The Effects of Nanostructuring on the Thermal Transport of Electronic Materials

摘要

Understanding the fundamental energy transport mechanisms in nanostructured materials is vital to the development of smaller, energy-dense systems. This is particularly important in materials used in high power density electronic systems and renewable energy platforms, where performance is directly tied to the thermal properties of constituent materials. By altering the nanostructure, the thermal properties can be tailored to meet various needs. Frequency Domain Thermoreflectance (FDTR), an optical pump-probe thermal characterization technique, is used to characterize the thermal properties of different electronic materials. An FDTR system was built as part of this project and validated with reference scans of known materials. This work thermally characterizes nickel titanium (NiTi), germanium telluride (GeTe), Bi2Te3/Bi2(TeSe)3 superlattices, and gallium nitride (GaN). NiTi is a candidate material for elastocaloric cooling and thermal energy storage applications. We show that increasing the grain size of NiTi significantly increases thermal conductivity on both sides of the phase change. We study the impact of film thickness on the thermal conductivity of the crystalline and amorphous phases of germanium telluride (GeTe). It was found that the mean free path of heat energy carriers is similar in both phases of the material. The application of a phonon scattering model to a thickness-dependent thermal conductivity dataset indicates that phonon boundary scattering is the predominant physical mechanism that limits thermal transport in thin-films of GeTe. Superlattices with alternating thin-films of Bi2Te3 and Bi2(TeSe)3 were thermally characterized. We find a large degree of thermal anisotropy and a significant increase in the in-plane thermal conductivity compared to bulk values, possibly due to a topological insulator effect. We report preliminary results of the thermal conductivity of seed-grown GaN films with a grain size gradient on partially etched substrates.