Materials Week '97: Tuesday AM Session

Focusing on physical metallurgy and materials, Materials Week '97, which incorporates the TMS Fall Meeting, features a wide array of technical symposia sponsored by The Minerals, Metals & Materials Society (TMS) and ASM International. The meeting will be held September 14-18 in Indianapolis, Indiana. The following session will be held Tuesday morning, September 16.

ALLOY MODELING AND DESIGN: Session III: Physical, Chemical, and Mechanical Properties of Alloys I

Session Chair: Dr. D.D. Johnson, Sandia National Laboratories, Livermore, CA 94551

EFFECT OF COMPOSITION AND STRUCTURE ON THE VOLTAGE OF Li-INTERCALATED COMPUNDS: G. Ceder, M.K. Aydinol, A.F. Kohan, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139

Lithium-intercalation compounds are an important part of rechargeable lithium batteries. Using the ab-initio pseudopotential method it is possible to predict the average voltage at which Li-capability occurs in metal oxides. We use this newly developed capability to systematically investigate the effects of metal ion, chalcogenide ion, and crystal structure on the intercalation voltage. For existing compounds, such as LiCoO₂ and LiMn₂O₄, the agreement with experiments is extremely good. In the lithium-metal-oxides, lithium is completely ionized with its valence charge transferred partly to the metal and partly to the oxygen ions. Charge transfer to oxygen seems to correlate well with high output voltage. We demonstrate how this can be used to design compounds with very high energy density.

9:10 am

VIBRATIONAL ENTROPY DIFFERENCE BETWEEN ORDERED AND DISORDERED ALLOYS: Axel Van de Walle, Gerbrand Ceder, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139

Previous experimental measurements seem to indicate that a significant portion of the entropy difference between ordered and disordered Ni₃A1 is due to vibrational effects. In order to assess this usually large importance of vibration, ab-initio computations of the phonon density of states were performed. The Full Potential LMTO method was used to compute the forces acting on each atom as function of their displacement. These results were used to fit spring constants in a Born-von Karman model. While the vibrational entropy of an ordered phase can easily be obtained from such a model, the entropy of a disordered phase requires a few more steps. The vibrational entropy obtained for different ordered structure were represented by a cluster expansion. The vibrational entropy of the disordered phase was then extrapolated using this expansion. These results were also compared to the ones obtained by linear response theory.

9:40 am

VIBRATIONAL ENTROPY DIFFERENCES BETWEEN hP24 AND FCC Co₃V BY HIGH TEMPERATURE INELASTIC NEUTRON SCATTERING: P.D. Bogdanoff , B. Fultz, Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA 91125; J.L. Robertson, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831

We report our measurements for the difference in vibrational entropy between the hP24 and fcc high temperature phases of Co₃V, as measured by high temperature inelastic neutron scattering. Previous work has shown that near the transition temperature the vibrational entropy of the hP24 phase is a very strong function of temperature, undergoing a 0.10 k_B/atom increase over the 200K below the transition. The anharmonic effects in the single phase regions mask the entropy difference between hP24 and fcc at the transition temperature. Earlier work done at Caltech measured the vibrational entropy difference between hP24 and fcc at 0.11+/-0.02 k_B/atom using low temperature calorimetry. Work supported by U.S. DOE DE-FG03-96ER45572.

10:10 am BREAK

10:30 am

NON-COLLINEAR MAGNETISM OF Fe-RICH FeNi ALLOYS: Y. Wang, Pittsburgh Supercomputing Center, Pittsburgh, PA 15213; G.M. Stocks, D.M.C. Nicholson, W.A. Shelton, Oak Ridge National Laboratory, Oak Ridge, TN 37831

The magnetic structure of Fe-rich FeNi alloys has long been a subject of great scientific interest and controversy. In this study, we attempt to understand an interesting phenomenon that the average magnetic moment of the alloys in the -phase (fee) decreases dramatically in the composition range near 70% Fe. Although the observation was made more than thirty years ago, the mechanism for this moment collapse is still controversial. In our approach, the non-collinear locally self-consistent multiple scattering (LSMS) method is applied to the magnetic structure calculation of large unit cell samples consisting of Fe and Ni atoms. The Fe and Ni atoms are randomly distributed on a fcc lattice. The moment directions are initialized to be randomly oriented, and then, as the self-consistent iterations proceed, are allowed to rotate to minimize the total energy. A stable magnetic structure of the alloy is determined by the final moment configuration. We compare our results with experiments and discuss the implication of our results for the INVAR mechanism.

11:00 am

EMBEDDED ATOM METHOD CALCULATIONS OF VIBRATIONAL THERMODYNAMIC PROPERTIES OF ORDERED AND DISORDERED Cu₃Au AND Ni₃Al: Dane Morgan, Jeffrey Althoff, Didier de Fontaine, University of California, Berkeley, CA 94720; Mark Asta, Stephen Foiles, D.D. Johnson, Sandia National Laboratories, Livermore, CA 94551

Recent work had suggested that vibrational effects play a significant role in determining some phase diagrams. In order to better understand these effects, we investigate the vibrational properties of disordered and ordered Cu₃Au and Ni₃Al using the Embedded Atom Method (EAM). We calculate vibrational thermodynamic quantities within the quasi-harmonic approximation. The vibrational entropy is found to be strongly dependent on volume and cell-internal relaxations. For fully relaxed structures the dependence on lattice decoration of the vibrational entropy is found to be significantly smaller than that suggested by recent experimental results. Research supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences.

11:30 am

COMPOSITION DEPENDENCE OF THE MAGNETIC MOMENT DISTRIBUTION OF Ni_cCu_1-c ALLOYS: G. Malcolm Stocks, D.M.C. Nicholson, W.A. Shelton, Oak Ridge National Laboratory, Oak Ridge, TN 37831; Yang Wang, Pittsburgh Supercomputing Center, Pittsburgh, PA 15213; I.A. Abrikosov, Physics Department, Uppsala University, S-75121 Uppsala, Sweden

We have studied the composition dependence of the magnetic structure of Ni_cCu_1-c alloys throughout the composition range for which these alloys are magnetic. The disordered solid solution phase is modeled using large (256-atom) super-cells and the calculations are performed using the first-principles locally self-consistent multiple scattering (LSMS) method utilizing the coherent potential approximation (CPA) boundary condition as proposed by Abrikosov et al. We present results for the composition dependence of the magnetic moments and the local environment dependent fluctuations in magnetic moments. We will focus on the extent of these fluctuations in the region of the critical concentration for the onset of magnetism. Research sponsored by the Division of Materials Science, Office of Basic Energy Research Sciences, and by the Division of Mathematical, Information, and Computation Sciences, Office of Computational and Technology Research, US DOE, under contract DE-AC05-96OR22464 with Lockheed Martin Energy Research Corp.