Temperature Dependences of the Density of Borate Glasses in Equilibrium States at Temperatures below the Glass Transition Point

The kinetics of density relaxation during isothermal treatment of samples of vitreous boron oxide and sodium borate glasses (containing 15, 20, and 30 mol % Na2O) cooled from high temperatures and after heating of the samples stabilized at low temperatures is investigated over a wide range of temperatures below the glass transition point T g. It is established that there exists a temperature T-s below which the density in the equilibrium state does not depend on the temperature, i.e., the temperature at which the supercooled liquid transforms into a new noncrystalline solid state. The distinguishing feature of this state is the absence of structural transformations after a change in the temperature. The temperatures of the transition of the supercooled liquid to the noncrystalline solid state and the ranges of lower temperatures at which the supercooled liquid can reach an equilibrium state within the limits of experimental error coincide with those previously obtained in the study of relaxation processes in these glasses by the small-angle X-ray scattering technique.