Thermal properties shock resistance

Non-reactive diluents. These diluents are low-viscosity materials which do not have any reactive sites and thus do not react with the epoxy systems. These diluents generally impart flexibility and improve the impact resistance, giving better thermal mechanical shock resistance. However, there is a sacrifice in physical strength, chemical resistance and high-temperature performance. Addition of 30 parts non-reactive diluent to 100 parts epoxy resin usually does not affect the physical properties of the system. Commonly used non-reactive diluents are nonyl phenol, furfuryl alcohol and dibutyl phthalate. 

SiHcon nitride (see Nitrides) is a key material for stmctural ceramic appHcations in environments of high mechanical and thermal stress such as in vehicular propulsion engines. Properties which make this material uniquely suitable are high mechanical strength at room and elevated temperatures, good oxidation and creep resistance at high temperatures, high thermal shock resistance, exceUent abrasion and corrosion resistance, low density, and, consequently, a low moment of inertia. Additionally, siHcon nitride is made from abundant raw materials. 

Thermal Properties. Many commercial glass-ceramics have capitalized on thek superior thermal properties, particularly low or zero thermal expansion coupled with high thermal stabiUty and thermal shock resistance properties that are not readily achievable in glasses or ceramics. Linear thermal expansion coefficients ranging from —60 to 200 x 10 j° C can be obtained. Near-zero expansion materials are used in apphcations such as telescope mirror blanks, cookware, and stove cooktops, while high expansion frits are used for sealing metals. 

Sihcones (qv) have an advantage over organic resias ia their superior thermal stabiUty and low dielectric constants. Polyurethanes, when cured, are tough and possess outstanding abrasion and thermal shock resistance. They also have favorable electrical properties and good adhesion to most surfaces. However, polyurethanes are extremely sensitive to and can degrade after prolonged contact with moisture as a result, they are not as commonly used as epoxies and sihcones (see Urethane polymers). 

The properties of high quaUty vitreous sihca that determine its uses iaclude high chemical resistance, low coefficient of thermal expansion (5.5 X 10 /° C), high thermal shock resistance, high electrical resistivity, and high optical transmission, especially ia the ultraviolet. Bulk vitreous sihca is difficult to work because of the absence of network-modifyiag ions present ia common glass formulations. An extensive review of the properties and stmcture of vitreous sihca is available (72). 

Vitreous sihca has many exceptional properties. Most are the expected result of vitreous sihca being an extremely pure and strongly bonded glass. Inert to most common chemical agents, it has a high softening point, low thermal expansion, exceUent thermal shock resistance, and an exceUent optical transmission over a wide spectmm. Compared to other technical glasses, vitreous sihca is one of the best thermal and electrical insulators and has one of the lowest indexes of refraction. 

MiscelDneous. Other important properties are resistance to thermal shock, attack by slag, and, in the case of refractories (qv), thermal expansion. For whiteware, translucency, acceptance of glazes, etc, may be extremely important. These properties depend on the clay mineral composition, the method of manufacture and impurity content. 

Porcelain enamels meet a variety of performance characteristics required for different appHcations. The common characteristics of all enamels include good adherence to the substrate and good thermal expansion fit to the metal. Specific properties depend on usage for example, acid and alkaH resistance, hot water resistance, abrasion resistance, thermal shock resistance, high gloss, high reflectance, specific color, heat resistance, and cleanabihty. 

Many of these properties depend upon others which may themselves be governed by yet other factors. Thus, as mentioned above, increased porosity usually gives better thermal shock resistance, but it may be necessary for reasons of watertightness to employ a body with a very low porosity. The size of an article is also closely related to the degree of thermal shock which it will withstand. For this reason it is very difficult to give accurate figures for the thermal shock resistance of stoneware bodies. In practice, if precautions are taken to heat up any stoneware articles slowly and evenly no trouble will be experienced. This is a matter on which the ceramic manufacturer should be consulted. 

On the other hand, following the development of hybrid combustor configurations that prevent operation of the catalyst module at temperatures above 900-1000 °C, the major drawback of metallic monoliths, namely the limited maximum operating temperature, has been overcome. Accordingly, honeycombs made of metal foils have been adopted in GT catalytic combustors in view of their excellent thermal shock resistance and thermal conductivity properties [9]. In addition, metallic substrates are a promising option for the fabrication of microcombustors. 

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