Mullite

Mullite has the formula 3Al203-2Si02. It does not exist in nature. It is suitable for high-temperature applications. It is used in furnace lining and in other refractory applications in the iron, steel, and glass industries. 

It is produced in two ways—by sintering and by fusing. Sintered mullite is obtained from a mixture of kyanite (Al20Si04), bauxite, and kaolin. Kyanite is a naturally occurring mineral found in metamorphic rocks. This mixture is prepared in the correct proportion. Then, it is sintered at temperature up to about 1600°C. The sintered product contains 85%-90% mullite, the balance being glass and cristobalite. 

In the second method, the required amounts of alumina and kaolin are fused in an electric arc furnace at about 1750°C. This method gives rise to a higher-purity mullite. The fused product contains 95% mullite, the remaining being a mixture of alumina and glass. 

High-purity oxides suitable for parts fabrication are produced mostly by chemical processes. The important oxides are discussed in the following text. 

It is one of the most widely used ceramic materials because of its unique properties, like high-temperature strength, thermal shock resistance, and low dielectric constant [249]. It is also a good refractory material, due to its high melting 

If firing is carried out above 1595°C, the highly refractory mineral mulhte then forms with an additional liberation of free silica that melts according to the following chemical reaction  

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Mullite fibers Multhiomyan Multiblock copolymers Multi chip modules... 

Lasdy, the importance of electroceramic substrates and insulators should not be overlooked. Here one strives to raise the breakdown strength by eliminating the interesting conduction mechanisms just described. Spark plugs, high voltage insulators, and electronic substrates and packages are made from ceramics like alumina, mullite [55964-99-3] and porcelain [1332-58-7]. 

Coming 9455 P-spodumene soHd solution mullite, 3 A12 02-2 Si02 low expansion, high thermal and mechanical stabiHty heat exchangers... 

Mullite Refractories. MuUite refractories are classified under ASTM C467. This brick must have an Al O content between 56 and 79% and contain less than 5% impurities. Impurities are considered metal oxides other than those of aluminum and siUcon. The hot-load subsidence is 5% max is 1593°C. 

High Alumina Refractories. The desired alumina content, from 100% to just above 45%, is obtained by adding bauxites, synthetic aluminosihcates, and synthetic aluminas to clay and other bonding agents. These refractories are used in kilns, ladles, and furnaces that operate at temperatures or under conditions for which fireclay refractories are not suited. Phosphate-bonded alumina bricks have exceptionally high strength at low to intermediate temperatures and are employed in aluminum furnaces. High alumina and mullite are used in furnace roofs and petrochemical apphcations. 

Sihca and aluminosihcate fibers that have been exposed to temperatures above 1100°C undergo partial conversion to mullite and cristobaUte (1). Cristobahte is a form of crystalline siUca that can cause siUcosis, a form of pneumoconiosis. lARC has deterrnined that cristobaUte should be classified as 2A, a probable carcinogen. The amount of cristobahte formed, the size of the crystals, and the nature of the vitreous matrix in which they are embedded are time- and temperature-dependent. Under normal use conditions, refractory ceramic fibers are exposed to a temperature gradient, thus only the hottest surfaces of the material may contain appreciable cristobahte. Manufacturers Material Safety Data Sheets (MSDS) should be consulted prior to handling RCF materials. 

Nickel—beryllium casting alloys are readily air melted, in electric or induction furnaces. Melt surface protection is suppHed by a blanket of argon gas or an alumina-base slag cover. Furnace linings or cmcibles of magnesia are preferred, with zirconium siUcate or mullite also adequate. Sand, investment, ceramic, and permanent mold materials are appropriate for these alloys. Beryllium ia the composition is an effective deoxidizer and scavenger of sulfur and nitrogen. 

Transmission electron micrographs show hectorite and nontronite as elongated, lath-shaped units, whereas the other smectite clays appear more nearly equidimensional. A broken surface of smectite clays typically shows a "com flakes" or "oak leaf surface texture (54). High temperature minerals formed upon heating smectites vary considerably with the compositions of the clays. Spinels commonly appear at 800—1000°C, and dissolve at higher temperatures. Quartz, especially cristobalite, appears and mullite forms if the content of aluminum is adequate (38). 

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