Abstract: A composite substrate is provided, which comprises a cordierite-anorthite-boron base composite material and has a microstructure with anorthite included in the cordierite phase. This substrate can be used in the form of a circuit substrate, because it has high-enough mechanical strength, a coefficient of thermal expansion close to that of silicon and a decreased dielectric constant and, moreover, can be sintered at a low temperature.

Abstract: A method for making mullite whiskers includes the steps of:preparing a mullite-intensive sol by mixing an alumina sol with a silica sol at a molar Al.sub.2 O.sub.3 /SiO.sub.2 ratio of 1.37-1.76,gelating said mullite-intensive sol by adding HF thereto at a molar HF/3Al.sub.2 O.sub.3 .multidot.2SiO.sub.2 ratio of 0.01-0.1,drying the gel at a temperature lower than 120.degree. C.; andcalcining the dried product in an open system at a temperature up to 1000.degree. C. and in a closed system at a temperature from 1200.degree.-1600.degree. C.

Abstract: A method for making composite mullite/cordierite ceramics comprises the steps of:preparing a mullite-intensive sol by mixing alumina and silica sols together at an Al.sub.2 O.sub.3 /SiO.sub.2 molar ratio of 1.37-1.76,preparing a cordierite-intensive sol by mixing alumina, silica and magnesia sols together,gelating a mixture of the mullite- and cordierite-intensive sols obtained at a weight ratio of 80:20-20:80calcinating the thus obtained gel at 1200.degree.-1400.degree. C., andcompacting and sintering the thus calcinated material at 1300.degree.-1450.degree. C.

Abstract: A process for preparing a zirconia-alumina base composite ultra-fine powder is described, including the steps of: mixing an aluminum salt or an alumina hydrate with an aqueous zirconium oxychloride solution, with or without metal salt acting as a stabilizer; mixing urea or a substance capable of generating ammonia by hydrolysis with the resulting mixture; and allowing the mixture to react at a temperature of 140.degree. to 300.degree. C. and a pressure of 4 to 150 kg/cm.sup.2 in a high-temperature and -pressure vessel. The reaction product is further dried, calcined and crushed to obtain an ultra-fine powder.