1981 Volume 89 Issue 1030 Pages 292-302
Growth of single crystals of alkaline earth metal hexaborides (CaB6, SrB6 and BaB6) in molten aluminum in an argon atmosphere has been investigated.
Results obtained are as follows.
The optimum growth conditions of single crystals:
The mixing atomic ratios of raw materials: for CaB6: (B/Ca)=3.20-5.70, (Al/Ca)=10.50-50.50, for SrB6: (B/Sr)=2.00-3.50, (Al/Sr)=25.98-77.94, and for BaB6: (B/Ba)=3.00-5.70, (Al/Ba)=34.81-76.28. The heating temperature and time: at above 1450°C for longer than 10h in any case.
Cubical single crystals composed of {100} faces, and needle-like and thick-plate-like crystals having well-developed (100) faces, respectively, were grown.
The lattice constants (a0) and densities (D) of single crystals:
CaB6: a0=4.15215±0.00008(Å), D=2.44±0.02g/cm3 SrB6: a0=4.19756±0.00009(Å), D=3.37±0.03g/cm3 BaB6: a0=4.26961±0.00008(Å), D=4.26±0.03g/cm3
Values of Knoop-microhardness determined on (100) faces of single crystals:
CaB6: 1840-1940kg/mm2 SrB6: 1680-1960kg/mm2 BaB6: 1770-2010kg/mm2
As-grown single crystals were used for the oxidation in air at temperatures between 700° and 1200°C. Oxidation began to proceed at 720°-730°C, and noncrystalline oxidation products were always obtained.
The oxidation proceeded in two stages. The initial stage of oxidation was found to be expressed by the general oxidation rate equation, (dw)n=kt, and apparent activation energies of CaB6, SrB6 and BaB6 single crystals calculated on the basis of the equation were 56kcal/mol, 55kcal/mol and 64kcal/mol, respectively.
