1. P. C. Kong, Y. C. Lau, and E. Pfender,Ceram. Trans. 1, 939 (1988).
2. G. I. Babat,J. Inst. Elect. Eng. (London) 94, 27 (1947).
3. T. B. Reed,J. Appl. Phys. 32, 821 (1961).
4. T. B. Reed,J. Appl. Phys. 32, 2534 (1961).
5. H. U. Ecken,High Temp. Sci. 6, 99 (1974).