Jioiioriiiaiion Center

National Bureau of Standards

Gaithersburg, Maryland 20899

NBSIR 87-3680

i %Sm.

j

v s v, i«c3 r Lf d

Preparation of Thin Film J

“ ‘ Polyvinylidene Fluoride Shock Wave

Pressure Transducers

Anthony J. Bur and Steven C. Roth

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Institute for Materials Science and Engineering

Polymers Division

Gaithersburg, MD 20899

December 1987

Annual Report

Contract No. N6092187WRW0082

J. W. Forbes, Contract Officer

Prepared for: Naval Surface Weapons Center

White Oak

Silver Spring, MD 20903

NBSIR 87-3680

PREPARATION OF THIN FILM

POLYVINYLIDENE FLUORIDE SHOCK WAVE

PRESSURE TRANSDUCERS

Anthony J. Bur and Steven C. Roth

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Institute for Materials Science and Engineering

Polymers Division

Gaithersburg, MD 20899

December 1987

Annual Report

Contract No. N6092187WRW0082

J. W. Forbes, Contract Officer

Prepared for: Naval Surface Weapons Center

White Oak

Silver Spring, MD 20903

U.S. DEPARTMENT OF COMMERCE, C. William Verity, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

ABSTRACT

Thin film polyvinyl idene fluoride transducers have been prepared by a poling

procedure which involves the repeated cycling of the applied electric field

over the ferroelectric hysteresis curve. Using this procedure, it is possible

to prepare a uniformly poled transducer and to establish, with precision, the

final remanent polarization. The remanent or permanent polarization is

calculated by measuring the total poling charge and subtracting from it that

which arises from the resistance and capacitance of the specimen. By this

method we were able to prepare ten gages with the same remanent polarization

with a precision of 0.5%. X-ray observations and measurements of density on an

unpoled specimen show that the film is biaxially oriented, contains both a and

/3 phase crystals, and is 50% crystalline. A poled specimen with a remanent

polarization of 5.7 /xC/cm2 showed a decrease in the a phase content with a corresponding increase in the /? phase, but the overall crystallinity remained

unchanged. We observe that polarization saturation is a function of the

maximum poling field and it is suggested that the existence of space charge at

the surface of the polar crystals presents an opposing local field to the

crystallites and moderates the ferroelectric switching over a wide range of

applied fields.

1
INTRODUCTION

The procedure which we have used to carry out the poling of polyvinyl idene

fluoride (PVDF) thin film gages is based on work performed by Bauer. 1 This

technique involves increasing the applied electric field in small increments,

and, at each field level cycling many times over the ferroelectric hysteresis

curve. In so doing each electrode spends approximately equal time in the

positive and negative state thereby producing a symmetric distribution of space

charge in the specimen. Since space charge affects the magnitude of the

internal electric field, a symmetric distribution of charge will result in a

symmetric distribution of permanently oriented dipoles. The symmetry of

permanent polarization in specimens poled by this procedure has been observed

by carrying out thermal pulse experiments. 2 In these experiments, 26 //m thick

poled specimens were observed to have uniform polarization throughout the

interior 16 pm but to possess a slight decrease in polarization for the 5 pm

distance nearest each surface. 2 Uniformity of the polarization is a

particularly important criterion for PVDF shock gages because a shock wave,

while traversing the thickness of the film, will produce an electric charge vs

time profile which mirrors the polarization vs thickness profile in the gage.

Another advantage of using the Bauer poling technique is that the final

remanent polarization of the gage can be precisely controlled. This control is

achieved by calculating the remanent polarization, Pr , in real-time and

adjusting the field in small increments to produce a gage with the desired

value of remanent polarization. The calculation of Pr involves correcting the

observed poling charge by subtracting from it that which is due to ohmic

conductivity and that which is associated with the capacitance of the PVDF chi:.

2
film. The data which we present here will show that we have been able to

fabricate shock gages whose remanent polarization was controlled within a

precision of 0.5%.

