PHYSICAL-PROPERTIES OF MOLTEN LITHIUM TETRABORATE

The physical properties of molten Li2B4O7 were studied for growing hig h-quality single crystals. We found that the density, rho, surface ten sion, gamma, and volume thermal expansion coefficient of the melt, bet a, varied as rho = 2.444-0.000414T g/cm3, gamma = 249-0.045T mN/m and beta = 2.1 x 10(-4) K-1, respectively, for temperatures of 1190 to 137 3 K. The temperature dependence of the viscosity did not show a simple exponential relationship. It was found that the flow unit volume at t he melting point (1190 K) was 10 times larger than that at 1373 K. The viscosity and density displayed relaxation after the starting materia l was completely melted, showing that a minimum of 15 h was required f or the melt to become stable at 1223 K. Our results suggest that for g rowing high-quality Li2B4O7 single crystals the melt should be held mo re than 15 h before the pulling process is started.