Cracking silica 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. 

The most widely used conventional chemical methods are pyrolysis [21-25] and catalytic cracking [13, 26-30], The latter yields products with a smaller range of carbon numbers and of a higher quality than products generated by the former method. Several types of solid acid catalysts, which are known to be effective for catalytic cracking (e.g. HZSM-5, HY and rare earth metal-exchanged Y-type (REY) zeolite and silica-alumina (SA)) were evaluated by catalyst screening tests and are listed in Table 6.1. The acidic... 

A variety of rocks may cause expansion and crack formation in concrete if used as aggregate in combination with Portland cement that contains amounts of alkalis considered acceptable for normal use. Such alkali-aggregate reactivity occurs in two forms as alkali-silica or alkali-silicate reactivity, and as alkah-carbonate reactivity, of which the former is much more common. 

Certain drying conditions produce gel particles of particular shapes. As a thin layer of gel dries, the shrinkage causes it to crack usually into scales or ribbons. A fibrous form of gel 5-25 microns wide and several inches long is obtained by drying a film of concentrated silica sol on an inert surface. The gel cracks into parallel ribbons especially when drying progresses along the surface in one direction (269). "Microballoons" or discrete hollow spherical particles are formed when water solutions of SiO, are spray-dried with a small amount of gas-former such as ammonium carbonate to inflate the droplets (270). 

Improvements in chemical processes are very often based on the discovery or development of new catalysts or adsorbents. One particularly exciting example in the field of zeolite catalysis is the replacement of the formerly used amorphous silica-aliunina catalysts in fluid catalytic cracking (FCC) of vacuiun gas oil by rare earth exchanged X-type zeoUtes [1]. This resulted in considerably improved yields of the desired gasoUne and, hence, a much more efficient utilization of the crude oil. Fiuther examples are the introduction of zeolite HZSM-5 as catalyst in the synthesis of ethylbenzene from benzene and ethylene [2], for xylene isomerization [3] and for the conversion of methanol to high-... 

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