Dielectric polarization mechanism ionic

Although sulfur vulcanization has been studied since its discovery in 1839 by Goodyear, its mechanism is not well understood. Free-radical mechanisms were originally assumed but most evidence points to an ionic reaction [Bateman, 1963]. Neither radical initiators nor inhibitors affect sulfur vulcanization and radicals have not been detected by ESR spectroscopy. On the other hand, sulfur vulcanization is accelerated by organic acids and bases as well as by solvents of high dielectric constant. The ionic process can be depicted as a chain reaction involving the initial formation of a sulfonium ion (XI) by reaction of the polymer with polarized sulfur or a sulfur ion pair. The sulfonium ion reacts with a polymer molecule by hydride... 

Dielectric relaxation study is a powerful technique for obtaining molecular dipolar relaxation as a function of temperature and frequency. By studying the relaxation spectra, the intermolecular cooperative motion and hindered dipolar rotation can be deduced. Due to the presence of an electric field, the composites undergo ionic, interfacial, and dipole polarization, and this polarization mechanism largely depends on the time scales and length scales. As a result, this technique allowed us to shed light on the dynamics of the macromolecular chains of the rubber matrix. The temperature as well as the frequency window can also be varied over a wide... 

Microwave radiation, as all radiation of an electromagnetic nature, consists of two components, i.e. magnetic and electric field components (Fig. 1.3). The electric field component is responsible for dielectric heating mechanism since it can cause molecular motion either by migration of ionic species (conduction mechanism) or rotation of dipolar species (dipolar polarization mechanism). In a microwave field, the electric field component oscillates very quickly (at 2.45 GHz the field oscillates 4.9 x 109 times per second), and the strong agitation, provided by cyclic reorientation of molecules, can result in an... 

In this section, a simple description of the dielectric polarization process is provided, and later to describe dielectric relaxation processes, the polarization mechanisms of materials produced by macroscopic static electric fields are analyzed. The relation between the macroscopic electric response and microscopic properties such as electronic, ionic, orientational, and hopping charge polarizabilities is very complex and is out of the scope of this book. This problem was successfully treated by Lorentz. He established that a remarkable improvement of the obtained results can be obtained at all frequencies by proposing the existence of a local field, which diverges from the macroscopic electric field by a correction factor, the Lorentz local-field factor [27],... 

Fig. 20 Mechanisms of polarity inversion in a dielectric, (a) Electric polarization (b) ionic polarization (c) orientation polarization.

The application of an electric field E across a linear dielectric material results in polarization P or the separation of positive and negative charges. The relative dielectric constant k is a measure of the capacity of a solid to store charge relative to vacuum and is related to the extent to which the charges in a solid polarize. Atomically there are four main polarization mechanisms electronic, ionic, dipolar, and space charge. 

Since DNA is a highly charged macromolecule surrounded by a layer of counter ions, it is more probable that the dielectric polarization of DNA arises from the polarization of the ion atmosphere. Various mechanisms of ionic polarization have been proposed. The theories of ionic polarization for a spherical particle suspension were reviewed and carefully discussed by Schwan (20, 21). Since DNA is a thin elongated molecule, those theories must be modified substantially. Various theories for ellipsoids are briefly reviewed here. 

MW-enhanced chemistry is based on the efficiency of interactions of molecules with waves by microwave dielectric heating effects. This phenomenon depends on the ability of materials to absorb MW radiation and convert it into heat. The electric component of the electromagnetic field has been shown to be the most important [22-24]. It results in two main mechanisms - dipolar polarization and ionic conduction. Irradiation of polar molecules at MW frequencies results in orientation of the dipoles or ions in the applied electric field (Scheme 4.1) [25]. 

In order to ascertain the origms and mechanisms responsible for the observed temperature behavior of die static dielectric permittivity, let us analyze the total dielectric relaxation behavior of ionic mieroemulsions. Dynamic aspects of the dielectric polarization can be taken into account... 

In order to understand this complex relaxation behavior of the microemulsions, it is necessary to analyze dielectric information obtained from the various sources of the polarization. For a system containing more than two different phases the interfacial polarization mechanism has to be taken into account. Since the microemulsion is ionic, the dielectric relaxation contributions are related to the movement of surfactant counterions relative to the negatively charged droplet interface. A reorientation of AOT molecules, and of free and bound water molecules, should also be mentioned in the list of polarization mechanisms. In order to ascertain which mechanism can provide the experimental increase in dielectric permittivity, let us discuss the different contributions. 

