1. K. Haberko, A. Ciesla andA. Proon,Ceram. Int. 3 (1975) 111.
2. M. A. C. G. Van DeGraaf andA. J. Burgraaf, in ?Advances in Ceramics?, Vol. 12 (The American Ceramic Society, Stuttgart, 1983) pp. 744?65.
3. H. Schubert, N. Claussen andM. Buhle, ??Ibid.in ?, pp. 766?73.
4. B. Fegley Jr, P. White andH. Kent Bowen,Amer. Ceram. Soc. Bull. 64 (1985) 1115.
5. P. Recio, C. Pascual, C. Moure, J. R. Jurado andP. Duran,Proc. Brit. Ceram. Soc. 38 (1986) 127.