Volatility silica formers

See, for example, E.J. Opila, Oxidation and volatilization of silica formers in water vapor. Journal of the American Ceramic Society 86 1238-1248,2003. 

E.J. Opila, N.S. Jacobson, D.L. Myers, and E.H. Copland, Predicting oxide stability in high-temperature water vapor, Journal of the Minerals, Metals, and Materials Society 58 22-28, 2006 I. Kvernes, M. Oliveira, and P. Kofstad, High temperature oxidation of Fe-13Cr xAl alloys in air/water vapor mixtures, Corrosion Science 17 237-52, 1977 H. Asteman, J.-E. Svensson, M. Norrell, and L.-G. Johansson, Influence of water vapor and flow rate on the high-temperature oxidation of 304L Effect of chromium oxide hydroxide evaporation. Oxidation of Metals 54 11-26,2000 J.M. Rakowski and B.A. Pint, Observations on the effect of water vapor on the elevated temperature oxidation of austenitic stainless steel foil. Proceedings of Corrosion 2000, NACE Paper 00-517, NACE International, Houston, Texas, 2000 E. Essuman, G.H. Meier, J. Zurek, M. Hansel, and W.J. Quadakkers, The effect of water vapor on selective oxidation of Fe-Cr Alloys, Oxidation of Metals 69 143-162,2008 E.J. Opda, Oxidation and volatilization of silica formers in water vapor. Journal of the American Ceramic Society 86(8) 1238-1248,2003. 

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. 

Gibson s method 4 is based on the principle that ammonium hydrogen fluoride effects the complete decomposition of beryl at a low temperature, even if the mineral is only coarsely ground. Much of the silica is volatilized as ammonium fluosilicate and the beryllium and aluminium converted first to fluorides, then sulfates. The former is separated by solution in (NH COs. 

However, it must be emphasized that there has to be some attempt to recognize the limitations of the method before any projection relating to the mineral composition of coal is possible. For example, the high temperature required for the ashing may result in the loss of the volatile constituents of the minerals or the mineral constituents will undergo a chemical change. In the former case, certain of the mineral elements will escape detection while in the latter case the constituents of clays or shale (to cite an example) will lose water of hydration or the carbonate minerals will lose carbon dioxide and the oxides so produced may even undergo further reaction with sulfur oxides or with silica to produce completely different mineral species ... 

Examples of volatile nonmetallic compounds are very familiar methane, ammonia, water. Examples of involatile compounds are silicon carbide (SiC) and silica (Si02). The former is manufactured by heating silica with graphite and is sold under the name carborundum. The commereial produet is black, but when pure it is colourless. It melts at about 2700 C. Pure silica also forms colourless crystals, melting to a colourless liquid at about 1600 °C and boiling at about 2400 C. Both compounds are insulators. Conductivity measurements have also been made on liquid silica, in which state it remains a poor conductor (cf salt-like compounds). 

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