Dielectric constant ferroelectric polymers

Vinylidene fluoride-trifluoroethylene (VF2-F3E) copolymers exhibit a ferroelectric-paraelectric phase transition, the first such case found for a synthetic polymer. In this transition, the electric polarization and piezoelectric constant of the film disappear above the Curie point (Tcurie)- The temperature dependence of the dielectric constant, , obeys the so called Curie-Weiss law ... 

Conducting polymers also can be utilized to form core-shell structures with high dielectric constant particles. Fang et al. used PANl to encapsulate barium titanate via in situ oxidative polymerization. They examined the influence of the fraction of BaTiOs particles on the ER behavior, and found that the PANl/ BaTiOs compo-sites-based ERFs exhibit a better ER effect than does pure PANl, which result might be due to the unique ferroelectric properties as well as the high dielectric constant of BaTiOs nanoparticles. 

Relaxor ferroelectric polymers are intimately related to the ferroelectric polymers described above. All known relaxor ferroelectric polymers are based on the P(VDF-TrFE) copolymer. As the name suggests, these polymers behave as relaxor ferro-electrics, which is distinguished by a broad peak in dielectric constant and a strong frequency dispersion [99,100]. There are two major limitations of the P(VDF)-based ferroelectric actuators. First, the electrically induced paraelectric-ferroelectric transition that allows for actuation only occurs at temperatures above the Curie... 

The temperature dependence of the dielectric constant of P2 is shown in Figures 12.9 and 12.10. The dielectric constant (e) suddenly increased in the temperature region of the SmC phase. This is the typical ferroelectric behavior of FLC, because the dielectric constant of the ferroelectric material is inversely proportional to the absolute temperature according to the Curie-Weiss law, i.e. e = C(T — Tq), where C is the Curie constant and Tq is the Curie temperature. At the same time, the dielectric constant increased with decrease in frequency. This tendency is due to the high viscosity of the LC conjugated polymer. 

In summary, polythiophene derivatives were synthesized by introducing a fluorine-containing chiral LC group into the side-chains. It was found that the effective conjugation length of the main chain increased after the spontaneous orientation of the LC side-chains. One of the polymers (P2) showed an enantiotropic SmC phase, which was characteristic of ferroelectricity. The LC temperature region of the SmC phase was as broad as 20 °C. This is the first case where the SmC phase has been observed in polythiophene derivatives. The ferroelectric behavior was confirmed by examining the temperature dependence of the dielectric constant of the polymer. 

Li Z, Grimsditch M, Xu X, Chan SK (1993) The elastic, piezoelectric and dielectric constants of tetragonal PbTiOs single crystals. Ferroelectrics 141 313-325 MarraSP, Ramesh KT, Douglas AS (1999) The Mechanical properties of lead-titanate/polymer 0-3 composites. Compos Sci Technol 59 2163-2173 Materials Data Sheets of APC International, Tokin, Ferroperm, Morgan Matroc, Siemens Mattiat OE (1971) Ultrasonic transducer materials. Plenum Press, Tokyo McLachlan DS, Blaszkiewicz M, Newnham RE (1990) Electrical resistivity of composites. J Am Ceram Soc 73 2187-2203... 

Lijie et al. reported the fabrication and characterization of BaTiOs/PVDF nanocomposites via the sol-gel method, in which nanosized BaTiOs particles with an average size of 50-100 nm were grown in situ in the PVDF matrix. BaTiOs is a ferroelectric ceramic widely used in capacitors and ultrasonic transducers. It was observed that the relative dielectric constant of nanocomposites increased in the frequency range of 5 x 10" to 3 x 10 Hz with increasing weight fraction of nanosized ceramic in the polymer matrix [214]. 

PVDF was discovered by Dr. Heiji Kawai in 1969 [32]. Although PVDF is a piezoelectric system, it is a ferroelectric cum piezoelectric material as explained earlier, with a Curie point of 103 °C. PVDF possesses various phases such as a, p, y, and 5, among which p-phase has the most responsive piezoelectric properties. Compared with all ferroelectric polymers, PVDF has a dielectric constant with a reasonable chemical and mechanical durability [4,32]. In general, the physical properties of PVDF make it the most valuable material for application in sensors. 

Here x" ia the linear susceptibility, while x and x are the second- and third-order nonlinear optical susceptibilities, respectively. The b related directly to the dielectric constant and refractive index of a material. Baaed on the symmetry, the effects appear only in a noncentrosymmetric medium where effects have no such restrictioos. Ferroelectric polymers have advantages compared with other organic materiab as they are ooncentrosymmetric materiab with large spontaneous polarization. 

Materials with a small dielectric constant, such as ferroelectric polymers, are best suited to large-area elements, and vice versa. 

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