PROPERTIES OF PVDF POLYMER FOR SONAR

R. H. Tancrell,* D. T. Wilson* and D. Ricketts**

* * *Raytheon Research Division, Lexington, MA 02173 Raytheon Submarine Signal Division, Portsmouth, RI 02871

ABSTRACT

Polyvinylidene fluoride (PVDF) polymer is evaluated for application in large sonar arrays. The piezoelectric coefficients and elastic constants of thick (550 urn) PVDF produced at Raytheon are measured, and the apparatus for making the measurements is described. It is shown, both theoretically and experimentally, that electrodes de- posited on the surface can stiffen the polymer and significantly alter the observed properties. The free-f ield frequency response for an experimental hydrophone is shown over a broad frequency range, f ee of spurious resonances. Stability to relatively high temperature (90°C) and to high static pressure (6.9 MPa = 1000 ps 1 are shown from experimental data.

Introduction

During the past few years, several lab- oratories have been exploring the use of polyvinylidene fluoride (PVDF) polymer for sonar systems, especially as hydrophones for large arrays. For these applications, where the sonar frequency is below 50 kHz, the thickness of the polymer is much smaller than an acoustic wavelength. Since sensitivity increases with the material's thickness, it is desirable to fabricate material as thick as possible. (As PVDF becomes thicker it is progressively more difficult to polarize because of the very high voltage required.) This paper discusses PVDF produced at Raytheon Research with a thickness of about 550 urn, which has sufficient output voltage to compete with the current ceramic piezo- electric technology. Unlike ceramics, however, PVDF has several unique properties which are especially attractive for large arrays. First, the material can be produced in large, uniform sheets by continuous manufacturing methods. Second, each element in the array can have a large area so as to "fill" the array, thereby

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intercepting as much acoustic energy as possible. Often in ceramic arrays, the elements are widely spaced relative to their size. Third, a large area hydro- phone acts as a filter to flow-induced noise and substantially reduces the effect of flow noise --- the noise produced by turbulence as a ship moves through the water --- without reducing sensitivity to sonar signals. Fourth, the design of a hydrophone is very simple geometrically and is easy to fabricate. And lastly, based on our current experience, the properties of the polymer are more uniform than ceramic materials.

In this paper, we document the important performance parameters of thick PVDF produced at Raytheon. We also discuss measurement methods for obtaining the intrinsic material properties of PVDF; electric, elastic and piezoelectric. In making material measurements, one has to keep in mind that polymers are much softer than ceramics -- their properties may be affected by the electrodes deposited on the surfaces, and by other materials laminated to them. These perturbing conditions are discussed. Also shown are practical environmental considerations: stability of the material to elevated temperature and to static pressure.

Basic Material Properties

Mathematical models for hydrophones are essential for effective design and well- defined perfomance. In order to numerical- ly evaluate the models, the intrinsic properties of the polymer must be indepen- dently measured beforehand. As a basis €or comparing different materials, a low frequency "f igure-of -mer i t" can be defined in terms of these basic properties. The total available power (voltage x current) is a meaningful quantity for this. The parameters of interest are:

E : permitivity, E ' + j E "

tan 6: l o s s tangent ( E " / & ' 1

0090-560718510000-0624 $1.00 0 1985 IEEE

piezoelectric matrix coefficient (coulombs per newton) piezoelectric coefficient under hydrostatic pressure Young's modulus (pascals), at constant electric field, (l/sii) elastic compliance (square meters/newton), at constant electric field "figure-of-merit'' defined as (voltage) (charge) (area)/( force)*

PVDF Piezo-Tensors

where t: thickness of material (meters) A : area of hydrophone (square meters)

The dielectric constant ( E ' / & ~ ) for PVDF has a value ranging from 12 to 7, depending on the processing technique (stretching and polarizing). Our PVDF typically has a value of 8.0. During processing, voids can be introduced into the PVDF. These are small microcracks between polymer strands which reduce the dielectric constant and make the material softer. The loss tangent has a value of 0.015 at 1 kHz. This ratio does not change with different processing, as the real and imaginary parts do: it appears to be intrinsic to PVDF itself.

