Dielectric transition

The dielectric transition is mainly given by the polarization of the space charge and to a lesser extent also by the mobility of charge carriers. 

The common features shared by the co mechanical and dielectric transitions tend to indicate that these two phenomena have the same origin, i.e. they arise from motions that involve the conjugated amide groups situated between the aromatic ring and aliphatic cyclodiamine. 

Studies of the electrical properties of polycrystalline AgN03 above and below room temperature have revealed the existence of a dielectric transition at — 35 °C with a maximum in the dielectric constant occurring at 110°C.203... 

In the majority of quasi-one-dimensional compounds the superconducting transition is not observed, since at higher temperatures there takes place a phase transition into a dielectric state. It has been suggested that suppression of the dielectric transition by impurities may be beneficial for producing superconductivity. This concept is based on the absence of the non-magnetic impurity influence on the superconducting transition in three-dimensional conductors. 

In a mean field approximation based on the electron interaction on different lines we obtain the following equation for the dielectric transition temperature (2) ... 

Then for the free path -t at which the dielectric transition temperature turns to zero we get the equation... 

Thus, at a sufficiently small free path the dielectric transition vanishes. 

The occurrence of the dielectric transition In quasi-one-dimensional conductors means that the electron interaction on different threads (V in formula (1) ) is larger than the amplitude of the pair jump ( W in formula (5) ). The dielectric transition can be suppressed by introducing a sufficiently large amount of impurities, but the superconducting transition in such a case will inevitably be suppressed too. And only exotic impurities may save the situation which either do not reverse the electron momentum, or favour the probability of the electron jump from one thread to another. 

Some quasi-one-dimensional conductors ( KCP and the quinolinium and acridizinium TCNQ salts) display an intrinsic disorder. Apparently such a disorder does not result in a short free path, since in all given compounds there is a dielectric transition (6). The electron scattering on the impurities seems to explain the absence of the dielectric transition in the quasi-one-dimensional inorganic polymer (SN)X. 

The growth rates of the a and phases in KNOg have been measured as a function of the number of transformations and the temperature during the a -> /3 and /5 -> a transitions. It was found that the transformation rate is influenced by recrystallization and relaxation processes. Dielectric transitions in MNOg (M = Li, Na, K, Rb, Cs, Ag, Tl, or NH4) have characteristic temperatures which depend on the degree of packing of ions in the crystal lattice. The same factor also influences the melting points of the compounds. 

Anmunetallic inorganic material can be a bulk material (e.g., a glass chip, a quartz chip, or a silicon chip which possesses a natural silicon oxide layer similar to glass) or a coating (e.g., a dielectric transition metal oxide coating on a chip). 

The texture transition can also be observed for smectic A liquid crystals with negative dielectric anisotropy [112]. In that case, the transition from a homeotropic into a planar texture occurs. The threshold of this, dielectric transition, can be modified (lowered) at the low frequencies of an applied field by the anisotropy of the electrical conductivity of a substance. 

PEP exhibits a single hrst-order transition that is its melting point. The relaxation temperature of FEP increases with the HFP content of the copolymer. Fluorinated ethylene-propylene has a dielectric transition at -150°C, which is unaffected by the monomer composition or crystallinity (specific gravity). 

Vinogradov AP, Sarydiev AK (1983) The structure of conducting channels and the metal-dielectric transition in composites Zhum Eksper Teor Fiz 85 1144-1151... 

In the case of polymer blends and composites, one or both phases may be dispenive. In such systems, in addition to the dielectric transitions, separate MWS transitions may be delect or some transitions may be distorted, even obscured, by the MWS effect. This problem has recently been thoroughly analyzed and checked experimentally (30]. 

Since in ferroelectric polymers ohmic oooductivity may be frequency dependent, the tan 6 represenution is not recommeiKled. The dielectric transition may be represented by the frequency-dependent permittivity related to the imaginary part of the actually measured admittance (V ) and by the frequency-dependent AC conductivity related to the real part V as... 

It is common practice to define the dielectric transition frequency by the maxinu of the c (co) or tan 6(u>) curves, although these are somewhat shifted with respect lo each other. Neither of these methods is correct, because the relaxation time distribution is usually nonsymmetric Tbc transition frequency may be defined as that corresponding lo Ibe derivative of the real part c by log(frequency). According to Eq. (19). ibis corresponds lo the reciprocal of the maximum of the relaxation time distribution. The first derivative is usually sufficient, e is affected by the ohmic conductivity only when in-lerfocial polarization is dominant in the frequency range studied. 

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