Dielectric interfacial polarization

The dielectric constant is a measure of the ease with which charged species in a material can be displaced to form dipoles. There are four primary mechanisms of polarization in glasses (13) electronic, atomic, orientational, and interfacial polarization. Electronic polarization arises from the displacement of electron clouds and is important at optical (ultraviolet) frequencies. At optical frequencies, the dielectric constant of a glass is related to the refractive index k =. Atomic polarization occurs at infrared frequencies and involves the displacement of positive and negative ions. 

At lower frequencies, orientational polarization may occur if the glass contains permanent ionic or molecular dipoles, such as H2O or an Si—OH group, that can rotate or oscillate in the presence of an appHed electric field. Another source of orientational polarization at even lower frequencies is the oscillatory movement of mobile ions such as Na". The higher the amount of alkaH oxide in the glass, the higher the dielectric constant. When the movement of mobile charge carriers is obstmcted by a barrier, the accumulation of carriers at the interface leads to interfacial polarization. Interfacial polarization can occur in phase-separated glasses if the phases have different dielectric constants. 

One aspect of the research will examine equilibrium aspects of solvation at hydro-phobic and hydrophilic interfaces. In these experiments, solvent dependent shifts in chromophore absorption spectra will be used to infer interfacial polarity. Preliminary results from these studies are presented. The polarity of solid-liquid interfaces arises from a complicated balance of anisotropic, intermolecular forces. It is hoped that results from these studies can aid in developing a general, predictive understanding of dielectric properties in inhomogeneous environments. 

We can further describe the polarization, P, according to the different types of dipoles that either already exist or are induced in the dielectric material. The polarization of a dielectric material may be caused by four major types of polarization electronic polarization, ionic (atomic) polarization, orientation polarization, and space-charge (interfacial) polarization. Each type of polarization is shown schematically in Figure 6.24 and will be described in succession. In these descriptions, it will be useful to introduce a new term called the polarizability, a, which is simply a measure of the ability of a material to undergo the specific type of polarization. 

The terms polarizability constant and dielectric constant can be utilized interchangeably in the qualitative discussion of the magnitude of the dielectric constant. The k values obtained utilizing dc and low-frequency measurements are a summation of electronic E, atomic A, dipole P0, and interfacial /, polarizations. Only the contribution by electronic polarizations is evident at high frequencies. The variation of dielectric constant with frequency for a material having interfacial, dipole, atomic, and electronic polarization contributions is shown in Figure 6.1. 

The heating effect relies upon dielectric polarization [1], itself containing components of electronic, atomic, dipolar, and interfacial polarization, of which the last two have timescales which allow them to contribute to the overall heating effect at these frequencies. The loss tangent, tan 5, consists of two components, s, the dielectric constant, and s", the dielectric loss, where... 

However, for the oxide electrode, such as SrRuOj, the structural similarity of the electrode and dielectric material allows a certain penetration of the dielectric polarization into the oxide electrode. As a result, the formation of the intrinsic low dielectric interfacial layer is effectively suppressed and film thickness independent dielectric constants are obtained, as reported by the author for the case of sputtered BST films on IrO electrodes and more recently reported by Toshiba researchers for the case of MOCVD BST films on SrRu03 electrodes. Under these circumstances dielectric constants are solely determined by processing conditions. The stoichiometric composition and good crystallization of the films are the two most important parameters for a high dielectric constant. 

The majority of interfacial polarization loss processes can be closely approximated by some modification of the Debye description of orientational dipole polarization in homogeneous media (13). The subject of interfacial polarization effects and the dielectric properties of many classes of heterogeneous systems have been reviewed by Van Beek (14). 

In this study, both the normal mode relaxation of the siloxane network and the MWS processes arising from the interaction of the dispersed nanoclay platelets within the polymer network have been observed. Although it is routine practice to observe the primary alpha relaxation of a polymeric system at temperatures below Tg, in this work it is the MWS processes associated with the clay particles within the polymer matrix that are of interest. Therefore, all BDS analyses were conducted at 40°C over a frequency range of 10 to 6.5x10 Hz. At these temperatures, interfacial polarization effects dominate the dielectric response of the filled systems and although it is possible to resolve a normal mode relaxation of the polymer in the unfilled system (see Figure 2), MWS processes arising from the presence of the nanoclay mask this comparatively weak process. 

Another type of polarization arises from a charge build-up in the contact areas or interfaces between different components in heterogeneous systems. This phenomenon is also known as interfacial polarization and is due to the difference in the conductivities and dielectric constants (see below) of the materials at interfaces. The accumulation of space charge is responsible for field distortions and dielectric loss and is commonly termed Maxwell-Wagner polarisation . 

