High-temperature applications, ceramics

C. A. Lewinsohn, M. Singh, and C. H. Henager, Jr., Brazeless Approaches to Joining of Silicon Carbide-based Ceramics for High Temperature Applications, Ceram. Trans., 138,201 -12 (2003). 

Ceramics Ceramic microfilters for commercial applications are almost always employed as tube-side feed multitube monoliths. They are also available as flat sheet, single tubes, discs, and other forms primarily suited to lab use. They are used for a few high-temperature applications, in contact with solvents, and particularly at very high pH. 

This competition between mechanisms is conveniently summarised on Deformation Mechanism Diagrams (Figs. 19.5 and 19.6). They show the range of stress and temperature (Fig. 19.5) or of strain-rate and stress (Fig. 19.6) in which we expect to find each sort of creep (they also show where plastic yielding occurs, and where deformation is simply elastic). Diagrams like these are available for many metals and ceramics, and are a useful summary of creep behaviour, helpful in selecting a material for high-temperature applications. 

Carbide-based cermets have particles of carbides of tungsten, chromium, and titanium. Tungsten carbide in a cobalt matrix is used in machine parts requiring very high hardness such as wire-drawing dies, valves, etc. Chromium carbide in a cobalt matrix has high corrosion and abrasion resistance it also has a coefficient of thermal expansion close to that of steel, so is well-suited for use in valves. Titanium carbide in either a nickel or a cobalt matrix is often used in high-temperature applications such as turbine parts. Cermets are also used as nuclear reactor fuel elements and control rods. Fuel elements can be uranium oxide particles in stainless steel ceramic, whereas boron carbide in stainless steel is used for control rods. 

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. 

Another area of success has been in applied materials research. Because of the integral nature of materials to advances in energy production and consumption, the laboratories have developed a number of toughened ceramics. When used as a replacement for steel, they will improve the energy performance characteristics of high-temperature applications for components of combined-cycle power plants and vehicle engines. 

Lithium compounds are used in ceramics, lubricants, and medicine. Small daily doses of lithium carbonate are an effective treatment for bipolar (manic-depressive) disorder but scientists still do not fully understand why. Lithium soaps—the lithium salts of long-chain carboxylic acids—are used as thickeners in lubricating greases for high-temperature applications because they have higher melting points than more conventional sodium and potassium soaps. 

Schafer W and Schmidberger R. Ca and Sr doped LaCr03 Preparation, properties, and high temperature applications. In Vincenzini P, editor. High Tech Ceramics. Amsterdam, the Netherlands Elsevier Science Publishers, 1987 1737-1742. 

Service temperature limitations must be considered in the use of composites, not only in the selection of polymer and process, but sometimes in the selection of the reinforcement as well. Composites cannot generally perform as well as metals or ceramics in very high temperature applications, but they can be made fire resistant to meet most construction and transportation codes. 

Recent research has explored a wide variety of filler-matrix combinations for ceramic composites. For example, scientists at the Japan Atomic Energy Research Institute have been studying a composite made of silicon carbide fibers embedded in a silicon carbide matrix for use in high-temperature applications, such as spacecraft components and nuclear fusion facilities. Other composites that have been tested include silicon nitride reinforcements embedded in silicon carbide matrix, carbon fibers in boron nitride matrix, silicon nitride in boron nitride, and silicon nitride in titanium nitride. Researchers are also testing other, less common filler and matrix materials in the development of new composites. These include titanium carbide (TiC), titanium boride (TiB2), chromium boride (CrB), zirconium oxide (Zr02), and lanthanum phosphate (LaP04). 

Due to the potential high-temperature application of nanocomposites, as well as the fact that metal-reinforced ceramic nanocomposites combine metal and non-metal phases in equilibrium, it is important to understand the oxidation resistance of such materials. Using the Ni-alumina system as an example, and following Sekino et al.,12 the partial pressure of oxygen required to prevent the formation of nickel spinel (NiAl204) from a two-phase mixture of Ni and A1203 can be described as 58,59... 

In the high-temperature range, ceramics also offer advantages. Interesting work has been performed by Wang et al. [69] on a ceramic micro reactor made of ceramic tapes, which might well be a future option depending on the application. 

Epoxy-phenolic adhesives were developed primarily for bonding metal joints in high-temperature applications. Their first major application was to join major aircraft components. They are also commonly used for bonding glass, ceramics, and phenolic composites. Because of their relatively good flow properties, epoxy phenolics are also used for bonding honeycomb sandwich composites. 

Prospects of Developing Fiber-Reinforced Ceramics for High Temperature Applications... 

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