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Sökning: swepub > Johansson Börje > Refereegranskat > (2010-2011)

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31.
  • Li, Chun-Mei, et al. (författare)
  • First-principles investigation of the composition dependent properties of Ni2+xMn1-xGa shape-memory alloys
  • 2010
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 82:2, s. 024201-1-024201-9
  • Tidskriftsartikel (refereegranskat)abstract
    • The composition dependent lattice parameter, phase stability, elastic moduli, and magnetic transition temperature of the Ni2+xMn1-xGa shape-memory alloys are studied by using the first-principles exact muffin-tin orbital method in combination with the coherent potential approximation. The lattice parameter and tetragonal shear modulus of the cubic L-21 austenite phase decreases linearly with increasing concentration x of excess Ni atoms. The heats of formation of both cubic L-21 and tetragonal beta''' phases and their difference increase with x, indicating decreasing stability of the cubic and tetragonal phases and increasing driving force for the L-21 to beta''' martensitic transition. Investigating the electronic density of states, we find that the Ni-induced decreasing phase stability can mainly be ascribed to the weakening of the covalent bonding between minority spin states of Ni and Ga. Using the computed parameters, the composition dependence of the martensitic transition temperature is discussed. The theoretical Curie temperature, estimated from the Heisenberg model in combination with the mean-field approximation, is larger for the beta''' phase than for the L-21 phase. For both phases, the Curie temperature decreases nearly linearly with increasing x.
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32.
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33.
  • Li, Chun-Mei, et al. (författare)
  • Interplay between temperature and composition effects on the martensitic transformation in Ni(2+x)Mn(1-x)Ga alloys
  • 2011
  • Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 98:26, s. 261903-
  • Tidskriftsartikel (refereegranskat)abstract
    • Martensitic transformation in Ni(2+x)Mn(1-x)Ga alloys is known to be controlled by the soft tetragonal elastic constant C' of the high-temperature austenitic phase. The temperature (T) and composition (x) dependence of C'(T, x) are calculated using the first-principles exact muffin-tin orbitals method. We show that the temperature factor of C' is dominated by the phonon-smearing term. The competition between the negative alloying effect (partial derivative C'/partial derivative x < 0) and the positive temperature effect (partial derivative C'/partial derivative T > 0) is found to lead to nearly constant C'(T(M)(x), x) at the critical temperature TM(x). We demonstrate that a proper account of the temperature and composition dependence of C'(T, x) is indispensable for reasonable theoretical TM(x) values.
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34.
  • Li, Chun-Mei, et al. (författare)
  • Site preference and elastic properties of Fe-, Co-, and Cu- doped Ni(2)MnGa shape memory alloys from first principles
  • 2011
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 84:2, s. 024206-
  • Tidskriftsartikel (refereegranskat)abstract
    • The site preference and elastic properties of Fe-, Co-, and Cu-doped Ni(2)MnGa alloys are investigated by using the first-principles exact muffin-tin orbital method in combination with coherent-potential approximation. It is shown that Fe atom prefers to occupy the Mn and Ni sublattices even in Ga-deficient alloys; Co has strong tendency to occupy the Ni sublattice in all types of alloys; Cu atoms always occupy the sublattice of the host elements in deficiency. For most of the alloys with stable site occupations, both the electron density n and the shear modulus C' can be considered as predictors of the composition dependence of the martensitic transition temperature T(M) of the alloys. The physics underlying the composition-dependent C' are discussed based on the calculated density of states.
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35.
  • Li, Sa, et al. (författare)
  • Dehydrogenation associated with Ti catalyst in sodium alanate
  • 2010
  • Ingår i: Journal of Physics and Chemistry of Solids. - : Elsevier BV. - 0022-3697 .- 1879-2553. ; 71:8, s. 1073-1076
  • Tidskriftsartikel (refereegranskat)abstract
    • Addition of small amount of Ti precursors to sodium alanate has recently been found to substantially improve the kinetics and thermodynamics of hydrogen sorption in sodium alanate. In spite of several attempts, a fundamental understanding of how the catalyst works has remained unattainable. Using first principles methods we have investigated the mechanisms for hydrogen desorption in this material. We conclude that Ti substituted at Al site is energetically most favorable. The small amount of Ti substitution does not introduce large lattice distortion. The Ti atom serves as a magnet that continues to attract nearby H atoms as the nearest ones are successively desorbed. The number of Al atoms near to the Ti site remains at four upon hydrogen desorption when Ti is substituted at the Al site. These results provide important new insight into the design of future catalysts for hydrogen storage materials.
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36.
  • Lu, Song, et al. (författare)
  • Composition and orientation dependence of the interfacial energy in Fe-Cr stainless steel alloys
  • 2011
  • Ingår i: Physica status solidi. B, Basic research. - : Wiley. - 0370-1972 .- 1521-3951. ; 248:9, s. 2087-2090
  • Tidskriftsartikel (refereegranskat)abstract
    • Using a first-principles quantum mechanical method, we calculated the (001) and (110) interfacial energies between the low temperature alpha and alpha' phases of Fe-Cr alloys as functions of chemical composition. Weshow that the interfacial energies and the interfacial energy anisotropy are highly composition dependent. In particular, the increasing interfacial energy anisotropy with decreasing compositional gap may induce different morphology of the decomposed phases for different compositions of the host alloys.
