Latent-heat based measurements of the melting curve of iron to the pressure of Earth’s inner core boundary

该奖项的全部或部分资金来自《2021年美国救援计划法案》(公法117-2)。地球的核心是一个巨大的金属球，位于我们脚下3000公里处。对其基本性质的了解将有助于地球科学家更好地了解固体地球的许多层(内核、外核、地幔和地壳)内热量和物质的运动方式。例如，液态金属在外核的运动会产生一个强大的磁场，保护表面的生命免受有害辐射的伤害，但驱动这种运动的力的大小尚不确定。另一个例子是，来自外核的热量可能会熔化地幔底部岩石中的一些矿物，但地幔底部的热流量和温度是未知的。这些不确定性的一个值得注意的原因是，关于地核的一个基本事实仍然高度不确定：它的温度。推断核心温度的最好方法依赖于一个简单的想法。当地球冷却时，主要是铁的液体外核冻结在固体内核上，这意味着液-固界面上的温度是主要是铁的液体材料的冻结温度，或者相当于熔化温度。因此，了解地核温度及其对整个地球的影响的起点是准确地确定铁在内外核边界存在的巨大压力下的熔化温度：330万大气压。即将到来的项目旨在通过使用一种新型的实验室测量来准确确定这一关键温度。除了这些科学影响，该项目还将吸引本科生参与，并使物理学家和材料科学家受益，他们研究涉及到熵变化的高压相变--从磁相变到超导相变，再到熔化。该团队还将在视频杂志JoVE上公布样品准备细节，以向广大受众清楚地传播他们的方法。当铁和硫化铁样品被快速加热时，将进行实验以确定潜热的吸收。样品将在钻石砧座中被压缩到高达330万大气压的压力，并用流经样品的短脉冲电流加热。这种脉冲焦耳加热程序通过限制从样品中逸出的热量来检测潜热，并通过将样品熔化的总时间限制在大约1至100微秒来限制化学反应。该项目的目标是(A)通过实现高达约100万个大气压的正负50K精度和高达330万个大气压的正负200K精度，显著提高高达330万个大气压的铁的熔化温度数据的准确性，(B)通过以正负50K的精度测量高达100万个大气压的Fe(0.9)S(0.1)的固相线温度，证明潜热技术可以扩展到铁合金的部分熔化，和(C)将新的测量结果同化到地核绝热模型和磁场形成和演化的流体动力学模型中。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

This award is funded in whole or in part under the American Rescue Plan Act of 2021 (Public Law 117-2).The Earth’s core is a massive ball of metal located 3000 km beneath our feet. Improved knowledge of its basic properties will help Earth scientists better understand the way heat and material move inside the solid Earth’s many layers (inner core, outer core, mantle, and crust). For example, motion of liquid metal in the outer core generates a powerful magnetic field that shields life at the surface from harmful radiation, but the magnitude of forces driving the motion are uncertain. Another example is that heat from the outer core might melt some minerals in the rocks at the base of the mantle, but the amount of heat flow and the temperature at the base of the mantle are unknown. One notable reason for these uncertainties is that a basic fact about the Earth’s core remains highly uncertain: its temperature. The best way to infer the core temperature relies on a simple idea. The liquid outer core, which is mostly iron, freezes onto the solid inner core as the Earth cools, meaning the temperature at the liquid-solid interface is the freezing temperature, or equivalently the melting temperature, of the mostly iron liquid material. Hence, the starting point for understanding the core’s temperature, and its effect on the entire Earth, is to accurately determine the melting temperature of iron when subjected to the enormous pressure that exists at the inner core-outer core boundary: 3.3 million atmospheres. The upcoming project aims to accurately determine this crucial temperature by using a new type of laboratory measurement. In addition to these scientific impacts, the project will engage undergraduate students, and benefit physicists and materials scientists studying high-pressure phase transitions that involve entropy changes - from magnetic transitions, to superconducting transitions, to melting. The team will also publish the sample preparation details in the video journal JoVE to clearly disseminate their methods to a wide audience. Experiments will be performed to identify the absorption of latent heat while samples of iron and iron-sulfide are rapidly heated. The samples will be compressed in diamond anvil cells to pressures as high as 3.3 million atmospheres, and heated with short pulses of electricity that flow through the samples. This pulsed Joule heating procedure allows latent heat detection by limiting the amount of heat escaping from samples, and also limits chemical reactions by restricting the total time during which the sample is molten to roughly 1 to 100 microseconds. The project aims to (a) dramatically improve the accuracy of the melting temperature data for iron up to 3.3 million atmospheres by achieving plus/minus 50 K accuracy up to approximately 1 million atmospheres and plus/minus 200 K accuracy up to 3.3 million atmospheres, (b) to prove that the latent-heat technique can be extended to partial melting of iron alloys by measuring the solidus temperature of Fe(0.9)S(0.1) up to 1 million atmospheres with plus/minus 50 K accuracy, and (c) assimilate the new measurements into models of Earth core adiabats and hydrodynamic models of magnetic field formation and evolution.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.