All ETDs from UAB

Advisory Committee Chair

Sergey Vyazovkin

Advisory Committee Members

Aaron Lucius

Eugenia Kharlampieva

Andrei Stanishevsky

Vinoy Thomas

Document Type

Dissertation

Date of Award

2021

Degree Name by School

Doctor of Philosophy (PhD) College of Arts and Sciences

Abstract

This research explores the effects of nanoconfinement, aggregate state type, and an inert gas pressure on the kinetics of thermally stimulated processes. The processes considered are solution crystallization and thermal decomposition. The kinetic effects are measured via differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and high pressure differential scanning calorimetry (HPDSC), combined with isoconversional kinetic analysis. The first chapter demonstrates that DSC can be applied to measure the kinetics of nonisothermal crystallization. The crystallization of potassium nitrate and ammonium perchlorate are from unsaturated and saturated aqueous solutions. An advanced isoconversional method demonstrates that the process is represented by negative values of the effective activation energy, which varies with the progress of crystallization. The saturated and unsaturated solutions have demonstrated distinctly different crystallization kinetics. In the second chapter, the crystallization of ammonium perchlorate from a methanol/water solution confined to native and organically modified silica nanopores is analyzed. The effects of nanoconfinement on the salt crystallization are primarily due to the surface interactions with the nanopores’ walls. The process in the native nanopores occurs at markedly lower temperature and with much larger heat relative to the bulk, which is linked to adsorption of the salt ions on the nanopores surface. The process in organically modified pores also demonstrates predominantly a surface effect, revealed in larger crystallization temperature and smaller crystallization heat relative to the bulk. This is proposed to be due to a phase separation of the methanol/water solution inside the hydrophobic nanopores, so that the crystallization occurs from the methanol phase. In the third chapter, TGA has been applied to probe the possible differences in decomposition of malonic (propanedioic) acid in three aggregate states: solid, liquid, and supercooled liquid. The activation energy and preexponential factor for all three states have been determined to be practically the same: E = 110 ± 10 kJ mol−1 and log(A/min−1) = 13 ± 1. The reaction order values have been similar for the liquid and supercooled liquid phases, but markedly smaller for the solid-phase decomposition. In the fourth chapter, the effect of an inert gas pressure on the kinetics of reversible thermal dehydration of lithium sulfate monohydrate (Li2SO4·H2O) has been probed via HPDSC. An increase in nitrogen pressure from 0.1 to 7 MPa shifts the reaction to higher temperature by more than 10 °C. Kinetic analysis indicates that the activation energy remains practically unchanged by changing pressure. The pre-exponential factor has been found to decrease by ~2 orders of magnitude with the pressure increase.

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