AFAPL-TR-74-16

Polymeric Pyroelectric Sensors

for Fire Protection

U.S. Department of Commerce

National Bureau of Standards

Institute for Applied Technology

July 1975

Technical Report AFAPL-TR-74-16

NBSIR 75-728

FINAL REPORT FOR PERIOD JUNE 1972

TO MAY 1973

APPROVED FOR PUBLIC RELEASE

DISTRIBUTION UNLIMITED

AIR FORCE AERO-PROPULSION LABORATORY

AIR FORCE SYSTEMS COMMAND

WRIGHT-PATTERSON AIR FORCE BASE, OHIO
NOTICE

When Government drawings, specifications, or other data are used for any purpose

other than in connection with a definitely related Government procurement operation,

the United States Government thereby incurs no responsibility nor any obligation

whatsoever; and the fact that the Government may have formulated, furnished, or in

any way supplied the said drawings, specifications, or other data, is not to be

regarded by implication or otherwise as in any manner licensing the holder or any

other person or corporation, or conveying any rights or permission to manufacture,

use, or sell any patented invention that may be in any way related thereto.

This report has been reviewed and cleared for open publication and/or public re- lease by the appropriate Office of Information (01) in accordance with AFR 190-17

and DOD 5230.9. There is no objection to unlimited distribution of this report to

the public at large, or by DDC to the National Technical Information Service (NTIS).

This technical report has been reviewed and is approved for publication.

FOR THE COMMANDER:

B. P. BOTTERI

Chief, Fire Protection Branch

Fuels and Lubrication Division

Air Force Aero Propulsion Laboratory

Copies of this report should not be returned unless return is required by securi

considerations, contractual obligations, or notice on a specific document.
Unclassified

SECURITY CLASSIFICATION OF THIS PAGE (When Data Entered) REPORT DOCUMENTATION PAGE READ INSTRUCTIONS

BEFORE COMPLETING FORM

1. REPORT NUMBER 2. GOVT ACCESSION NO. AFAPL-TR-74-16

3. RECIPIENT'S CATALOG NUMBER

4. TITLE (and Subtitle) POLYMERIC PYROELECTRIC SENSORS FOR FIRE

PROTECTION

SJ- TVPP i t > c OF\j r RFPADT ncrurr i Aoc Dr Ptniuu Di nn f~r\\/CDrn U V t. r< t. U Final

6. PERFORMING ORG. REPORT NUMBER

7. AUTHORfsj 8. CONTRACT OR GRANT NUMBERfsJ

9. PERFORMING ORGANIZATION NAME AND ADDRESS 10. PROGRAM ELEMENT. PROJECT, TASK AREA a WORK UNIT NUMBERS

30480761

11. CONTROLLING OFFICE NAME AND ADDRESS

Air Force Aero Propulsion Laboratory

Fire Protection Branch (AFAPL/SFH

Wriqht-Patterson AFB , Ohio 45433

12. REPORT DATE

July 1975

13. NUMBER OF PAGES

27

14. MONITORING AGENCY NAME ft ADDRESSf// different from Controlling Office) 15. SECURITY CLASS, (of this report) UNCLASSIFIED

15a. DECL ASSI Fl C ATI ON/ DOWN G R ADI N G SCHEDULE

16. DISTRIBUTION STATEMENT (of this Report)

ADDroved for Public Relpa^p* ni<;tri hufir>n un 1 -im-i t-oH

17. DISTRIBUTION STATEMENT (of the abstract entered In Block 20, if different from Report)

18. SUPPLEMENTARY NOTES

19. KEY WORDS (Continue on reverse side if necessary and identify by block number) Heat detector; infrared detector; optical radiation sensor; polinq; Dolymer;

polyvinyl fluoride; polyvinylidene fluoride; pyroelectric effect; relative

spectral response; ultraviolet detector

20. ABSTRACT (Continue on reverse side If necessary and identify by block number) This report summarizes the results of a program to develop optical radiation

sensors for fire detection using polymeric pyroelectric sensing elements. Emphasis has been placed on techniques for constructing such devices, including

methods of enhancing the pyroelectric response. Materials investigated included polyvinyl fluoride (PVF) and polyvinylidene fluoride (PVF9

). Six sensors were developed and delivered to the sponsor.

2

continued on reverse DD 1 JAN 73 1 473 EDITION OF 1 NOV 65 IS OBSOLETE

SECURITY CLASSIFICATION OF THIS PAGE (When Data Entered)
SECURITY CLASSIFICATION OF THIS P ACE(When Data Entered)

13. ABSTRACT, CONTINUED

Highlights reported include achievement of a value of SDecific detectivity,

8 1/2-1 ' D*, of 2 x 10 cm- Hz • W and the construction of a set of sensors useful over a wavelength range extending from the ultraviolet to the infrared.

Methods of characterizing the performance of sensor elements are described.

SECURITY CLASSIFICATION OF THIS PAGEfWhen Data Entered)
FOREWORD

This is the final Technical Report prepared by the Instrumentation

Applications Section of the National Bureau of Standards. The effort was sponsored by the Air Force Aero-Propulsion Laboratory, Air Force Systems

Command, Wright-Patterson AFB, Ohio, under MIPR No. FY! 455720041 2 for the

period June 1972 to May 1973. The work herein was accomplished with FY72

Air Force Aero-Propulsion Laboratory Directors' funds. The work was accomplished under Project 3048, Task 304807, Work Unit 30480761 (Polymeric Pyroelectric Sensors for Fire Detection), with Mr. Terry M. Trumble

as Project Engineer. Mr. Julius Cohen and Mr. Seymour Edelman of the

National Bureau of Standards were technically responsible for the work.

