Mullite materials

The formation of mullite at ca 1000 °C is supported by the XRD data shown in Figure 12. Thus, based on DTA, TGA and XRD, one might decide that phase pure mullite without carbon contamination is produced by pyrolysis of precursor to 1000 °C. This then would represent a processing goal for making mullite materials. Also, it appears to be similar to the SiC precursor results, where nanocrystalline, phase pure SiC is obtained... 

Kiln furniture must be made of materials resistant to considerable temperature cycling in particular, during fast firing the changes in temperature are quite rapid (heating rate up to 30 — 40 ""C min ). A low thermal expansion coefficient and/or high thermal conductivity are therefore required. These requirements are best met by cordierite-mullite materials up to 1300 ""C and SiC-based materials for higher temperatures. 

Maitra S., RahamaA. n, SarkaA. r, Tarafdar A.. Zirconia-mullite materials prepared from semi-colloidal route derived precursors. Ceramics International, 2006,32 201-206... 

Applications of CBED for solving Problems in Ceramics Some Examples and Application to Zirconia/Mullite Materials... 

In the case of advanced ceramics and powders of nanometric size, the use of the classical selected area diffraction Patterns (SADP) by using different apertures in the electron path in the TEM is very short. In these cases and similarly for Zr02/mullite materials, the very small size of intragranular zirconia formed in the reaction sintered process inside the matrix, reaching 50-300 nm sizes, makes it fully necessary to use higher resolution in the electron diffraction analysis. 

In the manufacture of refractory and porcelain bodies, the desirable mullite is formed from the chemically similar minerals andalusite, kyanite and dumortierite, which decompose into mullite and silica at cone 13, cone 12 and cone 6, respectively. To break up siUimanite in the same way, cone 20 is required. Sintered and electro-fused synthetic mullites (including zirconium mullite) are used in kiln furniture and refractories for the glass and steel industries. The theoretical chemical analysis (wt%) of mullite and five typical mullite materials are as follows ... 

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. 

There are on the market a number of bricks made from electri-caUy fiised materials, such as fused mullite, fused alumina, and fused magnesite. These bricks, although high in cost, are particularly suitable for certain severe conditions. 

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. 

Note that these three minerals, which are common in the Earth s crust, are not stable at ambient pressure at high temperatures. At ambient pressure, mullite (3Al203-2Si02), is usually found in refractory materials based on these minerals. 

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... 

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. 

You have supplies of kaolin, silica, and mullite as raw materials. Using kaolin plus either silica or mullite, calculate the batch composition (in weight percent) necessary to produce a final microstructure that is equimolar in silica and mullite. 

Bauxitic Kaolins andMullites. Deposits of bauxitic kaolins, kaolins having aluminous minerals, have been discovered that have alumina contents between 50 and 70%. These materials are made into refractory aggregates called calcines, grog, clinker, or grain. In addition to sdectivdy mined deposits, synthetic compositions can be prepared from kaolin and alumina and other minerals to produce compositions of desired alumina and mineralogical content. These synthetic mullites are readily available in the form of sintered and fused aggregates. 

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