Novel Routes to Tune Thermal Conductivities and Thermoelectric Properties of Materials.

摘要

The goal of this project was to find novel routes to tune thermal conductivities and thermoelectric properties of materials mainly composed of relatively abundant light elements like boron, carbon, nitrogen, silicon, sulfur, oxygen, etc. Focus was on the control of the atomic network structure and symmetry in crystal structure of inorganic materials to lead to dramatic effects in the properties. One particularly studied system in the work was the recently discovered potential n-type counterpart (YB22C2N as a representative) to p-type boron carbide, which is one of the few thermoelectric materials with a history of commercialization. As a byproduct of attempting further control of the network structure by utilizing atomic bridging sites, we have discovered an anomalous vanadium doping effect in YB22C2N that increased the electrical conductivity by 50,000% while simultaneously increasing the Seebeck coefficient by 220%. Selective transition metal doping was also clearly demonstrated to modify the crystal structure of a borosilicide, leading to significant increase of the thermoelectric power factor and change of sign in the Seebeck coefficient. Such demonstration of the possibility to modify the crystal structures can expand the possibilities of borides as functional compounds. A series of indium-free novel TCO compounds with novel crystal structures, has revealed exceptionally low thermal conductivity through its homologous nature and mixing of the cation sites, and exhibited thermoelectric properties indicated to be better than the extensively studied IZO, as an n-type oxide. Furthermore, as an example of material design utilizing particular crystal structures, a unified description for crystal structures of another new homologous series in the gallium zinc Silicon or germanium based cage compounds with low thermal conductivity were also found to reveal enhanced properties through small substitution of the network atoms. A potential replacement material to boron carbide, having related crystal structure, boron sulfide B6S1-x, with S-S atoms bridging the boron clusters, was revealed to exhibit similar large thermopower at high temperatures, with much lower processing temperatures than boron carbide. To summarize, by focusing on control and design of crystal structures, novel promising thermoelectric materials with low thermal conductivity were discovered or powerful methods for modification were demonstrated in the borides, silicides and oxides.