Controlling the Formation and Stability of Alumina Phases

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…


A. Thin film deposition – sputtering
1. Basics
2. The plasma
3. Magnetron sputtering
4. Radio frequency sputtering
5. Reactive sputterin
6. Kinetic energies of sputtered particles
7. Nucleation and growth of thin films
a. The substrate structure
b. The content of the deposition flux
c. The energy supplied to the growth surface
8. The deposition system
B. Film and plasma analysis
1. X-ray diffraction
2. Transmission electron microscopy
3. Nanoindentation
4. Elastic recoil detection analysis
5. Energy-resolved mass spectrometry
A. The Hohenberg-Kohn theorems and the Kohn-Sham equations
B. The exchange-correlation potential
1. The local density and generalized gradient approximations
C. Treating the electrons
1. Pseudopotentials and plane waves
D. Computational details
1. DFT setup used in this work
2. The supercell
3. Stability calculations
4. Temperature in DFT calculation
5. Electronic structure
A. Properties of alumina
1. Alumina polymorphs: a conundrum
2. Properties of α-alumina
3. Properties of θ-alumina
4. Properties of γ-alumina
B. The alumina research field
1. Alumina as a wear-resistant coating
2. Growth of alumina – role of energetic bombardment
3. Growth of alumina – role of surfaces
4. Theoretical studies of alumina
5. The effect of doping on alumina phase stability
A. Thin film growth and characterization (Papers I and II)
B. Plasma analysis (Papers III and IV)
C. DFT calculations on alumina phase stability (Papers V and VI)

Author: Andersson, Jon Martin

Source: Linkoping University

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