EXPERIMENTAL APPARATUS AND PROCEDURE

The PVDF Film The PVDF film which we used for our experiments was manufactured

by Rhone - Poulenc

,

3 and was obtained from a selected master batch which is kept

at Sandia National Laboratories by R. Graham. 4 The thickness of this film is

26 ^m. Our x-ray observations and measurements of density indicate that the

material is biaxially oriented, contains both nonpolar a and polar

crystalline phases, and has a crystallinity of 50%. The as-received material

has a density at room temperature of 1.8060 gm/cm3

.

In cooperation with J. Forbes and D. Tasker of the Naval Surface Weapons

Center5

, an electrode pattern was established which minimizes the mutual

inductance (and resultant ringing effects at high frequencies) of the leads to

the gage sensitive area. This pattern is shown in Figure 1 and was deposited

on the film using a vacuum deposition technique in two steps. First, chromium

was deposited on the film, and second, gold was deposited on top of the

chromium. The thickness of the chromium layer was 20 to 30 nm and the

thickness of the gold layer was approximately 100 nm. The central circular

area, which is 0.5 cm in diameter, is the sensitive gage area.

The Poling Procedure Poling of the PVDF film is carried out at room

temperature with the film immersed in fluorinert, a dielectric fluid from the

3
3M Co.. 3 During poling, a pressure of 3.1 x 106 Pa (450 psi) is applied to the

central area of the film by a hydraulic ram. The purpose of the applied normal

1 force is to maintain the smoothness of the film surface which will wrinkle if

left unconstrained. A cross section of the poling cell is shown in Figure 2.

Poling commences at a low applied field, 1.15 MV/cm (3000 V for these gages).

Using a bipolar power supply, the field is cycled sinusoidally between + and -

1.15 MV/cm at a frequency of 0.05 Hz. After one hour of cycling, the field is

increased by 0.4 MV/cm and cycling continues for an hour at this new field

level. This procedure continues up to 2.75 MV/cm after which the incremental

increase is lowered to 0.2 MV/cm. The current vs field hysteresis curve is

monitored using a programmable digital oscilloscope. The corrected hysteresis

curve and remanent polarization are obtained using software which subtracts the

current due to ohmic conductivity and specimen capacitance from the observed

total current. (Details regarding the current correction calculation are

contained in the Appendix.) As the desired remanent polarization is

approached, a linear extrapolation is used to determine the field increment

which will yield the specified value of Pr

. The absolute error in the

measurement of Qr , the remanent polarization charge, is estimated to be ± 3%,

but owing to the difficulty in calculating the effective area of the electrode,

the error in the measurement of Pr is estimated to be ± 10%. The relative

precision with which Pr can be obtained from gage to gage is discussed below.

Density Measurements These experiments were carried out using the fluid

displacement method for which the displacement fluid was water at 23° C. The

weighing balance was a Cahn Electrobalance whose output was observed on a strip

4
chart recorder. 3 The weight of the sample in the fluid was observed to reach

equilibrium in approximately 2 hours. The poled samples, for which we made

density measurements, did not have the same electrode configuration as the

gages but had rectangular aluminum electrodes which were 1 cm x 6 cm. When

poling was complete, the aluminum electrodes were removed by placing the

specimen in a solution of sodium hydroxide. This specimen was then used for

both density and x-ray measurements. The uncertainty in the density

measurement is estimated to be ±0.0005 gm/cm3

.

X-ray Measurements Wide angle x-ray measurements were made using CuKa

radiation. X-ray patterns were recorded in transmission so that the (001)

reflections of the phase and the (002) reflections of the a phase could be

observed. Scans of the reflected x-ray intensity were made over a range of 29

from 30 to 46° . The ratio of intensity of the (001) reflection to the a

(002) reflection, weighted by their relative structure factors, was used to

calculate the relative amounts of a and fi phase in the specimen. 6 Because

these films are biaxially oriented in the manufacturing process, the 29 scans

were made for orientations of the specimen at 0, 45, and 90° with respect to

the plane of the incident and reflected x-ray beam. An average intensity ratio

was calculated from these three observations . The uncertainty in the

measurement of the defined average intensity ratio is estimated to be T 10 %.