This report presents results of a preliminary study. A number of

uncertainties remain that should be considered in more detail. Therefore,

the conclusions are not necessarily those that would be reached in a full

complete investigation.

iii
POLYMERIC PYROELECTRIC SENSORS

FOR FIRE PROTECTION

Table of Contents

1. Introduction 1 1.1 Background 1 1.2 Objectives and Tasks 1 2. Experimental Development 2 2.1 Materials Selection 2

2.2 Poling Methodology 3

2.3 Experimental Investigation of Poling Parameters 4

2.4 Characterizing Sensor-Element Performance 5 2.5 Other Investigations 8

3. Deliveries 9

4. Summary and Conclusions 10

5. Recommendations for Further Work 11 6. References 12

iv
List of Figures

Figure 1: Block Diagram of Apparatus Used to Monitor Poling

Effectiveness 14

Figure 2: Poling Fixture 15

Figure 3: Relative Response of a PVF 2 Sample as a Function

of Poling Temperature for Three Poling Fields 16

Figure 4: Relative Response of a PVF2 Sample as a Function

of Time After the Poling Field is Removed 17

Figure 5: Relative Polymer Pyroelectric Response as a Function of the Reciprocal of the Poling Field 18

Figure 6: Relative Polymer Pyroelectric Response as a Function

of the Reciprocal of the Poling Field 19

Figure 7: Relative Polymer Pyroelectric Response as a Function

of the Reciprocal of the Poling Field 20

Figure 8: Relative Polymer Pyroelectric Response as a Function

of the Reciprocal of the Poling Field 21 Figure 9: Relative Polymer Pyroelectric Response as a Function

of the Reciprocal of the Poling Field 22

Figure 10: Circuit of Voltage Preamplifier Used in Relative Pyroelectric

Response Measurements 23

Figure 11: Relative Spectral Response of a Mounted PVF Sensing

Element as a Function of Wavelength 24

Figure 12: Relative Spectral Response of a Mounted PVF 2 Sensing

Element as a Function of Wavelength 25

Figure 13: Sensor Construction 26

Figure 14: Circuit of Preamplifier Used in Blackbody Determinations

of Specific Detectivity, D* RR 27

v

POLYMERIC PYROELECTRIC SENSORS

FOR FIRE PROTECTION

Julius Cohen and Seymour Edelman

1. INTRODUCTION

Radiation sensors have proved to be among the most rapidly responding devices

for the detection of flames. The application of radiation sensors to aircraft

has been limited primarily to engine nacelle installations, where temperatures

vary from 90 to 430°C. Crew areas, cargo compartments, baggage compartments,

and passenger areas have had little, if any, protection. The work reported

was undertaken at the request of the sponsoring agency to develop low-cost,

highly sensitive sensors with high specificity for use in these critical

aircraft areas. Such sensors would also have extensive application in groundsupport facilities.

Polymeric sensors proposed to meet program requirements should contribute to

the realization of inexpensive, reliable radiation-sensing systems for

Air Force use.

1.1 Background

Certain polymers, e.g., polyvinyl fluoride (PVF) and polyvinyl idene fluoride

(PVF2 ), may be made to exhibit significant pyroelectric activity by means of

a poling process. Usually poling consists of applying an intense electric

field at an elevated temperature and cooling to room temperature with the

field still applied. When the pyroelectrically active material is subjected

to a small temperature change, charges produced on the surfaces are detectable as a voltage (open-circuit configuration) or as a current (short-circuit

configuration). Thus, the pyroelectrically active polymer may be used as a detector of electromagnetic radiation. Polymeric radiation sensors can operate at room temperatures, require no biasing, have an extremely broad

spectral response, and have a short response time compared to most other

types of thermal detectors.

Polymeric pyroelectric sensors have advantages over conventional crystalline

pyroelectric detectors because they are non-hygroscopic (and therefore easily

handled), chemically inert, flexible, simply fabricated into large-area devices,

and made from relatively inexpensive materials. These properties render polymeric sensors attractive for fire-protection applications.

1.2 Objectives and Tasks

The statement of work establishing the project defined the objective as "To

design and build operable pyroelectric sensors capable of being used for

detecting emission from combustion processes at 4.5, 7.2, and 10 microns, as well as in the 200 to 300 nanometer region."

-1-
The effort was to consist of the following tasks:

(1) Experimental selection of polymeric materials susceptible to treatment

for developing significant pyroelectric activity;

(2) Processing of selected polymeric materials to induce significant pyroelectric activity — i.e., by means of poling — with a view towards

achieving maximum pyroelectric activity for each specific material;

(3) Development of instrumentation for reliable measurement of detectivity;

and

(4) Construction of sensors together with measurement of detectivity and

relative spectral response for each device.

The goal was to produce sensors with values of specific detectivity D*, equal

to or greater than 10 8 cm * Hz 1 /

2 • W" 1

.

2. EXPERIMENTAL DEVELOPMENT

2.1 Materials Selection

The pyroelectric responses of a number of polymers were compared qualitatively. This operation was a rapid screening to select the best readily

available material for development for thermal radiation sensors. Selection was restricted largely to commercially available materials because

preparation of material in the large quantities needed for adequate testing would have been prohibitively expensive and time-consuming. As a first

cut, materials were selected containing a significant number of permanent

dipoles capable of being oriented. So-called crystal 1 inity was not considered a necessary property, although most of the materials selected had this

property. The materials investigated included polyethylmethacryl ate (PEMA),

fluorinated ethylene propylene (FEP), cellulose acetate (CA), polyvinyl

chloride (PVC), polyvinyl fluoride (PVF), polyvinyl idene fluoride (PVF 2 ),

and special research samples of polyphosphazenes (PPZ) prepared at no cost

to the project by a commercial laboratory. Although each material was selected for various properties, there was a principal reason for selecting

each. Specifically, PEMA was chosen because it softens at a relatively low

temperature, which may simplify poling; FEP was chosen because of its large

dipole moment; CA was tried because it is a derivative of wood previously

found by others to have piezoelectric properties [l]f; PVC, PVF, and PVF 2 were chosen because these materials possess a comparatively large number of

permanent dipoles per unit volume and exhibit substantial piezoelectric

activity [2,3], and PVF and PVF 2 in addition exhibit substantial pyroelectric

activity [4,5,6,7,8]; PPZ was tried because it is also a polar material.

