Shock resistance

It also has potential use in ceramic and glass formulas, as the oxide has a high melting point and imparts shock resistance and low expansion characteristics to glass. 

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. 

Eoamed polystyrene sheet has exceUent strength, thermal resistance, formabUity, and shock resistance, as weU as low density. It is widely known for its use in beverage cups, food containers, building insulation panels, and shock absorbent packaging. Polystyrene products can be recycled if suitable coUection methods are estabUshed. Eoamed polystyrene sheet can also be easily therm oformed (see Styrene plastics). 

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. 

The interelectrode insulators, an integral part of the electrode wall stmcture, are required to stand off interelectrode voltages and resist attack by slag. Well cooled, by contact with neighboring copper electrodes, thin insulators have proven to be very effective, particularly those made of alumina or boron nitride. Alumina is cheaper and also provides good anchoring points for the slag layer. Boron nitride has superior thermal conductivity and thermal shock resistance. 

Because of its high modulus of elasticity, molybdenum is used in machine-tool accessories such as boring bars and grinding quills. Molybdenum metal also has good thermal-shock resistance because of its low coefficient of thermal expansion combined with high thermal conductivity. This combination accounts for its use in casting dies and in some electrical and electronic appHcations. 

Materials and Reactions. Candle systems vary in mechanical design and shape but contain the same genetic components (Fig. 1). The candle mass contains a cone of material high in iron which initiates reaction of the soHd chlorate composite. Reaction of the cone material is started by a flash powder train fired by a spring-actuated hammer against a primer. An electrically heated wire has also been used. The candle is wrapped in insulation and held in an outer housing that is equipped with a gas exit port and rehef valve. Other elements of the assembly include gas-conditioning filters and chemicals and supports for vibration and shock resistance (4). 

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). 

Abrasives. Abrasive materials are either bonded or coated. Bonded phenoHc abrasives have superior strength and shock resistance compared to sintered ceramic compositions. The higher stabiUty permits higher rotational speeds for resin-binder wheels however, temperatures are lower than with ceramic wheels. 

There are important figures of merit (5) that describe the performance of a photodetector. These are responsivity, noise, noise equivalent power, detectivity, and response time (2,6). However, there are several related parameters of measurement, eg, temperature of operation, bias power, spectral response, background photon flux, noise spectra, impedance, and linearity. Operational concerns include detector-element size, uniformity of response, array density, reflabiUty, cooling time, radiation tolerance, vibration and shock resistance, shelf life, availabiUty of arrays, and cost. 

Sihcon nitride has good strength retention at high temperature and is the most oxidation resistant nitride. Boron nitride [10043-11 -5] has excellent thermal shock resistance and is in many ways similar to graphite, except that it is not an electrical conductor. 

The resistance against thermal spalling of fireclay and high alumina brick is indicated in Table 5. No standard test has been adopted for basic brick. Refractories composed of 100% magnesia exhibit poor thermal shock resistance, which is improved by addition of chrome ore. So-called direct bonded basic brick, composed of magnesia and chrome additions, exhibits good thermal shock resistance. 

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. 

Cordierite [12182-53-5] Mg Al Si O g, is a ceramic made from talc (25%), kaolin (65%), and Al O (10%). It has the lowest thermal expansion coefficient of any commercial ceramic and thus tremendous thermal shock resistance. It has traditionally been used for kiln furniture and mote recently for automotive exhaust catalyst substrates. In the latter, the cordierite taw materials ate mixed as a wet paste, extmded into the honeycomb shape, then dried and fired. The finished part is coated with transition-metal catalysts in a separate process. 

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