Calculation of Specimen Crystallinity The crystallinity of the specimen was

calculated by combining the x-ray and density data. If we let fa , f^ , and fA

be the volume fraction of a, /?, and amorphous phases in the specimen, and if we

define k to be the ratio of the weighted x-ray intensities of the and a

5
phases, then it follows that

fa + f/? + fA = i>

k = SI^/Ia = £

p

/fa .

fcc Va + f fA VA

where S is the relative structure factor of the /?(001) and the a(002)

reflections, va , v^ , and vA are the specific volumes of the a, /3, and amorphous

phases; vs is the measured specific volume of the specimen (the reciprocal of

the density) . From these equations we have

vs (k + 1) - va

- fA = (1)

vA (k + i) - va

- kv^

va

" Vs f« = — —> ( 2 ) vA (k + 1) - va

- kv^

f/? = kfa . (3)

Using va = 0.5197 cm3 /gm, v^ = 0.5074 cm3 /gm, and vA = 0.5980 cm3 /gm, room

temperature values for these quantities, 7 and S = 3, 6 we are able to calculate

the volume fraction of each phase if vs is known.

The crystallinity, x > is defined as x = fa + or X = 1 - ^a • We note from

equation (1) that x is relatively insensitive to changes in k and we estimate

the uncertainty in our measurement of x to be - 0.01.

RESULTS AND DISCUSSION

Polarization Measurements In Figure 3, we show uncorrected current vs poling

6
field hysteresis curves which were obtained for four maximum poling fields in

the range 1.10 to 2.70 MV/cm. The electric field was varied sinusoidally at

0.05 Hz for each curve, and each is a single cycle representative of the

approximately 180 cycles made at each maximum field level. The peak in the

current for each cycle occurs during ferroelectric switching. The total

current is the sum of the displacement plus ohmic currents. Thus, in terms of

the current density,

j = Jd + j P = edE/dt + dP/dt + E/p, (4)

where p is the resistivity, e is the dielectric permittivity, E is the electric

field, and P is the polarization. In order to obtain dP/dt, corrections are

made by subtracting from the measured j a quantity equal to edE/dt + E/p. In a

hysteresis plot of j vs E, the effect of the ohmic current is to rotate the

curve counterclockwise about the origin, and the effect of the capacitance

current is to add to j in accordance with dE/dt. The software which we

developed to make the corrections contains the following criteria: (a) the

slope of the current vs field curve equals zero when P(E) = constant, i.e. at

saturation; and (b) the value of the current equals zero for P(E) = constant.

A typical corrected current curve is shown in Figure 4. Upon integration of

the current hysteresis curve, we obtain the QP (E), the polarization charge

hysteresis curve of Figure 5 form which we calculate a remanent polarization of

6 . 00 pC/cm2

.

The specified remanent polarization for the group of ten gages which we

7
prepared was 6.00 /xC/cm2

. In order to realize the value of the poling field

which will yield the target remanent polarization for each gage, we made a

linear extrapolation of the remanent polarization vs poling field data from

lower values of Pr to 6.00 jxC/cm2

. The results, shown in Table I, indicate the

precision with which we are able to control the remanent polarization, in this

case with a standard deviation of ±0.5%.

