Direct measurements of thermal conductivity of materials at high pressures and high temperatures using the flash laser heating method with a Pockels cell

Abstract:

At the Earth’s core-mantle boundary, thermal conduction through iron-bearing silicates and iron alloys controls heat flow and therefore exerts an important control on the geodynamo. Direct measurements of thermal conductivity of Earth minerals, for instance, ferropericlase, at extreme pressures and temperatures are very challenging; the available experimental data are limited and inconclusive, causing geodynamical studies to rely on theoretical calculations and/or large extrapolations.

In the present work, we demonstrate an optimized technique to perform direct measurements of thermal conductivity of materials in diamond anvil cells at high pressures and high temperature. We used a continuous 100-Watt infrared laser to heat the sample from both sides to a desired temperature and a pulsed 300-Watt infrared laser to create a microsecond long heat wave from one side of the sample. Thermal conductivity can be determined by measuring temperature of each side of the sample as a function of time and fitting the results with finite element (FE) calculations. Temperature is measured radiometrically, using a streak camera coupled to a single-stage grating spectrograph. KCl was used as pressure transmitting medium. Ir was used for absorption of the laser if the absorption of the sample was small. The laser pulse was truncated to a square wave by a Pockels cell, which simplified and improved the accuracy of FE calculations. Furthermore, we significantly improved FE analysis by introducing automatic procedures based on python codes, enabling the fitting parameter optimization. Fast measurements of thermal emissions allowed us to resolve the heating response through the sample, enabling direct measurements of thermal conductivity of the Earth’s lower mantle minerals and Fe alloys in the core.

This complex system can be used for pressures in the range of 5 to 150 GPa for materials with any laser absorption properties. We used this system to obtain the values of thermal conductivity of ferropericlase, iron-bearing bridgmanite, and iron-silicon alloys which correspond well with literature data albeit mainly measured at room temperature.