Silicate ceramics

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. 

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. 

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)...

Although the ceramic raw materials have to be transformed into ceramics at relatively high temperatures, particularly silicate ceramic materials require a very low energy consumption for their production (Table 1). Part of the reason for this is that natural raw materials are used, which save energy. On the other hand, the example of carbon-fibre composites shows that high-performance materials can be extremely expensive. 

In the same year, 1986, the ormocers were presented to the public. These coatings are sprayed in a thickness of nm onto window glass and darken this when the sun shines on it. In the 1990s, these nanoscale powders were also used in silicate ceramics applied to tiles, sanitaryware and tableware articles or roofing tiles, they make sure that dirt particles caimot attache themselves to the surface. Special compositions even have an anti-bacterial effect. 

Silicate ceramics were still classified according to their microstmcture, density (water absorption) and colour. Under Other silicate ceramics , refractories and cordierite were also found. 

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