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- Thore, Andreas
(författare)
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Phase stability and physical properties of nanolaminated materials from first principles
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The MAX phase family is a set of nanolaminated, hexagonal materials typically comprised of three elements: a transition metal (M), an A-group element (A), and carbon and/or nitrogen (X). In this thesis, first-principles based methods have been used to investigate the phase stability and physical properties of a number of MAX and MAX-like phases.Most theoretical work on MAX phase stability use the constraint of 0 K conditions, due to the very high computational cost of including temperature dependent effects such as lattice vibrations and electronic excitations for all relevant competing phases in the ternary or multinary chemical space. Despite this, previous predictions of the existence of new MAX phases have to a large extent been experimentally verified. In an attempt to provide a possible explanation for this consistency, and thus help strengthen the confidence in future predictions, we have calculated the temperature dependent phase stability of Tin+1AlCn, to date the most studied MAX phases. We show that both the electronic and vibrational contribution to the Gibbs free energies of the MAX phases are cancelled by the corresponding contributions to the Gibbs free energies of the competing phases. We further show that this is the case even when thermal expansion is considered.We have also investigated the stability of two hypothetical MAX-like phases, V2Ga2C and (Mo1-xVx)2Ga2C, motivated by a search for ways to attain new two-dimensional MAX phase derivatives, so-called MXenes. We predict that it is possible to synthesize both phases. For x≤0.25, stability of (Mo1-xVx)2Ga2C is indicated for both ordered and disordered solid solutions on the M sublattice. For x=0.5 and x≥0.75, stability is only indicated for disordered solutions. The ordered solutions are stable at temperatures below 1000 K, whereas stabilization of the disordered solutions requires temperatures of up to 2100 K, depending on the V concentration.Finally, we have investigated the electronic, vibrational, and magnetic properties of the recently synthesized MAX phase Mn2GaC. We show that the electronic band structure is anisotropic, and determine the bulk, shear, and Young’s modulus to be 157, 93, and 233 GPa, respectively, and Poisson’s ratio to be 0.25. We further predict the magnetic critical order-disorder temperature of Mn2GaC to be 660 K. We base the predictions on Monte Carlo simulations of a bilinear Heisenberg Hamiltonian constructed from magnetic exchange interaction parameters derived using two different supercell methods: the novel magnetic direct cluster averaging method (MDCA), and the Connolly-Williams method (CW). We conclude that CW is less computationally expensive than MDCA for chemically and topologically ordered phases such as Mn2GaC.
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| 2. |
- Ehteshami, Hossein
(författare)
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Finite temperature properties of elements and alloy phases from first principles
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- First principles calculations are usually concerned with properties calculated at temperature 0 K. However, the industrially important materials are functioning at finite temperatures. To fill such a gap a first-principles based modeling of free energy has been developed in this thesis and finite temperature properties of different phases of Fe and Mn have been calculated and contrasted with available experimental data.In particular, using partitioning of the Helmholtz free energy, thermophysical properties of paramagnetic Fe have been reported. The heat capacity, lattice constant, thermal expansion and elastic moduli of γ- and δ-Fe show a good agreement with available experimental data. In the case of α-Fe, we observe a good agreement for elastic moduli and thermal expansion with experiments but the heat capacity is not well-reproduced in the calculations because of the large contribution of magnetic short-range which our models are not capable of capturing.α- and β-Mn theoretically pose a challenge for direct simulations of thermodynamic properties because of the complexity of magnetic and crystal structure. The partitioning of free energy has been used and thermodynamics of these phases have been derived. The obtained results show a good agreement with experimental data suggesting that, despite the complexities of these phases, a rather simple approach can well describe their finite temperature properties. High temperature phases of Mn, γ and δ, are also theoretically challenging problems. Employing a similar approach to Fe, thermophysical properties of these high symmetry phases of Mn have been reported which also show good agreement with available experimental data.The point defect and metal-self diffusion in titanium carbide (TiC), a refractory material, have been investigated in the present work. The common picture of metal-vacancy exchange mechanism for metal self-diffusion was shown to be unable to explain the experimentally observed values of activation energy. Several new clusters of point defects such as vacancies and interstitials have been found and reported which are energetically lower that a single metal vacancy. In a subsequent study, we showed that some of these clusters can be considered as mediators of metal self-diffusion in TiC.Evaluation of structural properties of Ti(O,C), a solid solution of TiC and β-TiO, from supercell approach is an extremely difficult task. For a dilute concentration of O, we show the complexity of describing an impurity of O in TiC using supercell approach. A single-site method such as the exact muffin-tin orbital method in the coherent potential approximation (EMTO-CPA) is a good alternative to supercell modeling of Ti(O,C). However, a study of Ti(O,C) using EMTO-CPA requires a further development of the technique regarding the partitioning of space. The shape module of EMTO has been modified for this purpose. With the help of the modified module, Ti(O,C) have been studied using EMTO-CPA. The results for the divacancy concentration and corresponding lattice parameter variations show good agreement with experimental data.
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