Fabrication Details of an Eddy Current Probe.. 28

10. Typical Eddy Current Decay Data 32

ix

The Eddy Current Decay Method

for Resistivity Characterization of

High Purity Metals*

A. F. Clark, V. A. Deason, J. G. Hust, and R. L . Powell

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

Characterization of high purity metals by

resistivity measurements at low temperatures

is discussed. In particular, the eddy current

decay method of resistivity measurement is

described in detail. The advantages and

limitations are presented along with the

theoretical basis of the method. Detailed

instructions are given for constructing and

operating the apparatus.

Key Words: Apparatus; characterization; eddy

current decay; electrical resistivity; high

purity metals; homogeneity; low temperature.

*This work was carried out at the National Bureau of Standards,

Boulder, Colorado, under the sponsorship of the National

Bureau of Standards, Office of Standard Reference Materials

(.NBS-OSRM) , Washington, D. C.

1
1. Introduction

The chemical and physical purity of metals can be characterized

by low temperature resistivity. As the temperature of a metal specimen is lowered, generally, the resistivity of the specimen decreases.

At a sufficiently low temperature the resistivity becomes essentially

independent of temperature. This value of resistivity, termed residual

resistivity, is characteristic of the type and concentration of chemical

impurities and physical imperfections in the specimen. Of course,

only those impurities and imperfections that act as electron scatter ers are important to resistivity. Resistivity characterization, perhaps,

finds its greatest usefulness in detecting variations in purity among

specimens of relatively homogeneous purity. These points will be

discussed in more detail later.

Electrical resistivities are frequently measured using 4-

terminal-potentiometric AC or DC methods. These methods all involve

the connection of two current leads and two potential leads to the specimen. From the measurement of current through and voltage across

the specimen one computes the resistance of the specimen; and the

resistivity is then determined from the resistance and the geometrical

form factor of the specimen. Often, the ratio of the resistivities at

two temperatures is desired instead of the resistivity at one temperature.

In this case, the form factor can be eliminated from the computation;

which also means that the specimen does not need to be fabricated into

some common uniform shape, such as a cylindrical rod. This is a

distinct advantage for specimens that are either difficult to machine or measure. The potentiometric methods are generally very useful for

specimens of reasonably high resistance. However, for very pure,

large diameter and, thus, low resistance specimens it is frequently

very difficult, if not impossible, to obtain an accurate measure of the

2
potential across the specimen. Potentials in the nanovolt range are not

uncommon. For example, the potential drop along a 3 cm long-1 cm2

copper specimen with a residual resistivity ratio, p(273 K)/p(4 K), of

15, 000 is about 1 nanovolt for a current of 1 amp.

A relatively new method, the eddy current decay method, of

measuring electrical resistivity and resistivity ratios has the advantage

of being most accurate in this range where the potentiometric methods

are least accurate. The eddy current decay method also has the added

advantage that no electrical connections to the specimen are needed.

This is especially important for pure specimens, to minimize the effects

of chemical diffusion of impurities into the specimen and to minimize

the effects of physical damage from mechanical strain. The measurement of metals such as mercury, potassium, sodium, and gallium is

facilitated by the elimination of lead attachment. In many cases, the

specimen can be left in the container in which it was prepared. The

eddy current decay method is least accurate for the higher resistance

specimens. Thus, the potentiometric method and the eddy qurrent

decay method of measuring resistivity are complementary methods for

the measurement of a wider range of resistivities than either can

accomplish separately.

This paper describes the theoretical basis of the eddy current

technique and gives detailed instructions for constructing and operating

an apparatus capable of monitoring eddy current decay over a wide

range of decay rates at cryogenic temperatures. Also included is a discussion of the theoretical basis for purity characterization of metals

by electrical resistivity at low temperatures.

3
2. General Description and Application

Briefly, the eddy current technique involves surrounding the

specimen with two concentric coils. A current is established in the

outer, primary, coil, and is allowed to come to a steady value. The

primary current is turned off and the collapsing field induces eddy

currents in the metal. These decaying eddy currents establish a decaying magnetic field that induces an emf in the inner, secondary, coil.

It can be shown that the rate of decay of these eddy currents is a function

of the specimen geometry, its magnetic permeability, and most

[ll

1 importantly, its resistivity. J The rate of eddy current decay is inversely proportional to the

resistivity and directly proportional to the specimen's cross sectional

area. Thus, as a specimen's resistivity increases or its dimensions

decrease, the time constant of the decay becomes shorter. At sufficiently short times the transient response of the measuring apparatus

will distort the eddy current signal. The response time of the system

is determined primarily by the characteristic decay of the primary and

secondary coils surrounding the specimen. These coils, when critically

damped, typically have time constants from 50 microseconds for the

smaller coils to several milliseconds for the larger coils. In practice,

this limitation frequently prevents one from determining specimen time

constants at temperatures much above that of liquid nitrogen or for

specimen diameters less than a few mm. Of course, a certain latitude

is possible by increasing the specimen size as the resistivity increases.

Even room temperature resistivities are measurable if the specimen is

large enough.

In addition to the problem of measuring very short time constants,

it is also difficult to measure very long time constants. With long time

constants the time rate of change of the magnetic field is small,

Figures in brackets indicate the literature references on page 36.
resulting in a very small induced voltage. At sufficiently small voltages,

the signal-to-noise ratio is too small to allow an accurate measurement.

Bean et al. ^ suggest moving the specimen in and out of the coil of a flux-meter at regular intervals to measure the magnetic field decay rate

of the specimen under these circumstances. Also flux gate meters or

[21 superconducting quantum interference devices may provide attractive

alternatives to coil type pickups for low level signals. We have had

good success in using a low temperature Hall effect probe to measure

the decaying magnetic field near the specimen. An advantage of the

Hall probe is that it directly measures the magnetic field rather than the

rate of change of the field. This eliminates the initial voltage spike

caused by the rapid initial decay of the field within the secondary coil.

Even with coil type sensors, one can improve the signal by using low

pass filters and signal averagers to eliminate most of the noise.

In the following sections we will consider the basic equation

governing the form of the eddy current decay and its characteristic

decay time. The relation between this decay time and resistivity will

be presented. This section is concluded with a discussion of some

useful applications of resistivity characterization measurements. One

[3] such application is described by Kasen.