Samples of these materials were processed by various mechanical, thermal, and

electrical means, or by a combination of these, in an effort to enhance pyroelectric activity. Some activity was developed in most of the samples tested.

The samples of PVF and PVF2 showed greatest activity. The outputs of two of

the special polyphosphazene samples too thick for convenient use as pyroelectric

"^Figures in brackets refer to references, section 6.

-2-
material were noticeably better than those obtained with any samples other

than PVF and PVF2 , although the samples lacked sufficient dielectric strength

to allow poling fields comparable to those used on the other samples.

(Further experimentation with the polyphosphazene formulation and with the

methods used to form the samples is warranted.) The pyroelectric response

found for PVF and PVF2 was so much greater than the activity found for other

polymers that all further efforts were devoted to these two materials. The

form in which they were available for sensor development was as thin films,

10.3-ym thick in the case of PVF and 6.0-yrn thick for PVF 2

.

2.2 Poling Methodology

Because the methods used to manufacture PVF and PVF2 are considered proprietary by some suppliers, it has not been possible to learn the past history

of the samples used in this project. In particular, little information on stretching modes and the possible application of electric fields during

processing has been available. Most polymeric materials as received from

commerical sources require poling in order to enhance existing pyroelectric

activity or to develop new activity to useful levels.

2.2.1 Uniaxial Stretching. For PVF 2 a preliminary step to the poling process

was in some cases uniaxial stretching. As used in this report, uniaxial

stretching refers to an intentional process applied to as-received material.

The polymer sheet as received was assumed to be predominantly in the a form,

that is, as described in one model, with molecules in a trans-gauch-transgauch-primed configuration in which the electric dipoles were approximately

parallel to the axis of the molecule. According to this model, uniaxial

stretching in the direction of the chain axis converts some proportion of

the molecules into a 6 form in which the chain is extended to be an almost

planar zig-zag, and the dipoles are therefore nearly perpendicular to the

chain axis and hence to the plane of the polymer film [9, 10]. The $ phase

is thought to be more amenable to poling as the dipoles of interest are preferentially aligned to be parallel or anti-parallel with the perpendicular

to the chain axis. The assumption is that an electric field applied to the

uniaxially stretched polymer tends to orient the dipoles, perhaps by rotation

of the chain axis.

2.2.2 Application of Electrodes. Apart from uniaxial stretching of certain

PVF 2 samples, a preliminary operation in the poling process was the vacuum deposition of metal -film electrodes. For the pyroelectric sensor work, a visually semi transparent nickel coating about 20-nm thick was deposited on the front surface of the sensor (i.e., the surface intended to be facing a source) with a 50- to 80-nm coating on the rear surface. These coatings

served both as electrodes and as absorbers. Both PVF and PVF 2 are transparent in the spectral region of primary interest. Absorbing layers were therefore required to convert a change in the amount of radiation incident

on the polymer into a change in the polymer temperature. The upper limit

to the thickness of the front-surface coating was determined by the

-3-
requirements that the amount of energy reflected away from the sensor be

minimized and that a reasonable proportion of the energy reach the rear- surface coating. The lower limit was determined by the requirements that some absorption take place in the front-surface coating and that the coating be electrically conductive, and by 1 imitations in the minimum thickness

that could be reproducibly deposited with available equipment. (Thickness

of the deposited layer was monitored with a commercial instrument during

the deposition process. The sensor head of this instrument was mounted

in the vacuum chamber at the same distance from the evaporative source as the polymer film and close to it; the measuring principle of the instrument

used depends on the shift in resonant frequency of a sensing crystal as the mass of the crystal is increased by the portion of evaporated material it

intercepts.

)

2.2.3 Poling. The poling cycle used consisted of applying the poling field

at room temperature, raising the temperature to some pre-determined maximum,

maintaining the maximum temperature (the poling temperature) for approximately

30 min, and cooling to room temperature before the field was removed.

Thermostatically controlled laboratory ovens provided heating. Poling temperatures used in this work ranged from 90 to 130°C, with room temperature about

20°C. Poling fields ranged from 300 to 800 kV • cm

-1

, applied across the

thickness of the polymer film. For some samples, multiple poling cycles were used with successively higher field strengths.

The poling process was monitored by measuring sensor-element response using

the arrangement shown in figure 1. This system is analogous to one that has

been used to monitor piezoelectric activity of polymers during poling [11].

Each polymer-film element was mounted in a fixture as shown in figure 2.

(The fixture was adapted from one developed by Phelan and Mahler [6]). Note

that in this design a gap is left between the rear electrode and the fixture

to minimize the tendency to arc around the edge. The field applied to the

sensing element acting as detector was supplied by a bias circuit. Radiation

incident on the polymer film from a 100-W zirconium arc lamp was modulated

at 10 Hz by a mechanical chopper. The amplitude of the signal developed

across the detector was measured with an oscilloscope.

2.3 Experimental Investigation of Poling Parameters

Experiments were conducted to provide an improved understanding of the role

of the two poling parameters, temperature and field.

