1. W. Ashcroft, ?Special Ceramics?, Vol. 6, edited by P. Popper (British Ceramic Research Association, Stoke-on-Trent, 1975) p. 245.
2. F.F. Lange, J. Amer. Ceram. Soc. 57 (1974) 84.
3. S.D. Hartline, R.C. Brandt, D.W. Richardson and M.L. Torti, ibid. 57 (1974) 190.
4. J.L. Iskoe, F.F. Lange and E.S. Diaz, J. Mater. Sci. 11 (1976) 908.
5. J.L. Henshall, D.J. Rowcliffe and J.W. Edington, ?Special Ceramics?, Vol. 6, edited by P. Popper (British Ceramic Research Association, Stoke-on-Trent, 1975) p. 185.