Ferroelectric substances

Above a specific temperature, the Curie temperature, a ferroelectric substance becomes paraelectric since the thermal vibrations counteract the orientation of the dipoles. The coordinated orientation of the dipoles taking place during the ferroelectric polarization is a cooperative phenomenon. This behavior is similar to that of ferromagnetic substances, which is the reason for its name the effect has to do nothing with iron (it is also called seignette or rochelle electricity). 

The electric susceptibility and dielectric constant of ferroelectric substances obey a Curie law dependence on temperature (Equation (9.13) ... 

Ferroelectric substances such as LiNb03 or BaTi03 were found to show anomalous photovoltaic effects (APV) of the order of 103-5 V83). A thin film of a ferroelectric polymer such as PVDF also was found to show APV effects84). Voc of 2.5 x 104 V and short circuit current (Isc) of the order of nA/cm2 were reported. Although the output is very small up to now1, it could be noticed as a specific photoeffect of a thin polymer film. 

There are certain crystals in which dipoles are spontaneously aligned in a particular direction, even in the absence of electric field. Such substances are called ferroelectric substances and the phenomenon is called ferroelectricty. The direction of polarisation in these substances can be changed by applying electric field. Baruion titanate (BaTi03), sodium potassium tartarate (Rochelle salt), and potassium hydrozen phosphate (KH2I04) are ferroelectric solids. If the alternate dipoles are in opposite directions, then the net dipole moment will be zero and the crystal is called anti-ferroelectric. Lead zirconate (PbZr03) is an anti-ferroelectric solid. 

Ferroelectric Behavior. Ferroelectric substances are those which have a stable, permanent polarization. They are of interest here because H bonding is one of the important ordering mechanisms responsible for permanent polarization. 

FIG, 193. Hysteresis loop of a ferroelectric substance (P — polarization, E — electric field intensity). 

The group of ferroelectric substances with a perovskite structure comprises numerous other compounds, such as KNbOj, KTa03, LaGaOj, etc. However, there also exist ferroelectric substances with other types of structures, e.g. CdNb207, LiTa03, WO3 and others. 

Ferroeleciric substances exhibit a hysteresis loop on the curves of the dependence of polarization on electric field intensity (Fig. 193). In the case of non-ferroelectric substances, this dependence is linear and its slope is related to the permittivity value. Figure 193 indicates that with ferroelectrics, the permittivity depends on the field intensity as well as on the direction of changes in the field intensity. When the electric field is cut oft, the dielectric shows remanent polarization (electrical analogy with ferromagnetic substances). 

Filling of polymers is a common technique to improve their properties. For examples, hydrogel can be filled with ferroelectric substances to obtain... 

Figures 4.5-3, 4.5-4, and 4.5-5 demonstrate how ferroelectric research has developed. Figure 4.5-3 indicates the number of ferroelectrics discovered each year for oxide (Fig. 4.5-3a) and nonoxide ferroelectrics (Fig. 4.5-3b). Figure 4.5-4 gives the total number of ferroelectrics known at the end of each year. At present more than 300 ferroelectric substances are known. Figure 4.5-5 indicates the number of research papers on ferroelectrics and related substances published each year.

Rg. r.5 -If Number of ferroelectric substances known at the end of each year. The solid line represents aU ferro-electrics, including hqnid crystals and polymers. For liquid crystals and polymers, each gronp of homolognes is connted as one substance. The dashed line represents ferroelectric hquid crystals and polymers alone. Figure prepared by Prof. K. Deguchi... 

Ferroelectric substances perovskites exhibit no permanent dielectric dipoles in the p electric phase (which belongs to the centrosymmetrical symmetry group /w3/w). The dipole moments appear in ferroelectric phase as a result of the spontaneous displacements of ions. Such phase transition is therefore called displacement-type phase transition. 

Some of the piezoeleetric materials are being used in applieations in the form of thin films of various thieknesses. The physieal situation is somewhat different from bulk materials. For example the electrode layer thickness is of comparable size with the thickness of piezoelectrically active film. If the film is deposited on the substrate the mechanical clamping is significant. Clamping could influence the film effective properties and the domain stracture for ferroelectric substances. Material coefficients are not usually completely available in the hterature because of the specific geometry and they depend on film thickness. Material properties of thin films are different from the bulk properties of the same material. Especially the size effect could result in loss of ferroelectricity below certain film thickness. Lower limit for the ferroelectricity is not generally known for all materials. 

The optical pattern may be observed for several months after switching off the dc field. The theoretical model is based on the consideration of a stabilizing role of the structural defects (dislocations) interacting with free charges in a FLC layer. The tilted layer structure responsible for such defects has been observed by direct X-ray investigation. The field-off domain structure has also been observed after the application of the electric field to the smectic A phase of a ferroelectric substance with very high spontaneous polarization [196]. The phenomenon is assumed to arise due to a break in the smectic layers (very similar to that just mentioned) induced by a strong electroclinic effect. 

In conclusion, electric field effects in liquid crystals is a well-developed branch of condensed matter physics. The field behavior of nematic liquid crystals in the bulk is well understood. To a certain extent the same is true for the cholesteric mesophase, although the discovery of bistability phenomena and field effects in blue phases opened up new fundamental problems to be solved. Ferroelectric and antiferroelectric mesophases in chiral compounds are a subject of current study. The other ferroelectric substances, such as discotic and lyotropic chiral systems and some achiral (like polyphilic) meso-genes, should attract more attention in the near future. The same is true for a variety of polymer ferroelectric substances, including elastomers. 

In this way. the VDF-TIFE copolymers with VDF 0-40% exhibit the X-ray pattern of the CL phase and the transition between the CL and HT phases occurs in a wide temperature region with ahnost no detectable hysteresis. By measuring the inversion current. Oka and Koizumi suggested that FIYFE behaves like a ferroelectric substance... 

Z. A. Gabrichidze, and E. A. Popova, 1964. Vibrational spectra of ferroelectric substances... 

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