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Effects of additives in α- and θ-alumina: an ab initio study
Linköping University, Department of Physics, Chemistry and Biology, Plasma and Coating Physics . Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Plasma and Coating Physics . Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Plasma and Coating Physics . Linköping University, The Institute of Technology.ORCID iD: 0000-0002-1744-7322
2004 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, Vol. 16, no 49, 8971-8980 p.Article in journal (Refereed) Published
Abstract [en]

It is of high fundamental and practical importance to be able to control the formation and stability of the different crystalline phases of alumina (Al2O3). In this study, we have used density functional theory methods to investigate the changes induced in the thermodynamically stable α phase and the metastable θ phase as one eighth of the Al atoms are substituted for different additives (Sc, W, Mo, Cr, Cu, Si, and B). The calculations predict that the additives strongly affect the relative stability between the two phases. Most tested additives are shown to shift the relative stability towards, and in some cases completely stabilize, the θ phase, while Cu doping is predicted to increase the relative stability of the α phase. The reasons for these effects are discussed, as are possible implications on the growth and use of doped aluminas in practical applications. In addition, the effects of the additives on bulk moduli and densities of states have been investigated.

Place, publisher, year, edition, pages
2004. Vol. 16, no 49, 8971-8980 p.
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-13581DOI: 10.1088/0953-8984/16/49/012OAI: oai:DiVA.org:liu-13581DiVA: diva2:20985
Available from: 2005-12-13 Created: 2005-12-13 Last updated: 2013-10-30
In thesis
1. Controlling the Formation and Stability of Alumina Phases
Open this publication in new window or tab >>Controlling the Formation and Stability of Alumina Phases
2005 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this work, physical phenomena related to the growth and phase formation of alumina, Al2O3, are investigated by experiments and computer calculations. Alumina finds applications in a wide variety of areas, due to many beneficial properties and several existing crystalline phases. 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 metastable γ and θ phases find applications as catalysts or catalyst supports, since their surface energies are low and, hence, they have large surface areas available for catalytic reactions.

The metastable phases are involved in transition sequences, which all irreversibly end in the transformation to the stable α phase at about 1050 °C. As a consequence, the metastable aluminas, which can be grown at low temperatures, cannot be used in high temperature applications, since they are destroyed by the transformation into α. In contrast, α-alumina, which is the only thermodynamically stable phase, typically require high growth temperatures (~1000 °C), prohibiting the use of temperature sensitive substrates. Thus, there is a need for increasing the thermal stability of metastable alumina and decreasing the growth temperature of the α phase.

In the experimental part of this work, hard and single-phased α-alumina thin films were grown by magnetron sputtering at temperatures down to 280 °C. This dramaticdecrease in growth temperature was achieved by two main factors. Firstly, the nucleation stage of growth was controlled by pre-depositing a chromia “template” layer, which is demonstrated to promote nucleation of α-alumina. Secondly, it is shown that energetic bombardment was needed to sustain growth of the α phase. Energy-resolved mass spectrometry measurements demonstrate that the likely source of energetic bombardment, in the present case, was oxygen ions/atoms originating from the target surface. Overall, these results demonstrate that low-temperature α-alumina growth is possible by controlling both the nucleation step of growth as well as the energetic bombardment of the growing film. In addition, the mass spectrometry studies showed that a large fraction of the deposition flux consisted of AlO molecules, which were sputtered from the target. Since the film is formed by chemical bonding between the depositing species, this observation is important for the fundamental understanding of alumina thin film growth.

In the computational part of the work, the effect of additives on the phase stability of α- and θ-alumina was investigated by density functional theory calculations. A systematic study was performed of a large number of substitutional dopants in the alumina lattices. Most tested dopants tended to reverse the stability between α- and θ-alumina; so that, e.g., Modoping made the θ phase energetically favored. Thus, it is possible to stabilize the metastable phases by additives. An important reason for this is the physical size of the dopant ions with respect to the space available within the alumina lattices. For example, large ions induced θ stabilization, while ions only slightly larger than Al, e.g., Co and Cu, gave a slight increase in the relative stability of the α phase. We also studied the stability of some of these compounds with respect to pure alumina and other phases, containing the dopants, with the result that phase separations are energetically favored and will most likely occur at elevated temperatures.

Place, publisher, year, edition, pages
Institutionen för fysik, kemi och biologi, 2005
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 987
Keyword
thin films, alumina, magnetron sputtering, phase stability, density functional theory
National Category
Other Engineering and Technologies not elsewhere specified
Identifiers
urn:nbn:se:liu:diva-5038 (URN)91-85457-71-X (ISBN)
Public defence
2005-12-15, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 09:15 (English)
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Available from: 2005-12-13 Created: 2005-12-13 Last updated: 2013-10-30

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Wallin, ErikAndersson, Jon M.Chirita, ValeriuHelmersson, Ulf

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