Dielectrics 1098 Subject

A normal dielectric may be characterized by Eq. (4.1) with the piezoelectric terms deleted. For an isotropic dielectric subject to uniaxial strain and a collinear electric field this equation takes the form... 

Such a situation has been thoroughly reported in solid state physics books on dielectrics subjected to external electric fields [3] and may be applied directly here to aqueous solutions. Consider, for example, a pair consisting of a cation and an anion (each of charge - - q and — q) that are freshly formed in vacuum. Suppose their centers are separated by a distance r. Then, Coulomb s law of electrostatics gives the change in electrostatic energy of attraction as... 

The interaction of an organic medium with the external electrical or optical field can be described within the framework of a dielectric subjected to an electric field. The induced dipole moment in a molecule due to the applied field is given by ... 

In this section we wish to show that an alternative form for the ponderomotive force, proposed by Helmholtz, can also be justified by statistical-mechanical methods and that its relation to the ponderomotive force and pressure derived in the previous section is the same as found from purely thermodynamical arguments.16 Helmholtz arrives at an expression for the ponderomotive force on the basis of macroscopic energy considerations for a dielectric subjected to reversible transformations. We are thus led to develop this part of the theory by considering a system in equilibrium. Since the system need not be uniform (due to the presence of a nonuniform external field), we shall divide it into a number of cells. Each cell contains a large number of atoms, but is sufficiently small to be considered macroscopically uniform. This means that... 

The third equation involves the so-called Kerr Constant B—the birefringence observed when polarized light of wavelength A traverses a 1-cm path through an otherwise isotropic dielectric subjected to a unit electric field applied at 90° to the light beam and at 45° to the plane of polarization. By the Langevin-Born orientation theory, B can be written as (24),... 

Calculate the power dissipated as a function of co for a Debye dielectric subjected to a sinusoidal voltage. co = 2nf... 

Tree formation n. The generation of a treelike void structure in a transparent plastic by electron bombardment at a point on the surface. The effect in acrylic blocks is dramatically decorative. Similar breakdown structures form in dielectrics subjected to strong electric fields, eventually penetrating the dielectric and causing a short circuit. Ku CC, Liepins R (1987) Electrical properties of polymers. Hanser Publishers, New York. Seanor DA (1982) Electrical conduction in polymers. Academic Press, New York. 

DIELECTRIC LOSS Electric energy transformed into heat in a dielectric subjected to a changing electric field. 

Dielectric Behavior of Adsorbed Water. Determination of the dielectric absorption of adsorbed water can yield conclusions similar to those from proton NMR studies and there is a considerable, although older literature on the subject. Figure XVI-7 illustrates how the dielectric constant for adsorbed water varies with the frequency used as well as with the degree of surface coverage. A characteristic relaxation time r can be estimated... 

It should be mentioned that as well as for metals the passivation of semiconductors (particularly on Si, GaAs, InP) is also a subject of intense investigation. However, the goal is mostly not the suppression of corrosion but either the fonnation of a dielectric layer that can be exploited for devices (MIS stmctures) or the minimization of interface states (dangling bonds) on the semiconductor surface [63, 64]. 

All electrical et iiipmeiil and devices connceled tin the system tire subject to higher dielectric stresses due to a higher elTectivc voltage (equation (2.. l)). 

Figure 6.1. (a) Atom not subject to external electric field. Centre of electron cloud and nucleus coincident, (b) Electron cloud displacement through application of external electric field, (c) Charged condenser plates separated by vacuum, (d) Condenser plates separated by dielectric... 

Nonlinear properties of normal dielectrics can be studied in the elastic regime by the method of shock compression in much the same way nonlinear piezoelectric properties have been studied. In the earlier analysis it was shown that the shape of the current pulse delivered to a short circuit by a shock-compressed piezoelectric disk was influenced by strain-induced changes in permittivity. When a normal dielectric disk is biased by an electric field and is subjected to shock compression, a current pulse is also delivered into an external circuit. In the short-circuit approximation, the amplitude of this current pulse provides a direct measure of the shock-induced change in permittivity of the dielectric. 

