Mullite sintered

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. 

Electron microprobe analysis of concentration profiles across sections of the sintered samples revealed the existence of concentration gradients, the mullite content decreasing with increasing distance from the surface of the bodies [Marple Green, 1993], SEM examination also revealed a microstructural effect the alumina grain size tended to increase from the surface of the... 

The abundance of mullite in the mullite/ZTA system increased with increasing infiltration time [Low et al., 1993]. The density (p) and mullite content of the sintered sample as a function of infiltration time are shown in Table 5.2. The results suggest that the infiltration process was time (t) dependent and diffusion-controlled with the infiltration front travelled as a function of t112. The content of mullite was greatest near the surface and decreased sharply towards the core of the sample. 

Figure 7.1 Typical thermal expansion trace kyanite (AljCVSiOj) + quartz (Si02) at 5°C/min. The a-0 quartz inversion is apparent at 573°C. Kyanite converts to mullite (3Alj03-2Si02) and residual glass starting at 1100°C, reaching a maximum rate at 1400°C [1]. The sharp contraction starting at 1100°C is interpreted to correspond to sintering. At 1320°C, the rapid formation of the less dense decomposition products of kyanite cause a temporary expansion [2].

In the sintering of AI2O3 a small amount of SiO, some 5%, is used to lower the sintering temperature from near 1600 to 1400°C due to a low melting eutectic at 1587°C. After sintering is complete, calculate the amount of mullite present in the sample. Explain why cooling this sample will be problematic. 

The conventional methods of mullite preparation are the following crystallization from melt, high-temperature sintering of oxides, high-temperature annealing of kaolin [32]. Mechanical activation of hydrated oxides brings to decrease of mullite synthesis temperature [32-39]. 

More specialized ceramics include refractory silicates such as mullite, Al6Si20i3, or binary oxides such as MgO, AI2O3, TiOy, or stabilized zirconia, which are fired and sintered, sometimes under pressure at 1500-2000 °C. A great deal of this refractory production involves the fabrication of low and medium density fire-bricks for ovens and furnaces. 

FIGURE 12.6 (a) Optically clear PLZT 65/35 ceramic sintered in PbO-enriched atmosphere (thickness 5 mm), (b) Translncent mullite ceramic with SiC fibers (diameter 12 pm). 

Colomban, P, and Vendange, V., Sintering of alumina and mullite prepared by slow hydrolysis of alkoxides the role of the protonic species and of pore topology, J. Non-Crystl. Solids, 147, 245, 1992. 

At the same time, very fine mullite will begin to form in the clay component in the 1000 —J 200 C range. Its formation and crystal growth then proceeds by reaction between the partially transformed clay component and the feldspar melt. The resulting acicular mullite crystals are several pm in length. Extensive sintering due to the melt proceeds simultaneously. 

Pask [29] reports that mullites with higher molar ratios of alumina to silica (i.e., >3 1) have been prepared by homogenous melting of the constituents above the liquids and subsequent quenching. As a note, mullites prepared by fusion are generally weaker than those produced by sintering [33]. 

Mullite powders obtained by various methods can first be crystallized near 1,200°C, and then sintered at temperatures below the eutectic. Highly pure mullite and mullite composites have been obtained by hot pressing below 1,300°C with this method [34],... 

Fig. 5 Scanning electron micrograph of 3 2 mullite. Specimen was sintered at 1,700°C, hot isostatically pressed at 1,600°C, and thermally etched. From [54]...

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