Metal Oxide Ceramics

Bricks, standard and specially shaped building blocks Naphthalene (moth) balls, / ara-chlorobenzene, chlorine cleaning tablets, swimming pool chemicals Disposable cutting tools, intricate components, steatite balls, capacitors, carbon contacts and diaphragms, metal ceramics, oxide ceramics... 

Reactions of nanoscale materials are classified with respect to the surrounding media solid, liquid, and gas phases. In the solid phase, nanoscale crystals are usually connected with each other to form a powder particle (micron scale) or a pellet (milli scale) see Figure 14.1. Two or more materials (powder or pellet) are mixed and fired to form a new material. The nanosized structure is favored, due to the mixing efficiency and high reaction rate. Alloys (metals), ceramics (oxides), cement (oxides), catalysts (metals and oxide), cosmetics (oxides), plastics (polymers), and many functional materials are produced through solid reaction of nanoscale materials. One recent topic of interest is the production of superconductive mixed oxides, where control of the layered stracture during preparation is a key step. 

Usually two approaches are used while fabricating the interconnect materials, i.e., ceramic and metallic. Ceramic oxides possess stability in an oxidising atmosphere but possess lower conductivities as compared to the metals. Usually ceramic oxides are used at higher temperatures and metals are used at lower temperatures. 

Metal to ceramic (oxide) adhesion is very important to the microelectronics industry. An electron transfer model by Burlitch and co-workers [75] shows the importance of electron donating capability in enhancing adhesion. Their calculations are able to explain the enhancement in adhesion when a NiPt layer is added to a Pt-NiO interface. 

Most talc sold to paper, ceramics, and other industrial customers is manufactured to specifications agreed to between the producer and consumer. In paper, properties such as color, abrasion, surface area, and tint ate most important, whereas in ceramics, oxide chemistry, fired color, pressing characteristics, and alkaH metal content ate mote important. There ate some military specifications for talc used in corrosive coatings (6) and for cosmetic talc products used for cleaning of personnel in chemical warfare zones (7). 

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. 

Special low fusing porcelain veneers are appHed to pure (unalloyed) titanium dental castings. It is important that firing be done either in a vacuum or inert atmosphere to protect the metal surface from excessive oxidation. The strength of the metal-ceramic bond is apparently adequate although the bonding is thought to involve primarily a mechanical rather than a chemical component. 

Fig. 6. Catalyst inhibition mechanisms where ( ) are active catalyst sites the catalyst carrier and the catalytic support (a) masking of catalyst (b) poisoning of catalyst (c) thermal aging of catalyst and (d) attrition of ceramic oxide metal substrate monolith system, which causes the loss of active catalytic material resulting in less catalyst in the reactor unit and eventual loss in performance.

The ceramic oxide carrier is bonded to the monolith by both chemical and physical means. The bonding differs for a ceramic monolith and a metallic monolith. Attrition is a physical loss of the carrier from the monolith from the surface shear effects caused by the exhaust gas, a sudden start-up or shutdown causing a thermal shock as a result of different coefficients of thermal expansion at the boundary between the carrier and the monolith, physical vibration of the cataly2ed honeycomb, or abrasion from particulates in the exhaust air (21) (see Fig. 6d). 

It is to be expected that tire conduction data for ceramic oxides would follow the same trends as those found in semiconductors, i.e. the more ionic the metal-oxygen bond, the more the oxides behave like insulators or solid elee-trolytes having a large band gap between the valence electrons and holes, and... 

Above 1000°C Refractory metals Mo, W, Ta Alloys of Nb, Mo, W, Ta Ceramics Oxides AI2O3, MgO etc. Nitrides, Carbides SiaN., SiC Special furnaces Experimental turbines... 

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. 

Precious metals and oxides platinised titanium, platinised niobium, platinised tantalum, platinised silver, solid platinum metals, mixed metal oxide-coated titanium, titanium oxide-based ceramics. 

Early work on superconductors concentrated on metals or metal mixtures (alloys). Niobium alloys are particularly good superconductors, and in 1973 a niobium alloy, Nb3Ge, was found to have Tc — 23 K, the highest known value for a metal superconductor. In 1986, a ceramic oxide with formula La2- Ba CuOq was found to show superconductivity at 30 K. Through intense research efforts on ceramic oxides, YBa2 C U3 Oj-, with Tc — 93 K, was discovered in 1987. 

Ceramic oxide superconductors have distinct atomic layers. The Cu-containing superconductors contain planes of copper and oxygen atoms, as the molecular view shows. These planes alternate with layers containing oxygen and the other metals that make up the superconductor. Superconductivity takes place in the Cu—O planes. 

C. N. R. Rao, B. Raveau, Transition Metal Oxides Structures, Properies, and Synthesis of Ceramic Oxides. Wiley, 1998. 

An idea to use polybasic hydroxy carboxylic acids in syntheses of oxides goes back to Pechini [3], Evaporating solutions of metal salts in citric acid at presence of ethylene glycol he obtained a polymeric resin as a precursor of target oxides. Then this process was extensively used to manufacture various ceramic oxide powders in several publications [4-8],... 

Aluminum is produced commercially by the electrolysis of cryolite, Na3AlF6, but bauxite, A1203, is the usual naturally occurring source of the metal. The oxide is a widely used catalyst which has surface sites that function as a Lewis acid. A form of the oxide known as activated alumina has the ability to adsorb gases and effectively remove them. Other uses of the oxide include ceramics, catalysts, polishing compounds, abrasives, and electrical insulators. 

Lanxide A process for making composites of metals with oxides. A molten metal reacts with an adjacent oxidant and is progressively drawn through its own oxidation product so as to yield a ceramic/metal composite. Fibres or other reinforcing materials can be placed in the path of the oxidation reaction and so incorporated in the final product. The Lanxide Corporation was founded in 1983 in Newark, DE, to exploit this invention. In 1990 it formed a joint venture with Du Pont to make electronic components by this process. Variations are Dimox (directed metal oxidation), for making ceramic metal composites, and Primex (pressureless infiltration by metal), for making metal matrix composites. 

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