Quantifying and Understanding Anharmonicity of Engineering Materials in Real and Reciprocal Space

PhD Student: Dipanshu Bansal

Advisors: Amjad Aref and Gary Dargush

Publication Year: 2015

Abstract: The MCEER research endeavor is broad and cuts across many engineering disciplines, including materials science. A recently completed project, which was supported in part by the Department of Energy, is described below. Dr. Bansal is now extending these developments as a Research Scientist at the Oak Ridge National Laboratory. He can be reached at bansald@ornl.gov.

The complex interactions between phonon-phonon, phonon-electron, phonon- magnon, and many other collective excitations and quasi-particles are often responsible for deviation of crystalline material from harmonic behavior, and a source of non-harmonic behavior. Understanding this intricate intercommunication within lattice structure is essential to developing a deeper knowledge of sophisticated compounds with unusual material properties. In this research project, the interplay of different excitations was investigated to quantify and better comprehend the non-harmonic dynamics of crystals in both real and reciprocal space for a wide range of engineering materials.

As a product of this research, a deeper understanding of anharmonicity has been developed using experimental inelastic neutron scattering measurements, first principle calculations, and thermodynamic principles. Topics explored include the dynamics of electron-phonon coupling in elemental metal niobium, phonon-phonon interactions in aluminum, the remarkably high thermoelectric performance in tin-selenide, and its comparison with similar group IV-VI compound tin-sulfide, the large negative thermal expansion for wide range of temperature in scandium tri-fluoride, and low frequency (MHz) – large wavelength (μm) excitations in four cubic single crystals (sodium-chloride, potassium chloride, pure copper, and copper zinc).

Important scientific contributions include the analysis of real space interatomic force constants using the ‘Phonon Analysis in Real Space’ (PARS) package, calculation of anharmonic entropy, internal energy, and vibrational free energy from volumetric thermal expansion coefficient and inelastic neutron scattering data based on thermodynamic principles beyond any harmonic/quasi- harmonic approximations, and the characterization of the linear elastic mechanical behavior of cubic crystals at a length scale of 100 nm to 1000 μm using wave propagation.

The PhD dissertation of Dr. Bansal will be published in Q4 of 2015 or Q1 of 2016 as an MCEER report, which will be downloadable at no charge from the MCEER website.