DSC thermal anomalies in the glass transition temperature domain for a boron oxyfluoride (BOF) glass

摘要

Heat capacity measurements by DSC in the glass transition domain of BO_(1.34)F_(0.32) glass have allowed the determination of two glass transition temperatures T_g on heating up to (T_g+80)K. This result is in good agreement with a previous structural characterisation of a BO_(1.32)F_(0.36) glass by NMR, thus demonstrating the appearance of new polyhedral species near T_g and the formation of BF_3 trapped in the glass matrix. The difference between the heat capacities of the solid and the liquid phases, 0.8 J/gK, reveals the fragile character of the melt.The structural, physical and chemical characteristics of B_2O_3 based glasses have been extensively studied because of the good glass forming character of this oxide.~(1) In the ternary phase diagram B-O-F, the only well known compounds are BF_3 and B_2O_3. When BF_3 and B_2O_3 are heated in a closed volume, a glass phase is formed. These BOF glasses have not only been characterised by NMR and mass spectroscopies, but their structures have been studied by computer simulation.~(1-3) Their exact compositions were determined by chemical and charged particle activation energy methods and are well represented by the following formula BO_(1.5-0.5x)F_X, x varying between 0.2 and 0.6.