Permittivities values

Experimental values are collected in the McClellan book (B-63MI40400) and in a review on dipole moments and structure of azoles (71KGS867). Some selected values are reported in Table 3. The old controversy about the dipole moment of pyrazole in solution has been settled by studying its permittivity over a large range of concentrations (75BSF1675). These measurements show that pyrazole forms non-polar cyclic dimers (39) when concentration increases and, in consequence, the permittivity value decreases. 

Thus, both transmissivity and permittivity values allow for the thickness to be avoided in subsequent analyses. 

A. Water and Tissue Water. The dielectric properties of pure water have been well established from dc up to microwave frequencies, approaching the infrared (3). For all practical purposes they are characterized by a single relaxation process centered near 20 GHz at room temperature. Static and infinite frequency permittivity values are, at room temperature, close to 78 and 5, respectively. Hence, the microwave conductivity increase predicted by Eq. (1) is close to 0.8 mho/cm above 20 GHz, much larger than typical low-frequency conductivities of biological fluids which are about 0.01 mho/cm. The dielectric properties of water are independent of field strength up to fields of the order 100 kV/cm. 

A relationship was then developed that defines the power densities at various carrier frequencies which would give the same average internal electric field intensity in the sample. These developments were necessary because the relative complex permittivity of the tissue and the wavelength of radiation change with carrier frequency. Finally, using the complex permittivity values reported by Foster et al. (10), relationships were derived for incident power densities at 50, 147,... 

Many ferroelectrics possess very high permittivity values which vary considerably with both applied field strength and temperature. The permittivity reaches a peak at the Curie point and falls off" at higher temperatures in accordance with the Curie-Weiss law... 

Other microwave dielectrics have been identified and developed into commerical products and those currently exploited are listed in Table 5.8. The modified neodymium titanates are particularly important because of their high permittivity values and the reduction in resonator size this offers. 

G.A. Smolenskii [20] was a pioneering researcher and at the time relaxors were being intensively studied in Russia. Since then they have attracted very considerable interest because of the following attributes. Firstly the low sintering temperatures and high peak permittivity values which are attractive for the... 

Ferroelectric materials above their Curie point behave electrostrictively and comparison of the electrostriction coefficient with d j2eP shows them to be of similar magnitude. This suggests that the large -coefficients shown by some ferroelectric materials are due to a combination of large electrostriction coefficients and large spontaneous polarization and permittivity values. 

Values for /n and fm are readily measured using an impedance analyser. Specific ( 33 ) coupling coefficient, compliance and permittivity values can be measured on appropriately dimensioned specimens from which the corresponding d and g coefficients can be calculated the following examples illustrate how this is accomplished. 

The lead-based perovskite relaxor ferroelectrics, Pb(B B")03, (see Section 5.7.2) have exceptionally high permittivity values and therefore, from Eq. (6.7) are expected to be strongly electromechanically active and especially suited to transducer applications. 

Ferroeleciric substances exhibit a hysteresis loop on the curves of the dependence of polarization on electric field intensity (Fig. 193). In the case of non-ferroelectric substances, this dependence is linear and its slope is related to the permittivity value. Figure 193 indicates that with ferroelectrics, the permittivity depends on the field intensity as well as on the direction of changes in the field intensity. When the electric field is cut oft, the dielectric shows remanent polarization (electrical analogy with ferromagnetic substances). 

A significant increase in permittivity of barium titanate dielectrics, required very small size capacitors, can be attained by utilizing the barrier phenomena whi are based on the formation of thin insulating layers in semiconducting BaTi< ceramics. These systems exhibit permittivity values of the order 10. ... 

Dielectric relaxation data for a 0.08 M Mg2S04 solution are shown in fig. 4.11. On the basis of an analysis of these data by Barthel and coworkers [29, 32], three relaxation processes may be discerned. The first one, involving the ion pair, occurs between permittivity values of 82.9 and 75.2 and involves a relaxation time of 181 ps. The second process, which is attributed to the slow reorientation of water clusters, takes place between the permittivity values of 75.2 and 8.4 with a relaxation time of 8.4 ps. Finally, the high-frequency process, which occurs between 8.4... 

The relative permittivity values normally encountered in crystals are rather small (Table 11.1). Some crystals, however, exhibit relative permittivity values many orders of magnitude higher than those in normal dielectrics. For example, one crystallographic polymorph of barium titanate, BaTiOs. has a relative permittivity, e, of the order of 20 000 (more values are given in Table 11.2.). By analogy... 

Some crystals, however, exhibit relative permittivity values many orders of magnitude higher than found in normal dielectrics. By analogy with magnetic behaviour, this behaviour is called ferroelec-tricity, and the materials are called ferroelectrics. Ferroelectrics also possess a spontaneous polarisation, Ps, in the absence of an electric field and a mechanical distortion. They are, therefore, a subset of pyroelectrics and, as such, all ferroelectrics are also pyroelectrics and piezoelectrics. The feature that distinguishes ferroelectrics from pyroelectrics is that the direction of the spontaneous polarisation, Ps, can be switched (changed) in an applied electric field. 

FIGURE 15. Real and imaginary permittivity values for muUite composites. Published with permission ofthe J. Am. Ceram. Soc. (37)... 

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