HEAT-CAPACITIES OF THERMALLY TREATED NA2O-3B(2)O(3) GLASSES ABOVE ANDBELOW T-G

Various contributions have been obtained from the heat capacities of ' 'as quenched'' and annealed Na2O-3B(2)O(3) glasses. It is shown that t he enthalpic release gives a major contribution to the DSC curves obta ined by reheating quenched glasses. Different relaxation processes are followed by comparing DSC scans of quenched and annealed samples in t he vitreous state and around the glass transition temperature, T-g. As a function of the annealing time at T = T-g- 30 degrees C, the specif ic heats at room temperature initially increase, due to the decrease o f the exothermic contribution from quenched-in defects, which are abse nt in a fully annealed glass; over a month-long time scale these heat capacities drop, due to a partial ordering, which decreases the config urational contribution. Such a decrease is related with the ''overshoo t'' which occurs above T-g and which attains a limiting value Delta H- g congruent to 14 J mol(-1) in a matter of days. The heat capacities a bove T-g relax towards a value of C-p of congruent to 2.3 J mol(-1) K- 1 after a few days of annealing, which is intermediate between the low C-p (similar to 1.8 J mol(-1) K-1) of a fast quenched (similar to 10( 3) K min(-1)) and that (similar to 2.9 J mol(-1) K-1) of a sample anne aled for one hour. These phenomena point to complex relationships amon g enthalpic relaxation, entropic relaxation and temperature in these g lasses.