Visualizing core melt percolation and glass structure under extreme using nanoscale transmission X-ray microscopy [electronic resource]
Keywords: Geological and Environmental Sciences.

Pressure has perhaps the greatest range of all physical variables, with 60 orders of magnitude from the vacuum of outer space to the interior of neutron stars. Most matter in the planet exists under extreme pressure compared to the Earth, which measures approximately 360 GPa at its core. Pressure shapes the Earth and other planets. Life on Earth's surface is profoundly affected by what happens at inaccessible depths. Pressure is a powerful and fundamental thermodynamic variable that can dramatically change structure and properties. It can turn a hill of coal into karats of diamond. It can also induce amorphization of crystalline solids and polymorphism in glass (Bridgman & Šimon 1994). Pressure also serves as a smooth and clean tuning parameter that could improve our basic understanding of existing materials at different levels of atomic and molecular interactions. High pressure studies have made significant contribution to our fundamental understanding of Earth science and materials science. This dissertation focuses on investigating core melt segregation behavior and amorphous materials structure by mainly employing a state-of-the-art high pressure synchrotron technique, nanoscale transmission x-ray microscopy (TXM). Our experimental studies have demonstrated that nanoscale TXM is a powerful 3D petrographic probe for non-destructive, high-resolution in-situ analysis of multiple minerals and amorphous phases synthesized under high-pressure and high-temperature conditions. Using nanoscale TXM coupled with a laser-heated diamond anvil cell, we image a marked transition in the shape of the iron-rich melt within a silicate perovskite matrix in 3-dimensional reconstructions of samples prepared at varying pressures and temperatures. We find that, as the pressure increases from 25 GPa to 64 GPa, the iron distribution changes from isolated pockets to an interconnected network. Our results indicate that percolation could be a viable mechanism of core formation at Earth's lower mantle conditions. In addition, nanoscale TXM can provide accurate in-situ pressure--volume (P--V) determination of amorphous materials in a diamond anvil cell (DAC), which enhances our understanding of the unusual behavior associated with the structure of disordered systems. Besides the P--V data brings critical constraints and validation to theoretical modeling and computational simulations. In this work, we conducted high-pressure volume measurements in situ on two types of amorphous materials: insulating silica glass, and metallic glasses. The density data measured in this study suggests that SiO2 glass may start with irreversible intermediate range order transformation. From 10 GPa the six-fold coordination structure gradually replaces the four-fold coordination structure and SiO2 glass behaves as a single amorphous polymorph with a six-fold coordination structure after 32 GPa. Compared to the strongly bound, tetrahedral network structure of SiO2 glass, metallic glasses made of metallic elements via non-directional metallic bonding are believed to possess some unique properties. With high pressure as a tuning parameter for probing the density of metallic glass, we demonstrate a crossover between fractal short-range (< 2 atomic diameters) and homogeneous long-range structures using in-situ x-ray diffraction, x-ray tomography, and molecular dynamics simulations. A specific class of fractal—the percolation cluster—explains the structural details for several metallic-glass compositions.