Oxide ceramics alumina

Recently, Vonau et. al [27] have suggested an all-solid reference electrode consisting of a sintered Ag/AgCl mixture, which is embedded in a solid melt of KCl inside a cylindrical hollow body of porous alumina oxide ceramics. The outer surface of the ceramics is coated with a chemical-resistant insulating layer. At the bottom end, a circular area is held free from the coating and acts as diaphragm. 

Interaction of polyetherpolyurethane with solvents and solubility parameter Polystyrene-hydrocarbon interaction parameters and solubility parameter Polycarbonate surface energies and interaction characteristics Surface heterogeneity of alumina oxide ceramic powders Surface energy distribution... 

Alumina, or aluminum oxide [1344-28-17, has a thermal conductivity 20 times higher than that of most oxides (5). The flexural strength of commercial high alumina ceramics is two to four times greater than those of most oxide ceramics. The drawbacks of alumina ceramics are their relatively high thermal expansion compared to the chip material (siUcon) and their moderately high dielectric constant. 

Although beryllium oxide [1304-56-9] is in many ways superior to most commonly used alumina-based ceramics, the principal drawback of beryUia-based ceramics is their toxicity thus they should be handled with care. The thermal conductivity of beryUia is roughly about 10 times that of commonly used alumina-based materials (5). BeryUia [1304-56-9] has a lower dielectric constant, a lower coefficient of thermal expansion, and slightly less strength than alumina. Aluminum nitride materials have begun to appear as alternatives to beryUia. Aluminum nitride [24304-00-5] has a thermal conductivity comparable to that of beryUia, but deteriorates less with temperature the thermal conductivity of aluminum nitride can, theoreticaUy, be raised to over 300 W/(m-K) (6). The dielectric constant of aluminum nitride is comparable to that of alumina, but the coefficient of thermal expansion is lower. 

The insulation around the central electrode is an example of a non-metallic material - in this case, alumina, a ceramic. This is chosen because of its electrical insulating properties and because it also has good thermal fatigue resistance and resistance to corrosion and oxidation (it is an oxide already). 

As observed by D. Johnson and J. Stiegler, "Polymer-precursor routes lor fabricating ceramics offer one potential means or producing reliable, cost-effective ceramics. Pyrolysis of polymeric metalloorganic compounds can be used to produce a wide variety of ceramic materials." Silicon carbide and silicon oxycarbide fibers have been produced and sol gel methods have been used In prepare line oxide ceramic powders, such as spherical alumina, as well as porous and fully dense monolithic forms. 

Figure 2. The proposed reaction of the bifunctional APS molecule when serving as an adhesion promoter or coupling agent for polyimide thin films on native-oxide ceramics such as silica and alumina.

In the following sections some examples are given of the ways in which these principles have been utilized. The first example is the use of these techniques for the low temperature preparation of oxide ceramics such as silica. This process can also be used to produce alumina, titanium oxide, or other metal oxides. The second example describes the conversion of organic polymers to carbon fiber, a process that was probably the inspiration for the later development of routes to a range of non-oxide ceramics. Following this are brief reviews of processes that lead to the formation of silicon carbide, silicon nitride, boron nitride, and aluminum nitride, plus an introduction to the synthesis of other ceramics such as phosphorus nitride, nitrogen-phosphorus-boron materials, and an example of a transition metal-containing ceramic material. 

Since the significant majority of the published literature on high temperature crack growth under static and cyclic loads is predicated upon experiments conducted on alumina and alumina matrix composites, the examples cited in the present review have centered around oxide ceramics and their composites. However, the implications of the results to other classes of ceramics, intermetallics, and brittle matrix composites are also described, wherever feasible, along with any available information in an attempt to illustrate the generality of the concepts developed here. 

As to ceramic membranes [3,4] the focus has been so frir in particular on amorphous porous aluminas and silicas. Other inorganics studied include titania, zirconia, non-oxide ceramics (carbides), and microporous carbons. 

The typical and most widely employed representative of oxide ceramics is sintered alumina on which it is convenient to demonstrate the characteristic features of this group of materials. 

The industrially most important oxide-ceramic material is sintered aluminum oxide. The raw materials used are so-called calcined alumina and melted corundum. 

In contrast with pure aluminum oxide ceramics, the raw materials used in the manufacture of alumina-rich refractory products are, for economic reasons, natural products. The choice of aluminum silicates cyanite, andalusite or sillimanite (chemical composition Al203-Si02) or low iron bauxite with an AbOj-content > 85% and a Si02-content < 10%, depends upon the aluminum oxide content required. Natural mixtures of alumina hydrates (bauxite) and kaolin with Al203-contents of 48 to 70% are fired to so-called mullite chamottes. To obtain still higher AbO -contents, industrially produced corundum has to be added. 

Compare oxide ceramics such as alumina (AI2O3) and magnesia (MgO), which have significant ionic character with covalently bonded nonoxide ceramics such as silicon carbide (SiC) and boron carbide (B4C see Problems 19 and 20) with respect to thermodynamic stability at ordinary conditions. 

Ceramic materials are also utilized in nuclear reactor components. Applications include insulation of pressure vessels with linings fabricated from silica and alumina-base ceramic bricks or fiber insulation pressure vessels made of prestressed concrete structures that enclose the entire reactor and secondary system wear-resistant surfaces produced by means of coatings such as chromium oxide or chromium carbide and shielding applications, which include materials such as concrete, graphite, and leaded glass ... 

The preparation of a mixed silica-alumina oxide by a microemulsion-mediated process has been reported . In this work, porous particles with a spherical morphology were obtained. This material can be used as a carrier for catalysts or as a precursor of ceramic materials. 

Until now, only a handful of ceramic fibers for use in CMCs, mainly fibers made from silicon carbide, have reached the market. While Ube Industries, Nippon Carbon, and, lately, Dow Corning produce SiC fibers, 3 M has developed some oxidic ceramic fibers (e.g. aluminoborosilicate, aluminosilica, or alumina) for this purpose. Since up to now for high-temperature applications only fibers made from SiC or SiBN3C are suitable, the most important features of these two classes will now be discussed. 

Since 1975 catalysts have been fitted to vehicles in the USA to control emissions, initially of HC and CO (oxidation catalysts), and latterly also of NOx (three way catalysts). The mode of operation of these catalyst systems in the USA and Japan is now well characterised (1). The catalysts typically comprise the precious metals platinum, palladium and rhodium, either singly or in combination, together with base metal promoters or stabilisers, supported on alumina pellets or alumina coated ceramic monoliths. Catalysts for the US market are designed to withstand 50,000 miles of road use and must be operated in conjunction with lead free fuel since they are poisoned by lead. 

Plain oxide ceramics do not enable economical machining of luckel base materials, due to their poor resistance to thermal shock and low firacture toughness. Alumina oxide with titanium-carbide, so-called mixed ceramics, was successful applied with cutting velocities up to 500 m/min for turning operations (Wiemann 2006). 

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