Turner, S.

2021

Abstract: Interest in the engineering of thermal process mechanisms has grown significantly in recentyears, particularly for applications involving thermal functionalization of a material for use as a thermalbarrier, thermoelectric converter, thermal diode, etc., which requires the control of thermalconductivity. The thermal manipulation and design of a material in turn requires knowledge of thefundamental transport properties of the elementary heat carrier particle, the phonon.This is easier said than done, however, since models that would allow us to comprehend heat transportin complex materials are still under development, and because we lack systematic energy andtemperature dependencies of experimentally measured phonon dispersions and lifetimes on a widerange of materials. This presents a barrier to our understanding of more complex and disorderedsystems, which are typically the ones needed to produce the thermal conductivity spectrum for theapplications listed above. With the introduction of the many more atoms per unit cell and/or thedisorder, we must now connect the dots between the effects of increasing structural complexity anddefects on the phonon spectrum, and, ultimately, on thermal conductivity.Within the context of overcoming these challenges, I present the phonon spectra in three families ofmaterials that exhibit different types of structural disorder and complexity. These contributionsinclude the inorganic type-I clathrate Ba7.81Ge40.67Au5.33, defined by its cage structure and manyatoms per unit cell; the equimolar high-entropy alloy FeCoCrMnNi, represented by a simple andaveraged monatomic FCC unit cell with significant chemical disorder; and cubic Yttria-StabilizedZirconia, characterized by an extended and correlated defect structure brought on by the inclusion ofoxygen vacancies. Towards this end, I have experimentally measured their phonon dispersions,lifetimes, and mean free paths using inelastic neutron and X-ray scattering techniques found at large-scale facilities. My results confirm the use of the ab-initio self-consistent phonon method calculations for clathrates,emphasizing the importance of quartic anharmonic terms in our understanding of both the hardeningof the lowest-lying optical branch with increasing temperature, and in the reproduction of the weaktemperature dependence of the lattice thermal conductivity. The FeCoCrMnNi alloy study establishesthe lattice dynamics of a random five-element alloy, demonstrating that the factor limiting phononlifetimes is associated to force-constant fluctuations. Finally, I revisit the lattice dynamics of cubicYttria-Stabilized Zirconia with higher resolution measurements, bringing new insight into the acoustic-optic interaction within the phonon spectrum, and to the linewidth broadening that results from theextended defect structure in this material.Through each of my case studies, I provide the energy dependence of a specific type of phononscattering mechanism. Then, by methodically detailing these features in each system, we can workupwards from their microscopic phonon properties to their macroscopic material properties, bringingus one step closer to understanding heat transport in complex and disordered crystalline systems.

Keyword(s): Energy (1st) ; Condensed Matter Physics (2nd)

1. TRISP (TRISP)

1. TRISP: Three axes spin echo spectrometer (SR5b)