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Controlling the Formation and Stability of Alumina Phases
Linköping University, Department of Physics, Chemistry and Biology, Plasma and Coating Physics . Linköping University, The Institute of Technology.
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 [en]
thin films, alumina, magnetron sputtering, phase stability, density functional theory
National Category
Other Engineering and Technologies not elsewhere specified
Identifiers
URN: urn:nbn:se:liu:diva-5038ISBN: 91-85457-71-X (print)OAI: oai:DiVA.org:liu-5038DiVA: diva2:20988
Public defence
2005-12-15, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 09:15 (English)
Opponent
Supervisors
Available from: 2005-12-13 Created: 2005-12-13 Last updated: 2013-10-30
List of papers
1. Microstructure of α-alumina thin films deposited at low temperatures on chromia template layers
Open this publication in new window or tab >>Microstructure of α-alumina thin films deposited at low temperatures on chromia template layers
2004 (English)In: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films, ISSN 0734-2101, E-ISSN 1520-8559, Vol. 22, no 1, 117-121 p.Article in journal (Refereed) Published
Abstract [en]

Radio frequency sputtering has been used to deposit -alumina (-Al2O3) thin films at substrate temperatures of 280–560 °C. The films are shown to be single phased and hard. Nanoindentation gives values of 306±31 and 27±3 GPa for elastic modulus and hardness, respectively, for a substrate temperature of 280 °C. Growth of the phase was achieved by in situ predeposition of a chromia template layer. Chromia crystallizes in the same hexagonal structure as -alumina, with a lattice mismatch of 4.1% in the a- and 4.6% in the c-parameter, and is shown to nucleate readily on the amorphous substrates (silicon with a natural oxide layer). This results in local epitaxy of -alumina on the chromia layer, as is shown by transmission electron microscopy. The alumina grains are columnar with grain widths increasing from 22±7 to 41±9 nm, as the temperature increases from 280 to 560 °C. This is consistent with a surface diffusion dominated growth mode and suggests that -alumina deposition at low temperatures is possible once initial grain nucleation has occurred. Results are also presented demonstrating chromia/-alumina growth on a technological substrate (Haynes230 Ni-based super alloy, Haynes International, Inc.).

Keyword
alumina, chromium compounds, sputtered coatings, indentation, elastic moduli, hardness, sputter deposition, transmission electron microscopy, epitaxial layers, grain size, surface diffusion, nucleation, wear resistant coatings, thermal barrier coatings
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-13578 (URN)10.1116/1.1636157 (DOI)
Available from: 2008-11-13 Created: 2008-09-29 Last updated: 2017-12-13
2. Phase control of Al2O3 thin films grown at low temperatures
Open this publication in new window or tab >>Phase control of Al2O3 thin films grown at low temperatures
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2006 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 513, no 1-2, 57-59 p.Article in journal (Refereed) Published
Abstract [en]

Low-temperature growth (500 °C) of α-Al2O3 thin films by reactive magnetron sputtering was achieved for the first time. The films were grown onto Cr2O3 nucleation layers and the effects of the total and O2 partial pressures were investigated. At 0.33 Pa total pressure and ≥ 16 mPa O2 partial pressure α-Al2O3 films formed, while at lower O2 pressure or higher total pressure (0.67 Pa), only γ phase was detected in the films (which were all stoichiometric). Based on these results we suggest that α phase formation was promoted by a high energetic bombardment of the growth surface. This implies that the phase content of Al2O3 films can be controlled by controlling the energy of the depositing species. The effect of residual H2O (10− 4 Pa) on the films was also studied, showing no change in phase content and no incorporated H (< 0.1%). Overall, these results are of fundamental importance in the further development of low-temperature Al2O3 growth processes.

Place, publisher, year, edition, pages
Elsevier, 2006
Keyword
Aluminum oxide, Chromium oxide, Sputtering, Ion bombardment, X-ray diffraction
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-14318 (URN)10.1016/j.tsf.2006.01.016 (DOI)
Note
Original publication: Andersson, J.M., Wallin, E., Helmersson, U., Kreissig, U. and Münger, E.P., Phase control of Al2O3 thin films grown at low temperatures, 2006, Thin Solid Films, (513), 1-2, 57-59. http://dx.doi.org/10.1016/j.tsf.2006.01.016. Copyright: Elsevier B.V., http://www.elsevier.com/ Available from: 2007-03-02 Created: 2007-03-02 Last updated: 2017-12-13Bibliographically approved
3. Molecular content of the deposition flux during reactive Ar/O2 magnetron sputtering of Al
Open this publication in new window or tab >>Molecular content of the deposition flux during reactive Ar/O2 magnetron sputtering of Al
2006 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 88, no 05, Art. No. 054101 JAN 30 2006- p.Article in journal (Refereed) Published
Abstract [en]

