The effects of annealing and aluminum substitution on the elastic behavior of alkali silicate glasses

In an effort to better understand the structural dynamics of glasses, we have measured the second-order adiabatic elastic moduli, and their pressure and temperature derivatives, of a sodium silicate and a sodium alumino-silicate glass. In both cases, we find increasing compressional and decreasing shear wave velocities with pressure. Substantial hysteresis is present in both glasses, especially in the shear-wave velocities. We also have studied the effects of annealing by measuring the elastic properties before and after annealing. We find that, in addition to previously observed increase in density and moduli, the pressure and temperature derivatives also increase, or become less negative. Comparison of the two glasses, which have nearly the same oxygen packing densities and unoccupied volumes, shows that substitution of aluminum for silicon increases the elastic moduli substantially. Shear modulus increases are accounted for by elimination of non-bonding oxygens, but bulk and Young's modulus changes are associated either with bond strengths in the network-forming tetrahedra, or with steric hindrance effects. In contrast, aluminum substitution does not affect the pressure derivatives of the moduli. The pressure derivatives are proportional to the unoccupied volume fraction in the glass, which is a measure of ion packing efficiency. This suggests that packing efficiency alone controls moduli pressure derivatives in alumino-silicate glasses.