SYNTHESIS AND CHARACTERIZATION OF ALUMINA OXIDES AS STAR DUST ANALOGS

Alumina has been found as presolar grains in meteorites with a crystal structure varying between amorphous and α-Al₂O₃ (corundum)_. Infrared spectroscopy provides a means of identifying crystal structure in space. We aim to produce a reference set of spectra for well characterized samples that include the metastable γ-, δ-, η-, and Θ-Al₂O₃ polymorphs, by three different methods. The first method was to use the Differential Scanning Calorimetry (DSC) from 0°C to 1500°C to create a schematic baseline using hydrous aluminate starting materials, including gibbsite (Al(OH)₃) and boehmite (AlOOH). The second method began with annealing Al(OH)₃ from 327°C to 1000°C and AlOOH from 300°C to 1200°C in air. Starting materials and annealing products were then analyzed using X-ray diffraction (XRD) and DSC.

The DSC results for unannealed Al(OH)₃ showed endothermic peaks at ~350°C and 550°C, attributed to dehydration, and an exothermic peak at ~1300°C, attributed to recrystallization. Results on annealed samples suggest phase changes occurring at 327°C, 550°C and 900°C, 1000°C, and 1500°C, producing mixtures of metastable alumina polymorphs. As temperature increases more hydrogen bonds were removed, forming more stable forms of Al₂O₃ from 900°C to 1500°C. Above 1500°C, only corundum remains.

Pecharromán et al. (1999, J Phys Chem B 103: 6160-6170) conducted similar experiments, and noted that the sequence of phase changes is pre-determined by the precursor mineral. They found that the sequence for AlOOH was γ→ δ→ Θ→ α, and the sequence for Al(OH)₃ was η→Θ→ α. In contrast, Lamouri et al. (2017, Boletín De La Sociedad Española De Cerámica Y Vidiro 56: 47-54) observed the transformation sequences of Al(OH)₃ was η→γ→χ→κ→α. These findings emphasize that there are still new discoveries to uncover about aluminas. Additional experiments and characterization of run products are ongoing. Characterizing refractory oxides such as alumina, may offer insight into future astronomical studies of presolar alumina grains.