Field-induced ferroelectricity

Fig. 13.25 Geometry for discussion of the electric field-induced ferroelectric-antiferroelectric transition. Antiferroelectric structure (a) in the zero field and ferroelectric structure at the field exceeding the F-AF transition threshold (b)...

Interestingly it was observed that the switching between the field-induced ferroelectric states under typically above 3V/p,m rectangular electric fields is either accompanied by rotation of the optic axis, or it takes place without any rotation of... 

Figure 2-10. Textures between slightly uncrossed polarizers of the transformed state in antiferroelectric (upper row) and in the field-induced ferroelectric state (lower row). Arrows indicate the polarizer -analyzer configurations for the textures beneath...

Since the issue of order/disorder versus (or with) displacive aspects has remained an active field of research, most of the chapters presented in this book are devoted to it. In addition, new fields of applications are reviewed, since material optimization has considerably enlarged this area. A new aspect of ferroelectricity has been discovered recently by the finding of isotope-induced ferroelectricity in the quantum paraelectric SrTiOa. Here conclusive ideas about its microscopic origin are still missing and also the experimental situation remains controversial, since the symmetry of the low-temperature phase is unclear. But, there seems to be stringent evidence that polar clusters are... 

Nakata M, Link DR, Takanishi Y, Takahasi Y, Thisayukta J, Niwano H, Coleman DA, Watanabe J, Iida A, Clark NA, Takezoe H (2005) Electric-field-induced transition between the polarization-modulated and ferroelectric smectic-CgPf liquid crystalline states studied using microbeam X-ray diffraction. Phys Rev E 71 011705... 

Lopatina and Selinger recently presented a theory for the statistical mechanics of ferroelectric nanoparticles in liquid crystals, which explicitly shows that the presence of such nanoparticles not only increases the sensitivity to applied electric fields in the isotropic liquid phase (maybe also a possible explanation for lower values for in the nematic phase) but also 7 N/Iso [327]. Another computational study also supported many of the experimentally observed effects. Using molecular dynamics simulations, Pereira et al. concluded that interactions between permanent dipoles of the ferroelectric nanoparticles and liquid crystals are not sufficient to produce the experimentally found shift in 7 N/ so and that additional long-range interactions between field-induced dipoles of nematic liquid crystal molecules are required for such stabilization of the nematic phase [328]. 

The electric field induced intrinsic strain for different crystallographic directions could be calculated from the shift in peak positions (see Figure 7.9). Figure 7.11 shows the result of the measurements for a rhombohedral pzt in [111] and [100] direction. Only one half of each cycle is shown for the sake of a clarity of the plot. The curve for the [100] direction reveals the typical shape of a butterfly loop for the electric field induced strain in ferroelectrics. However in [111] direction, which is parallel to the spontaneous polarization, strain is significantly smaller. From both curves, the so-called unipolar strain can be evaluated as the strain induced at the maximum electric field Emax with a reference to the remanent state (E = 0). The calculation gives strain values in [111] direction of 0.02% and for the [100] direction 0.15%. The observations are in good agreement with theoretical calculations made by Du et al. [22],... 

Pyro- and Piezoelectric Properties The electric field application on a ferroelectric nanoceramic/polymer composite creates a macroscopic polarization in the sample, responsible for the piezo- and pyroelectricity of the composite. It is possible to induce ferroelectric behavior in an inert matrix [Huang et al., 2004] or to improve the piezo-and pyroelectricity of polymers. Lam and Chan [2005] studied the influence of lead magnesium niobate-lead titanate (PMN-PT) particles on the ferroelectric properties of a PVDF-TrFE matrix. The piezoelectric and pyroelectric coefficients were measured in the electrical field direction. The Curie point of PVDF-TrFE and PMN-PT is around 105 and 120°C, respectively. Different polarization procedures are possible. As the signs of piezoelectric coefficients of ceramic and copolymer are opposite, the poling conditions modify the piezoelectric properties of the sample. In all cases, the increase in the longitudinal piezoelectric strain coefficient, 33, with ceramic phase poled) at < / = 0.4, the piezoelectric coefficient increases up to 15 pC/N. The decrease in da for parallel polarization is due primarily to the increase in piezoelectric activity of the ceramic phase with the volume fraction of PMN-PT. The maximum piezoelectric coefficient was obtained for antiparallel polarization, and at < / = 0.4 of PMN-PT, it reached 30pC/N. 

R. Amaranatha Reddy, M.W. Schroder, M. Bodyagin, H. Kresse, S. Diele, G. Pelzl and W. Weissflog, Field-induced switching of chirality in undulated ferroelectric and antiferroelectric SmCP phases formed by bent-core meso-gens, Angew. Chem. 117(5), 784-788, (2005). doi 10.1002/ange.200461490... 

R.F. Shao, P.C. Wilhs and N.A. Clark, The field induced stripe texture in surface stabihzed ferroelectric hquid crystal cells, Ferroelectrics 121(1), 127-136, (1991). doi 10.1080/00150199108217616... 

The above peculiarities of relaxor ferroelectrics appear due to random electric field, induced by two factors. First one is substitutional disorder in cations positions, which leads to local shift of ions from their equilibrium positions. Second one is the presence of vacancies and other unavoidable defects. The shifted ions can be considered as random site electric dipoles, which are the sources of constant and alternating sign electric field (see Sect. 1.4.1). The former tends to order the system... 

L and dependence on temperature and film thickness in Fig. 3.24. It follows from Fig. 3.24a, c, that at fixed temperature the average polarization L decreases for the film thinning (compare curves 1-5). Built-in field smears the temperature of phase transition and susceptibility maximum its influence increases with the films thinning (see Fig. 3.24b, d). Moreover, the order parameter behavior for the thinnest possible films resembles that for thin films of ordered ferroelectrics with the thickness less than critical one (compare the curves 5 in Fig. 3.24c, d with Fig. in the paper [54]). Built-in field induces order parameter in the film with maximal disorder, see dotted curves 5 in Fig. 3.24a, b. However hysteresis loops is absent on these curves so that the behavior resembles that of electret state. 

Ren, X. (2004) Large electric-field-induced strain in ferroelectric crystals by point-defect-mediated reversible domain switching. Nat. Mater., 3, 91—94. 

Actually, such bowl phases are still to be found. However, polar achiral phases have been observed in the so-called polyphilic compounds [8]. The rod-like molecules of these compounds consist of distinctly different chemical parts, a hydrophilic rigid core (a biphenyl moiety) and hydrophobic perlluoroalkyl- and alkyl-chains at opposite edges. Such molecules form polar blocks that, in turn, form a polar phase manifesting pyroelectric and piezoelectric properties with a field-induced hysteresis characteristic of ferroelectric phases. 

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