Activity of nickel oxide in alkali borate melts

The activity/concentration relationships of NiO in R2O-B2O3 melts (R = Li, Na or K) have been studied by saturating glass melts contained in a nickel crucible under different partial pressures of oxygen. A pure boric oxide melt dissolves very little NiO (solubility less than 0.001 wt %); on adding R2O the solubility increases and reaches a maximum around 20 mol % for Li2O and Na2O, and 30 mol % for K2O; thereafter the solubility of NiO decreases with increasing R2O content of the melt, the maximum decrease being in K2O-B2O3 melts. It is suggested that the low solubility of NiO in low- and high-alkali borate melts is due to poor donor capacity of BΦ3/2 groups and high activity of oxide ions respectively. NiO in alkali borate melts produces a non-ideal solution.