The Crystalline Structure of Aluminium Oxide

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Aluminium oxide is a crystalline mineral which occurs naturally in corundum, ruby’s, sapphires and emeralds. It is a widely used refractory, ceramics and cutting tool material. It is also used as a fire retardant/smoke suppressant.

The crystalline structure of aluminium oxide is usually a trigonal Bravais lattice with a space group of R-3c. Oxygen ions are arranged in an almost face-centered cubic packing on the interstitial sites of the crystal. The interstitial oxygen anions occupy two-thirds of the octahedral space.

A trigonal crystal is the most stable type of alumina polymorph, containing only the stoichiometric g-Al2O3. The crystalline oxide has an O:Al ratio of 1.6-1.7 (refs. 21,22,53) and a density of about 3.97 g/cm3.

Stoichiometry in alumina oxide grown from bauxite has been reported to be super-stoichiometric on Al(100) substrates. On Al(111) the stoichiometry is 0.7-0.8 on the surface of the oxide layer and 1.5 on the centre.

Molecular dynamics simulations of low pressure oxidation and aluminium deposition using a nominal density of oxygen gas can be used to study the structural evolution of oxides, junctions and the formation of nanocrystals on aluminium surfaces6,21. However, the oxidation process itself can be highly time-consuming and difficult to reproduce accurately.

We present a novel method for analyzing the kinetics of oxidation by explicitly modelling the individual events which lead to the formation of an oxide and aluminium junction using molecular dynamics (MD). The simulated oxidation process starts with the growth of the initial oxygen layer on the aluminium surface. It then continues to deposit oxygen atoms and aluminium atoms as the oxide layer develops until the tri-layer junction structure is completed. This method allows us to investigate the trends in the oxidation process and properties of the final junction structures in great detail, a key step in understanding the chemistry of alumina and its applications.

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