Transition temperatures, silicon carbides

High Temperature. The low coefficient of thermal expansion and high thermal conductivity of silicon carbide bestow it with excellent thermal shock resistance. Combined with its outstanding corrosion resistance, it is used in heat-transfer components such as recuperator tubes, and furnace components such as thermocouple protection tubes, cmcibles, and burner components. Silicon carbide is being used for prototype automotive gas turbine engine components such as transition ducts, combustor baffles, and pilot combustor support (145). It is also being used in the fabrication of rotors, vanes, vortex, and combustor. 

In the following sections some examples are given of the ways in which these principles have been utilized. The first example is the use of these techniques for the low temperature preparation of oxide ceramics such as silica. This process can also be used to produce alumina, titanium oxide, or other metal oxides. The second example describes the conversion of organic polymers to carbon fiber, a process that was probably the inspiration for the later development of routes to a range of non-oxide ceramics. Following this are brief reviews of processes that lead to the formation of silicon carbide, silicon nitride, boron nitride, and aluminum nitride, plus an introduction to the synthesis of other ceramics such as phosphorus nitride, nitrogen-phosphorus-boron materials, and an example of a transition metal-containing ceramic material. 

Attempts are still being made to produce improved bolometers operating at or near room temperature. These include vanadium oxide metal-semiconductor transition devices [8.7], bismuth-lead layers [8.8], metallic nickel [8.8a], and aluminium [8.8b], silicon carbide [8.8c], and doped barium titanate ceramic [8.8d] elements. New designs of radiometers and power meters using thermistor bolometers have been described [8.9-11]. Microelectronic techniques have been used to produce fast but sensitive (NEP 10 WHz at 25 MHz and 100 pm wavelengths) uncooled for improved bolometers [8.11a]. 

Aluminum nitride and silicon nitride, like other refi tory carbides and nitrides, have die ability to deform plastically to some degree above the ductile-to-britde transition temperature. Below that temperature, they are intrinsically brittle (for discussion, see Sec. 4.3 of Ch. 4). 

Balat, M. et al.. Active to passive transition in the oxidation of silicon carbide at high temperature and low pressure in molecular and atomic oxygen. Journal of Materials Science, 1992. 27(3) 697-703. 

Narushima T, Goto T, Yokoyama Y, Takeuchi Y, IguchiY, Hirai T, Active-to-passive transition and bubble formation for high-temperature oxidation of chemically vapor-deposited silicon carbide in CO-CO2 atmosphere , J Am Ceram Soc, 1994 77(4) 1079-1082. 

The formation of junctions in carbon nanotubes is important for use in electronic application. The SWNTs have been shown to react with silicon and transition metals and form metal carbide nanorods and nanoparticles at high temperature under high vacuum. A heterojunction interface with metal carbide at the tips of SWNTs is possible. Silicon and metal substrates such as Ti and Nb have been used with SWNTs to form long SiC, TiC, and NbC nanorods respectively. A small number of SWNTs with partial reaction are found to have coimected to SiC nanorods (Figure 15). The formation of such a carbide heterojunctions in carbon nanotubes with... 

Such reactions are known to occur at about 2S0°C for monoaminosilanes reacted with a second type of amine [IS]. Amines incorporated in a polysilazane network require higher temperatures for the transamination because of steric hindrance and the necessity to cleave-rebuild simultaneously two Si-N bonds. Transition metal catalysts can reduce the temperature requirements. In early studies [16] of the transition metal-catalyzed polymmzation of silazanes, we observed the catalytic cleavage of Si-N bonds at temperatures below 1S0°C and the ability to metathesize silicon nitrogen bonds. The transamination reactivity occurs at temperatures below that of the carbonization and carbidization of the organic groups. 

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