Optical Study of Thermal Conductivity of the Mantle Minerals at High Pressure and Temperature

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Knowledge of thermal conductivity and thermal diffusivity of the Earth's minerals under extreme conditions is important for understanding the physical and chemical processes and their evolution in the Earth. The rate of the heat transport through the mantle is crucial for the existence and stability of the Earth's magnetic field. The temperature distribution inside the Earth's mantle depends on the rate of heat transfer by convection, conduction, and radiation. An understanding of these processes requires knowledge of the thermal conductivity as a function of pressure and temperature. In this project, the investigators propose to determine the thermal conductivity of the Earth's minerals under conditions of high pressure and temperature by direct optical measurements of its lattice and radiative contributions. The technique introduced in this proposal for measurements of the lattice thermal conductivity utilizes a periodic front surface temperature variation (measured by the spectroradiometry) of a metallic absorber surrounded by the material of interest and exposed to a pulsed laser radiation. They extract the thermal diffusivity of minerals by fitting the experimental results to the model finite element calculations. To extract the radiative part of thermal conductivity, optical properties of minerals are measured in near infrared, visible, and ultraviolet spectral ranges. The optical spectra are being measured in a wide range of pressure-temperature conditions (up to 130 GPa and 4000 K) to address the temperature effects and also the effects of the spin-pairing and perovskite- postperovskite transitions on the radiative heat transfer of the rock forming minerals in the lower mantle. Silicate perovskite and magnesiowustite, which are the two dominant phases of the Earth's lower mantle, are being studied; single crystals, grown of pre-synthesized materials with a composition close to that in the Earth's mantle, are being used as samples. The thermal conductivity of the postperovskite phase will also be measured on material synthesized by the laser heating. These experimental data give a direct estimation of the radiative and conduction parts of the thermal diffusivity, so they can be utilized in models of the thermal processes in the Earth, thus providing a crucial test of these models and our current understanding of the Earth's evolution. The results of this work have broad impact in various other fields including physics and chemistry, biology and soft matter, materials science and technology, and materials under extreme environments for advanced energy systems since all of them would benefit from the development of in situ technique for measurements of thermophysical parameters under extreme conditions. The recently designed versatile optical system for Raman, optical and IR spectroscopy is being adapted for in situ thermal conductivity measurements under extreme conditions of high pressure and temperature with external and laser heating. This is adding new features to the optical facility in Washington, DC, which is available for use by the broader high pressure research community, including visiting researchers and students, through a number of NSF-supported programs such as COMPRES and the Carnegie Summer Intern Program, as well as the DOE-supported CDAC high-pressure center headquartered at Carnegie. A range of students, including area high school students, undergraduates, graduate students, and postdoctoral associates, benefit from the scientific training at Carnegie and elsewhere through participating in cutting-edge science that is being developed throughout the course of this work.