Heat capacities and thermal behavior of alkali borate glasses

The heat capacities of selected glasses in the five alkali borate systems have been measured over a range of high temperatures which includes the respective glass transition regions. The heat capacities per gram atom at 350°K show little variation with composition while those at the low temperature ends of the glass transition, T_g-, show some systematic variations. When compared with the respective 3R values, the heat capacities at T_g- range from about 0.75 for B₂O₃ to values in excess of unity for various alkali borate compositions. The present data on heat capacity have been combined with previous data on the elastic moduli, densities and thermal expansion coefficients to evaluate the Grüneisen constant, γ, for each composition for temperatures around ambient. For B₂O₃, γ has a low value of about 0.25. It decreases with additions of Li₂O and increases with additions of the other alkali oxides.