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Sökning: db:Swepub > Kungliga Tekniska Högskolan > (2002) > Johansson Börje

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1.
  • Apazidis, Nicholas, et al. (författare)
  • An experimental and theoretical study of converging polygonal shock waves
  • 2002
  • Ingår i: Shock Waves. - 0938-1287. ; 12:1, s. 39-58
  • Tidskriftsartikel (refereegranskat)abstract
    • An experimental investigation was carried out to explore the possibility of producing converging polygonal shocks in an essentially two-dimensional cavity. Previous calculations by Apazidis and Lesser (1996) suggested that such configurations could be produced by reflecting a cylindrical outgoing shock from a smoothly altered circular boundary, the alteration having n-gonal symmetry. In the experiments the outgoing shock was produced by a spark discharge which yielded shocks in the Mach number range from 1.1 to 1.7 at a radius just prior to the reflection. Polygonal shocks were observed as predicted by using a modified form of geometrical shock dynamics, derived in the above paper. In addition, the modified theory was used to calculate the results of an experiment carried out by Sturtevant and Kulkarny (1976). The results of the numerical calculations were found to be in substantial agreement with both experiments, suggesting that the modifications in geometrical shock dynamics for non-uniform flow ahead of an advancing shock are useful in the case of shock focusing. The experiment. also showed that the polygonal shapes were stable in the examined range of shock Mach numbers. a result that may be of importance for a number of practical situations in which shock focusing is present.
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2.
  • Belonoshko, Anatoly B., et al. (författare)
  • Molecular dynamics study of phase transitions in Xe
  • 2002
  • Ingår i: Journal of Chemical Physics. - 0021-9606. ; 117:15, s. 7233-7244
  • Tidskriftsartikel (refereegranskat)abstract
    • A full account is given of our recent theoretical discovery [A. B. Belonoshko, R. Ahuja, and B. Johansson, Phys. Rev. Lett. 87, 165505 (2001)] of the fcc-bcc transition in Xe at high pressure and temperature. The interaction model and method for calculating phase boundaries are exhaustively tested by independent methods. The model was carefully checked against experimental data and results of ab initio molecular dynamics and it was found to perform very well. The two-phase method employed for finding the melting transition was compared with the robust thermodynamic approach and was found to provide data in exact agreement with the latter. The deviation of the calculated melting curve from the experimental one is quite tolerable at low pressures. After a reinterpretation of the experimental data, our results are also in good agreement with recent diamond anvil cell experiments. At a pressure of around 25 GPa and a temperature of about 2700 K, we find a triple fcc-bcc-liquid point. The fcc-bcc boundary is calculated without reference to the experimental data, in contrast to our previous work, and found to be in nice agreement with previous calculations as well as with the experimental data points, which, however, were interpreted as melting. Our finding concerning the fcc-bcc transition is confirmed by the direct molecular dynamics simulation of the fcc, bcc, and liquid phases in the same computational cell. In this simulation, it was observed that while the fcc phase melts, the bcc structure solidifies. Since Xe is a typical rare-gas solid, the fcc-bcc transition can now be expected for a number of other van der Waals systems, first of all in Ar and Kr. Our finding suggests, that the transition from close packed to bcc structure might be more common at high pressure and high temperature than was previously anticipated. The performed thorough test of methods and models in this study leads us to suggest that the original interpretation of experimental results is erroneous.
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3.
  • Dewhurst, J. K., et al. (författare)
  • Lattice dynamics of solid xenon under pressure
  • 2002
  • Ingår i: Physical Review Letters. - 0031-9007. ; 88:7
  • Tidskriftsartikel (refereegranskat)abstract
    • We use density-functional perturbation theory to obtain the phonon spectrum of fcc xenon under pressure. Thermodynamic properties obtained within the quasiharmonic approximation are in fair to good agreement with experiment at zero pressure. The transition pressure from the fcc to hcp phase is predicted to occur at 5 GPa. The fcc structure is found to be dynamically stable up to a pressure of 100 GPa, beyond which the phonon modes at the X and L symmetry points soften. We attribute the observed sluggish kinetics of the fcc-hcp transition to the small energy difference between the phases as well as to the high dynamical stability of the fcc phase.
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4.
  • Grechnev, G. E., et al. (författare)
  • Electronic structure, magnetic, and cohesive properties of LixMn2O4 : Theory
  • 2002
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 65:17
  • Tidskriftsartikel (refereegranskat)abstract
    • The volume dependent electronic structure of the spinel-type lithium manganese oxides LixMn2O4, x=0,0.5,1, is studied ab initio by employing a full-potential electronic structure method. The electronic structure, total energies, open-circuit voltage, and magnetic moments were obtained for various spin configurations of LixMn2O4 in the cubic spinel structure and the low-temperature orthorhombic structure. The effect of magnetic ordering on the band structure and structural stability has been investigated and an antiferromagnetic ordering proved to be the ground state of the LixMn2O4 spinels. Our calculations show that the manganese majority t(2g) d band is filled for all LixMn2O4 compounds studied, and the filling of the minority t(2g) band is expected in the lithiation process. The lithium intercalation potential, bulk modulus, magnetic moments, and optical properties are calculated within the itinerant band approach and are found to be in good agreement with available experimental data, indicating, that the density-functional theory provides reliable electronic structure of the LixMn2O4 system. The effect of the orthorhombic distortion on electronic structure and magnetism of LiMn2O4 was investigated, and our calculations do not show a substantial charge ordering at the structural transition from the cubic spinel to the orthorhombic structure, as proposed earlier. Instead, the low-temperature orthorhombic structure is found to possess the lowest energy via a Jahn-Teller distortion driven by the d band.
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7.
  • Holm, B., et al. (författare)
  • Theory of the ternary layered system Ti-Al-N
  • 2002
  • Ingår i: Journal of Applied Physics. - 0021-8979. ; 91:12, s. 9874-9877
  • Tidskriftsartikel (refereegranskat)abstract
    • We present calculations on the physical properties of Ti4AlN3 obtained from first principles, as well as some general observations concerning this group of materials. We further report mechanical data obtained through numerical simulations for some representative phases, and elaborate on the issue of high pressure stability. By examining the density of states of Ti4AlN3, we conclude that it is a semimetal, and we predict an opening up of the band gap around the Fermi level with increasing pressure.
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9.
  • Li, S., et al. (författare)
  • Pressure-induced phase transitions of KNbO3
  • 2002
  • Ingår i: Journal of Physics : Condensed Matter. - 0953-8984. ; 14:44, s. 10873-10877
  • Tidskriftsartikel (refereegranskat)abstract
    • We studied crystal structures of potassium niobate (KNbO3) at high pressure by means of first-principles self-consistent total-energy calculations within the local density approximation using the full-potential linear muffin-tin orbital method. For the first time, we have calculated the atomic equilibrium volume, bulk modulus, total energy, and transition pressures for KNbO3, covering the full pressure range for which the above-mentioned experiments have been done. Two pressure-induced transitions are derived theoretically, namely one from orthorhombic (I) to cubic structure at around 13.2 GPa and a second from cubic to orthorhombic (II) structure at a pressure of 39.7 GPa. This fully confirms the recent experiments by Kobayashi et al (Kobayashi Y, Endo S, Ashida T, Ming L C and Kikegawa T 2000 Phys. Rev. 615819).
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