1. M. N. Rich:U. S. Patent No. 1741955, 1929.
2. W. Rohn:Z. Metallic, 1924, vol. 16, pp. 275–77.
3. W. Baukloh and G. Henke:Z. Anorg. Allg. Chem., 1937, vol. 234, pp. 307–10.
4. C. Royère:Rev. Int. Hautes Temp. Refract., 1973, vol. 1.10, pp. 241–49.
5. P. V. Gel’d and O. A. Esin:Z. Prikl. Chim., 1950, vol. 23, pp. 1351–56.