1. Parr, N. L., and May, E. R. W., Proc. Brit. Ceram. Soc., 7, 81 (1967).
2. Godfrey, D. J., J. Brit. Interplanetary Soc., 22 (5), 353 (1969).
3. Crivelli-Visconti, I., and Cooper, G. A., Nature, 221, 755 (1969).
4. Godfrey, D. J., Metals and Materials, 10, 305 (1968).
5. Aveston, J., Nature, 226, 146 (1970).