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A theoretical investigation of Tin+1AlCn and Mn2GaC MAX phases: phase stability and materials properties
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
2014 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis presents theoretical research on MAX phases (M=transition metal, A=A-group element, X=carbon and/or nitrogen), with focus on predictions of phase stability as well as of physical properties.

The first part is an investigation of the phase stability of the MAX phases Ti2AlC, Ti3AlC2, and Ti4AlC3 at elevated temperatures, where the former two phases have been obtained experimentally. Phase stability calculations of MAX phases usually do not take temperature dependent effects such as electronic excitations and lattice vibrations into consideration due to significantly increased computational cost. The results have nevertheless so far been quite accurate, with good agreement between theory and experiments. Still, the question whether the inclusion of temperature into the calculations could significantly alter the results as compared to previous 0 K calculations needs to be investigated, since this has bearing on the reliability of future predictions of the stability of not yet known MAX phases. However, it is shown that for Tin+1AlCn, the different temperature dependent effects largely cancel each other. The results therefore suggest that to go beyond 0 K calculations for phase stability predictions of MAX phases is motivated only for borderline cases.

The second part investigates the Mn2GaC MAX phase, which was recently predicted from theoretical phase stability calculations and subsequently synthesized. As a new member of the MAX phase family as well as being one of the first known MAX phases to exhibit magnetism, it is of interest to explore its physical properties. Here, we have used firstprinciples calculations to determine the electronic, vibrational and elastic properties. Analysis of the electronic band structure indicates anisotropy in transport properties, while the electronic and phonon density of states shows that the relative orientation of the Mn magnetic moments over the Ga layers affects the distribution of the electronic and vibrational states for both Mn and Ga.

The Voigt bulk, Voigt shear, and Young's modulus is also investigated, together with the Poisson's ratio, the elastic anisotropy, and the  machinability via two machinability indices. In relation to experimental results of the moduli of other M2AC phases, the Voigt bulk and shear moduli are concluded to be fairly low, 157 and 93 GPa, respectively, while the magnitude of the Young's modulus at 233 GPa is intermediate. The Poisson's ratio, which is 0.25, on the other hand, is comparatively high. The phase is shown to be elastically quite isotropic, and, just as other M2GaC phases, also machinable. As all here investigated properties are affected by the choice of magnetic spin configuration, the results show the importance of identifying the correct magnetic ground state in future theoretical work on magnetic MAX phases.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2014. , 30 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1693
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:liu:diva-111955DOI: 10.3384/lic.diva-111955ISBN: 978-91-7519-181-2 (print)OAI: oai:DiVA.org:liu-111955DiVA: diva2:762339
Supervisors
Note

The series name of this thesis Linköping Studies in Science and Technology Licentiate Thesis is incorrect. The correct name is Linköping Studies in Science and Technology Thesis.

Available from: 2014-11-11 Created: 2014-11-11 Last updated: 2015-01-29Bibliographically approved
List of papers
1. Temperature dependent phase stability of nanolaminated ternaries from first-principles calculations
Open this publication in new window or tab >>Temperature dependent phase stability of nanolaminated ternaries from first-principles calculations
2014 (English)In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 91, 251-257 p.Article in journal (Refereed) Published
Abstract [en]

Methods based on first-principles calculations have proven effective for predicting the thermodynamic stability of materials that have not previously been considered. However, the vast majority of these predictions are based on 0 K calculations, which means that little is known about the effects of temperature on their accuracy. This causes considerable uncertainty with respect to stability predictions of new hypothetical phases. In this work we combine first-principles calculations with an optimization procedure to calculate the phase stability as a function of temperature for Ti2AlC, Ti3AlC2 and Ti4AlC3 MAX phases with respect to their most competing phases in the Ti-Al-C phase diagram, in a temperature interval from 0 to 2000 K. To model nonzero temperatures, we include effects from the electronic and vibrational free energies to the Gibbs free energy for all relevant competing phases. We show that, due to a mutual cancellation of the temperature dependent energy terms, the results of neither the harmonic nor the quasiharmonic calculations differ significantly from the calculated 0 K formation energies. We thus provide a plausible explanation for the success of previous 0 K predictions, an explanation which also serves as evidence for the hypothesis that the phase stability in many materials systems is primarily governed by the 0 K energy terms.

Place, publisher, year, edition, pages
Elsevier, 2014
Keyword
First-principles; Phase stability; Ternary carbides; Harmonic approximation; Quasiharmonic approximation; Density functional theory
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-109360 (URN)10.1016/j.commatsci.2014.04.055 (DOI)000339129100032 ()
Available from: 2014-08-15 Created: 2014-08-15 Last updated: 2017-12-05Bibliographically approved
2. First-principles calculations of the electronic, vibrational, and elastic properties of the magnetic laminate Mn2GaC
Open this publication in new window or tab >>First-principles calculations of the electronic, vibrational, and elastic properties of the magnetic laminate Mn2GaC
2014 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 116, no 10, 103511- p.Article in journal (Refereed) Published
Abstract [en]

In this paper, we report the by first-principles predicted properties of the recently discovered magnetic MAX phase Mn2GaC. The electronic band structure and vibrational dispersion relation, as well as the electronic and vibrational density of states, have been calculated. The band structure close to the Fermi level indicates anisotropy with respect to electrical conductivity, while the distribution of the electronic and vibrational states for both Mn and Ga depend on the chosen relative orientation of the Mn spins across the Ga sheets in the Mn–Ga–Mn trilayers. In addition, the elastic properties have been calculated, and from the five elastic constants, the Voigt bulk modulus is determined to be 157 GPa, the Voigt shear modulus 93 GPa, and the Young's modulus 233 GPa. Furthermore, Mn2GaC is found relatively elastically isotropic, with a compression anisotropy factor of 0.97, and shear anisotropy factors of 0.9 and 1, respectively. The Poisson's ratio is 0.25. Evaluated elastic properties are compared to theoretical and experimental results for M 2 AC phases where M = Ti, V, Cr, Zr, Nb, Ta, and A = Al, S, Ge, In, Sn

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2014
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-111954 (URN)10.1063/1.4894411 (DOI)000342833700023 ()
Available from: 2014-11-11 Created: 2014-11-11 Last updated: 2017-12-05Bibliographically approved

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Thore, Andreas

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