The high temperature mechanism

Above ca. 400—450 °C abstraction of a hydrogen atom from alkyl radicals by oxygen to yield the conjugate alkene and hydroperoxy radical 

At these temperatures, the hydrogen peroxide so formed suffers appreciable homogeneous decomposition to give to OH radicals [153] 

In addition, in the temperature range 450—650 °C pyrolysis will begin to compete effectively with the oxidation of alkyl radicals [38]. 

Pyrolysis of ethyl radicals was found to be relatively unimportant although the reverse is true for larger alkyl radicals such as n-propyl, isobutyl and tert-butyl. Indeed, Sampson estimated that almost half the isobutane consumed gives radicals whose fate is pyrolysis [155]. On the other hand, radical—radical reactions are important and many of the minor products are believed to be formed from further reactions of ethoxy radicals formed in reaction (52) 

It is extremely difficult from direct studies of hydrocarbon oxidation to unravel the contributions made to the overall mechanism at these temperatures by the additive, abstractive and pyrolysis routes. Baldwin, Walker and their co-workers have recently developed a new technique, however, which has overcome this problem to some extent and has also yielded quantitative Arrhenius parameters for many elementary reactions [108,157—161]. This technique utilizes the fact that the kinetics of slowly reacting hydrogen and oxygen mixtures in aged boric acid coated 

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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. 

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. 

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). 

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. 

The mechanism of conversion, which is exothermal in the direction o -> p, involves dissociation and recombination, during which the nuclear spins re-couple, parallel or anti-parallel, in equilibrium proportions. This occurs, for example, on collision (the high temperature mechanism) and probably in chemisorption (Eley and Rossington, 1957), when the atoms are separated by going into different lattice sites and subsequently recombine with nuclear spins oppositely coupled. 

The DB-procedure was optimised in respect with the kinetic requirements and the high-temperature mechanical properties of the Ni-superalloy. From the kinetic point of view, the bonding temperature should be over 1000°C when alumina and transition metals are directly bonded [6]. The bonding procedure was always carried out in high vacuum, better than 2-10 mbar (0.2 mPa). The typical thermal and axial compression cycles are presented in Fig.la. It was experimentally found that the ambient bonding temperature is 1100"C or less due to the fast creep of the superalloy beyond this. The compression for the tests was selected as 10 MPa in ceramic-metal joints and 20 MPa in ceramic-ceramic joints [6]. 

This paper outlines the effect of composition on oxynitride glass formation and properties that crucially affect the high temperature mechanical behaviour of silicon nitride based ceramics. 

The shock tube investigations of the hydrogen-oxygen reaction described in section 2.2 have revealed the significant qualitative features of the high-temperature mechanism considered above, viz. exponential growth of intermediate and product concentrations, the induction... 

The effect of the addition of zirconia on the high-temperature mechanical behaviour is to delay the onset of plasticity to 1100°C and to decrease the strain rates in creep (Pysher and Tressler, 1992 Lavaste et al., 1995). The mechanisms proposed have been the pinning of the grain boundaries by the intergranular zirconia particles and more recently the modification of the AP+ diffusion rates at the alumina/alumina grain boundaries by the presence of Zr + and Y + ions. However, these mechanisms are less... 

Differences of the high-temperature mechanical behavior of these materials are shown in terms of stress-rupture curves in Fig. 3.1-122. 

The thermal curve in Figure 15.4 shows the flexural storage modulus and loss properties of a rigid, pultruded oriented-fibre-reinforced vinyl ester composite. Since the flexural modulus and Tg increase dramatically with post-curing, the test can be used to evaluate the degree of cure, as well as to identify the high-temperature mechanical integrity of the composite. 

The maximum temperature at which iron aluminides have sufficient strength for structural application (600-650 C) is significantly lower than temperatures at which they have maximum resistance to corrosion (>900 °C). One approach to boosting the high-temperature mechanical properties has been the development of oxide dispersion-... 

Both perfluoroethylvinyl ether (PEVE) and per-fluoropropylvinyl ether (PPVE) are effective in improving the high temperature mechanical properties of TFE/ HFP polymers. Perfluoromethylvinyl ether (PMVE) does not produce such improvements due to the small size of its pendent ether group that cannot efficiently disrupt the crystallization of the polymer chain (Table 5.6). 

Durand F, Rouby D, Fantozzi G, Allard B, Dumas D. Characterization of the high-temperature mechanical behavior of carbon materials. Carbon. 1994 32(3) 857-65. 

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