1. Matsushita Electric Industrial Co Ltd, Japanese Patent 77 01402 (1977).
2. G. Goodman, in ?Advances in Ceramics?, Vol. 1, edited by L. M. Levinson (American Society, Columbus, 1981) pp. 215?231.
3. R. Wernicke, in ?Advances in Ceramics?, Vol. 1, edited by L. M. Levinson (American Ceramic Society, 1981) pp. 261?271.
4. J. M. Haussonne andJ. Lostec, French Patent 83 12995 (1983).
5. R. Mauczock andR. Wernicke,Philips Tech. Rev. 41 (11/12) (1983/1984) 338.