Ceramics, sintered mullite

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). 

Fig. 3 Microstructure of a solid-state sintered mullite ceramic with a relative density >99%. The ceramic had equiaxed grains with a grain size of -1 pm and no apparent glassy phase...

P. Boch and J. P. Giry, Preparation and Properties of Reaction-Sintered Mullite-Zr02 Ceramics, Mater. Sci. Eng., 71,39-48(1985). 

She e/ alP have determined, using XRD, the major phases present in porous SiC ceramics sintered at different temperatures for 4 h. At 1400°C, porous SiC ceramics consist mainly of SiC, cristobalite and a- ALO3, but slight mullite peaks can be found. When the temperature increases to 1450°C, the mullite peaks are obvious. At 1500 and 1550°C, the amount of a- AljOj decreases abruptly and more extensive mullitization occurs. 

With the CaO additions has been obtained a ceramic of mullite + zirconia + anorthite showing Kic values of 4.5 MPa m / in which HREM has been carried out [25]. The observation by conventional TEM depicts twinned crystals in both zirconia grains and anorthite precipitated from the glassy phase produced via reaction sintering. There are two types of zirconia composing the microstructure of these ceramics, viz intergranular with 1-3 yum size located between the mullite crystals and intragranular with very small (0.1-0.5 um size) rounded crystals. 

The sol-gel method makes it possible to prepare refractory materials at considerably lower temperatures than the conventional method of sintering and melt-quenching. Fully sintered mullite ceramics of the composition 3AI2O3 2Si02 are prepared by sol-gel method (Pask, 1987 Ismail, 1987). Alumina-sihcarefractory fibers (Sosman, 1988) and Si02 fibers (Taneda, 1988) are also prepared. 

K. Das and G. Banerjee, Mechanical Properties and Microstmctures of Reaction-Sintered Mullite-ZrOa Composites in the Presence of an Additive- Dysprosia,7. Ear. Ceram. Soc., 20,153-157 (2000). J.S. Moya and M.l. Osendi, Effect of ZrOa (ss) in Mullite on the Sintering and Mechanical Properties of Mullite/ZrOa Composites, 7. Mat. Sci. Lett., 2, 599-601 (1983). 

Zhou XB, Qiu F, Penchal Reddy M, Han YH, Lee J, Huang Q. Comparative study of conventional and microwave sintered mullite fibers a structural study. Advances in Applied Ceramics Structural, Functional and Bioceramics 2015 114 139-43. 

BOC 90] BOCH P., CHARTIER T. and RODRIGO, High purity mullite by reaction sintering , Mullite and Mullite Matrix Composites, Ceramic Transactions, Vol. 6, The Am. Ceramic Society, p. 353,1990. 

Prochazka, S., Wallace, J.S. and Claussen, N., Microstructure of Sintered Mullite-Zirconia Composites. J. Amer. Ceram. Soc. 66 C125-126 (1983). 

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. 

Sacks, M. D., Bozkurt, N., and Scheiffele, G. W., Fabrication of mullite and mul-lite-matrix composites by transient viscous sintering of composite powders, J. Am. Ceram. Soc., 74(10), 2428 (1991). 

Katsuki H, Furuta S, Komameni S. Formation of novel ZSM-5/porous mullite composite from sintered kaolin honeycomb by hydrothermal reaction. JAm Ceram Soc 2000 83(5) 1093-1097. 

The use of MW sintering to fabricate transparent ceramics is relatively less popular currently. Transparent ceramics that have been processed by using MW sintering include AI2O3 [150-152], YAG [153], AlON [154], and mullite [155]. MW sintering has been shown to be an effective technique to produce transparent ceramics as compared with the conventional sintering technologies. 

---