Lossy dielectric material

In a microwave cavity loaded with a lossy dielectric material (such as a ceramic), the power is absorbed mainly by the casket (specimen enclosure) and the specimen. The losses in the cavity wall can typically be ignored such that Eq. (3) can be written as... 

The penetration depth Pa is defined as the distance from the surface of a lossy dielectric material at which the incident power drops to 37% (1/e). Skin depth is, in fact, equal to twice the penetration depth. 

In this chapter we aim to provide a working background for the practical materials scientist or engineer who wishes to apply IS as a method of analysis without needing to become a knowledgeable electrochemist. In contrast to the subsequent chapters, the emphasis here will be on practical, empirical interpretations of materials problems, based on somewhat oversimplified electrochemical models. We shall thus describe approximate methods of data analysis of IS results for simple solid-state electrolyte situations in this chapter and discuss more detailed methods and analyses later. Although we shall concentrate on intrinsically conductive systems, most of the IS measurement techniques, data presentation methods, and analysis functions and methods discussed herein apply directly to lossy dielectric materials as well. 

Figure 1.2. Interaction of microwaves with different materials (a) - electrical conductor, (b) - isulator, (c) - lossy dielectric.

The penetration depth (Dp) of the materials used to denote the depth at which the power density of microwave irradiation is reduced to 37% (i.e., 1/e) of its initial value at the surface of the material. It is proportional to the wavelength of the radiation and depends on the dielectric properties of the material. For lossy dielectrics (e"/e 1) the... 

If the dielectric material is not homogeneous but could be regarded as an association of several phases with different dielectric characteristic, new relaxation processes could be observed. This relaxation processes called Maxwell-Wagner processes occur within heterogeneous dielectric materials. An arrangement comprising a perfect dielectric without loss (organic solvent) and a lossy dielectric (aqueous... 

Dielectrics. These are materials with properties that range from conductors to insulators. There is within this broad class of materials a group referred to as lossy dielectrics, and it is this group that absorbs... 

When a piece of material is exposed to microwave irradiation, microwaves can be reflected from its surface (when the surface is conducting as in metals, graphite, etc.), can penetrate the material without absorption (in the case of good insulators), and can be absorbed by the material (lossy dielectrics). Thus, heating in microwave ovens is based upon the ability of some liquids and solids to absorb and to transform electromagnetic energy into heat. Microwave radiation - as every ra-... 

Dissipation Factor. An important property of dielectric materials, dissipation factor is a measure of lossiness of a dielectric. When an ac signal is impressed on a capacitor or a capacitive load, the voltage and current are ideally out of phase by 90°, and no power is dissipated in the capacitor. However, in all real capacitors the phase difference is somewhat less than 90°, and power is dissipated. The angle between the current and voltage vectors is designated 8 (delta), and the tangent of delta is known as the dissipation factor. 

The real part is the magnetic permeability whereas the imaginary part is the magnetic loss. These losses are quite different from hysteresis or eddy current losses, because they are induced by domain wall and electron-spin resonance. These materials should be placed at position of magnetic field maxima for optimum absorption of microwave energy. For transition metal oxides such as iron, nickel, and cobalt magnetic losses are high. These powders can, therefore, be used as lossy impurities or additives to induce losses within solids for which dielectric loss is too small. 

The dielectric constant has, thus far, been treated as a single number. Such a treatment is equivalent to assuming that e is a static property of a material. Some of the energy of an applied electric field is, however, dissipated. Dissipation occurs when energy is lost to the "internal motions" of the material, which are defined as motions of the atoms from which the material is built [5,6]. This "lossy" component of the response of a material to an electric field is usually expressed in terms of the imaginary component of the complex quantity ... 

The characteristics of the ESR cavity (Fig.l) implies the basic restrictions in the construction of an electrochemical cell for ESR-measurements. Like any other probes the electrochemical cell is to be mounted in the center of the resonant cavity where the magnetic field has its maximum. Lossy samples like electrolyte solutions have to be restricted into the z-direction to avoid high dielectric absorption which lowers the quality-factor of the cavity and the sensitivity of the measurement or makes the measurements impossible at high absorption. Therefore a flat cell with a 0.3 to 0.5 mm thickness of the solution layer must be used. The cell has to be made of quartz because of the "sucking in effect of that material which improves the sensitivity by a factor of 2. This geometry gives the limitations of the electrochemical conditions low electrolyte volume, high cell resistance and small electrodes. For the last fact even further restrictions exist. 

The held distribution within the load which is contained in a multi-mode applicator depends not only on dielectric permeability or dielectric loss, but also on size and location of the load within the applicator. In this respect, a multi-mode device is best suited to very lossy loads occupying relatively large volumes (more than 50%) of the applicator. For low and medium loss matericds occupying less than about 20% of the applicator s volume, the temperature rise in the material, at best, will be a non-uniform one and, at worst, potentially damaging hotspots will occur as the result of extremely high local helds. 

Two characteristics that make syntactic foam an efficient buoyancy material are high compressive strength and low density. The low density of the syntactic foam allows buoyant lift to be designed into the insulation system. As an example, Cuming Microwave Corporation makes a series of syntactic foams (low loss dielectric as well as treated lossy materials), which are ultralight and high strength and can be adjusted to specifications. These materials are available in different forms uncured, pack-in-place, cured sheet, and molded to shape. The pack-in-place variety has the consistency of a snowball, which can be readily packed into complex shapes. 

Low circuit complexity Precise matching of impedance often needed Minimizing signal losses essential Small circuit element sizes often essential Only 1 or 2 layers High feature accuracy needed Low/uniform dielectric constants needed Very high circuit complexity Tolerant of impedance mismatches Tolerant of lossy materials Small circuit element sizes desirable Many signal and power layers Moderate feature accuracy needed Dielectric constant secondary... 

Losses in ceramic dielectrics may arise from ion migration, ion vibration and deformation, and electronic polarization. Figure 3.2 shows the loss contributions as a function of frequency for a lossy material. 

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