Yuta Shimoyama

Thermoelastic properties of liquid Fe-C revealed by sound velocity and density measurements at high pressure

Carbon is one of the possible light elements in the cores of the terrestrial planets. The P-wave velocity (VP) and density (ρ) are important factors for estimating the chemical composition and physical properties of the core. We simultaneously measured the VP and ρ of Fe-3.5 wt% C up to 3.4 GPa and 1850 K using ultrasonic pulse-echo method and X-ray absorption methods. The VP of liquid Fe-3.5 wt% C decreased linearly with increasing temperature at constant pressure. The addition of carbon decreased the VP of liquid Fe by about 2% at 3 GPa and 1700 K and decreased the Fe density by about 2% at 2 GPa and 1700 K. The bulk modulus of liquid Fe-C and its pressure (P) and temperature (T) effects were precisely determined from directly measured ρ and VP data to be K0,1700K = 83.9 GPa, dKT/dP = 5.9(2), and dKT/dT = −0.063(8) GPa/K. The addition of carbon did not affect the isothermal bulk modulus (KT) of liquid Fe but it decreased the dK/dT of liquid Fe. In the ρ–VP relationship, VP increases linearly with ρ and can be approximated as VP (m/s) = −6786(506) + 1537(71) × ρ (g/cm3), suggesting that Birchs law is valid for liquid Fe-C at the present P–T conditions. Our results imply that at the conditions of the lunar core, the elastic properties of an Fe-C core are more affected by temperature than those of Fe-S core.

Compressional and shear wave velocities for polycrystalline bcc-Fe up to 6.3 GPa and 800 K

Abstract The cores of the Earth and other differentiated bodies are believed to be comprised of iron and various amounts of light elements. Measuring the densities and sound velocities of iron and its alloys at high pressures and high temperatures is crucial for understanding the structure and composition of these cores. In this study, the sound velocities (vP and vS) and density measurements of body-centered cubic (bcc)-Fe were determined experimentally up to 6.3 GPa and 800 K using ultrasonic and X-ray diffraction methods. Based on the measured vP, vS, and density, we obtained the following parameters regarding the adiabatic bulk KS and shear G moduli of bcc-Fe: KS0 = 163.2(15) GPa, ∂KS/̸P = 6.75(33), ∂KS/∂T = –0.038(3) GPa/K, G0 = 81.4(6) GPa, ∂G/̸P = 1.66(14), and ∂G/∂T = –0.029(1) GPa/K. Moreover, we observed that the sound velocity–density relationship for bcc-Fe depended on temperature in the pressure and temperature ranges analyzed in this study and the effect of temperature on vS was stronger than that on vP at a constant density, e.g., 6.0% and 2.7% depression for vS and vP, respectively, from 300 to 800 K at 8000 kg/m3. Furthermore, the effects of temperature on both vP and vS at a constant density were much greater for bcc-Fe than for ε-FeSi (cubic B20 structure), according to previously obtained measurements, which may be attributable to differences in the degree of thermal pressure. These results suggest that the effects of temperature on the sound velocity–density relationship for Fe alloys strongly depend on their crystal structures and light element contents in the range of pressure and temperature studied.