Surface modification of polyvalent element-containing glasses

Seven soda-lime silicate glasses, each of which contains one of the following polyvalent metals: Fe, Mn, Cu, Ce, Ti, V, and Cr, are oxidized in air and reduced in H2/N2 (1/99) at their respective glass transition temperature for some period. A crystalline oxide surface layer is created on the glasses (except the vanadium-bearing glass) under the oxidizing condition, since the metallic ions are oxidized from lower to higher valence state, and thereby calcium ions diffuse outward and react with oxygen ions. In contrast, a silica-rich surface layer is created on the glasses under the reducing condition, since sodium and calcium ions diffuse inward. It is found that the extents of both outward and inward diffusions strongly depend on the type of the polyvalent ions for the same conditions of heat-treatment. Out of the seven polyvalent metals studied in this work, copper induces the highest extent of both the inward and outward diffusion, and hence, the thickest surface layer of both amorphous silica and crystalline alkaline earth oxides. The oxide layer lowers the onset temperature of the primary crystallization. The silica-rich surface layer enhances the chemical resistance of the glass in a hot basic solution.