Relaxation of longitudinal ultrasonic waves in some tellurite glasses

The longitudinal ultrasonic absorption in binary (100x)TeO2–xNb2O5 and ternary (100x)TeO2–0.5xNb2O5–0.5xLi2O, tellurite glass systems, was measured using pulse echo technique at ultrasonic frequencies 2, 4, 6 and 8 MHz in the temperature range from 200 to 280 K. The relaxation spectra or the shape of maximum peaks showed the presence of well-defined broad peaks at various temperatures depending upon the glass composition and operating frequency. The maximum peaks shift to higher temperatures with increasing frequency suggesting some kind of relaxation process. This process has been interpreted as a thermally activated relaxation process which arise when ultrasonic waves disturb the equilibrium of an atom moving in a double-well potential in the glass network. Results showed that the mean activation energy of the process is strongly dependent on the modifier content. The dependence of the maximum peaks on composition was analyzed in terms of an assumed loss of standard linear solid type, with low dispersion, and a broad distribution of Arrhenius type relaxation with temperature independent relaxation strength. The relaxation strength and deformation potential were determined experimentally and theoretically.