Ferroelectric effect

Ferroelectric effect, smart materials exhibiting, 22 709-710, 721t Ferroelectric LCDs, 15 115 Ferroelectric materials, 11 94—98. 

FERROELECTRIC EFFECT. The phenomenon whereby certain crystals may exhibit a spontaneous dipole moment twhich is called ferroelectric by analogy with ferromagnetic—exhibiting a permanent magnetic moment). The effect in the most typical case, barium manate. seems to he due to a polarization catastrophe, in which the local electric fields due lo the polarizuiion itself increase faster than die elastic restoring forces on the ions in Ihe crystal, thereby leading to an asymmetrical shift in ionic positions, and hence lo a permanent dipole moment. Ferroelectric crystals... 

This is called the ferroelectric effect, by analogy with ferromagnetism Equation (2.48) is analogous to the Curie-Weiss law. A few crystalline materials do indeed display ferroelectric behaviour, but the phenomenon is rare. 

Materials of particular interest are the perovskite oxides BaTiOs-SrTiOs (BST) and PbZrOs-PbTiOs (PZT) solid solutions as well as the layered perovskites based upon SrBi2Ta209 (SBT). Since the ferroelectric effect requires... 

The ferroelectric effect is an electrical phenomenon. Parhcular materials, including the ternary oxides (Ba,Sr)Ti03, Pb(Zr,Ti)03 and (Bi,La)Ti03, exhibit a spontaneous dipole moment which can be switched between equivalent states by an external electric held. Ferroelectric thin hlms are of importance for the production of nonvolahle ferroelectric random access memory devices (FeRAM) °. Two possibilities to synthesize such mixed metal oxides are given by the CVD and ALD methods. Table 10 shows the preparation methods of such materials synthesized from metal enolates recently. 

The first report of a ferroelectric effect by a metallomesogen was for an open-book palladium(II) complexes (Figure 44) of type discussed above, where the chirality was introduced in the bridging carboxylate [14a]. The H NMR spectra identified that the complex was a mix of isomers trans-AR,R (34%), trans-AR,R (34%) and cw-R,R (32%). Ferroelectric switching was demonstrated, although at a much slower rate ( 1 s) than in organic liquid crystal systems, due to the high viscosity of the material. 

Several of the described eondis phases are CD glasses (Sect. 4.2-4 ami 4.6) with different paths into the glassy states. Some of these CD glasses seem to have no corresponding, stabte eondis phase, but are produced as intermediates in the path from melt to stabte erystal [polypropylene]. The ferroelectric effect of poly(vinylidene fluoride), polytrifluOToethylene and its copolymers seems coimected to eondis glass and disorder transitions (Curie temperatures). Condis states seem also to have enhanced chain mobility on mechanical (teformation. 

A ferroelectric crystal does not normally show any observable polarisation, because the domain structure leads to overall cancellation of the effect. PolycrystaUine ceramics would be expected to be similar. In order to form a material with an observable polarisation the crystals are poled. This process involves heating the crystals above the Curie point, Tc, and then cooling them in a strong electric field. The effect of this is to favourably orient dipoles so that the crystal or polycrystaUine ceramic shows a strong ferroelectric effect The majority of ferroelectric materials used are, in fact, polycrystal-Une. 

Dielectrics can be self-polarizing this is the ferroelectric effect. These ceramics are used in capacitors because of their high dielectric constant. 

An important point now emerges the requirement that the piezoelectric, pyroelectric and ferroelectric effects are restricted to non-centrosymmetric crystals implies that these physical phenomena should not be observed in a polycrystalline solid. This is because the individual grains of a polycrystalline body will polarise in random directions that will cancel overall. This was changed by the discovery, in 1945, of a way to endow polycrystalline ceramic articles with ferroelectric properties. 

Fig. 13.11 Deformed Helix Ferroelectric effect. Scheme of observation of the effect (a) and the picture of distortion of the hehcal stmcture (b) in the zero, positive and negative field. P and A are polarizers and analyser, ITO means indium-tin oxide electrodes, Iq and I are intensities of incoming and outgoing beams. Note that at E = 0 the hehx is harmonic, for 0 < l l < anharmonic and for l l > l J unwound...

Helix Distortion and Deformed Helix Ferroelectric effect... 

Toshio Mitsui is an emeritus professor of Osaka University. He studied solid state physics and biophysics at Hokkaido University, Pennsylvania State University, Brookhaven National Laboratory, the Massachusetts Institute of Technology, Osaka University and Meiji University. He was the first to observe the ferroelectric domain structure in Rochelle salt with a polarization microscope. He proposed various theories on ferroelectric effects and biological molecular machines. 

J. Valasek (1920, 1921,1922,1924) discovered the ferroelectric effect of a single crystal in about 1921. Jaffe, who went to the United States in 1935, assisted Cady in the study of Rochelle salt. Rochelle salt was often used in various transducers because it has a great piezoelectric effect. In spite of being a piezoelectric crystal with the highest electromechanical coupling coefficient, Rochelle salt is of limited use because it is soluble in water, deliquescent and some parameters do not have a suitable temperature characteristic. 

Relatively few applications have utilized the ferroelectric effect in ceramics. Ferroelectric ceramics have been widely employed because of the other properties that they display, however. Their dielectric, piezoelectric, and pyroelectric properties have led to their use in capacitor, actuator and other piezoelectric applications, and infrared detection devices. Again, the most widely used materials are the lead-based ABO perovskite compounds. 

Table 8.7 shows that the parameters of the prototype light valve (CdS-nematic) are much worse than that of the a Si-FLC device. The operation speed of the latter comes closer to the solid electrooptical crystal modulator (PROM), but with a considerably higher resolution. Liquid crystal light valves on a Si-FLC operate using the Clark-Lagerwall mode [21], the electroclinic eflFect [22], or the deformed helix ferroelectric effect [24]. The operation speed in the two mentioned cases could be 10-100 times faster than mentioned in Table 8.7. 

Another special case of pyroelectricity is ferrodastidty. In these materials, the direction of the spontaneous polarization can be changed by applying a mechanical stress. In some crystals such as gadolinium molybdate [Gd2(Mo04)3 GMO] the ferroelastic effect is coupled with a ferroelectric effect (Bohm and Kiirten, 1973). 

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