Electric polarization, switching

Figure 1.27 Polarization switching current of TGBF single crystal with decreasing electrical field.

Fig. 7 Reproducible polarization reversal in TbMn2Os by magnetic fields, (a) Dielectric constant versus applied magnetic field at 3 and 28 K. (b) Change of the total electric polarization by applied magnetic fields at 3 and 28 K. (c) Polarization flipping at 3 K by linearly varying the magnetic field from 0 to 2 T. The results clearly display highly reproducible polarization switching by magnetic fields (from ref. 46).

Ferroelectrics are a class of materials possessing a spontaneous electric polarization, which can be switched between crystallographically defined directions by... 

Figure E38-2. Apparatus for this experiment. (A) cover plate. (B) electrode. (C) baffles. (D) acetate strip. (E) agarose layer. (F) sample spot. (G) electrode plugin. (H) buffer. (I) leveling screws. (J) electrical connections. (K) power supply. (L) amperes. (M) volts. (N) coarse adjust. (O) fine adjust. (P) main switch. (Q,S) output plugs. (R) pilot light. (T) polarity switch.

If the electric field switches polarity in a time that is much shorter than an ion s residence time on a site, the average energies of the two sites become equivalent (i.e.. the bias no longer exists — the sites become energetically degenerate). 

Note Electroosmotic flow is reversed upon switching electrical polarity. Figure 30.1. Typical configuration of the Lasagna process. 

All above means that the ferroics can be regarded as a general notation for the materials, where at T < (so-called low-temperature phase) some reorientable physical quantities (order parameters) spontaneously appear. Latter order parameters can be of vector (spontaneous electric polarization, spontaneous magnetization) or tensor (second order tensor like spontaneous deformation or higher order tensors like elastic moduli and piezoelectric coefficients) nature. In low-temperature phase ferroics can usually split into domains, their switching being possible by the external fields. 

To discuss the electric field switching we add the field term = —PE to (Eq. (13.4)) and make minimisation of free energy with respect to the total polarization P = Ps + Pin that includes the spontaneous and the field induced terms P = Ps + Pin- Then we obtain dgsIdP = —E or... 

In Fig. 4.9 a hypothetical measurement of the relative intensity /lei. is shown in dependence of the time t. The switching time r is measured in the range between 10 and 90 % of the maximum signal and is thus denoted as Xio go. The switching time Tio-90 is linked to the Spontaneous electric polarization Ps and the rotational viscosity Y9 via the Equation [17, 18]... 

Unfortunately, the only method to gain the rotational viscosity is to measure the switching times. However, in literature the values of the diverse types of viscosity often only differ marginally and mainly depend on the temperature. Thus, it is possible to make a rough estimation of the spontaneous electric polarization by... 

Ferroelectric materials are a subclass of pyro- and piezoelectric materials (Fig. 1) (see Piezoelectric Polymers). They are very rarely foimd in crystalline organic or polymeric materials because ferroelectric hysteresis requires enough molecular mobility to reorient molecular dipoles in space. So semicrystalline poly(vinylidene fluoride) (PVDF) is nearly the only known compoimd (1). On the contrary, ferroelectric behavior is very often observed in chiral liquid crystalline materials, both low molar mass and poljuneric. For an overview of ferroelectric liquid crystals, see Reference 2. Tilted smectic liquid crystals that are made from chiral molecules lack the symmetry plane perpendicular to the smectic layer structure (Fig. 2). Therefore, they develop a spontaneous electric polarization, which is oriented perpendicular to the layer normal and perpendicular to the tilt direction. Because of the liquid-like structure inside the smectic layers, the direction of the tilt and thns the polar axis can be easily switched in external electric fields (see Figs. 2 and 3). 

Reorientation Dynamics of a Ferroelectric Side-Chain Liquid-Crystalline Polymer in a Polarity-Switched Electric Field... 

Figure 2-24. Polarization geometry and niesogeti alignment of the investigated FLCP in the polarity-switched electric field (A polarity +. B polarity -).

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