Ceramic mullite

The commercially important aluminosilicate ceramic mullite (Al6Si20i3) can readily be produced by heating either monophasic (homogeneous) or diphasic gel precursors. The reactivity and reaction paths of both types of mullite precursor have been... 

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]. 

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. 

Swollen clay materials are soft and easy to mould. They serve to produce ceramic materials. High quality fire-clay has a high kaolinite content. Upon firing, the intercalated water is removed first at approximately 100 °C. Then, beginning at 450 °C, the OH groups are converted to oxidic O atoms by liberation of water, and after some more intermediate steps, mullite is formed at approximately 950 °C. Mullite is an aluminum aluminosilicate, Al(4 )/3[Al2 Si,05] with x 0.6 to 0.8. 

Use of the ceramic honeycomb packing structure in the recuperator keeps fuel and air substantially isolated as they travel through the recuperator. Various ceramic materials such as cordierite, mullite, alumina and silicon carbide can be used to fabricate honeycomb beds. While metallic materials have the potential to be used in honeycomb bed, corrosion resistance is a major issue... 

R. Hill, D. Wood, Apatite-mullite glass-ceramics, J. Mater. Sci. Mater. Med. 6 (1995) 311-318. 

The nebulization was also employed to generate composite powders for specific applications, such as in ceramics, by hydrolyzing with water vapor droplets containing Al(5ec-OBu) and silicon methoxide in the atomic ratio Al/Si = 3. This ratio of alkoxides was chosen in order to produce mullite, which was achieved by calcination of the resulting amorphous particles at rather high temperatures (up to I400 C) (52). In another approach a mixed Al-Mg-Si ethoxide was first synthesized, and then nebulized and hydrolyzed as usual (77). Depending on the experimental conditions, the powders calcined at 500 C exhibited structures of pure cordierite, or mixed with forsterite. In all of these described cases the nebulization yielded spherical but polydisperse particles. 

Most structural PMCs consist of a relatively soft matrix, such as a thermosetting plastic of polyester, phenolic, or epoxy, sometimes referred to as resin-matrix composites. Some typical polymers used as matrices in PMCs are listed in Table 1.28. The list of metals used in MMCs is much shorter. Aluminum, magnesium, titanium, and iron- and nickel-based alloys are the most common (see Table 1.29). These metals are typically utilized due to their combination of low density and good mechanical properties. Matrix materials for CMCs generally fall into fonr categories glass ceramics like lithium aluminosilicate oxide ceramics like aluminnm oxide (alnmina) and mullite nitride ceramics such as silicon nitride and carbide ceramics such as silicon carbide. 

Other Aluminosilicates, Transparent mullite glass-ceramics can be produced from modified binary Al C —Si02 glasses (21). In these materials, the bulk glass phase separates into tiny alumina-rich droplets in a siliceous matrix. Further heat treatment causes these droplets to crystallize to mullite spherulites less than 0.1 Jim in size. When doped with ions such as Cr3+, transparent mullite glass-ceramics can be made to absorb broadly in the visible while fluorescing in the near-ii (22,23), thereby making them potentially useful for luminescent solar collectors. 

The hardness parameters specified in Table 8.1 apply to glazed bodies whose hardness is usually 0.16-0.25 degrees on the Mohs scale lower than that of unglazed material. The 7-8 hardness interval for porcelain materials given by many authors is incorrect. It is specific to materials ranked under special ceramics of steatitic or mullite-corundum type (Tables 4.4.4, 5.6, 5.7). 

Mullite, 3 AI2O3 2 SiC>2, a high-temperature ceramic being considered for use in engines, can be made by the sol-gel method. 

FIGURE 10. TGA of OOPS derived mullite precursor. The shown ceramic yield is only 23 wt% (vs. ca 29 wt% theory) owing to traces of excess ethylene glycol and triethanolamine retained in the sample to provide processability... 

In the manufacture of engines turbine blades, bearings and plungers are made of ceramics. The materials used here are cordierite, Li-A1 silicate, mullite, titanate, SiC, A1203, Zr02, certain nitrides as well as particles- and fibre-reinforced composites and layered composites. 

Some of the most common combinations used in the development of new ceramic composites involve the use of silicon carbide, silicon nitride, aluminum oxide, silicon dioxide, and mullite (a form of aluminum sulfate (Al2[S04]3). Each of these compounds can he used either as the reinforcement or as the matrix in a composite. 

Yoon, C.K. and Chen, I.W. Superplasticity of two-phase ceramics containing inclusions - zirconia mullite composites , J. Am. Ceram. Soc. 73 (1990) 1555-1565. 

Marple, B.R. Green, D.J. (1991) Mullite/alumina particulate composites by infiltration processing III, Mechanical properties. J. Am. Ceram. Soc. 74, 2453-2459. 

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