TABLE I

Remanent Polarization and Poline Field Measi

Gage Pr xxC/cm^- E MV/cm

1 6.00 2.78

2 5.98 2.77

3 6.00 2.84

4 5.99 2.79

9 5.98 2.21

10 6.00 2.77

11 6.09 3.19

12 5.98 2.77

13 5.99 2.85

14 5.99 2.70

Average 6.00 ± 0.5% 2.77 ± 8%

In Table I, we also note that the maximum poling field, which is necessary to

achieve a polarization of 6.00 /xC/cm2

, has a standard deviation much larger

than that for the remanent polarization. We attribute this to the presence of

different amounts of internal space charge within each gage. We view the space

charge as residing at the surface of the polar crystallites and presenting an

internal field to the crystallite which opposes or diminishes the effect of the

applied poling field. Considering that a distribution of space charge exists

in each specimen, it follows that there is a distribution of local field

8
strengths. As a consequence, the poling field which is necessary for achieving

a given remanent polarization will be a function of the particular space charge

distribution within each gage. Since the space charge is an uncontrolled

parameter during the processing of this film, it is not possible to prepare

gages with identical remanent polarization simply by subjecting them to the

same maximum poling field.

Another observation which we attribute to the presence of space charge is the

dependence of the saturation polarization on the maximum poling field. As

implied by the definition of saturation, one would expect that the saturation

value of the polarization would remain constant as the maximum poling field

increases. But for PVDF, this is not the case. As we increment the maximum

poling field to a higher level, the saturation polarization also increases.

Two phenomena occur on the molecular scale during poling: (a) dipoles within

the (3 phase crystallites orient in the direction of the applied field if they

experienced a field equal to or greater than the coercive field for PVDF

(approximately 1.0 MV/cm) , and (b) the a phase crystallites are converted to a

polar a phase at fields near 1.0 MV/cm and subsequently to (3 phase crystals at

higher field strengths. 8 As the poling field increases, the opposing effect of

the space charge is overcome and more crystallites participate in the

ferroelectric switching. Those crystallites which remain unswitched or

unconverted to the polar a and /? phases at a given poling field will remain so

until the field reaches the appropriate value.

We have made repeated attempts to prepare gages with remanent polarization

greater than 8 juC/cm2

. These attempts have all met with failure because of

9
electrical breakdown which occurs at poling fields in the range from

3.0 to 3.5 MV/cm. Seven out of eight gages failed at the edge of the electrode

indicating that the electrode design and/or the electrode material are the

source of the breakdown problem. Fabrication of gages using different

electrode materials and employing sputtering rather than vacuum deposition may

improve the breakdown strength of this design.

X-rav Measurements In Figures 6 and 7, scans of the x-ray intensity vs 29 are

shown for three orientations of the control (unpoled) specimen and for a poled

specimen whose remanent polarization was 5.7 /im/cm2

. At zero degree

orientation, the ratio of the intensity of the ft (001) reflection to the a

(002) reflection is the largest of the three orientations. This indicates

that, during processing, the principal stretching and extension of the film

has occurred in a direction coincident with zero degrees. Such an

interpretation is based on the well known observation that the ft phase is

formed by extensional strain. 9

The largest difference in the x-ray data obtained from poled and unpoled

specimens is seen at the 90° orientation. Here, a significant increase in the

ft phase intensity at the expense of the a phase intensity has occurred for the

poled specimen. Thus, the poling field for this specimen, 2.6 MV/cm, was large

enough to produce some conversion from the a to the ft phase. It is interesting

that, at zero degrees, there is no difference in the x-ray scan between the

poled and unpoled specimens. The reason for the lack of conversion from the

a phase to ft phase along this direction is unclear.

10
From the x-ray measurements we calculate a value of k by taking a weighted

average of the k values obtained from the three orientations, i.e.

<k> = 0.25ko + 0.5k45 + 0.25k90 . This average k, shown in Table II, is used in

equations (1) through (3) to calculate the volume fractions of the a, /3, and

amorphous phases.

TABLE II

Density and X-Ray Results

Specimen Dens i tv e/cm3 <k> 4 u £a X

Control 1.8060 3.5 11% 39% 50% 50%

5 . 7/xC/cm2 1.8099 5.3 8% 43% 49% 51%

Density Measurements The results of the density and x-ray measurements on two

specimens are shown in Table II. The increase in density with increasing

remanent polarization is associated with the conversion of the lower density a

phase crystals to higher density f) phase crystals. Within the limits of

experimental uncertainty, we observe that the crystallinity remains constant.