2. 1 Eddy Current Decay Time

In 1959, Bean et al.

^

described the eddy current decay technique for measuring the electrical resistivity of metals. The method

involves setting up eddy currents in the metal, and then observing their

decay. The equation describing the decay of a magnetic field in a metal

is

5
which relates the decay of the magnetic flux density, B, to the resistivity, p, and magnetic permeability, [j. . Generally eq. (1) is applied

only if the metal is non-magnetic, so that u, = u . However, Bean

o postulates that the technique will work even with magnetic materials,

provided the magnetic field is allowed to fall only to a value for which

the metal is still saturated. A further assumption in eq. (1) is that the

[41 resistivity is isotropic, but J. E. Neighbor has discussed a modification of the technique whereby the complete resistivity tensor can be

determined from eddy current data. If the specimen is polycrystalline,

one can still use eq. (1) by assuming an averaged resistivity.

For specimens of uniform cross - section and uniform isotropic

-t/T

resistivity, the decay of an axial B field is given by B = C(e 1 +

-t/Ta -t/i", 4 , . , e + e +...). Because Tj_ > t3 > t3

. . . , this becomes a simple exponential after a sufficiently long delay. Thus,

-t/T B^ Ce ' (2)

where C is a constant determined by the specimen's geometry, the

initial field, and other properties, and t is a constant proportional to

p _1

. The constant of proportionality is dependent on the size and shape

of the specimen as discussed in the next section. Most published solutions to eq. (1) assume an infinite length for the specimen. If the ratio

Arp et al. have computed the error as a function of delay time due

to neglect of the higher order terms. Some experimental results have

been obtained that are in conflict with these calculations; however, it is presently not known whether these instances represent shortcomings

in the theory or experimental procedure. Inadequacies in experimental

procedure that may cause such behavior are discussed in a later section

The authors feel that for most specimens a delay of three time constants

is sufficient to reduce the higher order terms so they can be neglected.

6
of length to diameter falls below about 8, then the measured time constant of the decay will be measurably smaller than the calculated value.

Moulder et al.^-l performed a series of measurements at this lab on a cylindrical copper rod that was successively reduced in length. The

secondary or pickup coil remained centered on the same material

throughout the experiment. The results of this experiment are given in

figure 1 as a correction factor for measured time constants when the

[7] L/D ratio is small. LePage et al. have proposed a functional fit of

this data, together with data of their own. Their expression takes the

form

T actual 1 measured 1 - exp [- 1 . 2(L/D)° *68

]

2.2 Resistivity— Decay Time Relations

The next step is to find the relationship between T , p, and the

geometry of the specimen. Bean et al.^ and Weinstein^ have derived

equations relating the eddy current relaxation time to the bulk resistivity for solid and hollow rods and rectangular parallelepipeds. These

equations are given in Appendix A. As an example, the equation for rodshaped specimens is T = 2.17 X 10~9 |ir2 /p. Using these equations one

can compute the resistivity from the measured time constant and the

specimen dimensions. Appendix B contains some values of T for

typical values of p and r for an aluminum specimen, at various

temperatures

.

Characterization of overall purity of metals also can be accomplished from the ratio of high temperature to low temperature resistivity. The high temperature resistivity is characteristic of the base

metal while the low temperature resistivity is characteristic of the

purity of the particular specimen. By measuring only the ratio of

7

resistivities it is possible to eliminate most of the geometrical dependence. With the four -terminal potentiometric methods one usually uses

the ice point (273 K) for a high temperature point and the helium point

(4 K) for a low temperature point. The eddy current decay apparatus

is, however, not well suited for measurement of any metal at the ice

point because of the high resistivity at this temperature. The resistivity

of many metals is, however, still mainly characteristic of the base

metal as low as liquid nitrogen temperatures (76 K). The resistivity is

significantly lower at 76 K than at 273 K and, therefore, the eddy current

decay method is more applicable at this lower temperature. Thus, one

can measure t(4 K)/t(76 K) and from this obtain p(76 K)/p(4 K) since

p a t" 1

. It is to be noted that the geometrical factor does not totally

cancel due to the effect of thermal expansion. For most metals the

dimensional change amounts to less than 1/2% between room temperature

and absolute zero. Any crystallographic phase change in the material

may cause more pronounced effects.

tivities, p (273 K) and p^76 K), 11

one can compute p(273 K)/p(4 K),

which is the value normally used for material characterization. This

computation is done by assuming Matthiessen 1 s rule, which is written

as

P(T) = P (T) + P(4 K) (4)

We then write

p(76 K) _ t(4 K)

p(4 K) ~ t(76 K)

P

t (76 K)

+ 1 P (4 K)

and

P(4K) = p (76 K)/[t(4 K)/t(76 K) -1] .

9
Thus

p(273 K) P

i (2?3 K)[T(4 K > /T < 76 K)~ l

] RRR = p(4 K)

= p.(76K)

+ 1 (5)

It should be noted that changing the lower temperature, generally, is

not critical since p is independent of T at sufficiently low temperature.

For metals of reasonable purity, p(273 K) is dominated by

[91 thermal, phonon, scattering and is essentially independent of purity.

Thus, by using the ratio of eq. (5), one not only eliminates the geometric

factor, but also refers the ratio to p(273 K), which is essentially constant for these reasonably pure metals. For example, since the nonthermal contribution to p(273 K) is p (4 K) = (p(273 K))(RRR)~ 1

, the

variation in p(273 K) for specimens of a metal having RRR > 1000 will

be ^ 0. 1%.

Thus, the RRR for well annealed samples is a direct measure

of the resistivity contributed by impurities as long as one can ignore

both magnetoresistive and size effect contributions. Copper of ordinary

electrolytic purity has a RRR of about 100. Measurements on copper

and aluminum having RRR in excess of 35, 000 have been made in this

laboratory. In the following two sections we discuss characterization

by resistivity measurement.

2. 3 Residual Resistivity as a Measure of Purity

For metals having a single major impurity, it is usually possible to use the RRR to estimate the impurity concentration level in

atomic percent. Even if the metal contains multiple impurities, one

can sometimes make a more detailed analysis of the impurity concentration levels. If the metal is sufficiently pure and the various

impurities do not interact, one can assume that the total impurity

contribution to the resistivity is the sum of the contributions of the

10
individual imparities. Thus, for a set of n impurities one can write

ptotal = Pi + p 3 + + pn (6)

where p^ is the contribution of the i**

1 impurity to the total resistivity.

