Ultrasound propagation in the sodium borosilicate glass system

Abstract

Measurements of ultrasound wave velocity and attenuation are used to investigate the structure of Na(_2)O-B(_2)O(_3)-SiO(_2) glasses. The propagation characteristics of longitudinal and shear waves between 1.3 K and 400 K at frequencies between 12 MHz and 60 MHz are dominated by a broad and intense acoustic loss peak whose height and position are frequency sensitive. Of the previously proposed models for the mechanism of the acoustic loss, which also occurs in other inorganic glasses, the ultrasound absorption is most consistent with a thermally activated structural relaxation involving the transverse vibration of an oxygen atom between two potential wells of equal or nearly equal depth in the plane of a non linear cation-oxygen-cation bond. An Akhieser type acoustic phonon-thermal phonon interaction is shown no to be responsible for the observed loss. An attempt frequency of l0(^13)Hz and a distribution of activation energies out to 12 K cal/mole but with a mean value of about 3 K cal/mole are found for the relaxation mechanism in the Na(_2)O-B(_2)O(_3)-SiO(_2) glasses. The absolute value and the temperature coefficient of ultrasound velocity, and the maximum acoustic loss are strongly dependent on the total Na(_2)O content of the glasses. Ultrasound propagation characteristics are also affected by phase-separation inducing heat treatment: the steady rise in the height of the loss peak and the complex behaviour of the ultrasound velocity with time of treatment suggest that structural rearrangement is still taking place in the individual glassy phases even after long periods of heat treatment. Also reported is the existence of a small acoustic loss peak at liquid helium temperatures in the Na(_2)O-B(_2)O(_2)-SiO(_2) glasses. This feature of the ultrasound absorption spectrum is characteristic of many tetrahedrally coordinated inorganic glasses. On the assumption of an Arrhenius activation process for this loss peak, an activation energy of 60 ± 15 cal/mole and an attempt frequency of 10(^10) to 10(^12) Hz is indicated.