SUMMARY AND CONCLUSIONS

By using the Bauer polarization procedure, we have been able to prepare ten

gages with a remanent polarization of 6.00 /iC/cm2 to a precision of 0.5%.

From density and x-ray measurements we calculate that the crystallinity remains

constant during poling but there is a significant conversion of a phase

crystallites to the polar ft phase. The observation that each gage requires a

unique maximum poling field to achieve the same remanent polarization is

attributed to the presence of different distributions and amounts of space

11
charge in each gage. The observed increase in the saturation polarization with 1

increase in the maximum poling field is also attributed to the effect of space

charge which resides at the surface of polar crystals and creates a local

electric field which diminishes the effect of the applied poling field. Our

attempts to increase the remanent polarization to values greater than 8.0

/zC/cm2 were not successful because electrical breakdown occurred at the edges

of the electrodes before reaching this level of polarization.

REFERENCES

1. F. Bauer, Ferroelectrics 49, 231 (1983).

2. A. S. DeReggi, National Bureau of Standards, Gaithersburg, MD, private

communication. For a discussion of the thermal pulse experiment see

A. S. DeReggi and M. G. Broadhurst, Ferroelectrics 7_ 3 , 351 (1987).

3. Identification of a commercial product is made to facilitate experimental

reproducibility and does not imply endorsement by NBS

.

4. R. Graham, Sandia National Laboratory, Albuquerque, New Mexico.

5. J. Forbes and D. Tasker , Naval Surface Weapons Center, White Oak, MD.

6. J. B. Lando, H. G. Olf, and A. Peterlin, J. Polymer Sci., Pt. A-l 4, 941

(1966).

7. K. Nakagawa and Y. Ishida, Kolloid Z. u Z. Polymere 251 . 103 (1973).

8. G. T. Davis, J. E. McKinney, M. G. Broadhurst, and S. C. Roth, J. Appl.

Phys. 49, 4998 (1978).

9. E. L. Nix, L. Holt, J. C. McGrath, and I. M. Ward, Ferroelectrics 32, 103

(1981)

.

12
APPENDIX

Data Analysis

The poling voltage is in the form of a high voltage sine wave and is applied

across the dielectric PVDF film at a frequency of 0.05Hz. The measured

variable is the current through the dielectric. A signal proportional to the

high voltage and a signal from a current amplifier are fed into a programmable

digitizing waveform recorder. Both signals are digitized with 12-bit

resolution. Fifty seconds of data are collected at a sampling rate of 20

samples/second (1024 data points). This results in an effective sampling of

approximately 400 data points per cycle.

Figure 3 shows the recorded raw data. The following is a summary of the

analysis program:

1. Both waveforms are multiplied by the appropriate proportionality constants

to give applied voltage (V) and measured total current (/iA) .

2. By dividing the applied voltage by the sample thickness (entered as a constant), the applied field, E, is computed (V/cm) . A smoothing of the

digitized data is performed (a 10 point smoothing) which should not result

in significant degradation due to the sinusoidal nature of the field.

3. A complete cycle is chosen for analysis.

4. The resistive component of the current is calculated and subtracted from the

total current, I T

. The resistive term is calculated by drawing a straight

line between I T (E=Emax ) and I T (E=Emin ) and storing the value of the slope.

The field is multiplied by the slope to give the resistive current, which is

then subtracted from the total current.

5. Next, the capacitive component of the current is calculated and subtracted

from the total current. To do this, we subtract an elliptically shaped

current vs field function (see reference 1) so that dl/dE = 0 at Emax and

Em ±

n

. The resultant current is the polarization current, I p

.

6. To calculate the remanent polarization, the I p curve is integrated to give

the polarization charge Qp as shown in Figure 5. The remanent polarization

expressed as charge per unit area is: Pr = Qp (E=0)/Ae , where Ae is the area

of the electrode.

From this analysis we can obtain a value of the resistivity and dielectric

constant of PVDF. Typical values are p = 6.0 x 10 12 ohm-cm and

k = e/e 0 = 17.5 at 0.05 Hz and room temperature.