Each of the piezoelectric coefficients is measured independently, as shown by the test configurations in Fig. 1. Measured values for the first two coefficients are given in the figure. The easiest quantity to measure is dh, shown at the bottom of the figure. Here, all three orthogonal force components are applied simultaneously simply by subjecting a sample to hydro- static pressure (usually at 50 Hz). As shown, the output is the net charge result- ing from all three forces. Because of the test's simplicity, we use dh as a convenient parameter for quality control as we fabricate material. On the other hand, the most difficult quantity to measure is d33 because a force must be applied to the surface along the "3" direction without constraining movement (strain) of the polymer in the other two directions. Usually, optical interferometer techniques are required. To our knowledge, no one has measured this quantity for thick (optically opaque) PVDF. The value of d33 in Fig. 1 is deduced from the three other measure- ments. (Note that the algebraic sign of d33 is opposite that of the other two coefficients.)

djl a FI

F. t

saet D en

TYPICAL VALUES

+ 14 pC/N

+ 2 pC/N

Q d 1 3 -34 pC/N

33 F,

(Difficult to measure) (deduced)

dh = d31 +d,, +d33 (Current stondord measurement)

-18 pC/N

(Hydrostatic pressure)

Measurement Techniques

PVDF is processed in large sheets, typically 0.4 m x 2.0 rn (14 inches x 80 inches). For experimental convenience, measurements are made on small samples taken from the large PVDF sheets. With small samples, the size of the test chambers can be kept small. Further, this procedure permits us to verify uniformity of a large sheet by cutting many samples across it. To reduce the inconvenience of opening the chamber to mount each sample separately, a special mount was designed, holding 15 samples at a time, connected to a coaxial switch which allows each sample to be selected sequentially, as shown in Fig. 2. The capacitance and the output charge sensitivity are displayed by two meters reading directly in pF and pC/N. The dielectric constant is calculated using the measured thickness of the sample.

The drive frequency is low enough (50 Hz) so that the chamber can be considered at equilibrium with a slowly oscillating

1985 ULTRASONICS SYMPOSIUM - 625

Table I

uniform pressure (ie, no standing waves). Two speakers on opposite sides of the chamber are driven in phase to minimize any vibrations in the chamber itself. The samples are immersed in an open container of oil to stabilize their temperature during the measurement. (We have experi- mentally demonstrated that the pressure in the oil is the same as that of the air above it. 1

Measurements of the dynamic elastic moduli were made using an impulse testing technique in which a cantilevered beam of the material is set into motion by an impulse of force applied at its tip. From the resonance curve (resonance frequency and width), we can determine the complex modulus. A l s o static measurements, using strain gauges, were made on electroded and unelectroded samples. The static and dynamic values were in good agreement for unelectroded samples. With static force apparatus, d3l and d32 were determined using 3 coulomb-meter, (Fig. 1). For all these measurements, the sample shape was cut in the form of a long narrow strip, where the length was a least 5 times larger than the width. Values are shown in Table I.

Effect of Stiffening Layers

A layer of metal deposited on the surface can have a dramatic effect on PVDF's elastic and piezoelectric properties, as can be seen in Table I. Even normal copper electrodes can affect the measurement. ~t is important, therefore, to use very thin electrodes (100 nm of sputtered copper, or silver paint) when attempting to measure the intrinsic properties of the polymer.

