Dielectric fundamentals

When a metal body is exposed to an electric field, free electrons are displaced by electric forces until the field in the body vanishes. In an ideal dielectric (dc conductivity is zero), there exists only bound charges (electrons, ions) that can be displaced from their equilibrium positions until the field force and the oppositely acting elastic force are equal. This phenomenon is called displacement polarization (electronic or ionic polarization). A dipole moment is induced in every atom or between ion pairs. The molecular dipoles can only be rotated by an electric field. Usually, their dipole moments are randomly oriented. In an external held, however, an orientation parallel to the field direction is preferred so that a dipole moment is induced. This process is called orientational polarization. 

In an alternating electric field, the displacement polarization leads to electric oscillations. This is a resonant process with resonant frequencies of lO -lO Hz for the electronic and of 10 -10 Hz for the ionic polarization. 

Orientational polarization is not a resonant process since the molecular dipoles have inertia. The response of the orientational polarization to a charge of the electric field is, therefore, always retarded. This process is called dielectric relaxation. The characteristic time constant of such a relaxation process—this is the time for reaching new equilibrium after changing the excitation—is called relaxation time (r). It is strongly temperature dependent, since it is closely related to the viscosity of the material. At room temperature, the relaxation times of the orientational polarization in crystals are of 10 -10 s. In amorphous solids and polymers, however, they can reach a few seconds or even hours, days, and years, depending on the temperature. 

Real dielectrics also contain charge carriers that can be moved by electric forces between potential walls, formed by non-ohmic or blocking contacts or internal boundaries. 

The dielectric (e ) and loss (e ) constants are important properties of interest because these two parameters, among others, determine the suitability of a material for a given application. Dielectric relaxations are studied to reduce energy losses in materials used in practically important areas of insulation and mechanical strength. 

---

Luminescence has been used in conjunction with flow cells to detect electro-generated intennediates downstream of the electrode. The teclmique lends itself especially to the investigation of photoelectrochemical processes, since it can yield mfonnation about excited states of reactive species and their lifetimes. It has become an attractive detection method for various organic and inorganic compounds, and highly sensitive assays for several clinically important analytes such as oxalate, NADH, amino acids and various aliphatic and cyclic amines have been developed. It has also found use in microelectrode fundamental studies in low-dielectric-constant organic solvents. 

Commercial dryers differ fundamentally by the methods of heat transfer employed (see classification of diyers, Fig. 12-45). These industrial-diyer operations may utihze heat transfer by convection, conduction, radiation, or a combination of these. In each case, however, heat must flow to the outer surface and then into the interior of the solid. The single exception is dielectric and microwave diying, in which high-frequency electricity generates heat internally and produces a high temperature within the material and on its surface. 

Dielectric dryers have not as yet found a wide field of application. Their fundamental characteristic of generating heat within the solid indicates potentialities for diying massive geometrical objects such as wood, sponge-rubber shapes, and ceramics. Power costs may range to 10 times the fuel costs of conventional methods. 

Since the harmonic disorders occur at higher frequencies than the fundamental (/, > /). they cause higher dielectric losses due to a higher skin effect. 

Thin polymeric films have important industrial apphcations (e.g., as protective coatings, lubricants, adhesives, dielectric or nonlinear optic devices, etc.) and pose many fundamental problems of film stability [1,2,4]. An important question, therefore, is whether these films break up and spontaneously dewett the substrate, resulting in the formation of droplets. The... 

The charge of a number of proteins has been measured by titration. The early experimental work focused on the determination of charge as a function of pH later work focused on comparing the experimental and theoretical results the latter obtained from the extensions of the Tanford-Kirkwood models on the electrostatic behavior of proteins. Ed-sall and Wyman [104] discuss the early work on the electrostatics of polar molecules and ions in solution, considering fundamental coulombic interactions and accounting for the dielectric properties of the media. Tanford [383,384], and Tanford and Kirkwood [387] describe the development of the Tanford-Kirkwood theories of protein electrostatics. For more recent work on protein electrostatics see Lenhoff and coworkers [64,146,334]. 

Because of these factors, the fundamental experimental information about the interaction of metastable atoms with semiconductors and dielectrics is meant for the reflection coefficients that are determined with the aid of beam methods and for the coefficients of heterogeneous deactivation which are evaluated under diffusion conditions. However, the data in this event are fairly scarce and conflicting. The results obtained by the methods of electronic beams do not agree with diffusion experiments. Thus, Allison et al [ 137] report that the coefficients dealing with... 

In dilute solutions it is possible to relate the activity coefficients of ionic species to the composition of the solution, its dielectric properties, the temperature, and certain fundamental constants. Theoretical approaches to the development of such relations trace their origins to the classic papers by Debye and Hiickel (6-8). For detailpd treatments of this subject, refer to standard physical chemistry texts or to treatises on electrolyte solutions [e.g., that by Harned... 

Limits on Particle Charging. The electrical charge carried by a particle resides on the surface. Thus, a fundamental upper limit for particle electrification may be computed by imposing the constraint that the electric field at the surface can not exceed the dielectric strength of dry air, Eb 30 kV/cm. According to this hypothesis, the upper limit upon surface charge density becomes... 

Mechanistic details of the microwave-induced oligomerization of methane on a microporous Mn02 catalyst were studied by Suib et al. [67], with emphasis on fundamental aspects such as reactor configuration, additives (chain propagators, dielectrics), temperature measurements, magnetic field effect, and reaction conditions. 

We have shown in this paper the relationships between the fundamental electrical parameters, such as the dipole moment, polarizability and hyperpolarizability, and the conformations of flexible polymers which are manifested in a number of their electrooptic and dielectric properties. These include the Kerr effect, dielectric polarization and saturation, electric field induced light scattering and second harmonic generation. Our experimental and theoretical studies of the Kerr effect show that it is very useful for the characterization of polymer microstructure. Our theoretical studies of the NLDE, EFLS and EFSHG also show that these effects are potentially useful, but there are very few experimental results reported in the literature with which to test the calculations. More experimental studies are needed to further our understanding of the nonlinear electrooptic and dielectric properties of flexible polymers. 

The fundamental mode of a uniform lossless dielectric waveguide and, in particular, an MNF exists independently of its thickness. However, in practice, wave-guiding is limited by losses due to material absorption and geometric nonuniformities. For a very thin MNF, the transmission loss is primarily determined by input and output losses, which, in practice, cannot be reduced significantly11 64. As an example, Refs. 11 and 13 theoretically explored an MNF with adiabatically... 

The formation ofC—C bonds between aromatic rings is an important step in many organic syntheses and can be accomplished by chemical, photochemical, or electrochemical means. As was noted earlier, fundamental considerations of the parameters for a dielectric which must be dealt with in designing a thermally stable, low-dielectric-constant polymer naturally lead one to consider rigid-rod, nonconjugated aromatic polymers containing no lossy functional groups. A structure such as poly(naphthalene) is a likely candidate. 

The new features of this Table are (i) the values calculated by me (1, 2 and 3) (ii) the recognition that the values quoted apply only over a range of m which depends on the nature of the solvent (iii) the k+p for styrene and EVE in solvents of low polarity are very similar. In my view none of these values and others in the literature are sufficiently reliable for any activation energies calculated from them to afford useful information. I have refrained from attempting a correlation of the rate constants with the dielectric constant of the diluent because in my view even the same cation in each different solvent is a different species, so that the fundamental hypothesis of theories of the Laidler type is not valid. 

---