Structure and Thermal Properties of High-Alkali Molybdenum-Containing Borosilicate Host Materials

To study immobilization properties of oxide borosilicate systems, we have assessed the effect of molybdenum on the structure, phase composition, and thermal properties of glass-ceramic materials prepared via rapid cooling of high-alkali melts in the Li2O–(Na2O–K2O)–B2O3–SiO2 system. The results demonstrate that the molybdenum-free materials consist of homogeneous borosilicate glass, which partially crystallizes during heat treatment (annealing). The only exception is a glass sample of the K2O–B2O3–SiO2 system, which has high thermal stability. The addition of molybdenum oxide to these materials in the high-temperature synthesis step initiates crystallization processes, which are most pronounced after heat treatment (annealing). In the materials containing one alkali metal cation in their composition, all of the added molybdenum oxide enters into the composition of the alkali metal molybdates embedded in a homogeneous amorphous highly polymerized borosilicate matrix. In the samples containing cations of two alkali metals in their composition, crystallization processes lead to the formation of a highly inhomogeneous crystalline material. The good homogeneity of the glass-ceramic materials prepared via rapid cooling of high-alkali molybdenum-containing melts of the Li2O–(Na2O–K2O)–B2O3–SiO2 system with one type of alkali metal cation in their composition allows them to be recommended for temporary immobilization of molybdenum oxide-rich radioactive waste.