Susanne M. Opalka

Thermodynamic modeling of the sodium alanates and the Na–Al–H system

Abstract The thermodynamic properties of the Al–Na and Na–Al–H systems have been assessed by combining the “calculation of phase diagram” approach with first-principles predictions. The Gibbs energies of the individual phases were thermodynamically modeled, where the model parameters were obtained from best fit optimizations to combined experimental and first-principles predicted finite temperature data. The first-principles thermodynamic predictions were based upon density functional theory ground state minimizations and direct method lattice dynamics. The predictions proved to be important adjuncts to the assessments whenever experimental measurements were lacking or not feasible. It was shown that the phase stability conditions of sodium alanates, NaAlH4 and Na3AlH6, were well described with the present models.

Experimental studies of α-AlD3 and α′-AlD3versus first-principles modelling of the alane isomorphs

The thermal phase behaviour of cryomilled α′-AlD3 and α-AlD3 was investigated by in situsynchrotron powder X-ray diffraction (SR-PXD), differential scanning calorimetry and first principles atomic modelling. In situ measurements showed that α′-AlD3 decomposes directly into Al and D2 at around 80 °C during heating at 1 °C min−1. At higher temperatures the transformation of α′-AlD3 to α-AlD3 was observed by DSC measurements at 5 °C min−1, and tentatively by in situSR-PXD at 1 °C min−1. Atomic modelling was carried out to investigate possible structural relationships and transformation pathways between the α- and α′-phase. Group–subgroup relation analyses and direct method lattice dynamics were used to rule out a possible displacive transformation pathway between the α′- and α-phases. The likelihood of a reconstructive transformation was demonstrated by partial transformation of an interface between α′ and α domains during elevated temperature molecular dynamics. Such an α′- to α-phase transformation may be possible when kinetic barriers can be overcome at elevated temperatures or during long time periods. These insights are also relevant to the transformation mechanisms of the β-AlD3 and γ-AlD3 isomorphs to the α-phase.

Thermodynamic evaluation of the Al-H system

The thermodynamic properties of the Al-H system were assessed using models for the Gibbs energy of the individual phases, including the metastable hydride AlH3 phase. The model parameters were obtained through optimization by best fitting to selected experimental data. Particular attention was paid to hydrogen solubility in liquid and face-centered-cubic (fcc) Al. It was shown that the hydrogen can be treated as an ideal gas under normal conditions. The hydrogen solubility in liquid and fcc Al can be described very well with a regular solution model for liquid and fcc. The present calculations show satisfactory agreement with most experimental data for hydrogen solubility in fcc Al and selected data for hydrogen solubility in liquid Al, qualifying the extension of this binary model to higher-order Al-H-bearing systems.

ICME Approach to Corrosion Pit Growth Prediction

Corrosion is a significant problem in a large number of aerospace and commercial applications. Prediction of expected pit size and distribution due to localized corrosion processes is essential in understanding product life. Until now, the approach to predict pit size has been based on statistical analysis of pits in exposure tests. In this work, a combined experimental and multi-scale modeling method is used to develop a physics-based approach to understanding the factors that affect pit growth. The integrated approach uses thermodynamic modeling to understand the pit solution chemistry, atomistic modeling to study the kinetics, and experimental electro-kinetic measurements to validate the model predictions. Each of these pieces feed into an analytical model of pit growth to determine the maximum theoretical size. Model assumptions, results and pit size predictions for model aluminum systems are discussed.

Advanced Water-Gas Shift Membrane Reactor

The overall objectives for this project were: (1) to identify a suitable PdCu tri-metallic alloy membrane with high stability and commercially relevant hydrogen permeation in the presence of trace amounts of carbon monoxide and sulfur; and (2) to identify and synthesize a water gas shift catalyst with a high operating life that is sulfur and chlorine tolerant at low concentrations of these impurities. This work successfully achieved the first project objective to identify a suitable PdCu tri-metallic alloy membrane composition, Pd{sub 0.47}Cu{sub 0.52}G5{sub 0.01}, that was selected based on atomistic and thermodynamic modeling alone. The second objective was partially successful in that catalysts were identified and evaluated that can withstand sulfur in high concentrations and at high pressures, but a long operating life was not achieved at the end of the project. From the limited durability testing it appears that the best catalyst, Pt-Re/Ce{sub 0.333}Zr{sub 0.333}E4{sub 0.333}O{sub 2}, is unable to maintain a long operating life at space velocities of 200,000 h{sup -1}. The reasons for the low durability do not appear to be related to the high concentrations of H{sub 2}S, but rather due to the high operating pressure and the influence the pressure has on the WGS reaction at this space velocity.