1. M. NAKAJIMA, K. MARUYAMA, K. ISOGAYA and K. YOSHIDA, in ?High tech ceramics?, edited by P. Vincentini (Elsevier Science, Amsterdam, 1987) p. 501.
2. J. G. LEE and I. B. CUTLER,Amer. Ceram. Soc. Bull. 54 (1975) 195.
3. M. PATEL, C. B. RAJU, A. K. RAY and A. KARERA, Indian Patent 590, DEL 86, Field 860708 (1986).
4. J. V. MILEWSKI, F. D. GAC, J. J. PETROVIC and S. R. SKAGGS,J. Mater. Sci. 20 (1985) 1160.
5. I. B. CUTLER, US Patent 3 754076, 21 August (1973).