Atomistic modelling of functional solid oxides for industrial applications : Density Functional Theory, hybrid functional and GW-based studies

In this Thesis a set of functional solid oxides for industrial applications have been addressed by first principles and thermodynamical modelling. More specificially, measurable quantities such as Gibbs free energy, geometry and electronic structure have been calculated and compared when possible with experimental data. These are crystalline and amorphous aluminum oxide (Al2O3), Zirconia (ZrO2), magnesium oxide (MgO), indiumoxide (In2O3) and Kaolinite clay (Al2Si2O5(OH)4). The reader is provided a computation tool box, which contains a set of methods to calculate properties of oxides that are measurable in an experiment. There are three goals which we would like to reach when trying to calculate experimental quantities. The first is verification. Without verification of the theory we are utilizing, we cannot reach the second goal -prediction. Ultimately, this may be (and to some extent already is) the future of first principles methods, since their basis lies within the fundamental quantum mechanics and since they require no experimental input apart from what is known from the periodic table. Examples of the techniques which may provide verification are X-Ray Diffraction (XRD), X-ray Absorption and Emission Spectroscopy (XAS and XES), Electron Energy Loss Spectroscopy and Photo-Emission Spectroscopy (PES). These techniques involve a number of complex phenomena which puts high demands on the chosen computational method/s. Together, theory and experiment may enhance the understanding of materials properties compared to the standalone methods. This is the final goal which we are trying to reach -understanding. When used correctly, first principles theory may play the role of a highly resolved analysis method, which provides details of structural and electronic properties on an atomiclevel. One example is the use of first principles to resolve spectra of multicomponentsamples. Another is the analysis of low concentrations of defects. Thorough analysis of the nanoscale properties of products might not be possible in industry due to time and cost limitations. This leads to limited control of for example low concentrations of defects, which may still impact the final performance of the product. On example within cutting tool industry is the impact of defect contents on the melting point and stability of protective coatings. Such defects could be hardening elements such as Si, Mn, S, Ca which diffuse from a steel workpiece into the protective coating during high temperature machining. Other problems are the solving of Fe from the workpiece into the coating and reactions between iron oxide, formed as the workpiece surface is oxidized, and the protective coating. The second part of the computational toolbox which is provided to the reader is the simulation of solid oxide synthesis. Here, a formation energy formalism, most often applied to materials intended in electronics devices is applied. The simulation of Chemical Vapour Deposition (CVD) and Physical Vapor Deposition (PVD) requires good knowledge of the experimental conditions, which can then be applied in the theoretical simulations. Effects of temperature, chemical and electron potential, modelled concentration and choice of theoretical method on the heat of formation of different solid oxides with and without dopants are addressed in this work. A considerable part of this Thesis is based upon first principles calculations, more specifically, Density Functional Theory (DFT) After Kohn and Pople received the Nobel Prize in chemistry in 1998, the use of DFT for computational modelling has increased strikingly (see Fig. 1). The use of other first principles methods such as hybrid functionals and the GW approach (see abbreviations for short explanations and chapter 4.5 and 5.3.) have also become increasingly popular, due to the improved computational resources. These methods are also employed in this Thesis.QC 2011020

Atomistic modelling of functional solid oxides for industrial applications : Density Functional Theory, hybrid functional and GW-based studies

