Dielectric constant of a material

With conventional heating, energy transfer occurs mainly through conduction and convection. With microwaves, the primary mechanism is dielectric loss4,52. The dielectric loss factor (loss factor, s") and the dielectric constant ( ) of a material are two determinants of the efficiency of heat transfer to the sample. Their quotient ( "/ ) is the dissipation factor (tan 8), high values of which indicate ready susceptibility to microwave energy. 

The permittivity of a material is the constant e in the rationalised expression, F = (0i02)/(47rer2), where F is the force between charges Q and 02 separated by a distance r. The permittivity of a vacuum <6 according to this definition is equal to 8.854 x 1012 kg m 3 s4 A2. The dielectric constant of a material is equal to the ratio between its permittivity and the permittivity of a vacuum, and is a dimensionless quantity. 

The capacitance hygrometer detects the change of capacitance, which is a function of plate area, plate spacing, and the dielectric constant of the material between the plates. The dielectric constant of a material has a unique value for each substance. Water, having a dielectric constant of 80, is a good candidate for this measurement. When used to measure the moisture content of solids, compensation is required for particle size, packing, and material density because they all affect capacitance. The same principle can also... 

Of central importance for understanding the fundamental properties of ferroelec-trics is dynamics of the crystal lattice, which is closely related to the phenomenon of ferroelectricity [1]. The soft-mode theory of displacive ferroelectrics [65] has established the relationship between the polar optical vibrational modes and the spontaneous polarization. The lowest-frequency transverse optical phonon, called the soft mode, involves the same atomic displacements as those responsible for the appearance of spontaneous polarization, and the soft mode instability at Curie temperature causes the ferroelectric phase transition. The soft-mode behavior is also related to such properties of ferroelectric materials as high dielectric constant, large piezoelectric coefficients, and dielectric nonlinearity, which are extremely important for technological applications. The Lyddane-Sachs-Teller (LST) relation connects the macroscopic dielectric constants of a material with its microscopic properties - optical phonon frequencies ... 

The dielectric constant of a material depends primarily on its polarizability and, hence, strongly depends on density. The three types of polarization that contribute to the dielectric constant are electronic, ionic, and orientational polarizations and are given by the Debye equation ... 

Dielectric constant (e ) and dielectric loss factor (e") are the two most important properties, which have a pronounced effect on the effectiveness of microwave processing of a material. The dielectric constant (s ) is a ratio of the permittivity of a substance to the permittivity of free space. The dielectric constant of a material gives the extent the material could concentrate the electric flux. It is an electrical equivalent of the relative magnetic permittivity (Regier and Schubert, 2001 Venkatesh and Raghavan, 2004). 

The dielectric constant of a material is a measnre of its polarizability in response to an electric field. This polarizability is the resnlt of reorganization of charge, which can be in the form of interfacial or space charge motion, ionic motion, dipolar motion, and electronic motion (see Figure 3.2.3) [14]. The timescale of the charge redistributions determines the frequency dependence of this contribution to the dielectric constant for a given material. In the case of ionic motion, the frequency range is up to 10 Hz. 

Figure 14.5 (a) Apparatus for measuring the dielectric constant of a material L is the inductance of the coil, h) The actual response of a nonideal dielectric to an applied voltage is such that the angle between the current and voltage is not 7t/2, but 7t/2 — (c) Equiva-... 

The average dielectric constant of a material which includes a small number of such spheroids is calculated by electrostatics (see ref. 68, for example) as... 

For a vacuum, s = 1.0000, values for some other materials are listed above. In practice the dielectric constant of a material is found by measuring the capacitance of a parallel-plate condenser using the material as the dielectric, then measuring the capacitance of the same condenser with a vacuum as the dielectric, and expressing the result as a ratio between the two capacitances. 

The dielectric constant of a material is defined as the ratio of the capacitance of a particular capacitor containing the material to that of the same capacitor when the material is removed and replaced by air, i.e., the ability of a substance to store electrical energy in an electric field. The dielectric constant varies with frequency and generally increases with temperature. 

Eq. (15) and (16) indicate that the dispersion component of the surface energy is determined by the polarizability of the molecule, while the polar component is determined by the molecule dipole moment. It is known that the dielectric constant of a material can be expressed as follows if the local field is assumed to be the mean field [lOlJ. 

The dielectric constant of a material depends on contributions to the polarizability by... 

The dielectric constant of a material varies with its temperature, frequency, and composition. The power P absorbed in the material is proportional to the loss factor e", the frequency/[Hz], and the electric field intensity E [V/m] ... 

The static permittivity or dielectric constant of a material under given conditions reflects the extent to which it concentrates electrostatic lines of flux. Technically, it is the ratio of the amount of electrical energy stored in a material by an applied voltage, relative to that stored in a vacuum. Similarly, it is also the ratio of the capacitance of a capacitor using that material as a dielectric, compared to a similar capacitor which has a vacuum as its dielectric. The dielectric constant values of water have been given, for example, by Fernandez et al. (1995). 

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