High-temperature mechanism

Methane oxidations occur only by intermediate and high temperature mechanisms and have been reported not to support cool flames (104,105). However, others have reported that cool flames do occur in methane oxidation, even at temperatures >400 ° C (93,94,106,107). Since methyl radicals caimot participate in reactions 23 or 24, some other mechanism must be operative to achieve the quenching observed in methane cool flames. It has been proposed that the interaction of formaldehyde and its products with radicals decreases their concentrations and inhibits the whole oxidation process (93). 

Nickel—Copper. In the soHd state, nickel and copper form a continuous soHd solution. The nickel-rich, nickel—copper alloys are characterized by a good compromise of strength and ductihty and are resistant to corrosion and stress corrosion ia many environments, ia particular water and seawater, nonoxidizing acids, neutral and alkaline salts, and alkaUes. These alloys are weldable and are characterized by elevated and high temperature mechanical properties for certain appHcations. The copper content ia these alloys also easure improved thermal coaductivity for heat exchange. MONEL alloy 400 is a typical nickel-rich, nickel—copper alloy ia which the nickel content is ca 66 wt %. MONEL alloy K-500 is essentially alloy 400 with small additions of aluminum and titanium. Aging of alloy K-500 results in very fine y -precipitates and increased strength (see also Copper alloys). 

Dynamic mechanical analysis provides a useful technique to study the cure kinetics and high temperature mechanical properties of phenoHc resins. The volatile components of the resin do not affect the scan or limit the temperature range of the experiment. However, uncured samples must be... 

Fusion Reactors. The development of fusion reactors requires a material exhibiting high temperature mechanical strength, resistance to radiation-induced swelling and embrittlement, and compatibUity with hydrogen, lithium and various coolants. One aUoy system that shows promise in this appHcation, as weU as for steam-turbine blades and other appHcations in nonoxidizing atmospheres, is based on the composition (Fe,Co,Ni)2V (30). 

Ceramic-matrix composites are a class of materials designed for stmctural applications at elevated temperature. The response of the composites to the environment is an extremely important issue. The desired temperature range of use for many of these composites is 0.6 to 0.8 of their processing temperature. Exposure at these temperatures will be for many thousands of hours. Therefore, the composite microstmcture must be stable to both temperature and environment. Relatively few studies have been conducted on the high temperature mechanical properties and thermal and chemical stability of ceramic composite materials. 

Creep Resistsince. Studies on creep resistance of particulate reinforced composites seem to indicate that such composites are less creep resistant than are monolithic matrices. Silicon nitride reinforced with 40 vol % TiN has been found to have a higher creep rate and a reduced creep strength compared to that of unreinforced silicon nitride. Further reduction in properties have been observed with an increase in the volume fraction of particles and a decrease in the particle size (20). Similar results have been found for SiC particulate reinforced silicon nitride (64). Poor creep behavior has been attributed to the presence of glassy phases in the composite, and removal of these from the microstmcture may improve the high temperature mechanical properties (64). 

In the sintering of such materials as silicon nindde, a silica-rich liquid phase is formed which remains in the sintered body as an intra-granular glass, but this phase, while leading to consolidation, can also lead to a deterioration in the high-temperature mechanical properties. 

Baudin C, Martinez R, Pena P (1995) High temperature mechanical behaviour of stoichiometric magnesium spinel. J Am Ceram Soc 78(7) 1857-1862... 

Sammes NM and Ratnaraj R. High temperature mechanical properties of La SrjCrj y Coy03 for SOFC interconnect. J. Mater. Sci. 1995 30 4523M526. 

Although the initiation step under purely thermally induced conditions such as those imposed by shocks has not been formulated, it is expected to be a reaction that produces O atoms. The high-temperature mechanism would then be reactions (8.105), and (8.124)-(8.126), with termination by the elimination of the O atoms. 

Singh, R.N. (1993), Interfacial properties and high temperature mechanical behavior of fiber reinforced ceramic fiber reinforced ceramic composites. Mater. Sci. Eng. A 166, 185-198. 

Non-whisker SiC fibers are also of imporfance in MMCs, and they are currently available in two commercial forms Tyranno and Nicalon . As with the whisker form, the primary advantages of SiC fibers is their oxidation resistance and high-temperature mechanical property retention relative to other fibers. The high-temperature strength of three commercially available Nicalon SiC fibers is shown in Figure 5.111. 

Just as in the case of the H2-D2 exchange on ZnO, two mechanisms are also discernible for the carbon monoxide oxidation [stage (b)] on nickel oxide below 300°C. There is a low-temperature mechanism operative between 100° and 180°C. characterized by a low activation energy of 2 kcal./mole and a high-temperature mechanism, above 180°C., with a higher activation energy of 13 kcal./mole. The kinetics are different and are respectively ... 

TPX is used in mold-releasing films, printed circuit mold-releasing materials, various containers, etc., because of its heat resistance, chemical resistance, mold-releasing properties, transparency and other properties (4). In application areas, which require mechanical strength, gas barrier properties and high temperature mechanical strength, among other properties, composites are needed. 

In the last stages of hydrocarbon oxidation, by both the low and high temperature mechanism, when the oxygen concentration is low, a new phenomenon appears—the pic darret. The methodical study of the reaction of propane and oxygen at various pressures, temperatures, and concentrations indicates three different aspects of the slow oxidation. When the pic d arret occurs, the analysis of some reaction products indicates an increase in the amounts of methane, ethane, acetaldehyde, ethyl alcohol, propyl alcohol, and especially isopropyl alcohol, and a decrease in the formation of hydrogen peroxide and olefin. All these results are explained by radical reactions such as R + R02 (or H02) ROOR - 2 RO oxygenated products and R + R - RR. 

This region of negative temperature coefficient can be quantitatively ascribed to the failure of the system to produce alkyl hydroperoxide as a product at the higher temperatures. Instead, with increasing temperature because of the reversibility of Reaction 1, the equilibrium concentration of alkyl peroxy radicals decreases in favor of alkyl radicals, and the high temperature mechanism supersedes the low temperature mechanism (Reactions 4 and 5). 

Rouxel, T., High temperature mechanical behavior of silicon nitride ceramics, J. Ceram. Soc. Jap., 109(6) 89-98 (2001). 

MgO-spinel clinker is obtained by the same procedure, but with a substantial proportion of free MgO (75-85%). MgO-spinel is produced by sintering at temperatures of 1600-1800°C and a dense product is obtained. MgO-spinel refractory has better high temperature mechanical properties in arc furnace roof application compared to magnesia-chrome refractories, but it has lower strength than a high-alumina refractory. 

Owen, D.M., and Chokshi, A.H., The high temperature mechanical characteristics of superplastic 3 mol% yttria stabilized zirconia , Acta Mater, 1998, 46, 667-79. 

Jimenez-Melendo, M., and Dominguez-Rodriguez, A., High temperature mechanical characteristics of superplastic yttria stabilized zirconia. An examination of the flow process , Acta Mater, 2000, 48, 3201-10. 

Poorteman, M., Descamps, P., Cambier, F., Plisnier, M., Canonne, V., Descamps, J.C., Silicon nitride/silicon carbide nanocomposite obtained by nitridation of SiC fabrication and high temperature mechanical properties, J. Eur. Ceram. Soc., 23, 2003, 2361-2366. 

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