SOUND-VELOCITY MEASUREMENTS IN LIQUID-IRON BY ULTRASONIC INTERFEROMETRY

Compressional wave velocity V(p) has been measured in liquid iron in t he temperature range 1823-1883 K and in the frequency range 5.4-11.8 M Hz for the first time using ultrasonic interferometry. The value of V( p) at the melting temperature is 3820 +/- 3 m s-1 which is comparable to previous measurements all using time-of-flight methods. The tempera ture derivative of velocity, dV(p)/dT, in this temperature range (-0.7 2 +/- 0.07 m s-1 K-1) and the lack of dispersion observed in this freq uency range are in agreement with previous studies. Our calculated val ue of the relaxed (adiabatic) bulk modulus K0 at the melting temperatu re is 102 GPa. Estimate of the average longitudinal relaxation time ta u1 indicates that the liquid iron is in a relaxed state in the present frequency range. Accurate measurement of attenuation is not yet possi ble with our present apparatus, but typical interferograms indicate a very low attenuation, which is qualitatively consistent with the predi ction from viscoelastic theory and seismological determinations of att enuation of Earth's outer core.