Ultrasonic studies of silicate melts

The ultrasonic velocity and absorption of 32 silicate melts have been determined as a function of temperature (1175–1925 K) and frequency (3–12 MHz). The samples include the components SiO 2 , TiO 2 , Al 2 O 3 , FeO, MgO, CaO, SrO, BaO, Li 2 O, Na 2 O, K 2 O, Rb 2 O, and Cs 2 O, and range from simple alkali silicates to multicomponent synthetic and natural melts. The relaxed sound speed varies systematically with substitution of the alkali and alkaline earth oxides, decreasing monotonically with increased cation radius. An algorithm for the compositional dependence of the ultrasonic velocity at , where x i,υ is the volume fraction of component i and is independent of composition, is capable of predicting relaxed melt velocities to within 4%. The temperature dependence of the sound speeds in the nondispersive region is quite small, ≈1 part in 10 4 K −1 . A simple linear model for the compositional dependence of the derived property (δ V /δ P ) T has an uncertainty of 13%, approximately equal to that expected from the propagation of errors involving sound speed, density, thermal expansion, and heat capacity. The contributions of the changes in sound speed and density with temperature are nearly the same in their influence on the temperature dependence of the bulk modulus of natural melts. The relaxation properties c / c 0 (ratio of the sound speed to the low‐frequency value) and αλ (absorption per wavelength) of all of the liquids studied are very similar when plotted against ωτ, the product of the angular frequency and the shear relaxation time. Both properties are consistent with τ equal to 1% of the product of the low‐frequency shear viscosity and bulk modulus. Dispersion begins when ωτ ≃ 0.1 and the limiting high‐frequency sound speed c ∞ is about 2.3 times c 0 . The product αλ attains a maximum value of about 1.4 when ωτ ≃ 0.8. The data cannot be explained by a single relaxation time in the melts; rather, a spectrum of relaxation times is indicated.