1. P. W. HAAYMAN, R. W. DAM and H. A. KLASENS, German Pat. 929350, 23 June 1955.
2. B. M. KULWICKI, in “Advances in Ceramics, Vol. 1, Grain Boundary Phenomena in Electronic Ceramics”, edited by L. M. LEVINSON (American Ceramic Society, Columbus, OH, 1981) p. 138.
3. H. NEMOTO and I. ODA, J. Am. Ceram. Soc.
63 (1980) 398.
4. S. K. SUNDARAM, J. Phys. Ds
23 (1990) 103.
5. O. SABURI, J. Phys. Soc. Jpn
14 (1959) 1159.