The resistivity contributions per unit impurity concentration of various

impurities in given metals, called specific resistivities, are usually

written as

R. = 9.IC. (7)

l li.

in units of p,Q cm/ atomic percent. C. is the impurity concentration in

atomic percent. Then we have

P.ill= R.C. (8)

and

n

ptotal = £ R.C. (9)

i=l 1 1

Even if the effect of a given impurity is unknown, one can often estimate

its effect from the known effect of another impurity by considerations of

relative valence, atomic volume, electronic configuration, crystal

[9] structure, etc.

As mentioned earlier, residual electrical resistivity arises

from the presence of electron scattering centers. Certain impurities

such as the transition elements are strong scatterers, and hence contribute strongly to resistivity. Some impurities, such as silicon, are

often present as oxides or other compounds that scatter electrons to a much lesser extent. If the impurities segregate to localized sinks such

as grain boundaries, then the effect on resistivity is reduced as compared to distributions of impurities in solid solution.

It should be clear, that there are many problems and uncertainties in applying residual resistivity data to the determination of

11
concentrations of specific impurities. Unless a great deal is known

about the history and composition of a given sample, little can be

ascertained, other than an estimate of overall purity. However, if the

impurity concentrations in a specimen are well known, or if a RRR

representative of the bulk material is available, then the eddy current

technique offers a very convenient method for tracing the effects of

various treatments on the physical perfection of the crystals or on the

segregation of impurities at specific sites.

2.4 Residual Resistivity as a Measure of Homogeneity

One application for which the eddy current method seems especially well suited is the comparison among specimens of relatively

homogeneous purity. For example, one could prepare a sufficient

number of identical specimens from the bulk material to get an adequate

sampling of local variations. Then, a random subset of these specimens

could be carefully analyzed to obtain a quantitative measure of the

average concentrations of the various impurities. If one then compared

the RRR of each specimen with the average RRR of the analyzed subset,

quantitative variations in purity could be traced throughout the bulk

material. Note that it is necessary to assume that the relative proportions of the various impurities remain unchanged if one wishes to

directly correlate variations in RRR with variations in the concentration

of a specific impurity. Similarly, if the most likely contaminants are

known, along with their specific resistivity and the RRR of the specimen,

then these data can be used as an independent check on the impurity

concentrations determined by other methods of analysis. In general,

purity predicted from RRR will tend to be somewhat higher than the

real value, since some impurities may not act as electron scatterers.

Thus, it is usually only possible to set limits of purity by this method.

12
Bean et al. comment that the pickup coil should be sensitive

mainly to the material it encloses, and that one can therefore look for

variations in resistivity or purity along a continuous rod or bar of metal

[71 However, LePage et al. state that their experiments with joined rods

of dissimilar metals show that the sensitivity to a step change in resistivity is very poor. The L/D correction discussed previously shows

that some rather long range dependencies exist and may explain this

poor sensitivity.

3. Specimen Preparation

Specimen preparation for RRR measurements is relatively

simple. Specimens of virtually any size or shape maybe used. However, to insure straightforward data analysis, the specimen should

have uniform cross - section and a length to diameter ratio of at least

eight. The former requirement insures an unambiguous solution to

eq. (1), although Bean describes a technique for interpreting data from

irregular samples. This technique, however, requires that t(76 K) is

measurable. The restriction on the L/D ratio is simply due to the fact

that most of the available solutions to eq. (1) assume specimens of

infinite length. If t(76 K) is measurable, the limitations on L/D disappear, as eq. (5) then can be used to eliminate the geometrical factors,

A further restriction concerns the minimum thickness of a given specimen. If the specimen becomes so thin that the electronic mean-freepath is comparable to the minimum specimen dimension, surface

scattering of electrons may become a significant contributor to the

resistivity. Unless this contribution can be calculated, thin specimens

should be avoided. This problem usually does not occur except with

very high purity metals and specimens of quite small diameter. For

example, a 4 mm diameter aluminum specimen with a measured RRR

of 16, 000 requires a size effect correction factor of about 10 percent.

13
If a specimen is to be representative of the bulk material, it is

essential that the surface of the specimen be free of contamination.

This is because of the r

2 (r = radius) dependence of the time constant,

which tends to give extra weight to the surface material. Thus, it is

wise to heavily etch each specimen to remove any surface contamination

This should be done after the specimen is shaped, but before it is

annealed. Various chemical and electrolytic etching procedures can be

found in the Metals Handbook^ ^ or in Teggart.^^ The etch should

leave the surface smooth and unpitted, preferably polished. If eddy

current analysis is to be used to estimate chemical impurity, it is

necessary to anneal the specimen to remove strains and dislocations;

resistivity determinations done on unannealed metals will usually contain appreciable contributions due to lattice defects. Annealing procedures can be found in the various metallurgical handbooks. It is better

to anneal in a vacuum to avoid possible effects due to the atmospheric

gasses. The cooling to room temperature at the end of the anneal

should be done very slowly; rapid cooling may introduce lattice defects.

The specimen is now ready for testing.

4. Experimental Apparatus

A schematic diagram of the eddy current decay apparatus is

shown in figure 2. The basic electronic equipment required consists of a current supply, amplifier, and an appropriate recording device. The

mechanical equipment necessary is a specimen holder probe and a container for the cryogenic fluid. It was indicated in an earlier section of

this paper that problems may be encountered when measuring either

very short time constants (approximately less than 1 ms ) or very long

time constants (approximately greater than 1 minute). The problem

associated with observation of short time constants is distortion of the

1*
15
eddy current decay signal by the transient response of the system. The

additional electronics such as (a) linear current shut off, (b) delay

circuit, and (c) signal attenuator shown in figure 2 result from the

necessity to minimize the effect of this distortion. In the following

section we discuss the basis for the design and selection of the electronic components. Later the specimen holders are described in detail.

4. 1 Electronic Equipment

To satisfy the above mentioned requirements at both short and

long time constants one must obtain a system which has a rapid, error

free transient response with a minimum of long term baseline drift.

