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导率和热扩散率对于了解地球的物理和化学过程及其演化很重要。通过地幔的热传输速度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、传导和辐射的传热率。要了解这些过程，需要了解作为压力和温度的函数的导热系数。在这个项目中，研究人员建议通过直接光学测量矿物的晶格和辐射贡献来确定地球矿物在高压和温度条件下的热导率。这项建议中介绍的测量晶格热导率的技术利用了被感兴趣的材料包围并暴露在脉冲激光辐射下的金属吸收体的周期性前表面温度变化(通过光谱辐射测量)。他们通过将实验结果与模型有限元计算进行拟合来提取矿物的热扩散率。为了提取导热系数的辐射部分，矿物的光学性质在近红外、可见光和紫外光谱范围内进行测量。在很宽的压力-温度条件下(高达130 Gpa和4000K)测量了光谱，以研究温度效应以及自旋配对和钙钛矿-后钙钛矿转变对下地幔造岩矿物辐射热传递的影响。人们正在研究地球下地幔的两个主要相--硅酸盐钙钛矿和镁钙钛矿；单晶是由预合成材料生长的，其成分与地球地幔中的成分接近。还将在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，从而对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。这项工作的成果对物理和化学、生物和软物质、材料科学和技术以及高级能源系统的极端环境下的材料等其他领域都有广泛的影响，因为这些领域都将受益于极端条件下热物理参数原位测量技术的发展。最近设计的用于拉曼光谱、光学光谱和红外光谱的多功能光学系统正在适应外部加热和激光加热的高压和高温极端条件下的原位热导率测量。这为华盛顿特区的光学设施增加了新的功能，通过一系列NSF支持的项目，如COMPRES和卡内基夏季实习生计划，以及美国能源部支持的总部设在卡内基的CDAC高压中心，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。一系列学生，包括地区高中生、本科生、研究生和博士后助理，通过参与在整个工作过程中发展的尖端科学，从卡内基和其他地方的科学培训中受益。

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.