Measured Constants €or Raytheon PVDF

PVDF with PVDF with very thin 40 um Cu electrodes electrodes

Elastic Constants

2.5 GPa 13. GPa 1

Piezoelectric Constants

d3 1 14. pC/N 2.2 pC/N

d32 2.2 pC/N 0.2 pC/N

d h -18. pC/N -19. pC/N

In an actual hydrophone, additional stiffening layers are often added to raise the resonance frequency of the structure, and they must be taken into account in determining the device's overall sensitivity . We have developed theoretical equations

showing how stiffener layers change the observed charge and modulus. We will discuss only two limiting cases in this paper, first, "bare" PVDF and, second, PVDF with extremely stiff layers. For "bare" PVDF the hydrostatic dh can be shown to be:

For the extreme case of infinitely stiff electrode layers, the output charge for the composite can be shown to yield an effective dhe of the form (derived from the constitutive equations, with the condition of zero strain at the surface):

dhe = d33 + ("1 d3* + [%] djl ( 3 ) c33 c33

where the cij's are the stiffness matrix components for the polymer. These c's are related to the compliance moduli by the matrix relation:

(4)

Note that the cij's are not identical to the Yii's, by definition.

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The main difference between Eqs. ( 2 ) and (3) is that the contributions of d32 and d31 are reduced for the composite, since the factors in brackets are less than unity. We cannot evaluate Eq. (3) numeri- cally, since all of the coefficients have not yet been measured. But we have determined experimentally that stiff copper layers increase PVDF's sensitivity by only abo t 1 dB. (Note: the superscripts on cij' and ci.D indicating constant electric field or electric displacement have been removed for simplicity; the difference is minor for PVDF because of its low electro- mechanical coupling.)

I Polymer

Examining the effect of stiffening layers in Table I, we see that the the copper layers have a much more pronounced effect on d31 than on dh. To understand this, we first examine the effective modulus of the composite (Ye). This has been derived and can be expressed as:

where the thicknesses (h) are defined in Fig. 3 and Yllm and Yllp are the moduli of the metal and the polymer, respectively. Physically, Ylle defined by E q . ( 5 ) is the weighted average of the polymer and copper moduli. Using a value of 100 GPa for the stiffness of copper, 40 um ( 1 . 6 mils), and 550 um PVDF, we obtain an effective stiffness for the composite of 15 GPa, much larger than that of the PVDF itself. Experimentally, we measure a value of 13 GPa, in reasonable agreement with the theory. Further, in comparing the output

charge ( Q ) for a composite and for very thin electrodes, we have shown theoreti- cally that they are in the ratio:

Equation (6) can be used to infer the intrinsic d3l when making measurements with practical electrodes. The measured values in Table I agree to first order with Eq. (6). Ceramic piezoelectrics, on the other hand, change very little when electroded, because the metal is not as stiff as the ceramic itself!

Note that stiffening layers affect d31e and dhe in different ways. For force only in the "1" direction, the observed charge is greatly reduced because most of the force is borne by the copper layer and the polymer strains very little. The stiffer the layers, the smaller is the composite's d31e. But for hydrostatic pressure, the layers have a different effect. For example, for very stiff electrodes the internal elastic constants instrinsic to the PVDF itself (Eq. ( 3 ) ) determine how much the internal d31 contributes to the total charge. The actual stiffness of the added layers is of secondary importance.

Temperature Stability

Even though PVDF does not have to operate at elevated temperatures for sonar applica- tions, its temperature stability is important because it may be exposed to h i g h temperatures during lamination or in storage (as in direct sunlight). In our process, we have been able to stabilize the piezoelectric properties to 90°C, as shown in Fig. 4 . These data were taken for one hour exposure at successively higher temperatures. Additional tests have shown that Raytheon's PVDF does not degrade in piezo-activity when exposed to 90°C for over 100 hours.

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1985 ULTRASONICS SYMPOSIUM - 627

The expansion coefficient of our PVDF has also been measured up to 85OC. A s shown in Fig. 5, the length in the stretch (' '1") direction initially expands but then begins to shrink above 556C, and the shrinkage rate increases rapidly above 7OoC. This appears to be caused by polymer chains in the amorphous phase changing their net orientation. Permanent changes in the piezoactivity (due to the 6-phase crystals) are not observable on this fine scale of fractions of 1% (0.OldB). The expansion coefficient is important for bonding and curing of epoxies at elevated temperatures; it has little effect on our PVDF's piezo-properties.