In one set of experiments, several samples were poled in successive cycles

with increasing field strength, without being removed from the poling oven. The pyroelectric response was measured as described in 2.2. Figure 3 shows

relative response as a function of temperature for three poling cycles for

one sample of PVF2. The response observed with a large field applied was found to comprise a large temporary and a small quasi -permanent effect.

Immediately after the field was removed after cooling to room temperature,

a large decay in response was observed, followed by a more gradual decay.

After several hours, a quasi -steady value was reached. The response as a function of time of a poled sample of PVF2 is shown in figure 4.

-4-
Another set of experiments was conducted in an attempt to determine the

poling conditions required to maximize pyroelectric response for both

PVF and PVF 2

. Several sets of sample sensing elements, each set taken

from a single piece of polymer as received from the manufacturer, were

poled in successive cycles with increasing field strength but with con- stant poling temperature for each set. After each cycle, the polymer

sensing elements were removed from the oven and set aside. After a minimum of 18 hours after poling, the pyroelectric response was measured

as described in 2.2.3. As mounted in a poling fixture, each sensing

element had an area of 1 cm2

. Measurement parameters were: measuring

frequency, 10 Hz; blackbody temperature, 500 K; distance from blackbody

aperture to sensor, 50 cm; and equivalent noise bandwidth, 1 Hz. The

resu

1 '.- or" these experiments are plotted in figures 5 through 9, which

show the logarithm of the relative pyroelectric response as a function of

the reciprocal of the poling field. Figure 5 shows the plot for four samples

of PVF2 poled at 90°C; figure 6, the plot for five samples of PVF 2 poled at

110°C; figure 7, the plot for five samples of PVF2 poled at 130°C; figure 8,

the plot for five samples of PVF poled at 90°C; and figure 9, the plot for

four samples of PVF poled at 110°C, For these measurements, a voltageresponding preamplifier was used to condition the signal from the mounted

sensing element. The circuit diagram of this instrument is given in

figure 10.

2.4 Characterizing Sensor-Element Performance

2.4.1 Spectral Response Measurements. Relative spectral response was determined with the aid of a monochromator. Figures 11 and 12 show relative

response as a function of wavelength, for mounted sensing elements of PVF

and PVF2 , respectively. The curve for PVF exhibits little dependence of

response on wavelength in the region of about 6 to 18 ym; at longer wavelengths, however, the response rises rapidly with increasing wavelength.

The response curve for PVF 2 is limited to wavelengths below 10 ym because

of excessive noise experienced at higher wavelengths. Some of the observed

noise probably results from the effect of air currents on the exposed surface

of the sensing-element surface, and isolation of the sensor element from such

disturbances, for example by means of an infrared transparent window, is

recommended

.

2.4.2 Pyroelectric Response Measurements. Pyroelectric response was measured

using a commercial blackbody-type radiation source operated at 500 K. An

aperture plate in front of the source defined the source area. The radiation

from the source was chopped before passing to the sensing element. For these

measurements, the sensing element- was mounted in a housing to form a sensor

package, shown in figure 13. Distance from blackbody aperture to sensing

element was typically 50 cm. Blackbody, chopping wheel, and sensing element

were mounted on an optical bench to facilitate alignment. A commercially

available, two-channel, phase-sensitive amplifier was used to measure the

sensing-element output with the signal controlling the chopping wheel serving

as reference. This lock-in type of amplifier was equipped with modular

preamplifiers; however, for certain critical measurements, a special

-5-
preamplifier was used. The reading from the meter driven by the phasesensitive detector circuit of the amplifier, together with instrument

settings and other measurement conditions, provided information from

which the pyroelectric responsivity, defined as the ratio of rms signal

(sensor-output) voltage to rms value of blackbody incident signal power

(on the sensor), referred to an infinite sensor load impedance and to

the terminals of the sensor, could be calculated.

Responsivity is usually expressed in units of V • W" 1 or A • W" 1

, depending on whether short-circuit current or open-circuit voltage measurements

of sensor output are made.

Another parameter characterizing sensor pyroelectric performance is the

specific detectivity, D*. This quantity involves noise measurement and

is given by the ratio of (1) the product of signal-to-noise ratio and the

square root of the effective noise bandwidth to (2) the product of the

rms blackbody power density and the square root of the sensor area. The

blackbody power density, Hg

B

, is given by the relation [12]:

U - e0

^ A "BB ~ ~^d 2~ BB'

where e is the blackbody emissivity, a is the Stefan-Boltzmann constant,

T is the temperature in degrees K, ATM is the effective radiating area of

the blackbody in square centimeters, and d is the distance from blackbody

aperture to sensor in centimeters. When the radiation is chopped, the

rms value of H

BB

is the product of H

BB

and a modulation factor, 0.45 for

square-wave chopping. The effective noise bandwidth depends in part on instrument settings and is introduced, as is area, as a normalization

factor. D* measured using the total irradiance from. a blackbody source may sometimes by indicated by D*„

B to distinguish this parameter from

D* which represents D* measured at a single wavelength. Conventionally,

the blackbody temperature and the measuring frequency are given in parentheses following the D* value. Although most measurements were made at

a chopping frequency of 10 Hz and with equivalent noise bandwidth adjusted

to be 1 Hz, measurements for critical determinations of D* were made at

1.6 Hz and with an equivalent noise bandwidth of 0.1 Hz. All measurements

for critical determinations of D* were made with an appropriately small

limiting aperture and the baffle system described in 2.4.3.1 and 2.4.3.2,

respectively. Figure 14 shows the circuit of the preamplifier used in

these measurements. To reduce noise, the sensing elements were isolated

from air currents. Specific detectivity was found to be on the order of

10 8 cm • Hz 1 ' 2 • W" 1 for the more active sensing elements made from as- received PVF2 (i.e., from material not uniaxially stretched] poled at

800 kV/cm and 110°C, with values as high as 2 x 10 8 cm • Hz 1 /

2 • W" 1 being determined. For small samples made from uniaxially stretched PVF2 » detectivities of up to 2.5 x 10 8 cm • Hz 1 ' 2 • W" 1 have been determined.

