Direct Measurements of Shear and Longitudinal Wave Velocities of Iron and Iron Alloys at High Pressure and High Temperature by Laser Ultrasonics Technique

了解矿物在高压下的弹性行为是建立地球结构模型的关键因素，因为有关地球内部的大多数信息来自地震学数据。因此，直接测量矿物在高温高压下的速度和其他弹性性质是理解地震信息的关键，使我们能够将其转化为地球内部矿物的化学成分、矿物学和温度。铁被认为是地球核心的主要成分，人们已经做出了相当大的努力来了解它在高压和高温下的性质。然而，铁在高温高压(HPHT)条件下的弹性行为的实验数据与理论存在差异。为了了解这一现象的原因，应直接测量铁和铁合金在高压和高温下的横波和纵波速度。该项目的主要目标是使用由夏威夷大学新开发并成功测试的激光超声(LU-DAC)来直接测量铁和铁-镍合金在室温到100 Gpa的高压和高温到3000K的高温下的纵向和剪切速度。这项研究的成功对地球物理科学和材料科学具有重要意义。对于地球物理科学来说，在类似于地球内部的隆起条件下现场直接测量铁和铁镍合金的剪切和纵向速度对于解释观测到的地震异常和了解该地区矿物的物理性质至关重要。LU-DAC系统的主要思想是利用超短纳秒脉冲和连续波激光在DAC中远程激发和检测高温高压下材料中的声波。用于非透明非晶态固体和熔体的LU-DAC点源-点-接收器技术的主要优点是，它不需要任何额外的数据(如高压下样品的厚度)来确定铁在高压下的弹性性质。LU-DAC技术的发展将为研究非透明功能材料在极端条件下的弹性行为提供一种简单、廉价、无损的技术；例如，在高压和高温下合成的磁性、超硬或摩擦学材料。这一研究结果将为推广这种测量方法以研究LU-DAC系统与激光加热相结合的高压高温条件下富铁矿物熔体和部分熔体的弹性提供依据。对富铁矿物弹性性质的了解将使对地球核-地幔界面的模拟更加逼真，并有助于阐明D“层和地核的性质和组成，以及可能通过该热点形成夏威夷群岛的情况。

Understanding of the elastic behavior of minerals under high pressure is a crucial factor for developing a model of the Earth's structure because most information about the Earth's interior comes from seismological data. Therefore, direct measurements of velocities and other elastic properties of minerals at elevated pressures and temperatures are keys to understand the seismic information, allowing us to translate it into chemical composition, mineralogy, and temperature of the minerals inside the Earth. Iron is thought to be the main constituent of the Earth's core and considerable efforts have been made to understand its properties at high pressure and high temperature. However, there are discrepancies between experimental data and theory on the elastic behavior of iron under high pressure and high temperature (HPHT) conditions. To understand the reason for this, the shear and longitudinal wave velocities of iron and iron alloys at high pressure and high temperature should be measured directly. The main goal of this project is to carry out direct measurements of both longitudinal and shear velocities of iron and iron-nickel alloys under high pressures to 100 GPa at room temperature and high temperature up to 3000 K using the laser ultrasonic (LU) coupled with a diamond-anvil cell (LU-DAC), which has been newly developed and successfully tested at the University of Hawaii. Success of the proposed research is of fundamental importance for geophysical science and materials science. For geophysical science, direct measurements of shear and longitudinal velocities of iron and iron-nickel alloys in-situ at elevated conditions similar to the Earth's interior are essential for interpreting the observed seismic anomaly and understanding physical properties of minerals in that region. The main idea of the LU-DAC system is to use ultra-short nanosecond pulse and continuous wave lasers for remote excitation and detection of acoustical waves in materials under high pressure and high temperature in DAC. The main advantage of the LU-DAC point-source-point-receiver technique for non-transparent amorphous solids and melts is that it does not require any additional data (such as thickness of the specimen under high pressure) to determine the elastic properties of the iron at high pressures. The development of the LU-DAC technique will provide a simple, inexpensive, non-destructive technique for studying the elastic behavior of non-transparent functional materials at extreme conditions; for instance magnetic, superhard, or tribological materials synthesized at high pressure and high temperature. The results obtained from the proposed research will provide the basis for extending such measurements to study the elasticity of melts and partial melts of iron-rich minerals at high pressure high temperature conditions when the LU-DAC system is combined with laser heating. The knowledge of the elastic properties of iron-rich minerals will allow a more realistic modeling of the Earth's core-mantle interface and shed light on the nature and composition of the D" layer and the Earth's core, and possibly on the formation of the Hawaiian Islands through the hot spot.