Fe3C的高温高压物性研究

Cosmochemical and seismic data indicate the earth’s core is consisted by iron and a small amount of light elements, details about the species and proportions of the light elements are crucial for us to understand the core dynamics, the accretion process of primal earth, the origin of the geomagnetic, and so on. Carbon is an important potential light element in the core, especially the inner core. Carbon in the core also affects the deep carbon circle of the earth. Comparing the experimental determined densities and sound velocities of iron-light element compounds under high temperature-pressure conditions with seismic data is an effective way to constrain the amount of light-elements in the core. For iron-carbon compounds, the available sound velocity data in the literatures have obvious discrepancy, and most of them are measured under room temperature. In this project, the densities and sound velocities of Fe3C will be measured in a pressure-temperature range more close to the core by the dynamic compression method. By comparing with static compression results, the thermodynamic parameters of Fe3C under high temperature-pressure conditions and the temperature effect on the sound velocity will be determined, which will help constraining the carbon amount in the core.

宇宙化学及地震波数据表明地核是由铁及少量轻元素构成，而轻元素的种类及含量对地核动力学、原始地球的吸积、地磁场的起源等问题至关重要。碳有可能是地核，特别是内地核中的重要轻元素之一。地核碳含量亦与地球深部的碳循环密切相关。将实验测得的铁-轻元素化合物在高温高压下的密度和声速与地震波数据比较，是约束轻元素含量的有效方法。但是目前对于铁-碳化合物声速数据的研究存在争论，且缺乏高压同时高温下的数据。本项目拟采用动高压实验方法，在更接近地核的压力和温度范围内研究Fe3C的密度和声速等物性，并与静高压数据比较约束Fe3C在高温高压条件下的热力学参数，研究温度对声速的影响，为限定地核内碳元素含量提供重要实验支撑。

使用大体积多面顶压机通过高温高压法合成了致密的块状Fe3C样品。使用二级轻气炮在65 -250 GPa范围了测量了Fe3C的雨贡纽数据。Fe3C的冲击波速度（Us）和粒子速度（Up）的关系为线性： Us=4.77（7）+1.60（2）Up。通过第一性原理模拟和热力学计算分别确定了极端条件下Fe3C的电子和晶格格林爱森参数。随后，使用Fe3C的热状态方程来约束内地核的碳含量。我们发现，如果碳是地核中唯一的轻元素，则需要2.9±0.4 %比重的碳来解释内地核的密度亏损。Fe3C的声速测量工作正在进行中，目前需要补充高压下的声速数据。