Thermal expansion and transition temperature of glasses in the systems BeO–Al2O3–B2O3 and MgO–Al2O3–B2O3

Researches were conducted for glasses in the systems BeO–Al₂O₃–B₂O₃ and MgO–Al₂O₃–B₂O. The following characteristics have been determined: thermal expansion coefficient within 20–300 °C, structural thermal expansion coefficient (STEC) and glass transition temperature. The discussed properties of Be-aluminoborate and Mg-aluminoborate glasses have been compared with those of Ca-, Sr- and Ba-aluminoborate glasses. TEC of studied glasses gets higher going from Be-aluminoborate glasses to Ba-aluminoborate ones, but T_g decreases under the same succession. The dependence of STEC on the nature of a given cation is more complicated. The pattern of dependence of the properties on composition is due to changing the boron coordination number with respect to oxygen, which is the main network-former, and to competition between the aluminium and boron for the oxygen brought by an ion-modifier. The difference between the effect of one ion-modifier and that of another is determined by decreasing the radius of an ion and increasing its electric field strength in the succession from Ba²⁺ to Be²⁺. The ions of Be and Mg can also act as network-former to some extent.