For PVF, typical detectivity values were on the order of 10 7 cm • Hz 1 /

2 • W" 1 for a sensing element poled at 500 kV/cm and 90°C.
A study of the uniformity of response across the surface of a sensing element

made from uniaxial ly stretched PVF2 indicated that detectivity values even higher than the highest value given above have been attained over small

regions. A laser beam about 1 mm in diameter was used to scan the sensitive

area of the sensor in small increments. It was found that on the order of

90% of the response was developed in an area no greater than that covered

by the beam. The entire sensitive area had a diameter of 1 cm. The suggestion was made that the variation of response with position was due to non- uniform uniaxial stretching.

2.4.3 Sources of Error-in Pyroelectric Response Measurements. The sources

of error investigated were imperfections in the simulated blackbody source

and stray radiation. The latter is a particularly crucial consideration for

pyroelectric detectors, and for thermal detectors in general, because their

broad spectral response makes them sensitive to radiation from relatively

low temperature sources.

2.4.3.1 As a first step, response of a sensor to a series of power levels,

which were determined by limiting blackbody apertures of different known

areas, was observed. Results showed large departures from linearity. To

investigate uniformity of emission over the aperture of the blackbody source,

measurements were made using a lead sulfide photoconductive detector. Since

this photon detector cuts off rapidly above approximately 3 urn, sensitivity

is confined to the hotter areas of the source. The need for concern with

stray radiation from objects that may be warmed by proximity to the source

such as the chopper and the limiting apertures is avoided.

Varying source-to-detector distance and comparing the measured response with

that predicted on the basis of the inverse square relation of power with

distance provided a relative calibration of the lead sulfide detector response

and confirmed the linearity of this response. Following this check, the

various apertures were successively introduced in front of the blackbody,

and the available power was measured with the detector. Results showed good

agreement with a linear relationship of power to aperture area over the range

of aperture diameters from 0.32 cm to 1.27 cm. No apertures with diameters

smaller than 0.32 cm were used. Emission through a limiting aperture of 1.59 cm was lower than that called for by the linear relationship. With no limiting aperture inserted, the radiation was emitted from the opening of the

blackbody itself. The nominal diameter of this opening was 1.6 cm. Emission

with no aperture, however, was much greater than that called for by the linear

relationship. The excess was attributed to radiation transmitted through the

plastic liner that serves to define the 1.6-cm opening in the blackbody.

Subsequent measurements of the response of pyroelectric sensors were performed

with limiting apertures of no greater diameter than 1.3 cm.

2.4.3.2 The existence of stray radiation also interfered with measurements of

pyroelectric sensor response. Three important sources of such radiation were

the aperture disk, the blackbody housing and support bracket, and the chopping

-7-
wheel blades. Cooling the aperture disk with forced air or water (coldstopping) is the method commonly used to eliminate stray radiation for

measurements of photon-detection response for wavelengths beyond 10 ym.

This method may not be practical for measurements extending into the far

infrared, especially when large-area sources of stray radiation are present, because the decrease in power density achieved by lowering the

temperature of the contributing stray sources is not sufficient to overcome the effect of the sources.

The technique used in these measurements to minimize the effects of stray

radiation from both the aperture disk and the blackbody (including bracket)

was as follows: a baffle plate with provision for mounting aperture disks

was constructed and positioned in front of the blackbody so that only the

cavity itself could be seen by the pyroelectric sensor. The chopping wheel was repositioned to be in the space between blackbody and baffle plate, so that chopping of the radiation took place behind the aperture and therefore

neither aperture nor baffle, even if slightly heated, could contribute to

producing a spurious sensor output.

A second baffle was constructed and positioned near the first to provide a degree of collimation to radiation from the chopping-wheel blades so that

when a blade was in line with the aperture, the sensor could see an area

of the blade only slightly larger than that of the aperture. A given spot

on a blade spends only a small fraction of the time required for one revolution of the chopping wheel in front of the blackbody cavity (when it is

heated by blackbody radiation) and is small compared to the blade. Each

such spot has the opportunity to cool to near room temperature before it

returns to a position in line with the blackbody cavity. That the baffle

system is effective was confirmed by the observation that reducing the size

of the aperture (and, therefore, the effective radiating area of the choppingwheel blades) did not have any significant effect on measured sensor responsivity , with all the other conditions held constant.

Following the introduction of the baffle system, measurements were made with

various apertures in place and with a pyroelectric sensor instead of the

photon detector. Agreement was obtained between measured sensor response and

that predicted on the basis of the change in blackbody aperture. Extrapolation

of the data indicated that at zero power the responsivity should be nearly zero,

further confirming that effects of stray radiation were not significant with

the baffles in place.

2.5 Other Investigations

2.5.1 Experiments with Ultraviolet-Absorbing PVF. Experiments were carried

out with special samples of ultraviolet-absorbing PVF to determine the suitability of this material for use as an ultraviolet sensor. Sensors constructed

from this material were found qualitatively to be more sensitive to radiation

in the ultraviolet than sensors constructed in the same manner from either

-8-
"ordinary" PVF (which exhibits nearly no response to ultraviolet) or "ordinary"

PVF 2 (slight response). Initial attempts to obtain quantitative responsivity

data with the aid of a calibrated ultraviolet lamp were not successful because

of poor signal-to-noise ratio as measured at the phase-sensitive amplifier. A

possible source of noise was long-wavelength radiation from the chopper blades.

