Low-Temperature Mechanisms

Common alloying elements include nickel to improve low temperature mechanical properties chromium, molybdenum, and vanadium to improve elevated-temperature properties and silicon to improve properties at ordinary temperatures. Low alloy steels ate not used where corrosion is a prime factor and are usually considered separately from stainless steels. 

Structural Properties at Low Temperatures It is most convenient to classify metals by their lattice symmetiy for low temperature mechanical properties considerations. The face-centered-cubic (fee) metals and their alloys are most often used in the construc tion of cryogenic equipment. Al, Cu Ni, their alloys, and the austenitic stainless steels of the 18-8 type are fee and do not exhibit an impact duc tile-to-brittle transition at low temperatures. As a general nile, the mechanical properties of these metals with the exception of 2024-T4 aluminum, improve as the temperature is reduced. Since annealing of these metals and alloys can affect both the ultimate and yield strengths, care must be exercised under these conditions. 

Stem, L.A. Kirby, S.H. Durham, W.B. (1998). Polycrystalline Methane Hydrate Synthesis from Superheated Ice, and Low-Temperature Mechanical Properties. Energy and Fuels, 12 (2), 201 -211. 

Table V. Effect of 6-Days Exposure of JANAF Bars to 90% RH at 77°F. on Low Temperature Mechanical Properties" of Two Polyurethane Binders Filled with 75% NH4CIO4 at —40°F. and with c = 0.74 min. 1...

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 ... 

Carbon dioxide was found to have no effect on the rate, r, in the higher temperature range. Similar information is missing for the low-temperature mechanism. 

The main features of this investigation are the following. First, a "deactivation process similar to that observed on the pure nickel oxide was found on the modified catalysis as well, with the same logarithmic law to represent its evolution with time. Second, the kinetic equations which were found to fit the data on pure nickel oxide also apply to the modified catalysts. Thus there is a low-temperature mechanism operative between 100° and 180°C. For all the samples assembled in Table II, the activation energies were practically the same, about 2 kcal./mole and essentially equal to the value for pure nickel oxide. This indicates that, for this particular mechanism of the reaction, the added ions and the semiconductivity changes do not affect directly the catalytic process. 

Methyl ethyl ketone is unique, in that long and irreproducible induction periods were observed on occasion, reaction ensued only after 7 hours and then was completed within 10 minutes. During the long induction period the only detectable product was methanol. No convincing reason can be advanced to account for this anomalous behavior. The virtual absence of ethylene from the products of the low temperature slow combustion of methyl ethyl ketone strongly suggests that the low-temperature mechanism proceeds almost exclusively by further oxidation of the radicals produced by hydrogen abstraction from the parent ketone. 

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). 

The C2 chemistry is more complex than the Ci chemistry, and it is less well examined. As was the case for methane, the C2 hydrocarbons are oxidized through different pathways at low and high temperatures. The low-temperature mechanism is even more complex than that of methane, and is not discussed in detail here. It shares characteristics both with the methane low-temperature mechanism and that for higher hydrocarbons, which we discuss in general terms in Section 14.3.3. ... 

The OOQOOH radical may isomerize further, similar to the reactions of RO2. The isomer-ized product decomposes into a ketohydroperoxide5 species and one OH radical. The keto-hydroperoxide is fairly stable below about 800 K, but at higher temperatures it decomposes to yield two additional radicals [426]. Thus it is not until this final decomposition step of the ketohydroperoxide that chain branching is finally achieved in the low-temperature mechanism, yielding three radicals from the initial peroxide radical. 

McGrath, T. E., Chan, W. G., and Hajaligol, R. (2003). Low temperature mechanism for the formation of polycyclic aromatic hydrocarbons from the pyrolysis of cellulose. J. Anal. Appl. Pyrolysis 66,51-70. 

Low-temperature mechanical properties in the series of block copolymers having either one or two phases appear essentially unaltered from those of the homopolymers. Intermediate relaxations may appear in these systems when two phases are present. 

Defects provide the dominant recombination path when their density is above about 10 cm" or when the temperature is higher than about 100 K. The recombination mechanism depends on the temperature and on the mobility of the carrier. The low temperature mechanism is discussed first. 

The relative rates of the high and low temperature mechanisms for propane and n-pentane... 

Considerable modification of the low temperature mechanism is necessary in order to explain observations made at higher temperatures. In competitive pairs of elementary steps, the reaction with the higher activation energy is progressively favoured as the temperature is increased. Decomposition processes become more important, and the high temperature oxidations show enhanced CO yields because of acyl radical decomposition [105]... 

The slow combustion [93] is measurable at 380 °C, but there is no low temperature mechanism, nor have cool flames been observed [45]. At 560 °C, in a flow system, mixtures of air and methyl formate ignite with explosive violence [47(a)]. The preflame reaction produces methane and methanol. 

Propyl formate is fairly readily oxidized, and there are apparently both high temperature and low temperature mechanisms [92, 93]. 

There is little evidence for a low temperature mechanism in the combustion of this fuel, although Fish and Waris [95] have demonstrated that below 350 °C acetaldehyde and organic peroxides are formed, while above 400 °C the main products Eire acetic and formic acids. Cool flames have not been observed [45]. 

The combustion of this ester is unusual in that it yields considerable amounts of hydrogen peroxide [97]. Other products include methyl acrylate, carbon monoxide and dioxide, methanol and formaldehyde. There is no low temperature mechanism, and no cool flames have been observed. 

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