Time constant ferroelectric

In the absence of walls the dynamic behavior may be described, as in nematics, by adding a viscous torque to the elastic and electric ones. For the field-off state, the distortion decays with the nematic time constant (see Eq. (34), where K should be substituted for 22)- The time of the response to an electric field can only be calculated for the simplest case when jU=0, Te=7 / e sin 2 E -EI), where E is defined by Eq. (69). Thus, the response of the smectic C phase is sin 2 times slower than that of the nematic phase. However, in experiments the same substance often responds faster in the smectic C phase than in the nematic one [159-161]. This may be due to the smaller value of Yi when the motion of the director is confined by the cone surface. The same phenomenon has been observed for the ferroelectric smectic C phase [162]. The domain-wall motion makes the dynamics of switching more complicated the field-induced wall velocity has been calculated by Schiller et al. [70]. 

Barium titanate has many important commercial apphcations. It has both ferroelectric and piezoelectric properties. Also, it has a very high dielectric constant (about 1,000 times that of water). The compound has five crystalline modifications, each of which is stable over a particular temperature range. Ceramic bodies of barium titanate find wide applications in dielectric amplifiers, magnetic amplifiers, and capacitors. These storage devices are used in digital calculators, radio and television sets, ultrasonic apparatus, crystal microphone and telephone, sonar equipment, and many other electronic devices. 

The ferroelectric sample is pre-polarized by measuring a complete hysteresis. The excitation voltage is then kept constant for the relaxation time at a particular voltage. The relaxed... 

The switching of the director in the surface stabilised ferroelectric liquid crystal cells (SSFLC) [8] has briefly been discussed in Section 13.1.2. Due to its importance for ferroelectric liquid crystal displays we shall discuss this effect in more detail. The geometry of a planar cell of thickness d is shown in Fig. 13.1.2. Now, the helical structure is considered to be unwound. We are interested in the field and time behaviour of the director or c-director given by angle cp(r), and this process is considered to be independent of z and y- coordinates. The smectic C equilibrium tilt angle 9 is assumed constant. 

The temperature dependence of the dielectric constant of P2 is shown in Figures 12.9 and 12.10. The dielectric constant (e) suddenly increased in the temperature region of the SmC phase. This is the typical ferroelectric behavior of FLC, because the dielectric constant of the ferroelectric material is inversely proportional to the absolute temperature according to the Curie-Weiss law, i.e. e = C(T — Tq), where C is the Curie constant and Tq is the Curie temperature. At the same time, the dielectric constant increased with decrease in frequency. This tendency is due to the high viscosity of the LC conjugated polymer. 

Class II dielectrics comprise the ferroelectrics. These materials offer much higher dielectric constants than Class I dielectrics, but with less stable properties with temperature, voltage, frequency, and time. The diverse range of properties of the ferroelectric ceramics requires a subclassification into two categories, defined by temperature characteristics. 

Wada S, Park SE, Cross LE, Shrout TR (1999) Engineered domain configuration in rhombohedral PZN-PT single crystals and their ferroelectric related properties. Eerroelectrics 221 147-155 Wallnofer W, Stadler J, Krempl P (1993) Temperature dependence of elastic constants of GaP04 and its influence on BAW and SAW devices. Proceedings of 7th European frequency and time forum, pp 653-657... 

Class 2 capacitors are based on ferroelectric materials with considerably higher dielectric constants such as those listed in Table 3.10. These materials exhibit shifts in dielectric constant as a function of time (aging). This phenomenon is a result of ferroelectric domain movement over time. A typical aging curve is shown in Fig. 3.27. The application of heat. 

Ferroelectric liquid crystals are a novel state of matter, a very recent addition to the science of ferroelectrics which, in itself, is of relatively recent date. The phenomenon which was later called ferroelectricity was discovered in the solid state (on Rochelle salt) in 1920 by Joseph Valasek, then a PhD student at the University of Minnesota. His first paper on the subject [1] had the title Piezo-Electric and Allied Phenomena in Rochelle Salt. This was at the time when solid state physics was not a fashionable subject and it took several decades until the importance of the discovery was recognized. Valasek had then left the field. Later, however, the development of this branch of physics contributed considerably to our understanding of the electrical properties of matter, of polar materials in particular and of phase transitions and solid state physics in general. In fact, the science of ferroelectrics is today an intensely active field of research. Even though its technical and commercial importance is substantial, many breakthrough applications may still lie ahead of us. The relative importance of liquid crystals within this broader area is also constantly growing. This is illustrated in Fig. 1,... 

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