Thermodynamic Activities in B 2 O 3 ‐SiO 2 Melts at 1475 K

Eight glass samples in the B 2 O 3 ‐SiO 2 system with compositions from 20 to 90 mol% B 2 O 3 were prepared. The equilibrium vaporization was studied by Knudsen effusion mass spectrometry at temperatures between 1450 and 1500 K. B 2 O 3 ( g ) was the most abundant boron‐containing species in the vapor; no silicon‐containing gaseous species were detected. Thermodynamic activities of B 2 O 3 in the liquid were determined at 1475 K. Thermodynamic activities of SiO 2 and integral excess Gibbs energies were estimated from the thermodynamic activities of B 2 O 3 . The thermodynamic data support the results obtained by other methods indicating the existance of a miscibility gap in the metastable liquid.