Silicate ceramics Silicates

Hydrogen fluoride reacts witlr metal carbonates, oxides, and hydroxides. Accumulation of these fluoride compounds can render valves and other close-fitting moving parts inoperable in a process system, causing possible equipment or process failures. Hydrogen fluoride also attacks glass, silicate ceramics, leather, natural rubber, and wood, but does not promote their combustion. 

Hydrofluoric acid must not be stored in glass or silicate ceramic containers. Stainless steel or PTFE are satisfactory for storage and handling. Nonferrous materials such as carbon, bronze, Monel or lead may be used for weaker acids. 

Refractive index values vary from 1.0003 for air to over 2.7 for some solid oxide ceramics. Silicate glasses have a much narrower range of valnes, from abont 1.5 to 1.9. The refractive indices (or indexes) of other materials can be fonnd in Appendix 9. 

The siliceous ceramics are produced from naturally occurring minerals that have been purified to only a limited extent, for instance by washing out soluble impurities or removing iron-containing contaminants magnetically. The purer oxides used in capacitors require more elaborate processing. The discussion starts with insulating components based on natural minerals. 

Steatite is a magnesium silicate ceramic body used as insulation in high- frequency equipment. The data given are abstracted from the paper by Norton (reference 2). 

From a scientific viewpoint, calling all room-temperature-setting materials as cements is a misnomer. Highly crystalline structures, such as phosphate ceramics, are synthesized by chemical reaction at room temperature. They are ceramics because of their crystaHine structure, while they are cements because they are formed at room temperature. We would classify such materials as CBCs. If silicates are used to form them, they will be called chemically bonded silicate ceramics. When phosphates are used to form them, they are chemically bonded phosphate ceramics (CBPCs). By using the acronyms CBC and CBPC, we avoid the debate over the words cements and ceramics as the last letter C will stand for either of them. 

A.S. Wagh and S.Y. Jeong, Chemically bonded phospho-silicate ceramics, U.S. Patent No. 6,518,212, 2003. P. Sivaprasad, K. Ramesh, and Y.P. Reddy, Optical absorption spectrum of nickel doped MgKP04-6H20, Solid State Commun., 73 [3] (1990) 239-241. 

Table I represents the weight variation in each experimental step and dehumidification efficiencies of the adsorbent-impregnated ceramic sheets. The interesting thing was that the dehumidification efficiency increased in a sequence of silica, magnesium silicate, aluminum silicate and titanium silicate-impregnated ceramic sheets. It is evident that the incorporation of aluminum and titanium ions improved the dehumidification efficiency of silica gel-based adsorbent, but the incorporation of Mg ions doesn t affect the dehumidification efficiency of silica gel-based adsorbent.

Figure 3 represents the N2 adsorption-desorption isotherms of the silica, magnesium silicate, aluminum silicate, titanium silicate-impregnated ceramic sheets. In all the adsorbent-embedded ceramic sheets, there were the well-developed micropores which gave the isotherms to the hysteresis phenomena at the relative pressures from 0.4 to 0.8. Also, it was shown that the incorporation of Ti and Al ions have the considerable volume of adsorbed N2, compared with the pure silica and magnesium silicate-impregnated ceramic sheets. 

Fig. 3. N adsorption-desorption isotherms of silica, magnesium silicate, aluminum silicate and titanium silicate-impregnated ceramic sheets.

One of the very important processes for the formation of glasses and ceramics is the sol-gel process. The first publications, by Ebetmen, appeared in 1844. The focus until today has been on the preparation of silicate ceramics. Mostly alcoxysilanes such as Si(OR)4 (R = CHj, CH2CH3,. ..) or RSi(OR)3 are used as molecular precursors. Remarkably, sol-gel processing allows for the formation of ceramic bulk materials, fibers, and coatings. ... 

The silicate ceramics include materials that vary widely in composition, structure, and use. They range from simple earthenware bricks and pottery to cement, fine porcelain, and glass. Their structural strength is based on the same linking of silicate ion tetrahedra that gives structure to silicate minerals in nature. 

Silicate ceramics are well suited for structural applications because of their strength, which originates in the partially ionic, strong silicon-oxygen bonds in the tetrahedral orthosilicate anion. This structural unit appears in naturally occurring minerals and clays, which are fashioned into ceramic pieces through sintering and densification processes. 

Chemical composition sihca alumina ceramic, aUcah alumino silicate ceramic Si02 - 55-65%, AI2O3 25-38%, FejOj - 0.5-5%... 

Fig. 6.7-S Scanning electron micrograph of silicate ceramic granules produced in an Evactherm preparation plant (courtesy Eirich, Hardheim, Germany)...

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