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37.
  • Lu, Song, et al. (författare)
  • Stacking fault energies of Mn, Co and Nb alloyed austenitic stainless steels
  • 2011
  • Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 59:14, s. 5728-5734
  • Tidskriftsartikel (refereegranskat)abstract
    • The alloying effects of Mn, Co and Nb on the stacking fault energy (SFE) of austenitic stainless steels, Fe-Cr-Ni with various Ni contents, are investigated via quantum-mechanical first-principles calculations. In the composition range (c(Cr) = 20%, 8 <= c(Ni) <= 20%, 0 <= c(Mn), c(Co), c(Nb) <= 8%, balance Fe) studied here, it is found that Mn always decreases the SFE at 0 K but increases it at room temperature in high-Ni (c(Ni) greater than or similar to 16%) alloys. The SFE always decreases with increasing Co content. Niobium increases the SFE significantly in low-Ni alloys; however, this effect is strongly diminished in high-Ni alloys. The SFE-enhancing effect of Ni usually observed in Fe-Cr-Ni alloys is inverted to an SFE-decreasing effect by Nb for c(Nb) greater than or similar to 3%. The revealed nonlinear composition dependencies are explained in terms of the peculiar magnetic contributions to the total SFE.
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38.
  • Luo, Hu-Bin, et al. (författare)
  • First-principles investigations of the five-layer modulated martensitic structure in Ni(2)Mn(Al(x)Ga(1-x)) alloys
  • 2011
  • Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 59:15, s. 5938-5945
  • Tidskriftsartikel (refereegranskat)abstract
    • In this paper, the five-layer modulated (5M) martensitic structures of Ni(2)Mn(Al(x)Ga(1-x)), with x = 0, 0.1 and 0.2, are investigated by the use of the exact muffin-tin orbital method in combination with the coherent potential approximation. The 5M martensite is modeled by varying c/a (shear) and wave-like displacements of the atoms on (1 1 0) plane (shuffle) scaled by eta according to Martynov and Kokorin (J. Phys. III 2, 739 (1992)). For Ni(2)MnGa, we obtain 5M martensite with equilibrium c/a of 0.92 and eta of 0.08, in reasonable agreement with the experiment results (0.94 and 0.06, respectively). c/a and eta are linearly coupled to each other. Al-doping increases c/a and decreases eta, but the linear c/a similar to eta coupling remains. Comparing the total energies of the 5M martensite and L2(1) austenite, we find that the martensite is more stable than the austenite. Al-doping increases the relative stability of the austenite and finally becomes energetically degenerated with the 5M martensite at an Al atomic fraction (x) of about 0.26. The relative phase stability is analyzed based on the calculated density of states. The calculated total magnetic moments mu(0) as a function of c/a exhibit a maximum around the equilibrium c/a. Al-doping reduces mu(0)
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39.
  • Luo, H. B., et al. (författare)
  • Theoretical investigation of the effects of composition and atomic disordering on the properties of Ni2Mn(Al1-xGax) alloy
  • 2011
  • Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 59:3, s. 971-980
  • Tidskriftsartikel (refereegranskat)abstract
    • The influences of composition and Mn-Ga/Al disordering on the magnetic and elastic properties of the Ni2Mn(Al1-xGax) Hensler alloy are investigated by the use of the first-principles exact muffin-tin orbital method in combination with coherent-potential approximation. The transition temperature from the completely disordered B2 phase to the ordered L2(1) phase, estimated by the use of Bragg-Williams-Gorsky approximation, is in excellent agreement with experiments. The site-occupancy in the partially disordered alloy is determined by comparing the free energies of the alloys with different site-occupation configurations. It is found that Mn atoms generally prefer to exchange with Al atoms for the alloy with low degree of disorder and low Ga concentration. With increasing degree of disorder and Ga concentration, Mn may exchange with Ga as well. The total magnetic moment of the completely ordered alloy exhibits a minimum around x = 0.7 due to the competition between the increasing magnetic moments of Mn atoms and the decreasing moments of Ni atoms with increasing x. For the partially disordered alloy, both total and local magnetic moments of Mn and Ni decrease linearly with increasing degree of disorder, and Ga atoms show significant magnetism. The shear moduli C of the alloys are calculated with respect to both x and the degree of disorder. The results show that C decreases with increasing x and degree of disorder. The dependence on composition and on the degree of disorder of the martensitic transition temperatures is discussed in terms of the calculated C.
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40.
  • Luo, Wei, et al. (författare)
  • Dynamical stability of body center cubic iron at the Earth's core conditions
  • 2010
  • Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 107:22, s. 9962-9964
  • Tidskriftsartikel (refereegranskat)abstract
    • Here, using self-consistent ab initio lattice dynamical calculations that go beyond the quasiharmonic approximation, we show that the high-pressure high-temperature bcc-Fe phase is dynamically stable. In this treatment the temperature-dependent phonon spectra are derived by exciting all the lattice vibrations, in which the phonon-phonon interactions are considered. The high-pressure and high-temperature bcc-Fe phase shows standard bcc-type phonon dispersion curves except for the transverse branch, which is overdamped along the high symmetry direction Gamma-N, at temperatures below 4,500 K. When lowering the temperature down to a critical value T-C, the lattice instability of the bcc structure is reached. The pressure dependence of this critical temperature is studied at conditions relevant for the Earth's core.
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