Chemical durability of lithium borate glasses

Abstract

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 Li 2O content in the binary B 2O 3Li 2O system, and to depend on the Li 2 X/B 2O 3 ( X = Cl, O) in the ternary B 2O 3Li 2O(LiCl)) 2 system. For glasses with high B 2O 3 contents (>70 m/o), the durability decreases with increasing chlorine concentration; while for low B 2O 3 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 B 2O 3 to ∼2.0 eV for high (LiCl) 2/B 2O 3 ratios. The results suggest that a glass with minimum B 2O 3 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 Li 2O in the binary B 2O 3Li 2O system, and at O/B∼1.7 for glasses in the B 2O 3Li 2O(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.