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，地核的主要成分是铁，它被认为是与约 10 wt% 的镍和一些轻元素（例如 S、Si、O、C、H）的合金，地核由固体组成。被液态外核包围的内部区域，铁在高压下的熔化温度为地核内的温度分布提供了重要的参考点，并影响与该区域相关的许多重要地球物理量：热流。跨核幔边界 (CMB)、CMB 上方热边界层内的温度梯度、相位关系以及候选相的预期地震波速度。更具体地说，是液体外部之间边界处的铁熔点。核心和固体内核提供了该界面处的温度上限，因为迄今为止的研究表明，所有可能的外核液体与相应的内核合金固体在纯铁的熔点或以下共存。在使用目视观察或同步加速器 X 射线衍射和理论方法，通过冲击压缩、电阻加热和激光加热在金刚石砧座中进行高压然而，铁相对于其合金的熔化曲线仍然不确定，尤其是。 PI 开发了一种使用 57Fe 同步加速器在高压下检测含铁材料固液相边界的新方法。穆斯堡尔光谱 (SMS)，也称为核前向散射。具有 1 meV 带宽的聚焦同步加速器辐射穿过金刚石砧室内的激光加热的 57Fe 样品，当熔化发生时，特征 SMS 时间特征消失。通过兰姆-穆斯堡尔因子，该因子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力学，与静态衍射测量相比，由于这种方法监测原子的动态，SMS 技术提供了一种新的、独立的高压下材料熔点测定方法。 82 GPa。与之前的研究相比，我们的数据定义了不同的熔化趋势，从而为在更高压力下进一步对铁和铁合金进行该方法提供了重要的理由。熔化实验将在至少 80 GPa 的压力下进行。合金和高于 80 GPa 的纯铁，在阿贡国家实验室先进光子源的 3-ID-B 区，通过限制这些铁合金在高 PT 条件下的相对熔化曲线，我们将提供一种新的方法。限制选择轻元素合金对铁熔化行为的影响，这些结果将代表着在理解地球型行星核心条件下富铁合金的熔化方面迈出的重要一步。为它们各自的内部提供必要的温度限制。