In this Thesis a set of functional solid oxides for industrial applications have been addressed by first principles and thermodynamical modelling. More specificially, measurable quantities such as Gibbs free energy, geometry and electronic structure have been calculated and compared when possible with experimental data. These are crystalline and amorphous aluminum oxide (Al2O3), Zirconia (ZrO2), magnesium oxide (MgO), indiumoxide (In2O3) and Kaolinite clay (Al2Si2O5(OH)4). The reader is provided a computation tool box, which contains a set of methods to calculate properties of oxides that are measurable in an experiment. There are three goals which we would like to reach when trying to calculate experimental quantities. The first is verification. Without verification of the theory we are utilizing, we cannot reach the second goal -prediction. Ultimately, this may be (and to some extent already is) the future of first principles methods, since their basis lies within the fundamental quantum mechanics and since they require no experimental input apart from what is known from the periodic table. Examples of the techniques which may provide verification are X-Ray Diffraction (XRD), X-ray Absorption and Emission Spectroscopy (XAS and XES), Electron Energy Loss Spectroscopy and Photo-Emission Spectroscopy (PES). These techniques involve a number of complex phenomena which puts high demands on the chosen computational method/s. Together, theory and experiment may enhance the understanding of materials properties compared to the standalone methods. This is the final goal which we are trying to reach -understanding. When used correctly, first principles theory may play the role of a highly resolved analysis method, which provides details of structural and electronic properties on an atomiclevel. One example is the use of first principles to resolve spectra of multicomponentsamples. Another is the analysis of low concentrations of defects. Thorough analysis of the nanoscale properties of products might not be possible in industry due to time and cost limitations. This leads to limited control of for example low concentrations of defects, which may still impact the final performance of the product. On example within cutting tool industry is the impact of defect contents on the melting point and stability of protective coatings. Such defects could be hardening elements such as Si, Mn, S, Ca which diffuse from a steel workpiece into the protective coating during high temperature machining. Other problems are the solving of Fe from the workpiece into the coating and reactions between iron oxide, formed as the workpiece surface is oxidized, and the protective coating. The second part of the computational toolbox which is provided to the reader is the simulation of solid oxide synthesis. Here, a formation energy formalism, most often applied to materials intended in electronics devices is applied. The simulation of Chemical Vapour Deposition (CVD) and Physical Vapor Deposition (PVD) requires good knowledge of the experimental conditions, which can then be applied in the theoretical simulations. Effects of temperature, chemical and electron potential, modelled concentration and choice of theoretical method on the heat of formation of different solid oxides with and without dopants are addressed in this work. A considerable part of this Thesis is based upon first principles calculations, more specifically, Density Functional Theory (DFT) After Kohn and Pople received the Nobel Prize in chemistry in 1998, the use of DFT for computational modelling has increased strikingly (see Fig. 1). The use of other first principles methods such as hybrid functionals and the GW approach (see abbreviations for short explanations and chapter 4.5 and 5.3.) have also become increasingly popular, due to the improved computational resources. These methods are also employed in this Thesis.QC 2011020

Structural Determination of (Cr,Co)7C3

Chromium is one of the most well-known WC grain growth inhibitors in cemented carbides. It is thus vital to understand and to be able to thermodynamically model the prevailing phase equilibria in the WC-Co-Cr system. To do this it is important that the lower order systems, such as the Co-Cr-C system, are correctly described. Previous investigations have shown that the M7C3 (M=Cr,Co,W) phase is the first carbide to form when Cr is added in excess to the WC+fcc-Co/liquid+graphite phase field. However, the exact structure of this phase has not been investigated and there are many proposed structures already for the binary Cr7C3 carbide, ranging from trigonal, via hexagonal to orthorhombic symmetry. Recent investigations show that the hexagonal structure belonging to the P63mc space group is the stable structure at 0 K. In the present study the binary Cr7C3 carbide and a mixed M7C3 carbide are investigated. The structures of both carbides and preferential positions for Co atoms in the mixed carbide are determined by XRD measurements in combination with ab initio calculations and Rietveld refinement.QC 20130902</p

Chemical Interactions Between Cemented Carbide and Difficult-to-Machine Materials by Diffusion Couple Method and Simulations

A simple and efficient diffusion couple method is utilized to study the chemical interactions between cemented carbide cutting tools and difficult-to-machine materials (Ti, Ti-6Al-4V, Ni, Inconel 718, Fe, and AISI 316L). The experimental results and simulations probe different chemical interactions between the cemented carbide and work materials. In particular, the formation of a thick TiC layer is observed at the cemented carbide/Ti and Ti-6Al-4V interface while eta-phase is formed at the interface between the cemented carbide and work materials Inconel 718, Fe and AISI 316L. Pure titanium and Ti-6Al-4V both interact strongly with the tool causing formation of TiC and dissolution of WC-grains. Experiments and diffusion simulations confirm bcc-W formation and progressive diffusion of W into bcc-Ti. For both Ti and Fe work materials a dense band of carbides (WC + eta or WC + TiC) forms at the interface, effectively inhibiting further diffusion. Ni does not form any stable carbide and lowers the eta-phase limit in terms of carbon content, wherefore diffusion can occur freely. The diffusion couple method used in this work, corroborated by DICTRA simulations should serve as a useful tool in the detailed analysis of worn tools where chemical wear is dominant