Date of Graduation

Fall 2016

Degree

Master of Science in Materials Science

Department

Physics, Astronomy and Materials Science

Committee Chair

Robert A Mayanovic

Keywords

aluminosilica, mesoporous, nanomaterial, molecular dynamics, simulations

Subject Categories

Materials Science and Engineering

Abstract

Periodic mesoporous materials have tunable pore sizes and high surface to volume ratios. Some of the most anticipated applications are those that call for energy harvesting in extreme environments, and these materials have a great structural stability to withstand the harsh conditions. In this work, the structural properties of mesoporous materials SBA-15 silica and Al-SBA-15 aluminosilica have been investigated by pressure dependent in situ small angle x-ray scattering (SAXS) using a diamond anvil cell (DAC) up to ~12 GPa in pressure. Hydrothermal measurements were also made in this manner to near supercritical water/steam conditions (to 255 °C and ~ 114 MPa) using the DAC. Analysis of the pressure dependent SAXS data yielded bulk modulus values of 12.0 +- 3.0 GPa and 34.7 +- 6.5 GPa for the SBA-15 silica and Al-SBA-15 aluminosilica respectively. The hydrothermal DAC experiment produced results detailing a small net swelling of 1-2% of the pore walls from the dissolution of water into the network structure. The Al-SBA-15 shows significantly greater hydrothermal stability than the SBA-15 silica. In addition, classical molecular dynamics simulations were performed on a series of silica and charge uncompensated aluminosilica amorphous glasses with varying percentage porosity, chemical composition, and onset of pressure at varying temperatures. The simulations were conducted from two types: 1) onset of pressure at the computer-glass transition temperature, and, 2) onset of pressure at room temperature. Within each type, simulations were varied by percentage porosity (0% to 60%) and by aluminum cation percentage (0% to 33%.) These simulations show a decrease in bulk modulus with respect to increasing percentage porosity that follows an exponential decay curve. This is consistent with experimental data from randomly porous materials. The bond angle analysis shows a unique bimodal distribution of Al-O-Al bond angles from the charge uncompensated aluminosilica. This is caused by edge sharing of adjacent tetrahedra due to local charge imbalance created by the substitution of the Al3+ ions.

Copyright

© Dayton Gage Kizzire

Open Access

Share

COinS