Ceramic powders silicate

For high-temperature applications, sauereisen cement (Omega CC cement) and zinc oxychloride (dental cement) are useful irreversible cements. Sauereisen cement is made by suspending ceramic powders in sodium silicate solution ( water glass ). This cement sets very hard and withstands temperatures up to 1000°C. Zinc oxychloride is made by mixing calcined zinc oxide powder with concentrated zinc chloride solution. One can also use a ceramic putty (Omega CC high-temperature cement), which must be cured at 180°C and is then serviceable up to 850°C. 

The classes of ceramic powder are divided into two categories (silicates and nonsilicates) and are discussed separately in the following sections. 

Chemically bonded ceramics constitute ceramics that are being formed due to chemical reactions. Often the precursor material is a ceramic powder (e.g. Ca-silicate or Ca-aluminate), which is "activated in a water-based liquid. A chemical reaction takes place in which the initial powder is partly or completely dissolved and new phases precipitate. The precipitated phases are composed of species from both the liquid and the precursor powder. The precipitates can be formed in situ in vivo, often in the nanoscale due to low solubility of the phases formed. The nanostructural chemically bonded bioceramics are especially found within the Ca-phosphate, Ca-aluminate and Ca-silicate systems. 

It must be noted that the reaction mechanisms in glass/glass-ceramic powders are usually of a very complex nature. Jacquin and Tomozawa (1995) addressed the complex sintering process and the various ways of controlling it in the fabrication of lithium silicate glass-ceramics. 

The interatomic bonds brought into play in silicate ceramics are typically iono-covalent (Si02 exhibiting a fine compromise, because its bonds are regarded as 50% ionic and 50% covalent), therefore these ceramics are almost always electrical insulators. The accentuation of the ionic nature yields hydrolysable compounds halides can be regarded as ceramic compounds, but the salt-marsh workers are not classified among the producers of ceramic powders ... 

Ceramic powders were obtained from a wide variety of sources and included alumina, hydroxyapatite, and lithium aluminum silicate. This latter sample was also used after surface modification with a silane coupling agent. Ceramic powders were also obtained by chloroform extraction of two proprietary tooth restorative composite materials viz. Estilux (Kulzer and Fotofil (Johnson and Johnson). Particle sizes and shapes were noted. 

The uses in the glass and ceramics industries reflect the diagonal relation between boron and silicon and the similarity of vitreous borate and silicate networks (pp. 203, 206 and 347). In the UK and continental Europe (but not in the USA or Japan) sodium perborate (p. 206) is a major constituent of washing powders since it hydrolyses to H2O2 and acts as a bleaching agent in very hot water ( 90°C) in the USA domestic washing machines rarely operate above 70°, at which temperature perborates are ineffective as bleaches. 

Ceramic materials are typically noncrystalline inorganic oxides prepared by heat-treatment of a powder and have a network structure. They include many silicate minerals, such as quartz (silicon dioxide, which has the empirical formula SiO,), and high-temperature superconductors (Box 5.2). Ceramic materials have great strength and stability, because covalent bonds must be broken to cause any deformation in the crystal. As a result, ceramic materials under physical stress tend to shatter rather than bend. Section 14.22 contains further information on the properties of ceramic materials. 

Silica, or silicon dioxide, occurs in various forms including chalcedony, which is a decorative material chert, which is used in abrasives flint, which is used in abrasives and ceramics jasper, which is used for decorative purposes quartz, which is a constituent of sand tripoli, which is found in scouring powders, polishers, and fillers cristobalite, which is used in high temperature casting and specialty ceramics diatomaceous earth, which is used in filtration processes and as a filler and finally, silica gel, which is used in dehydrating and drying. Note, however, that the material of concern is silica, and not silicates, which are relatively harmless derivatives of silica, nor silicones, synthetic materials used especially as lubricants. Neither silicates nor silicones cause proliferative conditions. 

Because calcium oxide is a fairly reactive powder, it forms calcium hydroxide when in contact with water. This reaction is exothermic and hence heats water during formation of the hydroxide. Because of this excess heat, it cannot directly be used to form phosphate ceramics by reacting it with an acid phosphate solution and must be used in a less soluble form as sparsely soluble silicate or hydrophosphate. In spite of this difficulty, because human bones contain calcium phosphate, there have been sufficient efforts in developing methods of forming biocompatible CBPCs of calcium phosphate by using partially soluble phosphates of calcium rather than using oxide itself. A similar approach may also be taken if one uses partially soluble silicate or aluminate of calcium. These routes are discussed in Chapter 13. 

The extraction of Be from its ore is attended by exposure to acid salts of the metal, particularly the fluoride (Bep2), the ammonium fluoride and sulfate (BeS04), and also to ber rllium oxide (BeO), and hydroxide [Be(OH)2], Exposure to the oxide also occurs in the casting of beryllium alloys and in operations with beryUia ceramics. In the manufacture of fluorescent powders, lamps, and sign tubes, there may be exposure to beryllium carbonate and to more complex salts, such as ZnMnBe silicate. Exposure to beryllium compounds encountered in the extraction of the metal... 

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