Thermal & Structural Analysis of Gibbsite derived Aluminum Oxides (Alumina)

Asymptotic Giant Branch (AGB) stars contribute a significant amount of dust and material to the local Interstellar Medium (ISM). The type of dust produced depends on the stellar C/O ratio, C/O>1 = carbon-rich, and C/O<1 = oxygen-rich. In order to form oxide dust, free oxygen atoms are needed, hence we focus on oxygen-rich stars. AGB stars pulsate and generate convection currents in their mantles. These convection currents dredge material up to the surface, where it is ejected from the AGB star as a gas. Aluminum Oxide (Alumina; Al₂O₃) is one of the few minerals that can condense at high temperatures (>1500K). These alumina grains are important because they can act as seeds or surfaces on which other minerals can form. In the past there has been some work with alumina, but these works focused mainly on amorphous and the most common/stable polymorph corundum (α-Al₂O₃); other alumina structures have not been well studied. For instance, in material sciences, amorphous alumina has been used as substrates and templates to deposit metallic materials at nanoscale, while sapphire (corundum) is used in optoelectronics as a substrate to grow wide band gap semiconductor thin films.

The overarching goal of our work seeks to apply what we know about alumina and its polymorphs to astronomical environments and help to better understand which crystal structures of alumina exist around AGB stars.

The focus of this current study is Gibbsite (Al(OH)₃)-derived alumina. The various polymorphs of these alumina are metastable; their crystal structure depends on their formation temperature. We anneal samples of Gibbsite at different temperatures, for a set amount of time (in this case 2 hours per sample). After annealing, the samples are cooled and stored in a desiccator until it is time to classify them as follows:

1. X-ray diffraction (XRD) allows us to determine the crystal structure evolution of each sample after each annealing session.

2. Differential scanning calorimetry (DSC) allows us to track the phases changes occurring in our samples as they are annealed, which we can correlate with our XRD results.

In the future, the resulting polymorphs will have their optical properties measured and these will be applied to radiative transfer modeling of dust shells around AGB stars. Combining the newly-determined thermal properties with the spectral features of these aluminas will be a powerful diagnostic tool for understanding dust formation.