Electrical and thermal conductivities of Fe–Ni–Si alloy under core conditions: A reevaluation

We present experimental results on the electrical resistivity (the inverse of electrical conductivity) of solid hexagonal close-packed (hcp) and liquid Fe–10 at.% Ni–22.5 at.% Si (Fe–11.8 wt% Ni–12.7 wt% Si) alloys under high-pressure and high-temperature conditions, corresponding to the Earth's outer core conditions, using a diamond anvil cell. We found minimal temperature dependence of the resistivity of the hcp Fe–Ni–Si alloy, indicating thermochemically-induced resistivity saturation. We also observed that the resistivity saturation reduced the extent of resistivity change during the melting transition. Based on our findings, we estimate an upper limit for the core resistivity of approximately 110 μΩcm (= 1.10 × 10⁻⁶ Ωm) at the top of the outer core, corresponding to a lower limit for the electrical and thermal conductivities of approximately 9.2 × 10⁵ Sm⁻¹ and 90 Wm⁻¹ K⁻¹, respectively. Such high core conductivity is unavoidable and must be accounted for in understanding the Earth's thermal evolution.

“Extrapolation to core conditions of laboratory observations of electrical resistivities of iron and its alloys has been hampered by lack of understanding of interactions between the effects of temperature, pressure and impurities.”—F. D. Stacey and O. L. Anderson (2001).