The importance of the solvent, in many cases an excess of the quatemizing reagent, in the formation of heterocyclic salts was recognized early. The function of dielectric constants and other more detailed influences on quatemization are dealt with in Section VI, but a consideration of the subject from a preparative standpoint is presented here. Methanol and ethanol are used frequently as solvents, and acetone,chloroform, acetonitrile, nitrobenzene, and dimethyl-formamide have been used successfully. The last two solvents were among those considered by Coleman and Fuoss in their search for a suitable solvent for kinetic experiments both solvents gave rise to side reactions when used for the reaction of pyridine with i-butyl bromide. Their observation with nitrobenzene is unexpected, and no other workers have reported difficulties. However, tetramethylene sulfone, 2,4-dimethylsulfolane, ethylene and propylene carbonates, and salicylaldehyde were satisfactory, giving relatively rapid reactions and clean products. Ethylene dichloride, used quite frequently for Friedel-Crafts reactions, would be expected to be a useful solvent but has only recently been used for quatemization reactions. ... 

The most common measure of polarity used by chemists in general is that of dielectric constant. It has been measured for most molecular liquids and is widely available in reference texts. However, direct measurement, which requires a nonconducting medium, is not available for ionic liquids. Other methods to determine the polarities of ionic liquids have been used and are the subject of this chapter. However, these are early days and little has been reported on ionic liquids themselves. I have therefore included the literature on higher melting point organic salts, which has proven to be very informative. 

An alternative method of studying the molecular motions of a polymeric chain is to measure the complex permitivity of the sample, mounted as dielectric of a capacitor and subjected to a sinusoidal voltage, which produces polarization of the sample macromolecules. The storage and loss factor of the complex permitivity are related to the dipolar orientations and the corresponding motional processes. The application of the dielectric thermal analysis (DETA) is obviously limited to macromolecules possessing heteroatomic dipoles but, on the other hand, it allows a range of frequency measurement much wider than DMTA and its theoretical foundations are better established. 

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-... 

For all common substances the temperature coefficient of t is negative that is to say, the temperature coefficient of 1/e is positive hence (11) represents a loss of entropy on subjecting the homogeneous dielectric to the field. 

The book contains rather complete reviews of papers published in the last 5-10 years in the USSR and abroad on various problems of filled polymers of differing nature. The discussion is centered on the physico-chemical problems of these complex systems, their structure, mechanical, rheological, dielectric and other properties in a word, important aspects of theory and technology of filled composites. We hope the topical nature of the subjects discussed and the selection of authors that appear in the book all help to throw more light on this area of science and technology. The interested reader will be able not only to appreciate the book as a source of additional literature or a snapshot of the state-of-... 

If the wire is to be used to carry much higher frequency currents, the design problem in geometry and plastic selection becomes more complicated. The dielectric constant and dielectric loss values for the plastics become important in the design. At a frequency of one megahertz the effect of the dielectric on the power transmission behavior of the wire is substantial and, even at frequencies of 10 to 100 kilohertz, the insulation on the wire must be considered in the design as a major electrical element in the circuit. More on the subject of insulation will be following this section. 

Dielectric loss The dielectric loss factor represents energy that is lost to the insulator as a result of its being subjected to alternating current (AC) fields. The effect is caused by the rotation of dipoles in the plastic structure and by the displacement effects in the plastic chain caused by the electrical fields. The frictional effects cause energy absorption and the effect is analogous to the mechanical hysteresis effects except that the motion of the material is field induced instead of mechanically induced. 

Dielectric analysis (DEA) measures the electrical properties of a material as it is subjected to a periodic electric field under various conditions. This technique provides quantitative information on the capacitance and conductance of the ma-... 

We can define the principal electrical properties of polymers in terms of four characteristics electrical resistance, capacitive properties, dielectric strength, and arc resistance. We can change the surface characteristics of a polymer by subjecting it to a corona discharge generated by a strong electrical field. Lastly, we must also consider the influence of other physical properties on the application of polymers in electrical applications. 

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