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Dr. Brent T. Fultz Professor of Materials Science and Applied Physics 239
Keck Lab B.S.
- MIT, 1975 |
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| Expertise Materials science, inelastic scattering of neutrons, metals and alloys, battery electrode materials. Field of Study The monograph by J. W. Gibbs, "On the Equilibrium of Heterogeneous Substances," provided the thermodynamic backbone for today's materials science. Our efforts at Caltech have been to extend the fundamentals of materials science in ways that Gibbs could not have understood. Atoms may move in vibratory motions about their positions of equilibrium, generally on sites of a crystal lattice. These high frequency vibrational movements can be best regarded as a part of the thermodynamic state of a material at high temperatures, and provide a vibrational entropy of the solid. The entropy of vibrations was not understood by Gibbs, of course, and vibrational entropy has been largely ignored by materials scientists until recently. Our original (and ongoing) effort was to measure the difference in in vibrational entropy between two materials having identical chemical composition, but having different crystal structures. We found significant differences in vibrational entropy, surprising much of the materials science community. More recently, we have concentrated on measuring vibrational spectra of solids, and how these phonon densities of states change with changes in the structure and chemical composition. Such work involves the inelastic scattering of neutrons to measure neutron energy losses to phonon excitations in solids. The field of inelastic neutron scattering is a broad one, and one that will grow in the U.S. with the construction of the Spallation Neutron Source. Fultz is the principal investigator on the ARCS project to build the inelastic neutron instrument at the SNS. Most of what we know about atom arrangements in materials comes from diffraction measurements, where an incident plane wave is directed into a sample and the angles and intensities of the outgoing diffracted waves are detected. Fultz's group is exploring a novel method of gamma-ray diffraction, where an incident gamma-ray is absorbed by identical nuclei in a crystal. A cooperative excitation of nuclei in the crystal, a "nuclear exciton," is generated. The decay of this nuclear exciton creates a new gamma-ray photon with the average angular distribution of a diffraction pattern. The physical process of scattering is fundamentally different from that of x-ray diffraction, and therefore offers new opportunities for studies of the atom arrangements in materials. For example, we are using the chemical spectroscopic information of Mössbauer spectra to control the phase and intensity of wave emission from selected nuclei in a sample. Commercial lithium-ion batteries use graphite intercalation compounds for the anode and a layered LiCoO2 structure for the cathode. We used electron energy loss spectrometry studies of the Li K-edge to determine that the lithium atoms are neutral when intercalated into graphite. This controversial result means that the name "lithium-ion" battery is incorrect. The LiCoO2 cathode material shows a distinctive set of dislocations and perhaps heterogeneities in the lithium concentration that are associated with these defect structures. We are investigating how local defect structures alter the voltage-current characteristics of a battery. For example, we seek to relate the distribution of lithium in these materials to the chemical potential of lithium at various states of charge. A range of research activities from the fundamental to the applied works well in materials research when there is synergism between both efforts. We think we have achieved this balance. Selected Publications M. E. Manley, B. Fultz, R. J. McQueeney, C. Brown, W. L. Hults, J. L. Smith, D. J. Thoma, R. Osborn, and J. L. Robertson, "Large harmonic softening of the phonon density of states of uranium", Phys. Rev. Lett. 86, 3076 (2001). A. Hightower, C. C. Ahn, B. Fultz, and P. Rez, "Electron Energy Loss Spectrometry on Lithiated Graphite", Applied Phys. Lett. 77, 238 (2000). B. Fultz, T. A. Stephens, E. E. Alp, M. Y. Hu, J. P Sutter, T. S. Toellner, and W. Sturhahn, "Atom clusters and vibrational excitations in chemically-disordered Pt3Fe", Phys. Rev. B 61, 14517 (2000). B. Fultz and H. N. Frase, "Grain Boundaries of Nanocrystalline Materials - their Widths, Compositions, and Internal Structures", Hyperfine Interactions 130, 81 (2000). Brent Fultz and James M. Howe, "Transmission Electron Microscopy and Diffractometry of Materials", (Springer-Verlag, Heidelberg 2001), 748 page textbook. ISBN 3-540-67841-7. |
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© 2007 California Institute of Technology. All Rights Reserved. last
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9 March, 2007
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