Abstract
The Earth’s core mainly consists of iron, and its thermal transport properties are of vital importance for our understanding of the thermal evolution and the dynamics of the core and the mantle. However, the reported values of thermal conductivity of iron at the core conditions are so far inconclusive. Although hexagonal closed-packed (hcp) iron is often studied as a proxy metal to investigate the physical properties not only of the inner core, but also the outer core, the anisotropy of the thermal conductivity of hcp iron has never been experimentally examined. Here we report the results of texture analyses by means of synchrotron X-ray diffraction experiments and thermal conductivity measurements on polycrystalline hcp iron up to 44.5 GPa and 300 K. These results indicate that the thermal conductivity of single crystal hcp iron along c axis is about 3-4 times higher than that along a axis, which could have partially caused the controversial values of the thermal conductivity of hcp iron at the Earth’s core conditions.
Highlights
At some point in the past, the Earth’s liquid iron alloy core began to crystallize at Earth’s center, resulting in the birth and growth of the solid inner core
Prior to the high-P experiments, we investigated the crystallographic preferred orientation (CPO) of bcc iron starting materials loaded into a DAC at ambient conditions
In hcp iron that was transformed from bcc foil, completely different CPO patterns were observed depending on the run: was aligned along the compression axis in Runs 2 and 7 (Figure 7A), whereas was aligned along the compression axis in Runs 3 and 4
Summary
At some point in the past, the Earth’s liquid iron alloy core began to crystallize at Earth’s center, resulting in the birth and growth of the solid inner core. Due to the experimental difficulty in measuring thermal conductivity at such extremely high P-T conditions, conventional studies have measured electrical conductivity (σ , the inverse of electrical resistivity) of iron and iron alloys, and converted it to the electronic thermal conductivity (κ el ) via the Wiedemann-Franz relation, culprit of the discrepancy in the estimated iron conductivities by Konôpková et al (2016) and Ohta et al (2016) Both studies used iron foil as a sample and compressed it to its thickness direction in a DAC. As analog materials of hcp iron, one can review reports of the conductivity anisotropy in other hcp metals that are stable at ambient conditions, which shows that the magnitudes of conductivity anisotropy differ from each other
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