Growth rates, alumina/silica

Important features of the selective oxidation process are shown schematically in Figure 1. The slow growth rates of alumina and silica, illustrated in the plot of parabolic rate constants versus temperature at lower right, makes the formation of one of these oxides as a continuous surface layer necessary for long term oxidation protection. This requires that the protective oxide be more stable thermodynamically than the more rapidly growing oxides. The plot of standard free energy of formation as a function of temperature at lower left shows that the Ni-Al system satisfies this condition. Alumina is stable, relative to NiO, even when the activity of aluminum in the alloy is very low. However, when the Al concentration is low the alumina forms as internal oxide precipitates and is non-protective allowing an external layer of NiO to form (illustrated in the cartoon at top). Therefore, a critical concentration of Al exists above which out-... 

Accelereated Oxidation - the alumina or silica is not continuous and significant amounts of the other component(s) of the intermetallic are present in the surface film. The overall oxidation rate is substantially faster than that for the growth of alumina or silica. 

In studies on the growth of Ti02 on silica and alumina from titanium isopropoxide and air, Lindbald et al. [37] found the growth rates to be similar on the two supports. The reaction temperatures were 160 °C on silica and 100 °C on... 

The reactions involved in the preparation of synthetic zeolites of the type described above are relatively complex. The silicate and the aluminum salts form their respective hydroxides at rates dependii on pH and concentration. There is no reason to assume a coprecipitation in the sense of an intermolecular mixing of the components. A more tenable concept is that-of micellar growth and intermicellar linkages which lead to gel formation, as proposed by Flank (28). Evidently, the alumina micelles or particles are sufficiently influenced by the surrounding silica So that they behave as an acid and thus combine with the positive ions present, in this case sodium ion, to give a ratio of one sodium ion to one aluminum ion as long as the amounts of alumina do not exceed 30 per cent by weight of the silica-alumina. 

The crystallinity of alumina formed by DMO infiltration into SiC particulate preforms follows the expected behavior of heterogeneous nucleation on SiC particles and large monocrystalline regions with [0001] parallel to the growth direction within the larger voids [83,86]. Manor et al. [82] found that the infiltration rate increased as the SiC particle size decreased. However, no systematic trend was found in another work [83], most probably due to widely dilferent silicon concentrations in the percolation channels resulting from reduction of the silica layer during infiltration. 

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