ULTRASONIC VELOCITY IN ALKALI METAL BORATE GLASSES

摘要

The two different velocities of longitudinal and transverse ultrasonic waves in alkali metal borate glasses have been measured by the pulse-echo overlap method at a frequency of 10 MHz and a temperature of 298 K over the glass-forming region of each binary system x_2M_2O·(1 - x_2)B_2O_3, where M stands for Li, Na, K, Rb and Cs, and x_2 denotes the mole fraction of M_2O. Each of the two velocities plotted against x_2 showed the following characteristics: the velocity in lithium borate glasses increased monotonously with increasing x_2 in the composition range studied in this paper (0 ≤ x_2 < 0.28); the velocity in two binary systems of sodium and potassium borate glasses showed a single maximum; the velocity in two binary systems of rubidium and caesium borate glasses showed a maximum, a minimum and another maximum in succession. The elastic property of the glasses is analysed in terms of the three structural units defined as BO_3 ≡ a, M~+BO_2O~- ≡ b and M~+ BO_4~- ≡ c, where O represents a bridging oxygen and O a nonbridging oxygen, on the assumption that these structural units have their respective elastic constants defined on the basis of a thermodynamic equation of deformed body.