1. Z. ZAKHARIEV and D. RADEV, National Conference on New technology and materials, Primorsko (1984) p. 205.
2. A. LIPP and K. A. SCHWETZ,Ber. Dt. Keram. Ges. 52 (II) (1975) 335.
3. K. PORTNOY, G. SAMSONOV and K. FROLOV,J. Prikladnoy Chimii 3 (1960) 577.
4. V. DJEMELINSKY, M. KOVALCHENKO and G. MAKARENKO,Poroshkovaia Metallurgia 2 (1973) 122.
5. N. SHERBAN, ibid.9 (1973) 57.