The importance of this statement is best understood by considering the

sequence of events beginning with current shut-off, progressing through

the primary current decay, and ending with the eddy current decay. At

the instant the current is shut-off a finite rate of decrease of the primary

current commences. The rate of decrease is determined by the characteristics of the power supply and the coils. With the decrease of

primary current the magnetic field within the primary coil also decreases. At the same time eddy currents are established within the

specimen.

The secondary coil senses the field change caused by eddy

current decay and that caused by the collapse of the magnetic field within

the gap between the secondary coil and the specimen. The field decay

within the gap generally is much more rapid than the eddy current decay

and, thus, results in an initial voltage spike induced in the secondary

coil. The magnitude and duration of this spike is determined by the

characteristic decay time of the primary- s econdary coil system. Thus,

there is no advantage in a current shut-off more rapid than this decay

time. This establishes the lower limit of time constants that can be

measured. This depends on the size of the coil used and is as high as

16
several milliseconds for our larger coils and as low as 10-50 microseconds for our smaller coils. The problem of switching off the current

rapidly with a minimum of noise has been troublesome. Purely mechanical switches are too slow and erratic and even mercury wetted

relays have sufficient contact bounce to give frequent difficulty. A fast,

relatively noise free, solid state switch was designed at this laboratory

to eliminate the problems encountered using mechanical switching.

At present, we use a high current (3 A) power supply, figure 3,

for series III probes (these probes are described in a later section) and

a low current (110 mA) power supply, figure 4, for the series I and II probes. The 110 mA power supply has a linear current shut-off feature

in the current switch. This is done to control the magnitude of the

initial voltage spike and to minimize ringing in the secondary coil. It

is accomplished by using a field effect transistor in its constant current

mode that is turned off with the voltage decay from an RC circuit. The

adjustable rate of current decay together with an adjustable shunt resistance across the primary coil allows one to produce a noise-free,

critically-damped decay of the current.

The linear current shut-off described in the previous section is

incorporated primarily to minimize the magnitude of the initial voltage

spike. However, this voltage spike is still significantly larger than the

eddy current decay signal to be measured. The voltage spike is typically from 3 to 1 5 volts and the eddy current decay signal is often less

than 50 \x V. Any amplifier adjusted to respond to 50 n V is likely to be

severely saturated by a signal of several volts. The effect of such

saturation is likely to continue for appreciable time after the initial

spike is gone, thus distorting the true eddy current signal. Even with

an amplifier carefully selected for rapid recovery it would be prudent

to attenuate the voltage spike to reduce saturation effects. The delay
18
19
circuit and signal attenuator shown in figure 2 and 4 accomplish this

attenuation. The delay circuit is designed to gate the signal attenuator

for times varying from 0. 3 ms to 100 s. This timing circuit also provides a trigger pulse to be used to start the sweep of the recording unit.

The effect of this attenuation on the signal is shown in figure 2. The

attenuator delay circuit is on the same chassis as the 110 mA current

supply. The 3 A current supply is on a separate chassis, but provides

a trigger pulse capable of activating either a scope or the attenuatordelay circuit in the 100 mA supply. The primary coil is connected to

the appropriate current supply, but the secondary coil is always connected to the secondary input of the 110 mA supply in order to make use

of the attenuator circuit. If the attenuator-delay circuit is not being

used, one merely shuts off the delay circuit. One limitation of this

delay circuit might be a serious defect in some applications. The minimum delay possible is about 0. 3 ms. Most specimens at elevated

temperatures or thin wires at any temperature might require shorter

delays. One solution is to redesign the delay circuit, as the limitation

is there, rather than in the attenuator. Another solution is to pass the

unattenuated signal directly into the amplifier, and trigger a scope delay

unit directly off of the initial spike. At these short specimen time constants, the signals due to the decay of the primary field and the decay

of eddy currents in the specimen are of comparable magnitude. Thus,

the attenuator is unnecessary for protection of the amplifier. There is

the additional problem of distinguishing between the eddy current decay

[121

signal and the primary current decay signal. Stern et al. have used

bucking coils or adjustable RC circuits to cancel the signal due to the

primary current.

For recording the signals, we use either a persistant display

oscilloscope or a pen recorder with a time base. The pen recorder

20
trace is initiated by an actuating relay in the delay circuit. We are

thus able to use the attenuator-delay feature of the 110 mA current

supply in conjunction with both scope and pen recorder.

A useful addition to the system is a time mark generator for

calibrating sweep rates. This calibration should be performed for each

sweep rate used. A calibration of the vertical axis is not so important,

because only relative values are required and the voltage is plotted in

arbitrary units. It is necessary, however, that the amplifier and

recording devices be linear, in response to variations in signal level.

4.2 Specimen Probe Assembly

The specimen probe assembly, figure 5, consists of a specimen

holder, a primary and secondary coil support, and a probe support to

allow measurement within a cryogenic dewar. Two main goals directed

the design of this assembly: first, to achieve interchangeability among

a wide variety of primary and secondary coils of differing sizes and

time constants and, second, to avoid the time consuming and wasteful

transfer of liquid helium. The first set of probes (Series I) accepts

specimens with diameters ranging up to 6 mm and fits into standard 25

to 50 liter liquid helium storage dewars. A second set of probes (Series

II) accepts specimens up to 1.7 cm in diameter and fits into a specially

modified storage dewar with a 2 cm ID neck. The third set of probes

(Series III) accepts specimens up to 2.8 cm in diameter and is designed

to be used with a conventional nitrogen shielded glass dewar with a

5 cm ID. The primary winding is placed in the nitrogen shield surrounding the inner dewar and the secondary is attached to the probe

assembly in the same manner as Series I and II. The larger primary

coils generally have longer characteristic time constants. This however

presents no series decay interference problems as the specimen decay

time also increases as the square of the specimen diameter. The probe

21
22
support is made to fit either Series I, II, or III probes. Series I and II are designed such that: the primary coil form plugs into a 5 pin connector mounted in the probe support, the secondary coil form plugs into a 3 pin connector mounted in the primary coil form, and the specimen

fits into the secondary coil form. The specimen is held in place and

the secondary is secured to the primary by an end cap and a wire that

passes through all three parts. The wire is twisted together to prevent

loss of the specimen. All secondaries of a j^iven series are interchangeable. Several secondaries are available so as to match the size and

eddy current decay time of various specimens. Figure 6 shows a collection of these probes.

