1. R. L. COBLE, U.S. Patent number 3026210 (1962).
2. W. D. KINGERY, H. K. BOWEN and D. R. UHLMANN, ?Introduction to Ceramics? 2nd edn (Wiley Interscience, New York, 1976) p. 648.
3. H. B. PROST,Amer. Ceram. Soc. Bull. 59 (1980) 206.
4. K. H. JACK,J. Mater. Sci. 11 (1976) 1135.
5. K. HIHARA and T. HIRAI, ibid.11 (1976) 593.