The dielectric properties of a material are determined by the polarizability of its molecules. There are three primary contributions to the electric polarization of a dielectrics electronic, ionic and dipole reorientation - related (Uchino, 2000). The intensity with which each mechanism occurs depends on the frequency of applied electric field. The electronic polarization causes a displacement of the electrons with respect to the atomic nuclei and can follow alternating field with the frequencies up to - lOi Hz. The ionic polarization relies on a displacement of the atomic nuclei relative to one another and responds up to lO - lO Hz. Both mentioned polarization mechanisms are related to the non-polar molecules. The third mechanism associated with the dipole reorientation is valid only in the case of polar molecules. It can follow with the frequency of alternating electric field up to 10 - lO Hz. The dielectric permittivity of a material represents the ratio of the capacitance of a plane condenser filled with the dielectric to that of the same condenser under vacuum and is to calculate from the expression ... 

Insulator and capacitor applications depend on the dielectric properties of ceramics, that is, on their polarization response to an applied electric field. The four polarization mechanisms which describe the displacement of charged species in ceramics are (1) electronic polarization—the shift of the valence electron cloud with respect to the nucleus (2) ionic or atomic polarization— movement of cation and anion species (3) dipolar polarization—perturbation of the thermal motion of ionic or molecular dipoles and (4) interfacial polarization—inhibition of charge migration by a physical barrier. Further discussion of polarization phenomena may be found in Reference 1. 

The role of inherent polarization and ionic transport effects in actuation mechanism of EAPap actuators are investigated. To physically investigate the actuation mechanism, several tests are performed. X-ray diffraction (XRD) spectra are compared before and after electrical activation and the possibility of crystalline structure change is observed. Dielectric property measurement indicates a dependence of the dielectric constant on fiber direction, as well as on frequency, humidity, and temperature. Thus, we conclude that piezoelectric effect and ionic migration effect are in the EAPap at the same time associated with dipole moment of cellulose paper ingredients. The amount of these effects may depend on environmental condition. 

Insulator Dielectrics. Insulator thick film dielectrics are multiphase materials. The electronic, ionic, and interfacial polarization mechanisms all contribute to the dielectric constant of glass-ceramic materials. Electronic polarization is directly proportional to the density of electrons in the glass-ceramic. Thus, dielectrics based on glasses containing oxides of high atomic number elements (e.g., lead) or high density exhibit high dielectric constants. 

Possible polarization types include electronic (Figure 18.32a), ionic (Figure 18.32h), and orientation (Figure 18.32c) not all types need be present in a particular dielectric. For alternating electric fields, whether a specific polarization type contributes to the total polarization and dielectric constant depends on frequency each polarization mechanism ceases to function when the applied field frequency exceeds its relaxation frequency (Figure 18.34). 

Figures 12a and 12b show the dielectric constant (c ) as a function of frequency of LNMO and LCMO ceramics at different temperatures. It can be observed that the dielectric constant of both ceramics decreases as frequency increases. The decrease in the dielectric constant with increase in frequency can be explained by the behavior on the basis of electron happing from Fe to Fe ions or on basis of decrease in polarization with the increase in frequency. Polarization of a dielectric material is the quantity of the contributions of ionic, electronic, dipolar, and interfacial polarizations [63]. At low frequencies, polarization mechanism is keenly observed at low frequencies to the time var)ing electric fields. As the frequency of the electric field increases, different polarization contributions are filter out under leads to the decrement in net polarization under dielectric constant. Similar behavior has also been reported by different investigators earlier in the literature [60, 64]. The physical, magnetic, and dielectric properties of LMNO and LCMO are summarized in Table 1.

Fig. 5.20. The shock-induced polarization of a range of ionic crystals is shown at a compression of about 30%. This maximum value is well correlated with cation radius, dielectric constant, and a factor thought to represent dielectric strength. A mechanically induced point defect generation and migration model is preferred for the effect (after Davison and Graham [79D01]).

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