The probe support is a thin walled stainless steel tube which

contains the primary and secondary coil leads and has a connector

mounted at each end. The upper connector is a 4 pin panel plug mounted

on a plastic plug. The lower connector is recessed within the tube, and

the primary coil form slides into the tube to mate with this connector.

The recessed plug is secured to the probe support tube with a screw.

To prevent loss of the primary coil form in a dewar, a screw fastens

the coil form to the support tube. In addition, we have found it advisable to cut a few small holes in the stainless steel tubing above the

recessed plug to control thermal oscillations. This also reduces the

time required to insert the probe into a dewar, and helps save liquid

helium.

To maintain a positive pressure within the dewar during insertion

and withdrawal of the probe, the simple device shown in figure 7 is used.

The probe can be held at any depth by tightening the O-ring fitting. The

vacuum hose insures an air-tight fit on the neck of the dewar, while the

rubber tubing allows one to pressurize the dewar with He gas. The

latter is useful in preventing entry of air when the probe is repeatedly

23

Standard ''2 O-Ring Fitting

Rubber Vacuum Hose

Neck of LHe

Storage Dewar

Probe Support Tube

Primary and Secondary Leads

Copper Tee Fitting

To Helium Cylinder Dur

Removal of Probe

Rubber Tubing

To Probe

Assembly

Figure 7. Storage Dewar Fitting for Probe System.

25
inserted and withdrawn from the dewar. A small rubber balloon can be

attached at a tee fitting in the tubing and thus provide a visual check on

pressure. The balloon also helps control thermal oscillations in the

dewar.

During repeated thermal cycling, the plastic casing on most coaxial cable tends to shatter. If the shields of adjacent cables touch,

ground loops may form that will increase noise in the circuit. To

prevent this, we lay each cable down the middle of a long strip of cloth

tape. The tape is then folded over the cable so that the sticky surfaces

meet. Cloth tape is little affected by thermal shock, and will provide

adequate insulation. These taped leads are fed down the support tube.

We have provided detailed drawings in figure 8 and 9 and assembly instructions (Appendix C) for a Series I probe assembly. For

larger speciments, one can simply scale up the given dimensions. There

are two limiting factors to be considered. If the larger probe assembly

does not fit into the neck of the largest dewar available or, if the amount

of liquid helium boiled off by the extra mass of the larger primary coil

and windings becomes excessive, the best solution is to build a primary

coil large enough to encircle a double walled nitrogen shielded cryostat.

The inner dewar would contain the secondary coil form, the specimen

and the cryogen. The primary could be immersed in liquid nitrogen

both to cool the primary and provide a heat shield for the inner cryogen.

Unfortunately, the advantages of the dip-probe assembly are lost,

because cryogens must be transferred into the inner dewar. The Series

III probes are built in this way. All the secondaries are wound with

resistance wire to reduce the resistance change between room temperature and cryogenic temperatures. Copper wire, for instance, has a resistance ratio of about 8 between room temperature and 76 K, and a

ratio of 100 between room temperature and 4 K. If the coil resistance

26

FABRICATION DETAILS OF AN EDDY CURRENT PROBE

1

i it

.25 • .70 —rI 425 D' .Jo a

1 1

'/8 Drill

f

Drill thru on Dia. and \ Top 0-80 Countersink So Flathead Screws (\)

Protrude OOl"PLUG HOLDER No.

19/ « '32 Norrt —

V

32 Nom

425 D. = 480 0.

PLUG SET No.l (Male)

"V Outl ine "Amphenol" 5 - Pin Mitiature Connector, Mole 126-010

S/UNom—

l9/32* Nom

\_ Outline "Amphenol" ®5-Pin Miniature

Connector, Female 126-011 PLUG SET No. I (Female)

I -f480 D- 425 D

1 -L

.75 -

--.45 -

•- .40 -

77

Drill thru on Dia. and Tap 2-56 \ V '/8

" Drill Countersink so Flathead . Screws Protrude OOl" (4) PLUG HOLDER No. 2

f .375 D. 450 D. —*— I _*55 ( 052) Drill to center line

Outline ITT Cannon

,

MCIIE-8-3-PN 021312-0000 1^

47 —

T 1

t .370 D

1 ©

PLUG SET No. 2 (MALE)

Ref. Item 8 Drilling Instructions

.46 D 0=

X (.397) Drill V

'/|6 Wide slot mru wall

Far side only

- 'V Wide x %z Deep x I " Long

Groove, For side only

PRIMARY COIL FORM

^6 Wide x

,/32

,, 0eep Groove

,

Do two places at 180°

^6 wide x 1^2 deep

Groove, Full length this Item

-41- t03J4

.13

Outline ITT Cannon MCI4E-8-3SN

(Use rubber insert only)

*/32 Nom.

~- 5/

e" Nom—

I

,v.v.<T]

I ^".Nom" ©

PLUG SET No. 2 (FEMALE)

'/le'Wide x '/j^' Deep x .14" Long

Do two places at 180°

Ref Item 8 Drilling Instructions

• 55 ( 052) Drill To center line

"A" Dio -As desired,('A"mo,

= 257") SECONDARY

"B"Dio -"A"Dk>. «04 nom wall COIL FORM

" Wide x

'/32" Deep x 53" Long

,

Do two places at 90° ,, „ , „ '/J Wide x 14 j Deep x 14" Long

,

ifwi.uatiuim inru -» \ i. n,,,tJ |. n_ ,-n, - - . Dia To match Items6a9 V 7I6 Wide x Deep x J4 Long, Do four ploces at 90°

Do two ploces a ISO" (j)

CAP

NOTES

All tolerances to provide a smooth sliding

fit on mating pieces.

All Items (except plugs) to be made from

linen rod.

Figure 9. Fabrication Details of an Eddy Current Probe.

28
remains high at all temperatures, a critically damped coil with a minimal time constant is easier to maintain. Resistance wire works

well because its resistance is not strongly dependent upon temperature.

To increase the winding density, the wire should be insulated with a minimum thickness of enamel. A thin insulation is also less likely to

crack under thermal shock. A high resistance wire should not be used

to wind the primary since the f R heating may create temperature

disturbances on the specimen as well as cause excessive liquid helium

boil off.

