Dr. Brent Fultz
California Institute of Technology,
Department of Applied Physics and Materials Science, Pasadena, California
Friday, August 30
"Vibrational Entropy of Materials"
Atom vibrations are usually the main source of entropy in materials. This has been known for about a century, but the differences in vibrational entropy between different phases are often overlooked. This talk will explain where vibrational entropy comes from, and how it can be understood from measurements by inelastic neutron scattering.
The harmonic model is a classic approach to understanding the vibrational thermodynamics of materials. At high temperatures, however, non-harmonic effects alter the thermodynamics of alloy phases. One effect is textbook "quasiharmonic" behavior, where vibrational frequencies are lowered as a crystal expands against its bulk modulus. Most materials show a second effect from phonon-phonon interactions (PPI). Dr. Fultz's group has been measuring these effects with inelastic neutron scattering, and calculating them with density functional theory. All these non-harmonic effects prove important for understanding vibrational entropy at elevated temperatures.
For cubic ScF3, the group found phonon modes where the quartic anharmonicity is so big that it dominates the potential energy of phonon modes responsible for negative thermal expansion. The negative thermal expansion could be calculated with a model of local quartic oscillators. The team's ongoing work with another material with negative thermal expansion, cuprite Ag2O, also has large anharmonicity, but its delocalized oscillations require a different approach to understand.
Brent Fultz received his undergraduate degree from MIT, and his PhD from the University of California at Berkeley in 1982. He was a Presidential Young Investigator, he received an IBM Faculty Development Award, a Jacob Wallenberg Scholarship, and the TMS EMPMD Distinguished Scientist Award of 2010.
With his friend, Professor J. Howe of the University of Virginia, he published a graduate-level textbook on diffraction and microscopy of materials (now in its third edition).
Fultz was the Principal Investigator of the ARCS spectrometer project at the Spallation Neutron Source, now complete and in its operations phase. He was the Principal Investigator of the software project Distributed Data Analysis for Neutron Scattering Experiments, DANSE, which focused on how computing can elevate the science of neutron scattering.