Ceramics defined

Calcium phosphate ceramics are ceramics with varying calcium-to-phosphate ratios. Among them, the apatite ceramics, defined by the chemical formula M,o(X04)6Z2, have been studied most. The apatites form a range of solid solutions as a result of ionic substitution at the XO , or Z sites. In general, apatites are nonstoichiometric and contain less than 10 mol of ions, less than 2 mol of Z" ions, and exactly 6 mol of XOJ" ions (Van Raemdonck et al., 1984). The species is typically a bivalent metallic cation, such as Ca, Sr +, Ba ", Pb ", or Cd2+. The XO species is typically one of the following trivalent anions AsO , VO, CrO , or MnO. The monovalent 7r ions are usually F", OH , Br , or Cj (Van Raemdonck et al., 1984). 

Pt(CO)2Cl2] is used to deposit thin films of metallic platinum on surfaces. Concentiated organic solutions of poorly defined platinum complexes of alkyl mercaptides or sulforesinates are used to coat ceramics and glass. 

Tableting, pressing, mol ding, and extrusion operations are commonly used to produce agglomerates of well-defined shape, dimensions, and uniformity in which the properties of each item are important and output is measured in pieces per hour (see Ceramics, ceramics processing Pharmaceuticals Metallurgy, powderp tallurgy Plastics processing).

The processes employed in manufacturing a ceramic are defined and controlled to produce a product with properties suited to a specific appHcation. Processing—microstmcture—property relationships are deterrnined by characterizing the ceramic raw materials, mixes, and the formed ceramic body intermittently during processing and after final thermal consoHdation. It is possible to modify and optimize processes to optimize properties and to identify and correct processing deficiencies when less than optimal properties are obtained. Examples of some process—microstmcture—property relations in advanced ceramics are outlined in Figure 4. 

Elastic Behavior. Elastic deformation is defined as the reversible deformation that occurs when a load is appHed. Most ceramics deform in a linear elastic fashion, ie, the amount of reversible deformation is a linear function of the appHed stress up to a certain stress level. If the appHed stress is increased any further the ceramic fractures catastrophically. This is in contrast to most metals which initially deform elastically and then begin to deform plastically. Plastic deformation allows stresses to be dissipated rather than building to the point where bonds break irreversibly. 

Fracture Toughness. The fracture criterion was defined by a critical value of the crack tip stress intensity, known as the fracture toughness. Ceramics often fail ia pure tension, designated mode I, and Kj replaces ia equation 6. Thus die appHed tensile stress at which fracture... 

Hardness is determined by measuring the penetration (depth or area) when a harder material, such as diamond, is pushed into the surface of the material of interest under a specified load. Tme hardness is defined as the force divided by the projected area. Vickers hardness tests, which employ a pyramid-shaped indentor, are frequently used to characterize ceramics however, Vickers hardness calculations normally employ total surface area rather than projected area (43). Measurements are made on the diamond impression shown in Figure 6. Vickers hardness is calculated using... 

The observed dielectric constant M and the dielectric loss factor k = k tan S are defined by the charge displacement characteristics of the ceramic ie, the movement of charged species within the material in response to the appHed electric field. Discussion of polarization mechanisms is available (1). 

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 ... 

The performance characteristics of ceramic sensors are defined by one or more of the foUowing material properties bulk, grain boundary, interface, or surface. Sensor response arises from the nonelectrical input because the environmental variable effects charge generation and transport in the sensor material. 

Superconductivity The physical state in which all resistance to the flow of direct-current electricity disappears is defined as superconductivity. The Bardeen-Cooper-Schriefer (BCS) theoiy has been reasonably successful in accounting for most of the basic features observed of the superconducting state for low-temperature superconductors (LTS) operating below 23 K. The advent of the ceramic high-temperature superconductors (HTS) by Bednorz and Miller (Z. Phys. B64, 189, 1989) has called for modifications to existing theories which have not been finahzed to date. The massive interest in the new superconductors that can be cooled with liquid nitrogen is just now beginning to make its way into new applications. 

Laser sampling is more a physical phenomenon than a chemical one. The energy of the laser is used to nonselectively ablate the sample. This insures homogeneous sampling of a physically defined area regardless of the nature of the components Solubilities are not a factor. This technique shows much promise for ceramics, glasses, and geologic samples. 

This property is very hard to define, as articles may be subjected to very varied forms of abrasion, and in general a given ceramic body will react quite differently to different types of abrasion. This is a question on which the manufacturer should be able to give considerable guidance. Many types of standard abrasion test have been proposed, but none has proved satisfactory and experience must continue to be the main guiding factor. 

Crockery preferably is made from ceramic materials, although it is brittle and can break rather easily. Properties of ceramics, such as resistance to absorb flavours and low heat conductivity, however, are superior compared to metals and plastics. Therefore, we defined a task to design crockery with improved mechanical strength. 

Model B was especially designed for methane conversion to ethylene [54, 55]. This reaction needs pre-heating to a defined temperature before reaction. This is achieved by ceramic heaters in the housing. In addition, the gases do not enter as a... 

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