1. N. L. Parr, G. F. Mastin andE. R. W. May, in ?Special Ceramics?, edited by P. Popper (Haywood, London, 1960) p. 102.
2. P. B. Noakes andP. L. Pratt, in ?Special Ceramics 5?, edited by P. Popper (British Ceramic Research Association, Stoke-on-Trent, 1972) p. 229.
3. R. F. Coe, R. J. Lumby andM. F. Pawson,ibid.in ? p. 361.
4. K. Nihara andT. Hirai,J. Mater. Sci. 13 (1978) 2277.
5. D. Chakraborty andJ. Mukerji,ibid. 15 (1980) 3051.