Ferroelectric crystals dielectric relaxation

Yamada et al. [9,10] demonstrated that the copolymers were ferroelectric over a wide range of molar composition and that, at room temperature, they could be poled with an electric field much more readily than the PVF2 homopolymer. The main points highlighting the ferroelectric character of these materials can be summarized as follows (a) At a certain temperature, that depends on the copolymer composition, they present a solid-solid crystal phase transition. The crystalline lattice spacings change steeply near the transition point, (b) The relationship between the electric susceptibility e and temperature fits well the Curie-Weiss equation, (c) The remanent polarization of the poled samples reduces to zero at the transition temperature (Curie temperature, Tc). (d) The volume fraction of ferroelectric crystals is directly proportional to the remanent polarization, (e) The critical behavior for the dielectric relaxation is observed at Tc. 

Relaxor ferroelectrics can be prepared either in polycrystalline form or as single crystals. They differ from the previously mentioned normal ferroelectrics in that they exhibit a broad phase transition from the paraelectric to ferroelectric state, a strong frequency dependence of the dielectric constant (i.e. dielectric relaxation) and a weak remanent polarization. Lead-based relaxor materials have complex disordered perovskite structures. 

Dielectric Relaxation Smectic Phases Ferroelectric C Phase Polymeric Liquid Crystals... 

Yasuda et al. performed dielectric relaxation measurements on metastable solid MBBA, using a parallel-plate capacitor. Glass plates with strips of transparent tin-oxide-conducting coating served as electrodes. The cell was mounted in a copper beryllium pressure vessel, pressurized with liquid isopentane. A similar setup was used for the study of ferroelectric liquid crystals. Ferroelectric liquid crystals have also been studied by Chandrasekhar and co-workers who used a sapphire cell setup in a high-pressure apparatus. The sample was sandwiched... 

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By simply quenching bom melt, the o form (crystal form II) is formed, which is not ferroelectric. In this polymer three dielectric transitions may be delected, the p transition at about —(KTC. Ok o. transition near —10 and the a, transition near 1S0 C The p transition exhibits an Arrhenius-like relaxation behavior with an activatioo enthalpy of SO kJAnol it is attributed to local motions in the amorphous as well as in the crystalline state. The a. tiansitioo has a WLF-type relaxatkm beluvior it is attributed to the glass Iransitioa of the amorphous part The a, tiansitioo is attributed to motioos in the crystalline phase, but most probably not within the lamellae but in the partially... 

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