Page 160 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer
P. 160

P1: GPB/GLT  P2: GQT Final Pages
 Encyclopedia of Physical Science and Technology  en012f-594  July 26, 2001  11:9






               668                                                                                Polymers, Ferroelectric


































                      FIGURE 6 DSC comparison of three VDF/TrFE/CTFE terpolymers: (a) sample 9, (b) sample 10, and (c) sample 11
                      in Table II.

               temperature for samples 1, 2, 3, and 10, and almost  the frequency increases are common features of relaxor
               disappearing for samples 4, 8, and 11. That indicates  ferroelectrics.
               a very small energy barrier in the phase transition and  As shown in Table II, most of the terpolymers show
               implies a smaller crystalline domain. The results are  high dielectric constants at ambient temperature. Samples
               consistent with the electric responses of the terpolymer  4 and 5 exhibit dielectric constants as high as 53.5 and 53.1
               under an electric field.                           (1 kHz), respectively, at 22 C. In sharp contrast, prior re-
                                                                                       ◦
                                                                 search reported that high dielectric constants (>25) only
                                                                 existed in the VDF/TrFE/CTFE terpolymers with a narrow
               C. Electric Properties
                                                                 range (18–22%) of TrFE content. In addition, the mechan-
               For electrical measurements, polymer films (∼30 µm  ical stretching of the polymer film, usually very important
               thickness) were prepared either by solution casting on  for increasing the dielectric constant in VDF/TrFE copoly-
               a glass slide from N,N-dimethylformamide solution  mers, is not necessary in these terpolymers. The effective
               containing 8–10 wt% polymer or melt-pressing polymer  orientation of dipoles under an electric field may be at-
                           ◦
               powder at 200 C. The polymer films were annealed at  tributed to the low phase transition energy which occurs
                  ◦
               110 C under vacuum for 5 hr. Gold (<1µm thickness)  at near-ambient temperature.
               was sputtered on both surfaces of the polymer film. The  The polarization hysteresis loop was measured by a
               dielectric constant was measured by an HP multifreque-  Sawyer–Tower circuit with a frequency range between
               ncy LCR meter equipped with a temperature chamber.  1  and  10  Hz.  Figure  8  compares  the  polarization  hys-
               Figure  7 shows  the  dielectric  constant  of  59.3/32.9/7.8  teresis loops of two terpolymers (samples 2 and 8) and
               VDF/TrFE/CTFE terpolymer (sample 10) and 57.3/31.2/  a 55/45 VDF/TrFE copolymer. Both terpolymers show
               11.5 VDF/TrFE/CTFE terpolymer (sample 11) during  significantly smaller hysteresis than the 55/45 copolymer,
               the heating–cooling cycles.                       which has the narrowest polarization hysteresis loop ob-
                 In general, the dielectric constant hysteresis during the  served in the copolymer samples. Sample 2 (VDF/TrFE/
               heating–cooling cycles is very small, and the dielectric  CTFE = 66/22.5/11.5 mole ratio) maintains a high polar-
                                                                                          2
               peak appears near the ambient temperature, well below  ization level (P max = 78 mC/m at E = 100 MV/m) and
               the dielectric peak observed in 55/45 VDF/TrFE copoly-  reduces both a coercive field (17.3 MV/m at P = 0) and
                                                                                             2
                       ◦
               mer (>60 C) with a big hysteresis loop. Diffuse dielectric  remanent polarization (25.2 mC/m at E = 0). Sample 8
               peaks and peaks shifting toward higher temperatures as  (VDF/TrFE/CTFE = 55.6/36.1/8.3 mole ratio) exhibits a
   155   156   157   158   159   160   161   162   163   164   165