Issues (vi) and (vil) both deal with the nature of the solvent they are also related to (v). Considering water, the spatial distribution of the molecules is in a very complicated way determined by solvent-solvent, solvent-countercharge and solvent-surface charge interactions. A detailed knowledge of this structure is required to quantify ion-ion correlations, ion-ion and ion-surface solvent structure-originated interactions and the local dielectric permittivity. Polarization of the solvent also contributes to the interfacial potential Jump or X POtential (secs. 1.5.5a and 3.9), which does not occur in Poisson-BoltzmEmn theory. 

Ionic double layers and double layers caused by interfacied poleirlzation occur together but are not Independent. Changing the surface charge will affect the polarization of adjacent solvent, so that x Is generally a function of a°. Specific adsorption of ions in the Stern layer is intimately coupled to the solvent structure and conversely. The inner layer capacitances Kj, K, C and are also coupled to the interfacial polarization via the local relative dielectric permittivities ej and e. ... 

T. Hanai, Theory of the dielectric dispersion due to the interfacial polarization and its application to emulsion, Kolloid-Z, 171, 23-31 (1960). 

Polymers are quite special dielectrics in that they can be highly insulating and have extremely low dielectric constants. For example, polyethylene can exhibit a dielectric constant as low as 2.2 and conductivity as low as 10 19 S/cm. These two characteristics make polymers very susceptible to another source of polarization—accumulation of virtual charge at the interface between the polymer matrix and any more polar or conducting phase, for example, water droplets. This interfacial polarization can dominate the dielectric characteristics of the polymer at low to intermediate frequencies. As a result, the detection of this polarization becomes an effective means of demonstrating the presence of two phases in the polymer. Even phases in the nanometer range will show this effect. A schematic of such polarization is shown in Figure 7-4. 

Interfacial polarization in biphasic dielectrics was first described by Maxwell (same Maxwell as the Maxwell model) in his monograph Electricity and Magnetism of 1892.12 Somewhat later the effect was described by Wagner in terms of the polarization of a two-layer dielectric in a capacitor and showed that the polarization of isolated spheres was similar. Other more complex geometries (ellipsoids, rods) were considered by Sillars as a result, interfacial polarization is often called the Maxwell-Wagner-Sillars (MWS) effect. 

In the interpretation of the loss factor tg 8, it is not easy to make a distinction between a dipole relaxation and the interfacial polarization. With metallic electrodes, both effects are superposed on the ionic part of the dielectric loss and not necessarily distinguishable from it. With blocking electrodes, the relative intensity of the dipole relaxation and the Maxwell-Wagner effect depends on the ratio of the thickness of the blocking layers and the zeolite pellets (15). 

Capacitively coupled electrodes are frequently employed in electrical measurements to study such effects as interfacial polarization, dielectric polarization, and high-frequency effects [38]. Capacitive electrodes are also sometimes used with semi-insulators to generate a field inside the specimen that is well-defined, and have been applied to the study of the initiation of PbN [39,40]. However, caution must be exercised in such an enterprise. A material with conductivity (ohm m)" is generally viewed as a good insulator. Yet it has an... 

However, generally in composites with conductive inclusions, ionic current and interfacial polarization could often mask the real dielectric relaxation processes in the low frequency range. Therefore, to analyze the dielectric process in detail, the complex permittivity e can be converted to the complex electric modulus M by using the following equation ... 

In chitin, CS and PVA complex dielectric spectra (Z" versus Z plot shown in Fig. 2.3), two different behaviors are identified (i) a typical semicircle at high frequencies, which corresponds to the bulk material signal and (ii) a quasilinear response at low frequencies associated with interfacial polarization in the bulk of the films and/or surface and contact effects [46, 47]. This low frequency part of the electrical response is easily influenced by imperfect contact between the metal electrode and the sample as it was previously tested elsewhere [5], there is no influence of gold contact on the polymer impedance spectra (high frequency part of the spectra corresponding to the bulk of the film) and it is discarded for further analysis. 

The same dielectric behavior in dry annealed and wet samples is developed above 110°C. These observations are very important since they suggest an extra relaxation process [16] in the absence of water, which is analyzed in detail in the next section. On the other hand, the linear response at low frequencies in wet and dry chitin can be associated with interfacial polarization in the bulk films and/or surface and metal contact effects [47]. To analyze the dielectric relaxation of chitin films is necessary to understand the nature of the low frequency part of dielectric spectrum. 

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