Behavior in an Alternating Electric Field

If a dielectric material is suddenly placed in an electric field, the permanent molecular dipoles in the dielectricum will attempt to orientate. The orientation occurs by a random process, that is, via diffusion or jumps. The applied electrical field, of course, influences the mean orientation more than the reorientation of the individual molecular dipoles. Since molecular dipolar reorientation is coupled with reorientation of molecules or molecular groups, the time required for macroscopic reorientation corresponds to that for the reorientation of the molecules or groups. 

The power loss depends on the phase difference between the alternating current produced by an applied alternating voltage. When the material behaves as a perfect dielectric, the phase difference between the alternating potential and the amplitude of the current is 90° and the power loss is zero. If current and voltage are in phase, then all of the electrical energy is converted into heat and the power output is zero. The ratio of power loss Nv to power output Nb is called the dielectric dissipation factor, tan 6  

The real power output and the power loss can also be given in terms of the real e and imaginary e dielectric constants (relative permittivities), respectively  

The glass transition temperature and other relaxation temperatures can be determined by investigating the behavior of polar macromolecules in an alternating electric field (see also Section 11.4.5). If the frequencies are low and the sample is above the glass transition temperature, then the dipoles 

The term s tan 5 is called the loss factor, and is not the same as the dissipation factor. Materials with a high s tan S are suitable for high-frequency-field heating, i.e., they can be welded in a high-frequency field. These materials are not suitable, on the other hand, as insulating materials for high-frequency conductors. Nonpolar plastics such as poly(ethylene), poly(styrene), poly(iso-butylene), etc., have low dielectric constants ( 2-3) and dielectric loss factors (tan (5 = 10 to 8 x As insulating materials they are of consi- 

---

Dielectric spectroscopy is a technique which allows one to evaluate the complex dielectric permittivity e = e — ie" as a function of frequency and temperature, where e is the dielectric constant and e" is the dielectric loss [3,12]. A schematic view of a dielectric spectroscopy experiment is shown in Fig. 21.3. A dielectric sample of thickness d and area A is subjected to an alternating electric field of angular frequency w. Through measurements of the complex impedance of the sample it is possible to experimentally determine e [12,18-20]. Dielectric spectroscopy is a very suitable method to study molecular dynamics in polymers above Tg. In this case, segmental motions of the polymeric chains give rise to the so called cc-relaxation process, which can be observed as a maximum in e" and a step-Uke behavior in e as a function of frequency. Both, the intensity of the a relaxation, and the frequency of maximum... 

The ferroelectric domain structure in the crystal is a reason of the non-linear behavior of the polarization as a function of an electric field. Dielectric hysteresis appears in the alternating electric fields. The dielectric hysteresis loop is one of the most important features of ferroelectric materials (Fig. 5.6). Hysteresis loop or domain structure observation could serve as a proof of ferroelectricity in the material. Due to the presence of domain wall structure, the remanent polarization Pr is always smaller than the spontaneous polarization Ps. 

The behavior of a polar dielectric in an electric field is of the same kind. If the dielectric, is exposed to an external electric field of intensity X, and this field is reduced in intensify by an amount SX, the temperature of the dielectric will not remain constant, unless a certain amount of heat enters the substance from outside, to compensate for the cooling which would otherwise occur. Alternatively, when the field is increased in intensity by an amount SX, we have the converse effect. In ionic solutions these effects are vciy important in any process which involves a change in the intensity of the ionic fields to which the solvent is exposed—that is to say, in almost all ionic processes. When, for example, ions are removed from a dilute solution, the portion of the solvent which was adjacent to each ion becomes free and no longer subject to the intense electric field of the ion. In the solution there is, therefore, for each ion removed, a cooling effect of the kind mentioned above. If the tempera-... 

Figure 6.4. Generic behavior of temperature dependence of permittivity components (e, 8") recorded for an amorphous polymer with considerable ionic conductivity. The higher the relaxation time of the mechanism, the higher the temperature range at which the corresponding signal appears in this isochronal recording. The signals present in this spectrum will shift to lower temperatures by decreasing the frequency of the alternating electric field.

GERF (discovered by Wen et.al.) has a yield stress up to 300 kPa in response to an electric field, which provides an alternative choice of digitally controllable microvalve that can respond within 10 pm [13, 43]. Its solid-like behavior sustains shear in the direction perpendicular to the applied electric field, the shear stress can be enhanced when the applied electric field increases, and its rheological variation is reversible upon removal of the electric field (Fig. 4). These marvelous features qualify GERF as an electric-fluid-mechanical interface for digital fluid control in microfluidics [55, 56]. 

---