Low-temperature growth of alumina

• Physical phenomena related to the growth of alumina, A12O3, are investigated by experiments and ab initio calculations. Alumina is a well studied material with applications in a variety of areas, due to its many beneficial properties. For example, the α and κ phases are widely used as wear-resistant coatings due to their hardness and thermal stability, while, e.g., the γ and θ phases find applications as catalysts or catalyst supports, since they have large surface areas.Alumina growth at low temperatures usually results in one of the metastable phases. These are involved in transition sequences, which all irreversibly end in the transformation to the thermodynamically stable α phase at about 1050°C. Thus, the metastable aluminas can be grown at low temperatures but cannot be used in high temperature applications, while formation of the stable α phase typically require high temperatures, prohibiting the use of temperature sensitive substrates.In the experimental part of this work, single-phase α-alumina thin films were grown at temperatures down to 280°C. This was achieved by pre-depositing a chromia template layer, which is shown to promote formation of α-alumina. The results demonstrate that low-temperature α-alumina growth is possible once initial nucleation has occurred.In the second part of this work, the effect of additives on the phase stability of α- and θ-alumina is investigated by density functional theory calculations. The studied alumina dopants are 5 at.% of Cr, Mo, Co, or As, which substitute for Al in the lattices, and 5 at.% of N or S, substituting for O. We predict that most tested dopants tend to reverse the stability between α- and θ-alumina, so that, e.g., Mo-doping makes the θ phase energetically favored. The exception is Co, which instead gives a slight increase in the relative stability of the α phase. The stability of some of these compounds is also studied by calculating their energies of formation from their constituents, i.e., related dopant oxides, pure metals, and molecules. The results show that the Cr-, Mo-, and Co-doped aluminas are higher in energy than phase separations into pure alumina and other phases, containing the dopants. Thus, the doped aluminas seem to be metastable and will most likely phase separate at high temperatures.

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NATURVETENSKAP  -- Fysik (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences (hsv//eng)

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