Heat capacities of thermally treated Na2O-3B(2)O(3) glasses above and below T-g

Various contributions have been obtained from the heat capacities of "as quenched" and annealed Na2O-3B2O3 glasses. It is shown that the enthalpic release gives a major contribution to the DSC curves obtained by reheating quenched glasses. Different relaxation processes are followed by comparing DSC scans of quenched and annealed samples in the vitreous state and around the glass transition temperature, Tg. As a function of the annealing time at T = Tg - 30°C, the specific heats at room temperature initially increase, due to the decrease of the exothermic contribution from quenched-in defects, which are absent in a fully annealed glass; over a month-long time scale these heat capacities drop, due to a partial ordering, which decreases the configurational contribution. Such a decrease is related with the "overshoot" which occurs above Tg and which attains a limiting value ΔHg ≅ 14 J mol-1 in a matter of days. The heat capacities above Tg relax towards a value of Cp of ≅ 2.3 J mol-1 K-1 after a few days of annealing, which is intermediate between the anomalously low Cp (∼1.8 J mol-1 K-1) of a fast quenched (∼103 K min-1) and that (∼2.9 J mol-1 K-1) of a sample annealed for one hour. These phenomena point to complex relationships among enthalpic relaxation, entropie relaxation and temperature in these glasses.