Dielectric ceramic materials

Kanai, H. et al. (1998) Effects of microstructure on insulation resistance degradation of relaxors, in Advances in Dielectric Ceramic Materials, Vol. 88, The Am. Ceram. Soc., 295-9. 

The useful properties of a dielectric ceramic material-that is, their figures of merit, and in particular for microwave applications (see Section 8.4)-can be described by ... 

When an electric field is applied to a dielectric ceramic material, the electrical charges within the material will shift, and a polarization is induced that is... 

Apart from the mechanical damage of the inherently brittle dielectric ceramic materials, several breakdown mechanisms have been identified that tend to cause a deterioration in electrical performance. 

Bouchard RJ (1999) Thick film technology an historical perspective, vol 100, Ceramic transactions dielectric ceramic materials. Wiley, Rochester, NY, pp 429-442... 

Cera.micA.bla.tors, Several types of subliming or melting ceramic ablators have been used or considered for use in dielectric appHcations particularly with quartz or boron nitride [10043-11 -5] fiber reinforcements to form a nonconductive char. Fused siHca is available in both nonporous (optically transparent) and porous (sHp cast) forms. Ford Aerospace manufactures a 3D siHca-fiber-reinforced composite densified with coUoidal siHca (37). The material, designated AS-3DX, demonstrates improved mechanical toughness compared to monolithic ceramics. Other dielectric ceramic composites have been used with performance improvements over monolithic ceramics (see COMPOSITE MATERIALS, CERAMIC MATRIX). 

Capacitors. Ceramic materials suitable for capacitor (charge storage) use are also dependent on the dielectric properties of the material. Frequently the goal of ceramic capacitors is to achieve maximum capacitance in minimum volume. The defining equation for capacitance is given by ... 

In the broad range of ceramic materials that are used for electrical and electronic apphcations, each category of material exhibits unique property characteristics which directiy reflect composition, processing, and microstmcture. Detailed treatment is given primarily to those property characteristics relating to insulation behavior and electrical conduction processes. Further details concerning the more specialized electrical behavior in ceramic materials, eg, polarization, dielectric, ferroelectric, piezoelectric, electrooptic, and magnetic phenomena, are covered in References 1—9. 

Dielectric Constant The dielectric constant of material represents its ability to reduce the electric force between two charges separated in space. This propei ty is useful in process control for polymers, ceramic materials, and semiconduc tors. Dielectric constants are measured with respect to vacuum (1.0) typical values range from 2 (benzene) to 33 (methanol) to 80 (water). TEe value for water is higher than for most plastics. A measuring cell is made of glass or some other insulating material and is usually doughnut-shaped, with the cylinders coated with metal, which constitute the plates of the capacitor. 

Power supply designers are usually aware that the most stable ceramic capacitance comes from materials dubbed COG material, also called NPO (for negative positive zero, referring to its near perfect temperature coefficient). But this is a low dielectric constant material, and unsuitable for modern miniaturization. So the common materials in use today are called X7R, X5R, and so on. There are others, starting with a Y or Z prefix, which no power supply designer worth his or her salt will ever use. 

Dielectrics, optics and magnetism (properties of certain ceramic materials)... 

Barium titanate (BaTiOj), a perovskite-type electro-ceramic material, has been extensively studied and utilized due to its dielectric and ferroelectric properties. The wide applications of barium titanates include multiplayer capacitors in electronic circuits, nonlinear resistors, thermal switches, passive memory storage devices, and transducers. In addition, barium titanate can be used for chemical sensors due to its surface sensivity to gas adsorption. 

Therefore the dielectric constant is changed with temperature and the resonant frequency will change with temperature, and the microwave devices cannot respond at a specific frequency if the dielectric materials in microwave frequencies show a large TCK and thermal expansion coefficient a due to the thermal expansion of dielectric materials and the temperature dependence of polarizability. In general, the a of dielectric ceramics, which is well known as the slope of the Cockbain equation, is about 10 ppm/°C. Therefore control of TCP can be achieved by adequate manipulations of TCK. It is an important requirement for practical applications to control the stable TCP, nearly zero, which is available to temperature-stable microwave devices. 

Silicon dioxide is the most widely used ceramic material in the semiconductor industry, and the majority of Si02 deposits in microelectronic circuits are formed by CVD. Silica layers are used as diffusion sources, intermetallic dielectrics, and dopant and etch barriers in the microelectronics industry. CVD of Si02 is also commonly used in manufacturing energy-efficient glass windows, surface coatings for fiber optics, and micromechanical applications. ... 

Electrical properties of glass ceramics are determined by the properties of both the crystalline phases and the residual glass. Electrical conductivity and dielectric loss (at low frequencies) are dominated by the concentration and mobility of alkali ions in the glass phase. The dielectric constant is dominated by the crystalline phase, especially when that phase consists of high dielectric constant materials such as ferroelectric crystals. The... 

In addition to the use of heterometal alkoxides, metal alkoxides are often associated with more easily available precursors such as acetates for the SG route to multicomponent oxides. A number of such alkoxide acetate precursors [e.g., MNb2(/i-OAc)2(/i-OR)4(OR)6 (M = Cd or Mg), PbZr3(/t4-0)(/i-0Ac)2(/i-OR)5(OR)5, and Gd2Zr6(/i4-O)2(pi-OAc)6(/t-OR)l0(OR)i0 (with R = i-Pr)] were characterized (564) by X-ray crystallography. Their hydrolytic studies indicate their potential use as precursors for the synthesis of electrooptical materials, for example, Pb(ScNb)03 (PSN), and dielectric ceramics, for example, [PbMg1/3Nb2/303] (PNM). 

The mechanisms by which microwave energy is absorbed by the entire processing system (the microwave applicator, the specimens, and the thermal insulation) are crucial to understand the microwave processing of ceramic materials. The microwave energy absorbed per unit time per unit volume, pabs (the powder density), can be related to dielectric properties of the material and to the applied electromagnetic field " ... 

Fig. 4 Schematic diagram showing the nature of reflection and absorption from (A) low loss dielectric, (B) metallic, (C) monolithic high loss dielectric, and (D) a composite or particulate ceramic with high dielectric loss. (From Sutton, W.H. Microwave processing of ceramic materials. Am. Ceram. Soc. Bull. 1989, 68, 376-386, Fig. 2, page 377. Reprinted with permission of The American Ceramic Society, www.ceramics.org. Copyright 1989. All rights reserved.)...

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