Chemical durability of lithium borate glasses

A series of lithium borate and lithium chloroborate glasses, some of which exhibit fast ion conduction, have been tested for their corrosion resistance to molten lithium at temperatures of 180 to 250°C. In all cases, the mechanism of corosion involved formation of crystalline reaction layers. The thickness of these layers increased parabolically with time, supporting a model involving diffusion-controlled chemical attack. The rate of growth of the reaction layer was found to decrease significantly with increasing Li₂O content in the binary B₂O₃Li₂O system, and to depend on the Li₂X/B₂O₃ (X = Cl, O) in the ternary B₂O₃Li₂O(LiCl))₂ system. For glasses with high B₂O₃ contents (>70 m/o), the durability decreases with increasing chlorine concentration; while for low B₂O₃ contents (>50 m/o), the addition of LiCl increases the durability of the glasses at modest temperatures. The apparent activation energies for the corrosion process depend on initial glass composition, and vary from ∼0.5 eV for pure B₂O₃ to ∼2.0 eV for high (LiCl)₂/B₂O₃ ratios. The results suggest that a glass with minimum B₂O₃ content, consistent with glass formability, will result in optimum resistance to molten Li attack. The same glasses have been tested for their durability in water, both buffered (pH=7) and unbuffered solutions at various temperatures. All glasses dissolved at a constant rate, suggesting a reaction-controlled mechanism of attack. A minimum in dissolution rate was found at about 25-30 m/o Li₂O in the binary B₂O₃Li₂O system, and at O/B∼1.7 for glasses in the B₂O₃Li₂O(LiCl)₂ system. For a constant O/B ratio, addition of chlorine results in a decrease of the durability of the glasses. The combined results are discussed with reference to current views of the structure of borate and chloroborate glasses with insights obtained from NMR studies and measurements of densities and glass transition temperatures.