Ultrasonic velocity in potassium borate glasses

Abstract

Ultrasonic velocity in potassium borate glasses denoted by the composition formula x 2K 2O· x 1B 2O 3 ( x 1 and x 2 are mole fractions of B 2O 3 and K 2O, respectively, and x 1 + x 2 = 1) is measured as a function of x 2 at a frequency of 10 MHz and at a temperature of 298 K by the ultrasonic pulse echo overlap method. The elastic property of these glasses is analyzed in terms of the three structural units defined as B Ø 3 ≡ a, K + B Ø 2O − ≡ b and K +B Ø 4 − ≡ c, where Ø represents a bridging oxygen and constants of these structural units are defined on the basis of the elastic internal energy due to deformation. It is shown numerically that the structural unit c increases the rigidity of the glass whereas the structural unit b decreases it. By use of the values of the three elastic constants, the fraction, N 4, of boron atoms in tetrahedral coordination is calculated as a function of x 2/(1− x 2). It is shown especially that non-bridging oxygen begins to form at x 2 = 0.065 and also the rate at which N 4 increases with increasing x 2 changes suddenly at x 2 = 0.20.