Dynamics of Pyroelectricity of a Copolymer of Vinylidene Fluoride with Trifluoroethylene
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One difficulty in studying the pyroelectricity is that it is composed of two components: the primary effect and the secondary effect. The temperature derivative of the electric displacement at no stress boundary condition can be written as
'RT )
=
\(-•)
+ (ODi ) (1x)
where Di, the dielectric displacement, T, the temperature, x, the strain, X, the stress, eij, the piezoelectric stress constant and a3 is the thermal expansion constant. The first term is the primary effect and the second term is the secondary effect that is a piezoelectric response coupled with the thermal expansion. Kepler et al.4', 5) studied thoroughly the pyroelectricity in polyvinylidene fluoride. They used a laser pulse to heat the sample instantaneously and observed the time development of the pyroelectric response. They discussed the separation of the primary and secondary
83 Mat. Res. Soc. Symp. Proc. Vol. 600 0 2000 Materials Research Society
effect. We also developed a system to measure the pyroelectric response as a function of time by irradiating a laser pulse.') In this paper, we report our recent results on a copolymer of vinylidene fluoride with trifluoroethylene by using a laser generated by a mode-locked laser system. We also measured the pyroelectric response curve at various temperatures and discuss the temperature dependence of the primary and secondary effects. EXPERIMENT
The sample was the copolymer of vinylidene fluoride VDF and trifluoroethylene TrFE of 75 mol% VDF content which was supplied by Kureha Chemical Industry Co., Ltd.
ND filter laser
The polymer was dissolved in 2-
butanone together with crystal violet which well absorbs light of wavelength used in this study. The concentration of the dye was 0.001 mol/dm- 3 in the film. After cast at PIN Photometer room temperature, the dyed polymer film
Oscilloscope Trigger
Fig. 1. Experimental System
was annealed at 145 ' C for 1 hour and, after
coated with gold on both surfaces, subjected to an ac field of 0.1 Hz at room temperature. The amplitude of the field was 100 MV/m. The electric field was removed at the maximum. The poled film was cut to 4 mm square. Typical thickness was 16 pm. A laser pulse of wavelength of 532 nm, the pulse width of 7 ns and the pulse 500 energy of 200 mJ was generated by a Q-switched frequency-doubled yttrium400 aluminum-garnet, YAG, laser (Surelite300 I, HOYA Continuum Co.). A neutral 0 density, ND, filter, was used to attenu- C. 200 ate the energy to 4 mJ. Also used was a laser pulse of 532 nm, 20 ps, 12 mJ 100 of a mode-locked Nd:YAG laser gener0 ator(PL2143, EKSPLA) which was atI -6 * -2 -8 * -4 tenuated to 1.2 mJ by an ND filter. 10,2 10,4 10-6 108 100 Figure 1 illustrates the experimental system.') A laser pulse was applied t/s to the sample through a gold electrode which was semitransparent to the light Fig. 2. Pyroelectric response curve at room temperaused. Thus, the absorption of the laser ture. The pulse width is 20 ps. pulse energy occurs at the electrodes and inside the polymer film. With using a PIN photometer signal as
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