Subject permittivity

To eliminate the ambiguities in the subject of electricity and magnetism, it is convenient to add charge q to the traditional I, m and t dimensions of mechanics to form the reference dimensions. In many situations permittivity S or permeabiUty ]1 is used in Heu of charge. For thermal problems temperature Tis considered as a reference dimension. Tables 2 and 3 Hst the exponents of dimensions of some common variables in the fields of electromagnetism and heat. 

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. 

Physical properties of the solvent are used to describe polarity scales. These include both bulk properties, such as dielectric constant (relative permittivity), refractive index, latent heat of fusion, and vaporization, and molecular properties, such as dipole moment. A second set of polarity assessments has used measures of the chemical interactions between solvents and convenient reference solutes (see table 3.2). Polarity is a subjective phenomenon. (To a synthetic organic chemist, dichloromethane may be a polar solvent, whereas to an inorganic chemist, who is used to water, liquid ammonia, and concentrated sulfuric acid, dichloromethane has low polarity.)... 

In particular, VF2/F3E copolymers have also been the subject of extensive research [6,17,96]. As an example to illustrate the dielectric behavior of these copolymers, the temperature dependence of the real and the imaginary part of the complex permittivity at two different frequencies (1 and 100 kHz) are shown in Figs. 23a and 23b respectively. The measurements correspond to the 60/40 copolymer. The data have been collected by using a sandwich geometry with gold evaporated electrodes [95]. Frequencies of 103 and 106 Hz have been used by employing a 4192 A HP Impedance Analyzer. From inspection of Fig. 23b... 

In comparison to ordinary dielectrics, the permittivities of the so-called ferroelectric materials are about 103 times larger. The ferroelectric material can be transformed into a new type of material called piezoelectric material by heating the ferroelectric above its Curie temperature and then cooling it in a powerful electric field. A piezoelectric crystal changes its polarization once subjected to a mechanical strain. As a result, it can deform mechanically under an electric field or produce electric impulses as a result of mechanical impulses. Currently, piezoelectric materials are widely used as force or pressure transducers with fast response times and very sensitive output. Permittivities of common dielectric and ferroelectric materials are given in Table 1.9. 

The dielectric behaviour of pure water has been the subject of study in numerous laboratories over the past fifty years. As a result there is a good understanding of how the complex permittivity t = E — varies with frequency from DC up to a few tens of GHz and it is generally agreed that the dielectric dispersion in this range can be represented either by the Debye equation or by some function involving a small distribution of relaxation times. 

Medium-permittivity ceramics are widely used as Class I dielectrics, and in order to be in this category they need to have low dissipation factors. This precludes the use of most ferroelectric compounds in their composition since ferroelectrics have high losses (tan S >0.003), particularly when subjected to high a.c. fields. 

The grain size of a ferroelectric ceramic has a marked effect on the permittivity for the size range 1-50 /mi (see Fig. 2.48). Below about 1 /mi the permittivity falls with decreasing grain size. An important factor leading to this behaviour is the variation in the stress to which a grain is subjected as it cools through the Curie point. 

Ion solvation in polar solvents is also an important aspect of the subject matter considered here. This is traditionally studied by measuring the Gibbs energy and enthalpy of transfer of a variety of electrolytes from water to another polar solvent. Single ion quantities are then derived on the basis of the TATB assumption. Study of these quantities for simple monoatomic ions like the alkali metal cations and the halide anions leads to the conclusion that specific molecular properties, namely, Lewis acidity and basicity, are important in ion solvation. On the other hand, the dielectric permittivity, a non-specific bulk property. 

The relation between relative permittivity and molecular properties is an important subject and for non-polar molecules the problem is easier to solve. The relation which was first derived by Clausius-Mossotti dates back to the nineteenth century,... 

The amount of charge at the interface depends on the field strength and the dielectric properties (conductivity and permittivity) of the particle and the electrolyte. However, there is a slight asymmetry in the charge density on the particle which gives rise to an effective or induced dipole across the particle. Note that if the field is removed the dipole disappears, it is induced . The magnitude of the dipole moment depends on the amount of charge and the size of the particle. For a spherical particle in an electrolyte subject to a uniform applied electric field, three cases can be considered ... 

Electrorheological Fluid A dispersion of microscopic particles suspended in a low permittivity, low conductivity liquid such that the dispersion s flow properties change in the presence of electric fields. Also termed ER fluids the electrorheological effect is also termed the Winslow effect. When an electric field is applied to an ER fluid, the polarizable particles become electric dipoles and can align to form chains and more complex structures as a result the fluid becomes more viscous and may form a gel. Example cornstarch dispersed in corn oil and subjected to an electric field gradient of about 10,000 V/cm. See also reference 18. 

A simple model of a nano-dimensional structure in the form of a neutral spherical SNc of radius a and permittivity si, embedded in a medium with permittivity ej, has been discussed elsewhere. An electron e and hole h with elfective masses and m i were assmned to travel within this SNc (we use r and rh to denote the distances of the electron and the hole, respectively, from the center of the SNc). We assume that the two permittivites are such that E2 ei, and that the electron and hole bands are parabolic in shape. In this model, and subject to these approximations and the effective mass approximation, the exciton Hamiltonian takes the... 

Between the diffuse layer and the interface lies the Stern layer, i.e., layer of ions, which are not subjected to Brownian motion. Two levels are identified within it the internal with unhydrated ions and external with hydrated ions. Most of ions in the Stern layer are hydrated, so they caimot approach too closely the mineral surface. Because of this Helmholtz plane is drawn through the centers of immobile hydrated ions, and the thickness of Stern layer 8 is assumed equal to half of the median radius of hydrated ions (about 2 A). Electrostatic field in such layer is defined by the charge of mineral s surface, on the one hand, and by the charge of Helmholtz plane, on the other. It characterizes the density of electric permittance, which, according to equation (2.98), is equal to... 

Bisphenol-A carbonate has been widely studied by dielectric [8-26], dynamic mechanical [27 31] and thermally stimulated depolarization (TSD) [10- 13 32 35] techniques. However, differences in the compositions of the materials studied, and in their thermal history and pretreatment, have led to apparently conflicting results being reported in the literature, as discussed in detail in a recent paper [6]. In the present study contour maps of complex relative permittivity for both basic and u.v.-resistant grades of LEXAN have been obtained over an extended range of experimental conditions using a single apparatus, with each grade of material subject to the same thermal history. 

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