Constant electrical resistivity of Ni along the melting boundary up to 9 GPa

Abstract

Characterization of transport properties of liquid Ni at high pressures has important geophysical implications for terrestrial planetary interiors, because Ni is a close electronic analogue of Fe and it is also integral to Earth's core. We report measurements of the electrical resistivity of solid and liquid Ni at pressures 3–9 GPa using a 3000 t multianvil large volume press. A four‐wire method, in conjunction with a rapid acquisition meter and polarity switch, was used to overcome experimental challenges such as melt containment and maintaining sample geometry and to mitigate the extreme reactivity/solubility of liquid Ni with most thermocouple and electrode materials. Thermal conductivity is calculated using the Wiedemann‐Franz law. Electrical resistivity of solid Ni exhibits the expected P dependence and is consistent with earlier experimental values. Within experimental uncertainties, our results indicate that resistivity of liquid Ni remains invariant along the P‐dependent melting boundary, which is in disagreement with earlier prediction for liquid transition metals. The potential reasons for such behavior are examined qualitatively through the impact of P‐independent local short‐range ordering on electron mean free path and the possibility of constant Fermi surface at the onset of Ni melting. Correlation among metals obeying the Kadowaki‐Woods ratio and the group of late transition metals with unfilled d‐electron band displaying anomalously shallow melting curves suggests that on the melting boundary, Fe may exhibit the same resistivity behavior as Ni. This could have important implications for the heat flow in the Earth's core.

Heat Flow in Earth's Core From Invariant Electrical Resistivity of Fe‐Si on the Melting Boundary to 9 GPa: Do Light Elements Matter?

“…Fe‐4.5Si is a ferromagnetic alloy where magnetism originates from a partially filled spin‐polarized 3d band. Before the onset of the Curie temperature ( T C ), ρ of the A2 phase in the experimental pressure range 3–9 GPa exhibits the expected T 2 dependence characteristic of strong spin scattering of conduction electrons (Campbell & Fert, ) similar to the behavior of pure Fe, Ni, and Co (Secco & Schloessin, ; Silber et al, , ; Ezenwa & Secco, ; Figure ). T C marks the temperature at which the Fe alloy loses its long‐range magnetic spin moments and can be generally identified as the temperature at which the resistivity behavior departs from T 2 dependence.…”

Electrical Resistivity of Iron Phosphides at High‐Pressure and High‐Temperature Conditions With Implications for Lunar Core's Thermal Conductivity

“…Previous studies indicate that the electrical resistivity of the solid metal and iron alloys (Fe‐Si, Fe‐C) decreases linearly with increasing pressures (Deng et al, ; Ezenwa & Secco, , ; Kiarasi & Secco, ; Silber et al, ; Zhang et al, ). Similar behavior was also observed for iron phosphides in this study (Figure ).…”

“…Another factor that may affect resistivity is hydrostaticity during compression. We used a similar pressure medium as Silber et al (, ): hard Al 2 O 3 plus boron nitride. Notably, the Curie temperatures and the pressure derivative of the resistance for Ni in Silber et al () are larger than the results from Sundqvist () and Decker and Chen () who used liquid pressure medium (petroleum ether and silicone oil, respectively) to provide hydrostatic condition.…”

“…The total resistivity contribution of the discs was estimated at ~1.5% for Fe 3 P and less than 1% for FeP and Fe 2 P at high‐temperature conditions. Using the methods described by Ezenwa and Secco (, ) and Silber et al (), four W‐Re thermocouple wires (0.3‐mm diameter, W5%Re–W26%Re) legs acted as electrodes during the resistance measurement, fully contacting with the molybdenum electrodes. One more thermocouple was used to monitor temperature, emplaced vertically in the center of the sample and attached to the outer of the ceramic tube.…”

“…With increasing pressure, the resistivity of solid iron and iron alloy decreases, yielding higher thermal conductivity. It is hard to say their melts may follow the same rule because experiments show that the electrical resistivity is invariant at the melting points of Fe at 6–12 GPa (Silber et al, ), Ni up to 9 GPa (Silber et al, ), and Co up to 5 GPa (Ezenwa & Secco, ). The thermal conductivity of solid Fe 3 P at ~5 GPa should be slightly higher than the calculation in Figure .…”