However, other noise sources were probably contributing to poor signal-tonoise ratio, and it was clear that a more powerful calibrated source was required. Such a source was not available within the time constraints of the

project, and no further work was done in the ultraviolet region.

2.5.2 Sensors Tailored for Specific Wavelengths. Experiments were conducted

to investigate the feasibility of constructing polymer-film sensors which

would respond only to, or primarily to, selected wavelengths. Results from

this experimentation together with consultation with experts in the deposition

of metal films and dielectric coatings led to the recommendation that a useful

approach would be to use a pass-band filter in front of the sensor and to

compensate as much as possible for the losses in the filter by coating the

sensing-element surface with an absorbing film. Gold black was selected as

an absorber, and trial coatings have been successfully deposited, both on the polymer directly and on nickel electrodes. Time did not permit further

work on tailored-response sensors.

3. DELIVERIES

Six sensors were designed, constructed, measured, and delivered to the sponsor.

One sensor is active in the 200- to 3C0-nm region. The other five are active

in the infrared region and can be peaked by the use of suitable filters.

Two PVF sensors with a measured D* (500 K, 1.6 Hz) of more than 10 7 cm • Hz 1 / 2 W

-1 were delivered early in the course of this project. Four sensors representing further advancement in development were delivered near the end of the

project; pertinent characteristics are given in table 1.

Table 1

Characteristics of Four Experimental Sensors Delivered to Sponsor

Material Area Specific Detectivity, D* (500 K, 1.6 Hz)

(cm 2

) (cm Hz 1 / 2 W1

) Ultraviolet-Absorbing 0.7 • t

PVF

PVF 2 1.0 6 x 10 7 PVF 2 1.0 9 x 10 7 PVF 2 1.0 2 x 10 8

-No value of D* could be determined, because of the presence of noise and

other experimental difficulties as described in 2.5.1. A definite response

was observed.

-9-
4. SUMMARY AND CONCLUSIONS

Films of PVF2 were poled by applying a static electric field (300 to 800 kV/cm) across the films at temperatures in the range 90 to 130°C followed by cooling

to room temperature with the field maintained.

Results generally suggest that the greater the poling field, the greater the

ensuing pyroelectric responsivity. However, for PVF2 the responsivity is

a sensitive function of both poling field and temperature. An intrepretation

of the hysteresis in relative response shown in figure 3 is that the relative

response is proportional to the alignment of dipoles. For the lower curve

(field of 300 kV/cm), it appears that a temperature greater than about 60°C

is required to obtain a significant alignment of the dipoles by the field.

The hysteresis of the curves is believed to be a manifestation of the increased

polarization produced by the applied field during the rising portion of each

temperature cycle.

An interpretation of the temporal response curve of figure 4 is that the

response observed with a large field applied comprises a large temporary

effect and a smaller quasi-permanent effect. Immediately on removing the

field (after cooling to room temperature), there is a very large decay in

response, followed by a more gradual decay. After several hours a nearly

steady value is obtained. The effects are presumed to result from increased

dipole alignment and from surface charges. The initial large decay upon

removal of the field is attributed to partial dipole realignment while the

gradual decay is attributed to charge leakage. The quasi -permanent effect

that remains is postulated to be produced by dipoles that retain their

orientation after the field is removed.

Examination of figures 5, 6, and 7 reveals an exponential relationship

between responsivity and reciprocal field, relatively small scatter between

samples from a single piece of material, higher responsivity after using

higher poling fields, and, apparently, more effective poling at 100°C than

at 90°C or 130°C, particularly at the higher fields. For PVF, examination

of figure 8 shows large scatter in responsivity between samples, while in

figure 9 the scatter shown is much less. Both figures show comparative

insensi tivi ty of responsivity to poling field above approximately 400 kV/cm

(reciprocal of field = 2.5 x 10 -6 cm/V) and 90°C. The responsivity attained

in this material by poling at relatively low field and temperature is com- parable to that of PVF 2 after poling at appreciably higher temperature and

higher fields. Within the limits of field, temperature, and number of

samples used in this study, optimum poling for PVF 2 appears to be obtained

with a field of 800 kV/cm (reciprocal of field = 1.25 x 10- 6 cm/V) and a temperature of 110°C for PVF, optimum poling conditions are a field of 350

to 800 kV/cm (reciprocal of field = 2.86 x 10- 6 cm/V) and a temperature of

no°c.

The most responsive PVF 2 samples were uniaxial ly stretched before electrodes

were applied. As discussed in 2.2.1, uniaxial stretching of g-phase material

is thought to orient molecular chains in the plane of the sample. It is

assumed that for 3-phase material the electric field applied normal to the
film plane after stretching orients the dipoles, resulting in polarization

which is retained when the specimen is cooled. For stretched and electrically

poled PVF 2 , pyroel ectric coefficients approaching those of triglycine sulfate,

a material commonly used for IR detectors, have been obtained.

Sensors containing polarized PVF or PVF 2 with sensitive elements 1 cm2 in area were constructed and D* (500 K, 1.6 Hz) was measured. A typical value of D*

for PVF 2 was about 10 s cm . Hz 1 / 2 • W1

, and for PVF, 10 7 cm • Hz 1 / 2 • W" 1

.

The relative spectral response of a PVF sensing element (figure 11) showed

little dependence on wavelength in the region to 6 to 18 urn; the limited

data obtained at higher wavelengths indicated that the response increased

rapidly with increasing wavelength, at least to 25 pm, the maximum studied.

The relative spectral response of a PVF 2 sensing element measured in the

region of 1 to 10 urn (figure 12) showed maxima and minima.

Sensors made with ultraviolet-absorbing PVF were found to respond in the

ultraviolet more strongly than either "ordinary" PVF or PVF 2 No quantitative measurements were made.

