Date of Award
8-1-2025
Degree Name
Master of Science
Department
Geology
First Advisor
Hummer, Daniel
Abstract
There is a lack of available data for the size-dependent controls of Al-O-H mineral stability, and factors like temperature and time will inevitably affect the crystallization sequence. Initial predictions indicate a crossover in stability between the bulk stable phase of corundum to γ-Al2O3 and Al-OH minerals like nordstrandite at the nanoscale. Crystallization experiments are conducted using a ~4.6 pH AlCl3-NaOH-H2O solution, heated at lengths of 3, 12, and 24 hours from 75-250°C. Precipitated solids are analyzed via XRD, FTIR, SEM-EDS, BET, and TEM methods. Lack of detection for predicted oxide phases implies this study operates within a hydrated system, due to higher temperature requirements for transformation to a dehydrated oxide structure. Results indicate the dominant phase at all 3 and 12-hr temperatures is a previously undescribed alumina compound with some Na and Cl and assumed isometric structure. Methods yield crystalline size estimations for the unknown phase that increase in size with longer heating times, but SEM/TEM images show the dodecahedral morphologies decrease in definition and size at higher temperatures. At 24-hr times, this phase is only detected up to 225°C. A transition occurs somewhere between 12-24 hr and 225-250°C from the unknown compound to boehmite as rods with widths <100 nm. Boehmite may form as the first Al-OH phase due to Ostwald step-rule, being closest to the initial unknown phase in total enthalpy and overpowering any present size-dependent controls at the nanoscale.
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