外地核物质的预加热冲击实验与理论计算研究

The determination of the composition and state of the outer core materials plays an important role in geoscience, e.g., in the formation mechanism of geomagnetic field. As it is difficult to obtain the laboratory pressure and temperature condition that can be directly compared to those in the outer core, the research on this problem has become a hot but difficult topic. The preheated shock experiments can break through the limit of strength of material on the pressure and temperature in the Diamond Anvil Cell (DAC) experiment, and can overcome the disadvantage of the shock wave experiment that can only get specific shock states from the ambient state. Thus, it provides a new approach to solve this problem. In this project, we focus on the existence of the light element of candidate Si in the outer core. Taking Fe-Si system as the research object, we design the experiments according to the temperature and pressure range of the outer core. The sample with different Si content would be preheated to high initial temperature and even melt. Through the comparison of the shocked density and sound velocity results with the seismology data, the content of Si in the outer core can be identified. For the Si-content identified Fe-Si sample, we determine the melting curve of the outer core Fe-Si sample by comprehensively analyzing the shock melting temperature results at different initial temperatures and the preheated shock simulations based on molecular dynamics. Then the temperature at the Earth's inner-core boundary and the temperature profile of the outer core can be anchored. This project research can provide new data and research technique for the study of the composition and state of the outer core materials, and provide data support for the design of new materials under the extreme conditions.

外地核物质组成和状态确定对地磁场形成机理等地学课题有重要意义，由于实验室中难以获得可与外地核环境直接对比的压力和温度条件，该问题已成为地学领域的热点和难点。预加热冲击实验能突破材料强度对静高压实验压力、温度的限制，又能克服常态出发动高压实验仅能获得特定冲击压缩状态的缺点，为该问题解决提供新的技术途径。本项目着重考虑外地核候选轻元素Si的存在，以Fe-Si体系为研究对象，按照外地核压力和温度范围设计实验，将不同Si含量样品预加热至较高初温甚至熔融状态，通过冲击压缩下密度、声速测量结果和地震学数据进行对比，限定外地核Si含量；针对限定后的Fe-Si样品，综合不同初温下冲击熔化温度实验结果与预加热冲击分子动力学模拟结果，获得外地核Fe-Si样品的熔化曲线，并限定内外核边界温度、外地核温度剖面。通过本项目研究，为外地核物质组成和状态研究提供新的数据和研究技术，为极端条件下新材料设计提供数据支持。

外地核的物质组成和状态研究对地磁场形成机理等地学课题有重要意义，但由于实验室中难以获得可与外地核直接、全面对比的压力和温度条件，该问题已成为地学领域的热点和难点。.本项目以Fe-Si体系为研究对象，采用异于传统从室温出发冲击加载的预加热冲击实验技术，结合理论计算，对外地核的物质组成和状态问题展开研究，所取得技术进步和新认识有：.（1）针对较高初温、复杂环境下的样品加热、精确控温和初始状态保持等需求，对预加热冲击实验进行了技术优化和能力提升，可在气炮/火炮靶室以1℃分辨率、3℃控温精度和1800℃以下温度范围对样品进行快速加热、精确控温，样品温度均匀性、初始状态均满足冲击实验研究需求，具备在已获检验的663-1554℃初温范围内测量得到反映样品Hugoniot状态方程和声速的冲击压缩下样品/窗口界面速度历史信号之能力；.（2）对Fe-9Si进行初温1100℃预加热冲击系列实验，冲击温度远高于室温出发的冲击加载状态，在外地核的压力、温度下密度、声速结果与地震学测量结果（PREM模型）相交，交点处密度值接近，说明Fe-9Si在相应压力、温度点的密度、声速与外地核物质基本一致；.（3）结合1100℃Fe-9Si的声速测量结果，采用基于能量平衡的热物理模型和广泛使用的Lindemann熔化定律，给出基于Fe-9Si的内外核边界温度5552℃；.（4）提出并实现一种“直接自由能插值”（DFEI）非谐自由能高效、准确计算方法，优化了高温高压晶体结构预测程序MUSE、高压相图分析程序PHASEGO，提出并实现了可模拟不同初始状态起始冲击加载、获得冲击熔化曲线的冲击熔化模拟方法，可以在合适的计算量下对预加热冲击实验结果进行合理地外推预测，在获得有效数据的同时降低研究成本；.（5）对不同Si含量Fe-Si合金的晶体结构搜索得到其在地核压力的稳定结构，对其稳定性、密度和声速等随压力、温度变化规律有初步认识。.这些技术进步为更深入开展外地核的物质组成、状态和性质问题研究提供了有效的实验技术和理论计算方法，相关研究结果为该问题的最终解决提供了有效数据积累。