Frequency Response

An experimental hydrophone is shown in the photo of Fig. 6. For this device, the PVDF is laminated between G-10 (fiberglass) boards and encapsulated in polyurethane. Figure 7 shows the free-field receiving response for normal incidence of the sonar wave. The response is very smooth, with no resonances in the band of interest (below 20 kHz). A peak appears at 70 kHz, and is the result of a resonance between the two stiffener plate "masses" and the polymer "spring." Three curves are displayed for different static pressures, ranging from 20 to 6,900 kPa ( 3 to 1,000 psi). The sensi- tiviy changes very little over this pressure range, confirming that the small voids in the polymer retain their integrity and do not collapse. To demonstrate the operat ion at different operating temperatures, data shown here were taken at a temperature of 3 ' ~ . Other data at room temperature, not shown, are very similar in shape and absolute sensitivity. These results confirm that the hydrophones have stable characteristics over the operating temperature range of many sonar systems.

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Fig. 7. E.rpeririieiltal freqrrerzcy response at :Y°C at i'arioii$ pressirres. Design of hydrophoiie I S

slightly different frorri that of Fig. t ; . Data taketi ut the iVoc a1 Research Laboratory Florida

628 - 1985 ULTRASONICS SYMPOSIUM

Summary

PVDF shows strong potential for hydro- phone applications and especially large arrays. Its piezoelectic properties are high enough to compete with established ceramic technology and its high hydrostatic constant and planar geometry make it suitable for hydrophones and arrays which operate in the hydrostatic drive mode. PVDF hydrophones have been demonstrated which are free of spurious resonances over a broad frequency range,

From a systems viewpoint, the most important consideration is the total electrical energy available from the hydrophone. The smallest detectable acoustic signal is not limited by thermal noise in the PVDF hydrophone but rather by extaneous acoustic noise in the sea itself ("sea-state noise"). PVDF can meet signal level requirements now, but higher piezoactivity would simplify system design. Temperature and pressure stability are presently adequate for array implementa- tion.

Since PVDF is elastically "soft" its measured properties are affected by even thin layers deposited on the surface. Theoretical and experimental evidence of these effects is shown in this paper. Modelling of PVDF hydrophones requires knowledge of both elastic and piezoelectric matrix coefficients, of which only the major coefficients are now known. Other coefficients must be measured to completely understand hydrophone design tradeoffs.

Bibliography

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2.

3.

4.

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7 .

8.

9.

D. Ricketts et al, "Measurement of Complex Sound Speed in Sonar Transducer Materials using Impulse Testing Techniques," J. Acoust. Soc. A m . , Suppl. 1, (1980).

E. L. Nix, et al, "Highly Drawn Poly(viny1idene fluoride 1 with Enhanced Mechanical and Electrical Properties," Ferroelectrics 32, 1 0 3 - 114 (1981).

D. Ricketts, "Polymer Hydrophones," Electronics Progress 3, 8-13 ( 1 9 8 2 ) .

H. Schewe, "Piezoelectricity of Uniaxially Oriented Polyvinylidene Fluoride," IEEE Ultrasonics Symposium Proc., Cat. 5. 82CH1823-4, pp. 519-524 (1982.

J. C. McGrath, et al, "Recent Measurements on Improved Thick Film Piezoelectric PVDF Polymer Materials for Hydrophone Applications, " Ferroelectrics 50, 339-346 (1983).

D. Ricketts, "Recent Developments in the USA in the Applications of PVDF Polymer in Underwater Transducers," Proc. Inst. Acoustics 6, 46-54 (1984). P. Pantelis, "Properties and Applications of Piezoelectric Polymers," Physics in Technology, 15, pp. 239-243 and p. 261 (1984).

D. Ricketts, "Transverse Vibrations of Composite Polymer Plates", J. Acoust. SOC. Am. 1_1, 1939-1945 (1985).

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