The deposition flux obtained during reactive radio frequency magnetron sputtering of an Al target in Ar/O2 gas mixtures was studied by mass spectrometry. The results show significant amounts of molecular AlO+ (up to 10% of the Al+ flux) in the ionic flux incident onto the substrate. In the presence of ~10–4 Pa H2O additional OH+ and AlOH+ were detected, amounting to up to about 100% and 30% of the Al+ flux, respectively. Since the ions represent a small fraction of the total deposition flux, an estimation of the neutral content was also made. These calculations show that, due to the higher ionization probability of Al, the amount of neutral AlO in the deposition flux is of the order of, or even higher than, the amount of Al. These findings might be of great aid when explaining the alumina thin film growth process.

Place, publisher, year, edition, pages
Institutionen för fysik, kemi och biologi, 2006
Keyword
alumina, dielectric thin films, sputter deposition, mass spectra
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-10452 (URN)10.1063/1.2170404 (DOI)
Note
Original publication: Andersson, J.M., Wallin, E., Münger, E.P. & Helmersson, U., Molecular content of the deposition flux during reactive Ar/O2 magnetron sputtering of Al, 2006, Applied Physics Letters, (88), 054101. http://dx.doi.org/10.1063/1.2170404. Copyright: American Institute of Physics, http://apl.aip.org/apl/top.jspAvailable from: 2007-12-17 Created: 2007-12-17 Last updated: 2017-12-14
4. Effects of additives in α- and θ-alumina: an ab initio study
Open this publication in new window or tab >>Effects of additives in α- and θ-alumina: an ab initio study
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.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-13581 (URN)10.1088/0953-8984/16/49/012 (DOI)
Available from: 2005-12-13 Created: 2005-12-13 Last updated: 2013-10-30
5. Ab initio calculations on the effects of additives on alumina phase stability
Open this publication in new window or tab >>Ab initio calculations on the effects of additives on alumina phase stability
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2005 (English)In: Physical review. B, Condensed matter and materials physics, ISSN 1098-0121, Vol. 71, no 014101, 014101- p.Article in journal (Refereed) 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.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-13582 (URN)10.1103/PhysRevB.71.014101 (DOI)
Available from: 2005-12-13 Created: 2005-12-13 Last updated: 2013-10-30
6. Energy distributions of positive and negative ions during magnetron sputtering of an Al target in Ar/O2 mixtures
Open this publication in new window or tab >>Energy distributions of positive and negative ions during magnetron sputtering of an Al target in Ar/O2 mixtures
2006 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 100, no 3, Art. No. 033305 AUG 1 2006- p.Article in journal (Refereed) Published
Abstract [en]

The ion flux obtained during reactive magnetron sputtering of an Al target in Ar/O2 gas mixtures was studied by energy-resolved mass spectrometry, as a function of the total and O2 partial pressures. The positive ions of film-forming species exhibited bimodal energy distributions, both for direct current and radio frequency discharges, with the higher energy ions most likely originating from sputtered neutrals. For the negative oxygen ions a high-energy peak was observed, corresponding to ions formed at the target surface and accelerated towards the substrate over the sheath potential. As the total pressure was increased the high-energy peaks diminished due to gas-phase scattering. Based on these results, the role of energetic bombardment for the phase constituent of alumina thin films are discussed.

Place, publisher, year, edition, pages
College Park, MD, United States: American Institute of Physics (AIP), 2006
Keyword
aluminium, sputter deposition, diffusion, mass spectra, high-frequency discharges, plasma materials processing
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-10472 (URN)10.1063/1.2219163 (DOI)000239764100014 ()
Note

Original publication: Jon M. Andersson, E. Wallin, E. P. Münger & U. Helmersson, Energy distributions of positive and negative ions during magnetron sputtering of an Al target in Ar/O2 mixtures, 2006, Journal of Applied Physics, (100), 033305. http://dx.doi.org/10.1063/1.2219163. Copyright: American Institute of Physics, http://jap.aip.org/jap/top.jsp

Available from: 2007-12-19 Created: 2007-12-19 Last updated: 2017-12-14

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