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Ab initio calculations on the effects of additives on alumina phase stability
Linköpings universitet, Institutionen för fysik, kemi och biologi, Plasma och beläggningsfysik. Linköpings universitet, Tekniska högskolan.
Linköpings universitet, Institutionen för fysik, kemi och biologi, Plasma och beläggningsfysik. Linköpings universitet, Tekniska högskolan.
Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska högskolan.
Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
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2005 (Engelska)Ingår i: Physical review. B, Condensed matter and materials physics, ISSN 1098-0121, Vol. 71, nr 014101, s. 014101-Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

The effects of substitutional additives on the properties and phase stability of - and -alumina (Al2O3), are investigated by density functional theory total energy calculations. The dopants explored are 5 at. % of Cr, Mo, Co, and As substituting for Al, respectively, N and S substituting for O, in the and lattices. Overall, the results show that it is possible to shift, and even reverse, the relative stability between - and -alumina by substitutional additives. The alumina bulk moduli are, in general, only slightly affected by the dopants but density of states profiles reveal additional peaks in the alumina band gaps. We also show that phase separations into pure oxides are energetically favored over doped alumina formation, and we present results on a number of previously unstudied binary oxides.

Ort, förlag, år, upplaga, sidor
2005. Vol. 71, nr 014101, s. 014101-
Nationell ämneskategori
Teknik och teknologier
Identifikatorer
URN: urn:nbn:se:liu:diva-13582DOI: 10.1103/PhysRevB.71.014101OAI: oai:DiVA.org:liu-13582DiVA, id: diva2:20986
Tillgänglig från: 2005-12-13 Skapad: 2005-12-13 Senast uppdaterad: 2013-10-30
Ingår i avhandling
1. Controlling the Formation and Stability of Alumina Phases
Öppna denna publikation i ny flik eller fönster >>Controlling the Formation and Stability of Alumina Phases
2005 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
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.

Ort, förlag, år, upplaga, sidor
Institutionen för fysik, kemi och biologi, 2005
Serie
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 987
Nyckelord
thin films, alumina, magnetron sputtering, phase stability, density functional theory
Nationell ämneskategori
Övrig annan teknik
Identifikatorer
urn:nbn:se:liu:diva-5038 (URN)91-85457-71-X (ISBN)
Disputation
2005-12-15, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 09:15 (Engelska)
Opponent
Handledare
Tillgänglig från: 2005-12-13 Skapad: 2005-12-13 Senast uppdaterad: 2013-10-30

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