Dielectric multiphase

With the MBR, individual components of multiphase systems can be heated at different rates according to differences in their dielectric properties. We have termed this, differential heating. For example, the aqueous phase of a water/chloroform system (1 1 by volume), heated more rapidly than did the organic layer and a tempera-... 

In multiphase flow metering, it is usually required to distinguish hydrocarbon from water. If the liquid phase is "oil continuous," the water fraction can be determined by dielectric constant measurement at microwave frequencies because the dielectric constant of dry hydrocarbon is on the order of 2 to 4 and that of water is 82. Naturally, density measurement can also distinguish water from oil. The next requirement is to distinguish the flow of liquid from the flow of gas in a system where the two will try to separate and travel at different velocities. Cross-correlation by nuclear techniques can measure the density of the stream twice (a short vertical distance apart) and correlate the fluctuations in density with time to determine velocity. Multiphase flow metering is a new and evolving technology,... 

In contrast to dielectric losses permittivity is not, in general, sensitive to small amounts of impurities and for homogeneous dielectrics values can be calculated as described in Section 2.7.1, and the various mixture rules allow good estimates to be made for multiphase dielectrics. For Ba- and Sr-based dielectrics having the perovskite structure the variation of permittivity with temperature, which determines rf (see Eq. (5.37)), can be correlated with the tolerance factor t (see Section 2.7.3) [13] providing guidance for tailoring ceramics to have xf = 0. 

Non-Debye dielectric relaxation in porous systems is another example of the dynamic behavior of complex systems on the mesoscale. The dielectric properties of various complex multiphase systems (borosilicate porous glasses [153-156], sol-gel glasses [157,158], zeolites [159], and porous silicon [160,161]) were studied and analyzed recently in terms of cooperative dynamics. The dielectric response in porous systems will be considered here in detail using two quite different types of materials, namely, porous glasses and porous silicon. 

This expression, however, only deals with a single molecular dipole and as many such dipoles would be required to describe the overall membrane dipole potential but as a mean-field expression, this term is practical and culmulatively offers the approximation of the estimated dipolar organisation shown in Fig. 4. A further complication, however, involves the solvent environment and this too is also often dealt with as a mean field or in a continuum manner. But the relative permittivity (or dielectric constant) (sr) cannot be considered to possess the same value throughout the multiphase system represented by a membrane in an aqueous medium. The permittivity profile has been measured to vary from about 78.5 in the bulk aqueous... 

In any multiphase liquid, stability is a paramount concern. Thermodynamics drives clumping of dispersed components, and this is sometimes enhanced by flow. However, tricks for stabilizing suspensions are as old as the inks of Egypt. Electrostatic and steric stabilization are the most common. By matching the dielectric prc effie s, sdrrie pafticle-fluid combinations can be found that are inherently stable. A more detailed discussion of suspension stabilization is deferred to Chapter 7. 

Large polarizations are seen to occur in multiphase polymer systems, principally due to the different conductivities in the dissimilar microdomains. Due to the low overall conductivities of these polymers, the polarizations occur at low, often subaudio, frequencies. In order to study these processes two computer-aided dielectric spectrometers have been developed. They differ in the type of excitation used A.C. versus step. Each... 

Closed-form expressions from composite theory are also useful in correlating and predicting the transport properties (dielectric constant, electrical conductivity, magnetic susceptibility, thermal conductivity, gas diffusivity and gas permeability) of multiphase materials. The models lor these properties often utilize mathematical treatments [54,55] which are similar to those used for the thermoelastic properties, once the appropriate mathematical analogies [56,57] are made. Such analogies and the resulting composite models have been pursued quite extensively for both particulate-reinforced and fiber-reinforced composites where the filler phase consists of discrete entities dispersed within a continuous polymeric matrix. 

The fundamental measurements of dielectric constant and resistivity in multiphase systems follow directly from methods used for solid systems (Curtis, 1915). The material resistivity (or electrical conductivity) together with the permittivity are useful parameters for calculating the charge relaxation time of the material. 

In this review we have chosen to highlight the pros and cons of dielectric spectroscopy applied to emulsified and related systems. The reason for the choice of these systems is that they represent multiphase systems containing phase boundaries or interfaces. It is shown that dielectric spectroscopy scanning over large frequency intervals (from some kilohertz up to several gigahertz) is extremely sensitive towards interfacial phenomena and interfacial polarization. Hence, there exist many possibilities for this... 

As we have seen, the highest dielectric permittivity is found in such solids which are heterogeneous on microscopic level. The effect can be even more drastic in multiphase systems, especially if one of the phases is water. In 1934 Smith-Rose [91] discovered that soils, which in dry state have s of 2-10, increase it by several orders of magnitude if impregnated with water (which has 80). Later, similar... 

Composite materials based on sol-gel technology are used by material scientists to achieve improved strength, stiffness and toughness, and improve optical and dielectric properties, or gain temperatine, and corrosion resistance [202,226]. However, in electrochemistry, and particularly in analytical electrochemistry, these attributes are important only for some specific applications, and electrochemists resort to composites for different reasons. In this section, we describe and exemplify some electrochemical properties that may be gained by electrode and membrane design based on heterogeneous multiphase materials. 

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. 

Polymer composites are multiphase materials containing, usually, inorganic fillers or reinforcing materials embedded in an anurphous or polycrystalline matrix. The dielectric properties of the inclusions arc, usually, very different from those of the nutrix. For ferroelectric appUcatioos, inorganic ferroelectric materials (e.g.. ceramics) are often used u fillers. 

A ceramic material is heterogenous it is polycrystalline and contains pores it is often multiphase, the phases being distributed at grain boundaries, as a result of a liquid phase sintering, or forming inclusions. We may ask ourselves in what measure it is still possible to speak of electrical conductivity, relative dielectric permittivity, piezoelectric constant, etc. of the material. Or, to put it plainly, does the notion of material have meaning The answer is experimental, as we will see in section 11.5.1. 

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