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

了解地球矿物在极端条件下的热传导率和热扩散率对于了解地球的物理和化学过程及其演变非常重要。地幔热传输速率对地球磁场的存在和稳定至关重要。地幔内部的温度分布取决于对流、传导和辐射的热传递速率。了解这些过程需要了解作为压力和温度函数的热导率。在该项目中，研究人员建议通过直接光学测量其晶格和辐射贡献来确定地球矿物在高压和高温条件下的热导率。在该建议中引入的用于测量晶格热导率的技术利用由感兴趣的材料包围并暴露于脉冲激光辐射的金属吸收体的周期性前表面温度变化（通过分光辐射度测量）。 他们通过将实验结果拟合到模型有限元计算来提取矿物的热扩散率。为了提取热导率的辐射部分，在近红外、可见光和紫外光谱范围内测量矿物的光学性质。光谱是在广泛的压力-温度条件下（高达130 GPa和4000 K）测量的，以解决温度效应以及自旋配对和钙钛矿-后钙钛矿转变对下地幔中岩石形成矿物的辐射热传递的影响。目前正在研究硅酸盐钙钛矿和镁橄榄石这两种地球下地幔的主要相;正在使用由组成接近地幔的预合成材料生长的单晶作为样品。还将在通过激光加热合成的材料上测量后钙钛矿相的热导率。这些实验数据给出了热扩散率的辐射和传导部分的直接估计，因此它们可以用于地球热过程的模型中，从而为这些模型和我们目前对地球演化的理解提供了关键的测试。这项工作的结果有广泛的影响，在各种其他领域，包括物理和化学，生物学和软物质，材料科学和技术，以及先进能源系统的极端环境下的材料，因为所有这些都将受益于极端条件下的热物理参数测量的原位技术的发展。最近设计的用于拉曼、光学和红外光谱的多功能光学系统正被用于在高压和高温的极端条件下进行现场热导率测量，并伴有外部和激光加热。这为华盛顿的光学设施增加了新的功能，该设施可供更广泛的高压研究社区使用，包括访问研究人员和学生，通过一些NSF支持的计划，如COMPRES和卡内基暑期实习计划，以及总部设在卡内基的DOE支持的CDAC高压中心。一系列的学生，包括地区高中生，本科生，研究生和博士后助理，从卡内基和其他地方的科学培训中受益，通过参与尖端科学，正在开发整个过程中这项工作。

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.