13

Figure 1

.

The electrode pattern for the shock wave pressure gages

The central circle in the pattern has a diameter of 0.5 cm and is

area of the gage. The dark and shaded regions emanating from the

are the electrical leads on opposite sides of the clear thin film

is shown,

the sensitivecentral are vi
Figure 2. A cross sectional diagram of the poling cell is shown. The flat

glass insulators impinge upon the face of the gage through circular openings in

the teflon holders; they transmit the normal force of the hydraulic ram to the

sensitive area of the gage. The teflon holders support the PVDF film and

electrical leads.
Total Current

Figure 3. Typical uncorrected current hysteresis curves are shown. The maxim

poling fields for these curves are 1.54, 1.92, 2.31, and 2.69 MV/cm. The pp ? current for the curve with a ^ =2.69 MV/cm is approximately 2.5 /iA.
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Figure 6. X-ray scans, taken in transmission, are shown for three orientations

of the unpoled PVDF film. These scans show a (002) reflections at 29 = 39° and

(001) refections at 29 = 35° . The angle of orientation is the angle between

the principal direction of strain in the film and the plane of the incident and

reflected x-ray beam.
1—"

r POLED PVDF

T

Fi gure 7. X-ray scans, taken in transmission, are shown for three orientations

of the poled PVDF film. These scans show a (002) reflections at 29 = 39° and

P (001) refections at 29 = 35°

.

The angle of orientation is the angle between

the principal direction of strain in the film and the plane of the incident and

reflected x-ray beam.
-

.

'
U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

2. Performing Organ. Report NoJ 3. NBSIR 87 3680-

Publication Date

4. TITLE AND SUBTITLE

"Preparation of Thin Film Polyvinyl idene Fluoride Shock Wave Pressure Transducers"

5. AUTHOR(S)

Anthony J. Bur, Steven C. Roth

8. PERFORMING ORGANIZATION (If joint or other than NBS. see in struction s) 7. Contract/Grant No.

NATIONAL BUREAU OF STANDARDS

U.S. DEPARTMENT OF COMMERCE

GAITHERSBURG, MD 20899

N6092187WRW0082

8

.

Type of Report & Period Covered

Annual Report

Jan. 1 , 1987-Sept. 30, 19*

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State, ZIP)

Naval Surface Weapons Center

White Oak

Silver Spring, MD 20903

10.

SUPPLEMENTARY NOTES

3 Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11.

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

bibliography or literature survey, mention it here) , , ,

. Thin film polyvinyl idene fluoride transducers have been prepared by a poling

procedure which involves the repeated cycling of the applied electric field over

the ferroelectric hysteresis curve. Using this procedure, it is possible to prepare

a uniformly poled transducer and to establish, with precision, the final remnant

polarization. The remnant or permanent polarization is calculated by measuring the

total poling charge and subtracting from it that which arises from the resistance and

capacitance of the specimen. By this method we were able to prepare ten gages with

the same remnant polarization with a precision of 0.5%. X-ray observations and

measurements of density on an unpoled specimen show that the film is biaxial ly

oriented, contains both a and 8 phase crystals, and is 50% crystalline. A poled

specimen with a remnant polarization of 5.7 yC/cm2 showed a decrease in the a phase

content with a corresponding increase in the phase, but the overall crystallinity

remained unchanged. We observe that polarization saturation is a function of the

maximum poling field and it is suggested that the existence of space charge at the

surface of the polar crystals presents an opposing local field to the crystallites

and moderates the ferroelectric switching over a wide range of applied fields.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by

density; ferroelectric hysteresis; piezoelectric shock gages; polymer transducers,

polyvi nyl idene fluoride; remnant polarization; x-ray

13. AVAILABILITY

X 1 Unlimited

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H Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

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P RlN T£ C :es

15. Pr.ee

_] Order From National Technical Information Service (NTIS), Springfield, VA. 22161
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