5. RECOMMENDATIONS FOR FURTHER WORK

The results reported here support the expectation that significant improvement

of sensor performance in regard to detectivity can be made. To achieve this

potential improvement, the following further work is recommended:

(1) Continue investigations of the effect of stretching on PVF and PVF 2 , particularly uniaxially stretched samples of the latter which have

exhibited very high responsivi ties in small areas only. Further

work is needed to develop techniques to permit stretching uniformly

over large areas. Methods of determining or identifying areas of

uniform stretch are required. An optical method suggested in the course of the project may have promise.

(2) Experiment with other potential sensor materials, such as the polyphosphazenes . A preliminary trial conducted earlier suggests the

possibility of responsi vities comparable to those measured for PVF

and PVF 2

.

(3) Identify and, if possible, reduce major sources of noise.

(4) Experiment with new methods of poling, such as pulsing the field or applying the field during the stretching operation.

(5) Improve detector construction to reduce stray capacitance and thus

increase responsi vity; also, improve preamplifier signal-to-noise

figures of merit.

-11-
Continue work on development of polymeric ultraviolet sensors; develop

adequate instrumentation to measure ultraviolet pyroelectric response.

Continue work on development of polymeric pyroelectric sensors having

selective wavelength responses.

6. REFERENCES

Bazhenov, V.A., Piezoelectric Properties of Wood (Consultants Bureau

Enterprises, Inc., New York, 1961).

Cohen, J., and Edelman, S. , Direct Piezoelectric Effect in Polyvinylchloride Films, J. Appl . Phys. 4_2_, pp. 893-894 (February 1971).

Cohen, J., and Edelman, S., Piezoelectric Effect in Oriented Polyvinyl

choloride and Polyvinyl Fluoride, J. Appl. Phys. 42_, pp. 3072-3074

(July 1971).

Cohen, J., Vezzetti , C.F., and Edelman, S., Polymeric Pyroelectric

Detectors, Proc. of the Special Meeting on the Physics of Detectors,

U.S. Naval Training Device Center, Orlando, FL, March 15, 1972,

pp. 113-117, (Infrared Information and Analysis Center, Willow Run

Laboratories, U. of Mich., Ann Arbor, MI, 1972).

Glass, A.M., McFee, J.H., and Bergman, J.G., Jr., Pyroelectric

Properties of Polyvinyl idene Fluoride and Its Use for Infrared

Detection, J. Appl. Phys. 42_, No. 13, pp. 5219-5222 (December 1971).

Phelan, R.J., Jr., Mahler, R.J., and Cook, A.R., High D* Pyroelectric

Polyvinyl Fluoride Detectors, Appl. Phys. Letters 1_9, No. 9, pp. 337-

338 (November 1971).

Fukada, E., and Takashita, S., Piezoelectric Effect in Polarized

Polyvinyl idene Fluoride), Jpn. J. Appl. Phys. 8, pp. 960-961

(July 1969).

Kawai, H., The Piezoelectricity of Polyvinyl idene Fluoride), Jpn. J.

Appl. Phys. 8, pp. 975-976 (July 1969).

Lando, J.B., Olf, H.G., and Peterlin, A., Nuclear Magnetic Resonance

and X-Ray Determination of the Structure of Polyvinyl idene Fluoride),

J. Poly. Sci. 4, Part A-l , pp. 941-951 (April 1966).

Zbinden, R. , Infrared Spectroscopy of High Polymers, pp. 29-30

(Academic Press, New York, NY, 1964).

Edelman, S., Grisham, L.R., Roth S.C., and Cohen, J., Improved

Piezoelectric Effect in Polymers, J. Acoust. Soc. Amer. 48, No. 5 (Part 1) pp. 1040-1043 (November 1970).
[12] Engineering Design Handbook - Infrared Military Systems, Part 1,

pp. 3-126 et seq, AMCP 706-127 (U.S. Army Material Command 1971).

-13-

USCOMM-NBS-DC
LU

IftJ &oo

coo

.j

_jo

COo

CO = sio

CO

CO

LUO

LU

LU

Li_ LU

CDO

Q_

q:ooo

LU

CO

CO<<

Q_<<

q:

cd<oo

CQ ox

COoz

COo

ZDo

CO

CD<O>

CDO

Q_

LU

CD

-14-
SEMI TRANS PARENT

^ELECTRODE

TO BNC CONNECTOR

Figure 2: Poling fixture, A vertical cross section is shown; the

fixture is radially symmetric with respect to a vertical

axis through the pin. the brass ring used to retain the

polymer film is a press fit into the housing, the lower

end of the brass pin is connected to the center conductor

of a bnc connector; the housing/ to the connector shield.

The broadened head at the upper end of the pin is weakly

bonded to the rear electrode/ which has a smaller diameter

than the upper electrode so that no electrical contact is

made between the rear electrode and the housing.

-15-
40 60 80 100

TEMPERATURE, °C

Figure 3: Relative response of a PVF2 sample as a function of

POLING TEMPERATURE FOR THREE POLING FIELDS, ARROWS

INDICATE THE DIRECTION OF THE CHANGE IN TEMPERATURE

FOR A GIVEN PORTION OF THE POLING CYCLE. THE RESPONSE

WAS MEASURED AS DESCRIBED IN THE TEXT,

-16-
-17-
200

1 •

2 A

4

5

I 2 3

RECIPROCAL OF FIELD, IO"*cm/V

Figure 5:, Relative polymer pyroelectric response as a function

of the reciprocal of the poling field. relative

I response is on a logarithmic scale. the four samples

were taken from a single piece of pvf2 film.

