1. R. F. COE, R. J. LUMBY and M. F. PAWSON, ?Special Ceramics 5?, edited by P. Popper (British Ceramic Research Association, Stoke-on-Trent, 1972) p. 361.
2. K. NIHARA and T. HIRAI,J. Mater. Sci. 13 (1978) 2277.
3. D. CHAKRABORTY and J. MUKERJI, ibid.15 (1980) 3051.
4. N. L. PARR, G. F. MASTIN and E. R. W. MAY, ?Special Ceramics?, edited by P. Popper (Haywood and Co. Ltd, London, 1960) p. 102.
5. A. TSUGE and K. NISHIDA,Amer. Ceram. Soc. Bull. 57 (1978) 424.