1. M. Sasaki and T. Hirai, in Proceedings of the First International Symposium on Functionally Gradient Materials, edited by M. Yamanouchi, M. Koizumi, T. Hirai and I. Shiota, (Functionally Gradient Materials Forum, Tokyo, 1990) p. 83.
2. M. Sasaki, H. Suzuki and T. Hirai, ibid.in, p. 231.
3. R. Naslain, ibid.in, p. 71.
4. K. Fritscher and W. Bunk, ibid.in, p. 91.
5. M. Takemura, A. Yoshitake, H. Hayakawa, T. Hyakubu and M. Tamura, ibid.in, p. 97.