Melting of compressed iron-alloys by monitoring atomic dynamics

通过监测原子动力学熔化压缩铁合金

• 批准号: 1316362
• 负责人: Jennifer Jackson
• 金额: $ 20万
• 依托单位: California Institute of Technology
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2013
• 资助国家: 美国
• 起止时间: 2013-09-01 至 2016-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1316362&HistoricalAwards=false
• 关键词: Melting; compressed; iron; alloys; monitoring

Seismological and cosmochemical observations indicate that the main constituent in Earth's core is iron, which is thought to be alloyed with ~10 wt% nickel and some light elements (e.g. S, Si, O, C, H). Earth's core consists of a solid inner region surrounded by a liquid outer core. The melting temperature of iron at high-pressure provides an important reference point for the temperature distribution within Earth's core and affects a number of important geophysical quantities related to this region: heat flow across the core-mantle boundary (CMB), the temperature gradient within the thermal boundary layer above the CMB, the phase relations, and expected seismic wave speeds of candidate phases. More specifically, the melting point of iron at the boundary between the liquid outer core and solid inner core provides an upper bound of the temperature at that interface, because studies thus far indicate that all plausible outer core liquids coexist with corresponding inner core alloy solids at or below the melting point of pure iron. Previously, melting studies of iron at high-pressures were performed by shock-compression, resistive- and laser-heating in diamond anvil cells using visual observations or synchrotron x-ray diffraction, and theoretical methods. However, the melting curve of iron with respect to its alloys remains uncertain, especially at pressures above 70 GPa. The PI has developed a novel metric for detecting the solid-liquid phase boundary of iron-bearing materials at high-pressures using 57Fe synchrotron Mössbauer spectroscopy (SMS), also known as nuclear forward scattering. Focused synchrotron radiation with 1 meV bandwidth passes through a laser-heated 57Fe-bearing sample inside a diamond anvil cell. The characteristic SMS time signature vanishes when melting occurs. This process is described by the Lamb-Mössbauer factor, a quantity that is directly related to the mean-square displacement of the iron atoms. Therefore, we measure the dynamics of the atoms in the material, in contrast to a static diffraction measurement. As this method monitors the dynamics of the atoms, the SMS technique provides a new and independent means of melting point determination for materials under high-pressure. We have successfully performed such melting investigations on pure iron up to 82 GPa. Our data define a different melting trend compared with previous studies, thus providing important rationale to proceed further with this method on iron and iron-alloys at higher pressures. The melting experiments will be conducted to pressures of at least 80 GPa for the alloys and higher than 80 GPa for pure iron, at sector 3-ID-B of the Advanced Photon Source at Argonne National Laboratory. By constraining the relative melting curves of these iron-alloys at high-PT conditions, we will provide a new constraint on the effect of select light-element alloying to iron?s melting behavior. These outcomes will represent a significant step towards understanding the melting of iron-rich alloys at conditions of terrestrial-type planetary cores and thus provide necessary temperature constraints of their respective interiors.

地震学和宇宙化学观测表明，地核中的主要成分是铁，铁被认为与约10重量%的镍和一些轻元素（如S，Si，O，C，H）形成合金。地球的核心由一个固体的内部区域和一个液体的外部核心所组成。铁在高压下的熔化温度为地核内的温度分布提供了一个重要的参考点，并影响了一些与该地区有关的重要地球物理量：穿过核幔边界（CMB）的热流，CMB上方热边界层内的温度梯度，相位关系，以及候选相位的预期地震波速度。更具体地，铁在液态外核和固态内核之间的边界处的熔点提供了该界面处的温度的上限，因为迄今为止的研究表明，所有合理的外核液体与相应的内核合金固体在纯铁的熔点或低于纯铁的熔点处共存。以前，铁在高压下的熔化研究是通过冲击压缩，电阻和激光加热在金刚石砧单元中使用视觉观察或同步加速器X射线衍射和理论方法进行的。然而，铁相对于其合金的熔化曲线仍然不确定，特别是在70 GPa以上的压力下。PI开发了一种新的度量方法，用于使用57 Fe同步穆斯堡尔光谱（SMS）在高压下检测含铁材料的固液相边界，也称为核前向散射。聚焦同步辐射与1毫电子伏带宽通过激光加热的57铁轴承样品内的金刚石砧室。当熔化发生时，特征SMS时间特征消失。这个过程用兰姆-穆斯堡尔因子来描述，这个量与铁原子的均方位移直接相关。因此，我们测量材料中原子的动态，而不是静态衍射测量。由于这种方法监测原子的动力学，SMS技术为高压下的材料熔点测定提供了一种新的独立手段。我们已经成功地对高达82 GPa的纯铁进行了此类熔化研究。我们的数据定义了一个不同的熔化趋势与以前的研究相比，从而提供了重要的理由，进一步进行这种方法在铁和铁合金在更高的压力。熔化实验将在阿贡国家实验室先进光子源的3-ID-B区进行，合金的压力至少为80 GPa，纯铁的压力高于80 GPa。通过约束这些铁合金在高PT条件下的相对熔化曲线，我们将提供一个新的约束选择轻元素合金化铁的效果？的熔化行为。这些结果将是理解富铁合金在陆地型行星核心条件下熔化的重要一步，从而为它们各自的内部提供必要的温度约束。