Oxidation of ceramics

The oxidation of ceramic composites often obeys parabolic rate kinetics, implying diffusion control. Consider again the oxidation of alumina/SiC composites, one of the few ceramic composite materials for which oxidation data is available. Typical plots of (mass gain/area)2 are shown in Fig. 8.6 for alumina reinforced with different volume fractions of SiC. The parabolic rate constants can be defined by the relation... 

K. L. Luthra, Oxidation of Ceramic Composites, in Corrosion and Corrosive Degradation of Ceramics, eds. R. E. Tressler and M. McNallan, Ceramic Transactions, Vol. 10, 1990, American Ceramic Society, Westerville, OH, p. 183. 

Oxidation of Ceramics Forming Multi-Component Scales. 374... 

The chemical reactions between carbon and at the interphase and the reactions between ZrB, ZrC, SiC, and O, at the porous layer lead to consumption by oxidation of ceramic matrix, and causes a decrease in the pore size because of the difference in the density of boron oxide, silica, and zirconia, with the boride and carbide ceramics ... 

Lead ll) oxide, PbO, exists in two forms as orange-red litharge and yellow massicot. Made by oxidation of Pb followed by rapid cooling (to avoid formation of Pb304). Used in accumulators and also in ceramics, pigments and insecticides. A normal hydroxide is not known but hydrolysis of lead(II) oxyacid salts gives polymeric cationic species, e.g. [Pb OfOH) ] and plumbates are formed with excess base. 

Mixing. The most widely used mixing method is wet ball milling, which is a slow process, but it can be left unattended for the whole procedure. A ball mill is a barrel that rotates on its axis and is partially filled with a grinding medium (usually of ceramic material) in the form of spheres, cylinders, or rods. It mixes the raw oxides, eliminates aggregates, and can reduce the particle size. 

Putile Ceramic Pigments. StmcturaHy, aH mtile pigments are derived from the most stable titanium dioxide stmcture, ie, mtile. The crystal stmcture of mtile is very common for AX2-type compounds such as the oxides of four valent metals, eg, Ti, V, Nb, Mo, W, Mn, Ru, Ge, Sn, Pb, and Te as weH as haHdes of divalent elements, eg, fluorides of Mg, Mn, Fe, Co, Ni, and Zn. 

Zirconium oxide is used in the production of ceramic colors or stains for ceramic tile and sanitary wares. Zirconia and siHca are fired together to form zircon in the presence of small amounts of other elements which are trapped in the zircon lattice to form colors such as tin—vanadium yellow, praseodymium—zircon yellow [68187-15-5] vanadium—zircon blue [12067-91 -3] iron—zircon pink [68412-79-3] indium—vanadium orange (105—108). 

Electrical and Electronic Applications. Silver neodecanoate [62804-19-7] has been used in the preparation of a capacitor-end termination composition (110), lead and stannous neodecanoate have been used in circuit-board fabrication (111), and stannous neodecanoate has been used to form patterned semiconductive tin oxide films (112). The silver salt has also been used in the preparation of ceramic superconductors (113). Neodecanoate salts of barium, copper, yttrium, and europium have been used to prepare superconducting films and patterned thin-fHm superconductors. To prepare these materials, the metal salts are deposited on a substrate, then decomposed by heat to give the thin film (114—116) or by a focused beam (electron, ion, or laser) to give the patterned thin film (117,118). The resulting films exhibit superconductivity above Hquid nitrogen temperatures. 

Directed Oxidation of a Molten Metal. Directed oxidation of a molten metal or the Lanxide process (45,68,91) involves the reaction of a molten metal with a gaseous oxidant, eg, A1 with O2 in air, to form a porous three-dimensional oxide that grows outward from the metal/ceramic surface. The process proceeds via capillary action as the molten metal wicks into open pore channels in the oxide scale growth. Reinforced ceramic matrix composites can be formed by positioning inert filler materials, eg, fibers, whiskers, and/or particulates, in the path of the oxide scale growth. The resultant composite is comprised of both interconnected metal and ceramic. Typically 5—30 vol % metal remains after processing. The composite product maintains many of the desirable properties of a ceramic however, the presence of the metal serves to increase the fracture toughness of the composite. 

For a large number of applications involving ceramic materials, electrical conduction behavior is dorninant. In certain oxides, borides (see Boron compounds), nitrides (qv), and carbides (qv), metallic or fast ionic conduction may occur, making these materials useful in thick-film pastes, in fuel cell apphcations (see Fuel cells), or as electrodes for use over a wide temperature range. Superconductivity is also found in special ceramic oxides, and these materials are undergoing intensive research. Other classes of ceramic materials may behave as semiconductors (qv). These materials are used in many specialized apphcations including resistance heating elements and in devices such as rectifiers, photocells, varistors, and thermistors. 

Cobalt(II) nitrate hexahydrate [10026-22-9], Co(N02)2 6H20, is a dark reddish to reddish brown, monoclinic crystalline material containing about 20% cobalt. It has a high solubiUty in water and solutions containing 14 or 15% cobalt are commonly used in commerce. Cobalt nitrate can be prepared by dissolution of the simple oxide or carbonate in nitric acid, but more often it is produced by direct oxidation of the metal with nitric acid. Dissolution of cobalt(III) and mixed valence oxides in nitric acid occurs in the presence of formic acid (5). The ttihydrate forms at 55°C from a melt of the hexahydrate. The nitrate is used in electronics as an additive in nickel—ca dmium batteries (qv), in ceramics (qv), and in the production of vitamin B 2 [68-19-9] (see Vitamins, VITAMIN B22)-... 

Cobalt pigments are usually produced by mixing salts or oxides and calcining at temperatures of 1100—1300°C. The calcined product is then milled to a fine powder. In ceramics, the final color of the pigment maybe quite different after the clay is fired. The materials used for the production of ceramic pigments are... 

The addition of oxides to ceramic bodies and to glasses to produce color has been known since antiquity (2). The use of iron and copper oxides predates recorded history. Cobalt was introduced into Chinese porcelain about 700 AD. Chromium compounds have been used since 1800 AD. 

DCMA Classification and Chemical Description of the Mixed Metal Oxide Inorganic ColoredPigments, 2nd ed.. Metal Oxides and Ceramic Colors... 

Sohd rocket propellants represent a very special case of a particulate composite ia which inorganic propellant particles, about 75% by volume, are bound ia an organic matrix such as polyurethane. An essential requirement is that the composite be uniform to promote a steady burning reaction (1). Further examples of particulate composites are those with metal matrices and iaclude cermets, which consist of ceramic particles ia a metal matrix, and dispersion hardened alloys, ia which the particles may be metal oxides or intermetallic compounds with smaller diameters and lower volume fractions than those ia cermets (1). The general nature of particulate reinforcement is such that the resulting composite material is macroscopicaHy isotropic. 

Various combiaations of ceramic—matrix composites have been manufactured at the research level. Their properties are given ia Table 1 for oxide-based matrices and ia Table 2 for aoaoxide matrices. Some commercial products are ideatifted for information only. Such identification does not imply recommendation or endorsement by NIST, nor does it imply that the products are the best available for the purpose. 

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