Aluminium Oxide As a Glass Modifier

Aluminium oxide is a common ingredient in sunscreens and cosmetics. It is also a component of many formulations of glass and ceramics.

It forms the amorphous phase in glass, which allows it to be cooled and refracted into new crystals without forming a crystal nucleus from the initial melt temperature (Tm) to the glass transition temperature (Tg). General glass-forming oxides have a ratio of glass transition temperature to melting point of 0.6732.

Alumina, however, has an exceptionally low glass forming ability and exhibits extremely wide gaps in the glass forming properties between Tg and Tm. It shows a Tg/Tm of 470 degC and a Tg/Tm 0.743 K, which is much lower than the Tg/Tm values observed for all the common glass-forming oxides.

In g-Al2O3 and l-Al2O3, we find broad ring size distributions with a large fraction of AlO5 and AlO6 units, OAl3 triclusters, OAl4 tetraclusters, and edge-sharing AlOn polyhedra. These features are inconsistent with Zachariasen’s rules, which require a high cation-oxygen coordination number for glass-forming behavior5, and suggest that Al2O3 is a non-glass-forming oxide rather than a glass modifier6.

We observe a void volume ratio of only 4.5% in g-Al2O3 and 5.5% in l-Al2O3, which is considerably lower than those of common glass-forming oxides, which have void volume ratios of 32%5. Moreover, Al2O3 exhibits highly densely packed structure, with a bond angle distribution that is topologically disordered according to the results of our previous study. These properties are consistent with the presence of a significant fraction of octahedral AlOn polyhedra, which are not present in g-SiO2 but rather resemble those found in a non-glass-forming liquid, such as O-Zr-O in l-ZrO237 and O-Er-O in l-Er2O341. These results indicate that alumina, unlike most other common oxides, has a very low crystalline morphology and a highly densely packed structure.

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