General Ceramic Applications

The mineral zircon ZrSiO is used for moulds in foundries. Its high thermal conductivity improves the coohng rate compared to other mould materials. Zirconium oxide is used in firebricks. It has very good temperature resistance and is, after sintering, also used for crucibles and other components for work at high temperatures. When zirconia is heated, a phase transformation occurs at a temperature of about 1200°C. 

The corresponding volume change makes the use of pure zirconia impossible for many appHcations. 

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Many ceramic applications are high value and small volume, so energy expenditure is high. Ferroelectric magnets, electronic substrates, electrooptics, abrasives such as silicon carbide and diamond, are examples. Diamond is found naturally, and made synthetically by the General Electric Company at high pressure and temperature. Synthetic diamonds for abrasives require less energy to make than the value in Table 4 nevertheless, the market is carefully divided between natural and synthetic diamonds. 

Non-oxide ceramic materials such as silicon carbide has been used commercially as a membrane support material and studied as a potential membrane material. Silicon nitride has also the potential of being a ceramic membrane material. In fact, both materials have been used in other high-temperature structural ceramic applications. Oxidation resistance of these non-oxide ceramics as membrane materials for membrane reactor applications is obviously very important. The oxidation rate is related to the reactive surface area thus oxidation of porous non-oxide ceramics depends on their open porosity. The generally accepted oxidation mechanism of porous silicon nitride materials consists of two... 

Colloidal silica was used in general foundry applications such as gunning mixes, ceramic mold facings for core boxes and the like, mold washes, and semi-permanent molds with renewable facings. It was also used in the production of wallboard (Figure 4). 

Grid supports are generally less expensive than gas-injection supports and can provide open areas as high as 70 percent of column cross-sectional area in ceramic applications (305) and 95 to 97 percent in metallic applications (74). Grid supports are commonly used for structured packing, where gas-injection supports are usually unsuitable, and where most of the disadvantages listed below do not apply. 

The reason for their extreme resistance can be explained as follows. Ceramics are compounds between metallic and non-metallic elements. Corrosion products are also ceramics. Hence, ceramic may be thought of as having already been corroded [2]. Contrary to the corrosion of metals, corrosion of ceramics, if at all they corrode, involves chemical dissolution. Metals corrode by electrochemical processes. Another form of corrosion is oxidation. Oxidation takes place in an oxidizing environment. In this respect, the oxidation resistance of materials is to be considered. When we consider ceramics for application in an oxidizing environment, the nonoxide ceramics may not be that resistant. Members of the ceramic spectrum of borides, carbides, nitrides, silicides, and so on tend to get oxidized when exposed to an oxidizing atmosphere. Herein, oxide ceramics are the most stable. In general, ceramics are more stable than metals in terms of their oxidation. 

The thermal properties that are of imporfance in ceramic applications are heat capacity, thermal expansion, and thermal conductivity. Heat capacity is a measure of the heat required for changing fhe femperature. It is therefore important in the heat treatment and use of ceramics. The economy of running a furnace is decided by the amount of heaf needed to reach a particular temperature. Thermal expansion becomes important in ceramic composites. The different constituents in a composite should have close thermal expansion coefficients in order to prevent accumulation of fhermal sfresses in the composite. Thermal conductivity is an undesirable property in the case of insulators. Generally, ceramics possess low thermal conductivity. 

Veiy pure. 70 to 80 percent alumina for high temperatures. Under reciucing conditions the iron in the ceramic is controlling, as it acts as a catalyst and converts the CO to CO9 plus carbon, which results in spalling. The choice among the three t)pes of castables is generally made by economic considerations and the temperature of the application. 

A more complicated, and more effective, mechanism operates in partially stabilised zirconia (PSZ), which has general application to other ceramics. Consider the analogy of a chocolate bar. Chocolate is a brittle solid and because of this it is notch-sensitive notches are moulded into chocolate to help you break it in a fair, controlled way. Some chocolate bars have raisins and nuts in them, and they are less brittle a crack, when it... 

Those basic matrix selection factors are used as bases for comparing the four principal types of matrix materials, namely polymers, metals, carbons, and ceramics, listed in Table 7-1. Obviously, no single matrix material is best for all selection factors. However, if high temperatures and other extreme environmental conditions are not an issue, polymer-matrix materials are the most suitable constituents, and that is why so many current applications involve polymer matrices. In fact, those applications are the easiest and most straightforward for composite materials. Ceramic-matrix or carbon-matrix materials must be used in high-temperature applications or under severe environmental conditions. Metal-matrix materials are generally more suitable than polymers for moderately high-temperature applications or for modest environmental conditions other than elevated temperature. 

A different type of low friction or low drag application is encountered with sliding doors or conveyor belts sliding on support surfaces. In applications like this the normal forces are generally quite small and the friction load problems are of the sticking variety. Some plastics exhibit excellent track surfaces for this type of application. TFEs have the lowest coefficient of any solid material and represent one of the most slippery surfaces known. The major problem with TFE is that its abrasion resistance is low so that most of the applications utilize filled compositions with ceramic filler materials to improve the abrasion resistance. 

In structural applications for plastics, which generally include those in which the product has to resist substantial static and/or dynamic loads, it may appear that one of the problem design areas for many plastics is their low modulus of elasticity. The moduli of unfilled plastics are usually under 1 x 106 psi (6.9 x 103 MPa) as compared to materials such as metals and ceramics where the range is usually 10 to 40 x 106 psi (6.9 to 28 x 104 MPa). However with reinforced plastics (RPs) the high moduli of metals are reached and even surpassed as summarized in Fig. 2-6. 

The number of oxides is large since most metallic elements form stable compounds with oxygen, either as single or mixed oxides. However, the CVD of many of these materials has yet to be investigated and generally this area of CVD has lagged behind the CVD of other ceramic materials, such as metals, carbides, or nitrides. The CVD of oxides has been slower to develop than other thin-film processes, particularly in optical applications where evaporation. 

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