The secondary coil form is divided by thin walls into several

segments. Each segment is wound so as to produce several small coils

connected in series, all wound in the same direction. This reduces

inter-turn capacitance, thus reducing the coil time constant. A similar

design would improve the characteristics of the primaries, but would

make machining and winding the coils more tedious.

5. Typical Experiment

This apparatus has been used extensively to study homogeneity

and purity of metals considered for use as standard reference specimens.

Measurements have been performed on several hundred specimens of

zinc, copper, silver, tin and other metals as part of the characterization

prior to the issue of a certificate of analysis by NBS-OSRM on a standard reference metal. To illustrate the general procedures involved

in the eddy current analysis of an actual specimen, we will describe

how the series of tin specimens was prepared and analyzed. About 60

specimens were machined out of a large supply of high purity material

that had been analyzed for impurities by various other techniques. The

objective was to detect local variations in purity within the bulk material

by comparing the RRR of specimens selected from various parts of the

29
lot. Each of the specimens was a cylinder 3 cm long by 1 cm in

diameter

Earlier work with similar material indicated that the surface

layers of these specimens were badly contaminated. An electrolytic

etch recommended by Teggart^^ was, therefore, used to remove up

to 2 mm from the diameter of each specimen. Four specimens were

etched at a time, and good uniformity was obtained by bubbling nitrogen

over the tin during the etch. Also, better results were obtained by

using a large electrolytic cathode of tin placed concentrically about the

specimens. The large cathode seemed to reduce the development of

surface irregularities. The etch required about 30.minutes. The

specimens were then thoroughly washed with water and ethanol.

To minimize the effect of physical imperfections the specimens

were annealed. To insure uniformity of treatment all 60 specimens

were annealed at the same time. The anneal took place in a vacuum at

200°C for one hour. The oven was allowed to cool very slowly to room

temperature

The specimens were rather large, so we used a Series II probe

assembly (figure 6) and the modified large neck helium dewar. Each

specimen was centered in the secondary coil and supported by an

insulating rod. To remove ice blocks and prevent the probe from

freezing in place, the neck of dewar was reamed with a close fitting

brass rod before each measurement. The O-ring support (figure 11)

was slid into place on the dewar neck and the assembly was lowered

until the pressure increased. When the pressure droped again, the

assembly was lowered in stages until the specimen was covered by

liquid helium.

The delay-attenuator circuit was used, and the delay was set

at about 5 seconds, based on earlier tin measurements. The primary

30
current was turned on at about 100 mA for 1 0 to 15 seconds to allow the

field to penetrate the specimen. Note that field penetration rate of the

specimen is the same as the decay rate. The current must remain on

long enough to assure essentially total penetration. This can be judged

both from the amplitude of the signal and the measured decay relaxation

time. Two decay traces and two signal base lines for each specimen

were recorded on a single photograph. Each insertion boiled off about

0. 1 liter of liquid helium.

The data were analyzed by measuring the height of the signal

above the baseline at regular time intervals and plotting the log of this

amplitude versus time. A typical plot is shown in figure 10. In general,

the amplitude can be measured in arbitrary units as only the slope is

necessary to obtain the relaxation time. The semilog plot was checked

for linearity. Non-linearity maybe caused by insufficient delay, baseline drift, incorrect allignment or placement of the baseline, or a variable specimen cross section. For the tin specimens, the time

constant of the decay in liquid helium was about 2.1 s, while at 273 K

the time constant was calculated to be 60|j,s, which gives a residual

resistivity ratio of 35, 000. This value varied among the different specimens, presumably due to variations in purity. These samples have a length to diameter ratio of 3, which requires a form correction factor

of about 1.07 according to figure 1. The revised RRR is then 37,000.

To evaluate purity variations, one must use equations 5 and 9 and compare measured RRR with the RRR predicted from the quantitative impurity

analysis. The statistical variation of the homogeneity, however, is

immediately interpretable . A variation in electronic purity of about

20% from the mean was observed.

31
3l.6mV

(Full Scale)

X

X

_J

<

iD

C/)

TYPICAL EDDY CURRENT DECAY DATA

TAvg.

= 0 749 m s

Est. Std. Dev. = 0.017

T=0-738 ms

T = 0 750 ms

T = 0.760 ms

ORIGINAL OSCILLOSCOPE DATA

0.2 0.4 0.6 0.8

TIME, ms

1.0 1.2 1.4

Figure 10. Typical Eddy Current Decay Data.

32
6. Precision and Accuracy

As described earlier, one may determine either resistivity or

residual resistivity ratio with this apparatus. The precision of these

measurements, generally, is dependent on the measurement of the

temperature, specimen dimensions, and decay time constant. The

accuracy of the measured resistivity values is dependent on the accuracy

of these variables, and is also dependent on the accuracy of the model

relating eddy current decay and resistivity.

The imprecision and inaccuracy of the temperature measurement are generally negligible. Usually the temperatures are based on

the use of fixed point baths, such as: ice-water, boiling liquid nitrogen,

and boiling liquid helium. If reasonable care is taken, the temperatures

of these fixed points are easily reproduced within 0. 1%.

Generally, specimen dimensions are sufficiently uniform and

large that the imprecision and inaccuracy of measurements are less

than 1%. In other instances the measurement uncertainty is determined

by the specific specimen in question; in any case, the errors are

readily estimated.

The imprecision and inaccuracy associated with the determination of the time constant of decay is strongly dependent upon the nature

of the recording equipment and the amount of time spent by the operator

in analyzing the data. We have chosen equipment and a procedure consistent with an imprecision and inaccuracy of less than 1%. At this

level of uncertainty the cost of equipment is reasonable and the

recording-analyzing time is of the order of a few minutes per specimen.

If the need arises this precision and accuracy can by improved at the

expense of equipment cost and measurement time.

33
The most difficult variable to assess from the standpoint of

accuracy is the model relating decay time and resistivity. Probably

the best estimates of this uncertainty come from direct comparison

with resistivity measurements from potentiometric methods. We have

made a few such comparisons but unfortunately such comparisons can

be made only in the overlap region of the two methods. In this region

neither method is at its best; therefore, these estimates are probably

somewhat higher than typical. The results of these comparisons are

listed in table 1. Based on these results we estimate total uncertainty,

including model uncertainty, to be about 3% for residual resistivity

ratio and 5% for resistivity.

