Thermal Characterization and Crystal Growth of the Germanium Telluride-Tin Telluride System

Thermoelectric materials, which can create an electrical current from a temperature gradient within the material, are an important means of generating electrical power in very remote and harsh environments. However, their deployment in terrestrial environments has been limited, due to two primary factors: the high cost of thermoelectric materials and their low energy conversion efficiency- too low for economic use in almost all applications. One method by which thermoelectric material energy conversion efficiency may be increased is by reducing material lattice thermal conductivity, or a material’s ability to conduct heat through the vibrations of its crystalline atomic lattice, which are called phonons. In support of that objective, this work presents a characterization of fundamental material properties that influence the lattice thermal conductivity of the germanium telluride-tin telluride (Ge1-xSnxTe) system, a promising thermoelectric material. The properties characterized include composition-dependent coefficients of thermal expansion, speeds of sound within the material, and elastic moduli. These were characterized using high-temperature X-ray diffraction, resonant ultrasound spectroscopy, and high-pressure X-ray diffraction, respectively. This work also presents an account of the synthesis of bulk single-crystal ingots of several compositions within the Ge1-xSnxTe system, which will be used for an in-depth investigation of Ge1-xSnxTe phonon characteristics by collaborators using inelastic neutron scattering.