Silicon high temperature application

While unaffected by water, styrofoam is dissolved by many organic solvents and is unsuitable for high-temperature applications because its heat-distortion temperature is around 77°C. Molded styrofoam objects are produced commercially from expandable polystyrene beads, but this process does not appear attractive for laboratory applications because polyurethane foams are much easier to foam in place. However, extruded polystyrene foam is available in slabs and boards which may be sawed, carved, or sanded into desired shapes and may be cemented. It is generally undesirable to join expanded polystyrene parts with cements that contain solvents which will dissolve the plastic and thus cause collapse of the cellular structure. This excludes from use a large number of cements which contain volatile aromatic hydrocarbons, ketones, or esters. Some suitable cements are room-temperature-vulcanizing silicone rubber (see below) and solvent-free epoxy cements. When a strong bond is not necessary, polyvinyl-acetate emulsion (Elmer s Glue-All) will work. 

PEI polymers exhibit minimal creep under load. For example, unreinforced Ultem 1000 maintains part dimensions over thousands of hours at room temperature at a loading of 34 N/mm2 (4930 psi). PEI resins are available with HDTs ranging to 220°C (Ultem 5000). PEI copolymers with silicone mbber allow for flame-retardant, high temperature applications such as plenum wire coatings (Sfltem). Reinforcement of PEI resins improves their temperature performance. Table 13 compares unreinforced and 20% glass-reinforced Ultem resins. 

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). 

As noted, the oxidation resistance of silicon nitride ceramics depends on the type and concentration of the sintering aids. In materials designed for high temperature applications the specific weight gain resulting from oxidation upon a 500-h air exposure at 1200°C and 1350°C is about 1—2 g/m2 and 2—4 g/m2, respectively. The kinetics of the oxidation process have been investigated (63,64) as has the corrosion resistance (65). Corrosion resistance is also dependent on material formulation and density. 

Hot-melt thermoplastic elastomer systems (23. 24) are also effective coating materials. These materials are generally based on copolymers that are comprised of hard (crystalline or glassy) and rubbery (amorphous) segments contained in separate phases. The hard-phase regions form physical cross-links below their crystallization or vitrification temperature, and the system therefore has elastomeric properties. The moduli and low-temperature characteristics of these materials can be tailored to compare reasonably well with silicone rubbers at -40 C. However, they are limited in high-temperature applicability because of enhanced creep or flow due to softening. 

Silicides represent the transition from intermetallics with predominantly metallic bonding to non-metallic compounds since silicon is no longer a metal, but a semiconductor. Nevertheless the silicides are often comprised within the intermetallics. Silicides were selected for high-temperature applications already in the past because of their potentially high oxidation resistance at highest temperatures [89-91]. Presently... 

Although most ceramics are thermal and electrical insulators, some, such as cubic boron nitride, are good conductors of heat, and others, such as rhenium oxide, conduct electricity as well as metals. Indium tin oxide is a transparent ceramic that conducts electricity and is used to make liquid crystal calculator displays. Some ceramics are semiconductors, with conductivities that become enhanced as the temperature increases. For example, silicon carbide, SiC, is used as a semiconductor material in high temperature applications. 

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). 

Silicon carbide net-shaped ceramics are candidate materials for high-temperature applications as engine and turbine parts and as cutting tools. They are also important for the construction of reactors which show a high chemical resistance. SiC material that reveals a well defined organization of its microstructure is also a goal in the development of microelectronic devices. 

Due to its wide band gap, silicon carbide is also an interesting semiconductor for high-temperature applications [193]. In order to get monocrystalline... 

Until now, only a handful of ceramic fibers for use in CMCs, mainly fibers made from silicon carbide, have reached the market. While Ube Industries, Nippon Carbon, and, lately, Dow Corning produce SiC fibers, 3 M has developed some oxidic ceramic fibers (e.g. aluminoborosilicate, aluminosilica, or alumina) for this purpose. Since up to now for high-temperature applications only fibers made from SiC or SiBN3C are suitable, the most important features of these two classes will now be discussed. 

Hydrated montmorillonite has water between the silicate-aluminate-silicate layers. The micas (e.g., muscovite) have potassium ions in comparable positions and also have aluminum substituting for silicon in about 25% of the silicate sites. Changes in the proportions of aluminum and silicon in either of these allow the introduction of other cations and the formation of a large number of minerals. The layered structures of some micas are pronounced, allowing them to be cleaved into sheets used for high-temperature applications in which a transparent window is needed. They also have valuable insulating properties and are used in electrical devices. 

The overwhelming majority of commercial polymers are organic, that is, they are based on carbon chemistry, and they therefore possess poor heat resistance, so they can be ruled out from true high temperature applications, other than sacrificial ones such as ablative shields. But there is no reason why polymers should always be organic in future (the family of silicone rubbers provide an example of inorganic polymers). 

The protective properties of rutile are fairly low compared to those of alumina or silica. Thus, the presence of TiN or TiC limits the high-temperature applications of the above composites. At small amounts and particle size of TiN in silicon nitride ceramics, a continuous silicate film can be formed, covering TiN particles and protecting them from further oxidation [178]. 

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