Boria formers

As structural ceramics find more applications in high temperature systems, oxidation and corrosion at high temperatures becomes an important field of study. In this chapter, the critical issues in this field have been surveyed. Ceramics have been classified according to the type of protective oxide they form. These include silica formers, alumina formers, boria formers, and transition metal oxide formers. Most of the literature covers silica formers since there are a number of near-term applications for these materials. Basic oxidation mechanisms, water vapor interactions, volatilization routes, and salt-induced corrosion were discussed for these materials. Less information is available on alumina-forming ceramics. However the rapid oxidation rate in water vapor appears to be a major problem. Boria formers show rapid oxidation rates due to the formation of a liquid oxide film and are volatile in the presence of water vapor due to highly stable Hx-By-Oz(g) species formation. Transition metal carbides and nitrides also show rapid oxidation rates due to rapid transport in the oxide scale and cracking of that scale. 

Figure 159 shows the unusual effect that a surface silica coating has on the MW distribution of the polymer. Like boria, silica creates a second, high-MW peak. Because these polymers were made with H2 in the reactor, it is likely that the silica-affected sites lost their ability to respond to H2, and this loss caused the appearance of the high-MW peak. The addition of a small coating of silica on Cr/alumina had a different effect from the addition of Al3+ ions to Cr/silica. Both treatments increased activity, but whereas the former increased the polymer MW, the latter decreased it. Again, on Cr/Si-alumina catalysts this may reflect, not the rate of (3-hydride elimination, but the influence of hydrogenolysis on the MW distribution. 

Boron phosphorous oxide shows a high catalytic activity for 1-decane oligomerization. The optimum composition of the oxide and pretreatment temperature are P/B = 1.1 and 873 K, respectively. For the reaction, acid sites stronger than ffo— —5.6 are effective. Although the surface area of boron phosphorous oxide is less than one-tenth those of silica alumina and alumina boria, the conversion of 1-decane on the former catalyst is much higher than those on the latter catalysts. 

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