Thermal and electrical properties of zinc oxide nanowires embedded in silica aerogel.

摘要

Thermoelectric materials that can convert heat to electricity are good candidates for energy sources considering that the majority of energy produced in the world is wasted as heat. The performance of a thermoelectric material is characterized by the thermoelectric figure of merit, ZT= S 2σT/κ, where κ, T, σ and S are, respectively, the thermal conductivity, temperature, electrical conductivity and Seebeck coefficient. To improve the efficiency of energy conversion with thermoelectric materials, large values of ZT are required. The major obstacle to this is attempting to increase σ and S while decreasing κ since these quantities are interrelated and cannot, generally, be controlled independently.;We propose a nanocomposite of ZnO nanowires embedded in silica aerogel to solve this problem. The silica aerogel was used as a lattice vibration cladding layer, providing a new path for heat carrying phonons scattered from the nanowire surface, which should decrease the lattice thermal conductivity without compromising the electrical performance of ZnO nanowires. Our experimental results demonstrated that the thermal conductivity of ZnO nanowire arrays was greatly reduced by adding silica aerogel as a cladding layer. In order to control the morphology and density of ZnO nanowire arrays, the growth mechanism of ZnO nanowires was investigated.;We thoroughly investigated the thermal and electrical properties of ZnO nanowire arrays. The measured thermal conductivities of ZnO nanowires and bulk ZnO indicate that boundary scattering is the dominant phonon scattering mechanism in this material. The electrical conductivity in Cr-ZnO Nanowire-Cr metal-semiconductor-metal structures was found to be determined by the reverse biased Schottky barriers present at the Cr/ZnO interface. This transport mechanism was not affected by the presence of N2 or air. Pressure was found to play an important role in the current-voltage characteristics of these nanowires due to the piezoelectric nature of ZnO, in agreement with theoretical calculations. These results indicate that a ZT of two is possible in this nanocomposite material.