The processing for the four samples was intended to

be identical: all were poled at the same time in

the same oven. poling temperature was 90°c. the

relative response may be considered to be proportional to the pyroelectric responsivity/ with the

same proportionality factor for figures 5-9, the

measuring frequency was 10 hz, the blackbody temperature 500 k, and the distance from the blackbody

aperture to the sensor was 50 cm. the equivalent

noise bandwidth was adjusted to be 1 hz.

-18-
200

fOO

w

O

O

uj

(O

z

o

CL

to

Id

a:>

UJcr

10

9 1^0

H § £

SAMPLE

NO.

0

6

7

8

9

JO

0

a

o

o

0

a

RECIPROCAL OF FIELD, I0" 6 Cm/v *

Figure 6; Relative polymer pyroelectric response as a function of

THE RECIPROCAL OF THE POLING FIELD, RELATIVE RESPONSE

IS ON A LOGARITHMIC SCALE, THE FIVE SAMPLES WERE TAKEN

FROM A SINGLE PIECE OF PVF

2 FILM, THE PROCESSING

FOR THE FIVE SAMPLES WAS INTENDED TO BE IDENTICAL: ALL

WERE POLED AT THE SAME TIME IN THE SAME OVEN, PoLING

TEMPERATURE WAS 110°C, THE RELATIVE RESPONSE MAY BE

CONSIDERED TO BE PROPORTIONAL TO THE PYROELECTRIC

RESPONSIVITY, WITH THE SAME PROPORTIONALITY FACTOR FOR

figures 5-9, The measuring frequency was 10 Hz, the

BLACKBODY TEMPERATURE 500 K, AND THE DISTANCE FROM THE

BLACKBODY APERTURE TO THE SENSOR WAS 50 CM, THE

EQUIVALENT NOISE BANDWIDTH WAS ADJUSTED TO BE 1 Hz,

•19-
200

100

>»

w.O

h. -

JQ

k.0

UJ

CDz

oCL

CO

UJ

<r

UJ>

s*

_j

UJ

EE

10

o $

o

®

SAMPLE

NO.

3

8

7

9

0

o

1

I 2 3

| RECIPROCAL OF FIELD, I0" 6 Cm/V

Figure 7: Relative polymer pyroelectric response as a function of

the reciprocal of the poling field, relative response

is on a logarithmic scale. the five samples were taken

from a single piece of 6-//m pvf2 film, the processing

for the five samples was intended to be identical: all

were poled at the same time in the same oven. poling

temperature was 130°c. the relative response may be

considered to be proportional to the pyroelectric

responsivity, with the same proportionality factor for

figures 5-9. The measuring frequency was 10 Hz, the

BLACKBODY TEMPERATURE 500 K, AND THE DISTANCE FROM THE

BLACKBODY APERTURE TO THE SENSOR WAS 50 CM. THE EQUIVALENT NOISE BANDWIDTH WAS ADJUSTED TO BE 1 Hz

-20-
200

I 2 3

RECIPROCAL OF FIELD, I0~ 6 Cm/V

Figure 8: Relative polymer fyroelectric response as a function of

the reciprocal of the poling field, relative response

is on a logarithmic scale, the five samples were taken

from a. single piece cf d~/|m pvf film, trie processing

for the five samples was intended to be identical; all were poled at the same time in the same oven, poling

temperature was 90°c, the relative response may be

considered to be proportional to the pyroelectric

responsivity, with the same proportionality factor for

figures 5-9, The measuring frequency was 10 Hz, the

BLACKBODY TEMPERATURE 500 K, AND THE DISTANCE FROM THE

BLACKBODY APERTURE TO THE SENSOR WAS 50 CM, THE EQUIVALENT NOISE BANDWIDTH WAS ADUUSTED TO 1 Hz.

-21-
200

toc

>»o

o

UJ00

z

o

CLCO

UJK

UJ>

_lUJ

or

100 -

10

I 2 3

RECIPROCAL OF FIELD, I0" 6 Cm/V

Figure 9: Relative polymer pyroelectric response as a function of

the reciprocal of the poling field. relative response

is on a logarithmic scale, the four samples were taken

from a single piece of 13~/zm pvf film. the processing

for the four samples was intended to be identical: all

were poled at the same time in the same oven. poling

temperature was 110°c , the relative response may be

considered to be proportional to the pyroelectric

responsivity with the same proportionality factor for

figures 5-9. The measuring frequency was 10 Hz, the

BLACKBODY TEMPERATURE 500 K, AND THE DISTANCE FROM THE

BLACKBODY APERTURE TO THE SENSOR WAS 50 CM. THE EQUIVALENT NOISE BANDWIDTH WAS ADJUSTED TO BE 1 Hz.

-22-
+ 9V

INo

I T &

Figure 10: Circuit of voltage preamplifier used in relative

pyroelectric response measurements.

-23-
O © <0 + CM

s*!un AJDJ4|qJD !Vdl03dS 3AIXVH3d

-24-

REAR ELECTRODE

SEMITRANSPARENT

ELECTRODE

INSULATING

PLUG

BRASS

HOUSING

BRASS RETAINING

RING

POLYMER FILM

GOLD FILM

BRASS PIN

b

TO BNC CONNECTOR

Figure 13: Sensor construction, A vertical cross section is shown;

the sensor is radially symmetric with respect to a

vertical axis through the pin, the brass ring used

to retain the polymer-film sensing element is a press

fit into the housing, the lower end of the brass pin

is connected to the center conductor of a bnc connector;

the housing/ to the connector shield. the top of the

enlarged part of the pin is flush with an identation

in the top surface of the insulating plug; an evaporated

gold film covers pin and plug/ except for a narrow ring

at the plug circumference. electrical contact to the

rear electrode is made via the gold film to the pin.

This design/ compared to that of figure 2/ provides improved

thermal isolation from the brass pin/ which otherwise serves

as a heat sink.

-26-