We are graduate students, postdoctoral fellows, and established scientists, located mostly on the second floor of the Keck Laboratory on the Caltech campus. Our lab equipment is often shared with other faculty in materials science, and with others on the Caltech campus. Conversely, we also make use of materials science facilities maintained by the Johnson Group on the third floor of Keck, and with other groups in the Dept. of Applied Physics and Materials Science.
We study materials at the atomic level where quantum mechanics is usually more important than classical mechanics, and where statistical mechanics is more useful than classical thermodynamics. We measure the positions of atoms, usually when they are located in materials with some disorder. Many of our studies use inelastic scattering to measure how atoms vibrate, and how atoms transfer electrons when they bond to their neighbors. These measurements are performed by scattering x-rays, neutrons, electrons, and gamma-rays from the atomic electrons or nuclei. We determine the energy and momenta transferred during the individual scattering events.* Much of this work requires a substantial computing effort, first to "reduce" the data to a manageable size, and then convert it to physical quantities. An even larger computing effort is then required to perform electronic structure calculations and simulations to predict the dynamical processes in materials, and how these processes cause scattering of neutrons or x-rays.
A bit before 1990, we pioneered a new direction of research into the entropy of materials. At the time it was well-known that heat goes into the vibrations of atoms, and this heat is responsible for much of the entropy of a solid. What was not known, however, was if differences in crystal structure, chemical composition, or local arrangements of atoms would alter the vibrational spectrum enough to affect the entropy significantly. We found these differences in vibrational entropy to be generally important for the thermodynamic stabilities of different solid phases. After a few years of disbelief, it is now accepted the details of vibrational entropy make major contributions to the thermodynamic stabilities of materials.**
Energy-storage materials offer opportunities for doing both pure science, applied science, and materials engineering. Some of our work addresses important national needs for energy technology. We have worked on the thermodynamics and kinetics of how lithium is stored in materials used for electrodes of rechargeable batteries. We are interested in how the enthalpy of the lithiation reaction depends on the electronic structure around lithium atoms, which sets the voltage. We study the entropy of lithiation, too -- where it comes from, and how it can be used to determine the health of battery electrode materials. More recently, we have been working on materials for hydrogen storage. We are studying the fundamental mechanisms of how hydrogen molecules are physisorbed on surfaces, which are less well understood than how hydrogen atoms are absorbed into a crystal structure of a metal hydride, for example. We hope to optimize these interactions to improve the hydrogen storage properties of practical materials.
Our group performs many experiments at national facilities that supply intense x-ray or neutron beams (see Laboratory and Links). This has led to collaborations with scientists who meet at national neutron sources, and we have some involvement in the operations of these facilities. Brent Fultz led a project to build a state-of-the-art neutron scattering instrument, ARCS, at the world's most powerful neutron source, the SNS. In the course of this work, we identified new opportunities for scientific computing. ARCS was followed by a national project to do more sophisticated neutron scattering science with the help of computing, Distributed Data Analysis for Neutron Scattering Experiments, DANSE. Graduate students have interacted with top scientists involved in the ARCS and DANSE programs; a helpful edge for doing innovative science now, and for pursuing a career in research later.
* The three images at top are from our scattering studies with neutrons, electrons, and gamma-rays. The electron scattering is primarily Bragg diffraction (with cases of double diffraction here), the neutron scattering shows phonon dispersions that emerge from overexposed but indexed Bragg diffractions, and the gamma-ray scattering shows Bragg diffraction controlled by hyperfine interactions.
** As an aside, the other important contribution, configurational entropy, originates from the randomness of placing different atoms on crystal sites. This was known since the time of Gibbs. Gibbs did not know about electrons, phonons, or their thermal excitations, however.
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