It is to be noted that, depending on the nature of the experiment,

the uncertainty in some of the variables, although present, becomes

unimportant. For example, if the residual resistivity ratio is desired,

such as for homogeneity studies, the specimen dimensions need not be

measured. The model inaccuracy, if it is constant with temperature,

also becomes less important. If measurements are done to determine

the effect of various heat treatments on a single specimen, both the

specimen dimension and the model errors are unimportant since they

are constant.

The development of a usable system and practical techniques

was greatly facilitated by the work of J. C. Moulder and M. B. Kasen.

The electronic circuit components designed and built in our laboratory

were primarily the work of J. C. Jellison. The financial support and

samples provided by R. Michaelis and the Office of Standard Reference

Materials, NBS, Washington greatly aided and encouraged our work.

34
Table 1. Comparison of Resistivity Measurements by

Eddy Current Decay and Potentiometric Methods

Specimen Diameter Residual Resistivity, nQ cm

Identification mm RRR Eddy Current Potentiometric

Ag 923D-Y1)* 1.44 1330 13. 1 11.1 ± 12%

Al (Cl-3)* 2. 38 356 9. 75 9.46 ± 4%

Au (T-l-1) 1.26 847 2. 78 2. 62 ± 5%

Cu (round

robin #2) 6. 04 100 15.4 15. 4 ± 0.5%

Cu (OFHC-AFC) 6. 05 100 15.5 14. 9 ± 5%

Cu (Stock 5 - annealed) 3X3 3600 0. 41 8 0. 477 ± 0. 1%

Cu (8A) 6. 04 / A 64 39. 4 36. 1 ± 7%

Cu (OFHC-O) 1 . 6 580 2. 59 2.56 ± 1%

** Cu (OFHC-V) 1 . 6 15 6 9 . 55 9. 61 ± 1%

Cu (Stock 6K-0) 1 . 6 51 00 0. 298 0. 290 ± 1%

Cu (Stock 6K-V) 1 . 6 3740 0. 408 0. 389 ± 1%

** Cu (Stock 7G-0) 1.6 5480 0. 275 0. 273 ± 1%

Cu (Stock 7G-V) 1.6 1360 1.11 1.10 ± 1%

Mo (3) 6.28 83 55. 9 51.9 ± 4%

W (118-7) 3. 03 74 71. 1 69. 0 ± 1%

W (1-2) 6. 35 52 67. 9 70. 7 ± 3%

RRR is the residual resistivity ratio p /p .

* These specimens were slightly bent during the potentiometric measurement. This would lower the resistivity and may be partly

responsible for the differences observed between the two measurement

methods

.

##Size effect corrections have been performed on these data. The correction is greater for the eddy current method than the DC method. At

a ratio of 5000 the eddy current correction is greater than the DC

correction by about 7% for this diameter copper wire. Proportionately

smaller corrections apply for the lower ratios.

35
7. References

1. Bean, C. P., DeBlois, R. W., and Nesbitt, L. B. , J. Appl. Phys.

30, 1976 (1959).

2. Zimmerman, J. E. , Thiene, P., and Harding, J. T. , J. Appl.

Phys. 41, 1572 (1970).

3. Kasen, M. B., Phil. Mag. 21, 599 (1970).

4. Neighbor, J. E. , J. Appl. Phys. 40, 3078 (1969).

5. Arp, V. D. , Kasen, M. B., and Reed, R. P., Technical Report

AFAPL-TR- 68-87, WPAFB, Ohio, February, 1969.

6. Moulder, J. C. , Clark, A. F. , and Gniewek, J. J., Measurement

of Effective Electronic Purity by the Eddy Current Decay Method,

Symposium on Trace Characterization - Chemical and Physical,

October 3-7, 1966, NBS, Gaithersburg, Md. (unpublished).

7. LePage, J., Bernalte, A., and Lindholm, D. A., Rev. Sci. Instrum.

29, 1019 (1968).

8. Weinstein, M. A., J. Appl. Phys. _3_3, 762 (1962); also Weinstein,

M. A., J. Appl. Phys. _3_7» 248 (1966); also Weinstein, M. A., J.

Appl. Phys. 37, 2548 (1966).

9. Meaden, G. T. , Electrical Resistivity of Metals, Plenum Press,

New York (1965).

10. Metals Handbook, 8th Ed. Vol. 11, Amer. Soc. for Metals,

Novelty, Ohio (1964).

11. Teggart, W. J. McG. , The Electrolytic and Chemical ' olishing of

Metals, Pergamon Press, New York (1959).

12. Stern, R. , Levy, M. , Kagiwada, R. , and Rudnick, I., Appl. Phys.

Lett. 2, No. 4, 80-81 (1963).

36
13. The use of trade names of specific products is essential to the

proper understanding of the work presented. Their use in no wayimplies any approval, endorsement, or recommendation by NBS.

14. Blatt, F. J., Physics of Electronic Conduction in Solids , McGrawHill, New York (1968).

3 7
Appendix A: Useful Formulae

To calculate the time constant t in seconds of a uniform specimen, given the resistivity p in Q cm and relative permeability |i

A. Circular cross section

T(r, p) = (2.17 • lO"9 )-^

where r = specimen radius in cm.

B. Rectangular cross section

r(a,b,p) = (1.27 • lO"9 )^ jf fh5

where a, b are the dimensions of the cross section in cm.

C. Hollow Elliptical Cylinders

For a circular cylinder of \i =1, a = inner radius in cm.

b = outer radius in cm.

T(a,b,p) = 10-9 g(a/b)b2 /p

g(a/b) is given in table below:

a/b g a/b g a/b g a/b g

0. 00 2. 173 0. 35 2. 102 0. 65 1. 587 0. 95 0. 3011

0. 05 2. 173 0.40 2. 059 0. 70 1. 431 0.96 0. 2430

0. 10 2. 172 0. 45 2. 001 0. 75 1. 253 0. 97 0. 1838

0. 15 2. 170 0.50 1. 926 0. 80 1. 052 0. 98 0. 1236

0. 20 2. 164 0. 55 1

833 0. 85 0. 827 0. 99 0. 0623

0.25 2. 153 0. 60 1. 720 0. 90 0. 576 1. 00 0. 000

0. 30 2. 133

38
II. To correct a measured time constant for end effects, use figure

1 to find the L./D correction factor, C, for the given specimen

T j

~ Ct corrected measured

III. To calculate RRR from T and T . determination

76 K 4 K

p. (273 K)[t(4 K)/t(76 K) -1]

RRR = ^?TKj + 1

IV. To correct a measured RRR for size effect

P p (1 +£ /d) measured bulk

where <£» is the electronic mean-free-path and d is the minimum

sample dimension. ^

V. To calculate the impurity contributions to resistivity, assuming

Mathies sen's Rule, and given the specific resistivities of the

impurities

.

n

p. = 2 R.C.

i i=l i i

th where R^ = specific resistivity of i impurity

(see reference 14)

th

= concentration of i impurity

n = number of impurity species

VI. To calculate various circuit parameters

A. Inductance, L in henries, of a solenoid of radius a in meters,

length -t, in meters with n turns per meter and permeability,

M- L = ira

2

m, n

2 \J& + a

2

- a]

39
B. Length of wire in an n layer, densely wound circular cylindrical solenoid of inner radius a, length t and wire radius b

length = niT-C — + ^

It is assumed that b is much smaller than both £ and a. C. Time constant of a pure L-R circuit

40
Appendix B: Tables of Typical Numerical Calculations

Table 1. Time Constant, t in seconds, of circular rod vs. Resistivity

and Diameter.

Resistivity

(fi cm)

8X10

1.7X10

-3

-6

10 -10

Diameter (cm)

.05 1.0

» 1- 2.5

1.7X10

8 X 10'

1.4 X 10-

-8

6. 8 X 10"6

3. 2 x lO"4

5.4

4.2 X 10" b

2. 0 X 10 -3

34

Table 2. Calculated resistivity and time constants of a 5 mm diameter

aluminum cylinder as a function of temperature (assume

RRR =* 10, 000).

temp. (K)

295

273

200

80

p ([afi cm)

2. 74

2.5

1.6

0. 25

0. 00027

T(M. s)

49. 6

54. 4

85.

544.

496, 000

41
Appendix C: Assembly Instructions for Series I Probes

Figures 8 and 9 illustrate the details of a Series I probe assembly.

The detailed fabrication procedure for this probe is given below.

Numbers given in parenthesis refer to part numbers in figures 8 and

,.[13]

[1 31 Primary: First assemble plug holder #2 (4). Teflon insulated- 36 gauge wires are soldered to the five pin plug (3). The wires

are fed through the 1/8 inch hole in the holder (4) and two are passed

through the #55 hole to be soldered to the primary leads. The remaining two leads are soldered to the male three pin plug (5). Both plugs

(3, 5) are glued in place (in an arbitrarily selected but standarized

orientation to insure interchangeability of probes) with plastic cement.

The cement is thinned with acetone for easier handling.

Next, the plug holder (4) is inserted into the primary coil form (6)

and the #55 hole is aligned with the lead groove. The plug holder (4) is

glued in place with thinned cement. The wires are laid along the groove

and soldered to the primary coil leads. These wires are held down with

a thin layer of cement. Excessive cement here can break the wires as

it dries or when it shrinks upon cooling. The latter problem frequently

goes undetected at room temperature but exhibits itself as an open

circuit upon cooling.

Probe Support: The primary and secondary leads are soldered

to a four pin connector at the top of the support tube, and are then

brought down the tube and connected to the internal recessed plug. Thes

leads should be sufficiently long to pass completely through the tube.

The excess is drawn up into a loop at the top, as the plug is recessed

into the tube.

42
Insulated wires are soldered to the male five pin plug (2) and

pushed through the 1/8 inch hole in the holder (1). To reduce noise

pick-up, the pair of primary leads should be twisted together to form

a tight double spiral. The secondary pair is similarly twisted. These

wires are connected to the appropriate coaxial leads. Each wire

should be insulated at the solder point by a bit of cloth tape or shrink

able tubing. Ordinary plastic tape does not hold well at low temperature. The plug (2) is aligned and glued with plastic cement. If a primary is plugged into the #1 holder (1), it is easy to slide the holder

into place in the probe support tube, and align the screw holes. To

standardize the holes in the probe support tube, a brass guide is made

with a 1/2 inch ID to slide over the support tube, and the screw holes

are drilled through appropriately placed holes in the wall of the guide.

Secondary: Teflon insulated #36 wires are soldered to the secondary plug (7) and twisted to reduce noise pickup. The leads are

passed through the #55 hole and along the groove in the secondary coil

form (8). The plug (7) is then aligned and glued in place. The leads

from the secondary windings are brought along the groove and soldered

to the plug (7) leads. Thin cement holds the leads at the bottom of the

groove. If desired, the sense of the primary and secondary windings

ce*n be noted and leads attached so as to always give signals of the same

polarity, no matter which coils are used. We have not done this as the

banana plug input to the recording device is easily reversed.

43

|f<3rm NBS-114A (1.71) U.S. DEPT. OF COMM. 1 . PUBLICATION OR REPORT NO. 2. Gov't Ac cess ion BIBLIOGRAPHIC DATA NBS SP 260-39 N°* 5H CCl

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Standard Reference Materials:

The Eddy Current Decay Method for Resistivity

Characterization of Hicrh Puritv Metals

5. Publication Date May 1972

6. Performing Organization Code

7.^uTr|pR(s

(i,^ ar^^ v ^ a. Deason, J. G. Hust, and R. L. Pow

8. Performing Organization

all

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

12. Sponsoring Organization Name and Address

Same as item 9.

13. Type of Report & Period CoveredFinal

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

PWilh ararf^pri GftX CIV^ LCJLlua7 a +LJ.i onLfll o"F —' J— 11J. nicrh M * 1 nuriJ- 1— _J rne 1LI«^;3 1 aCI J-1 ^O bvJw* resistivity measurements at low temperatures

is discussed. In particular, the eddy current

decay method of resistivity measurement is

described in detail. The advantages and

limitations are presented along with the

theoretical basis of the method. Detailed

instructions are given for constructing and

operating the apparatus

.

17. KEY WORDS (Alphabetical order, separated by semicolons) . ... Apparatus; characterization; eddy current decay; electrical resistivity;

high purity metals; homogeneity; low temperature.

18. AVAILABILITY STATEMENT

UNLIMITED.

1 1 FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE

TO NTIS.

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

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22. Price

55 cents

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