Charge transport and thermoelectric properties of ZnO-carbon nanomaterials composites

Abstract

Recently, thermoelectric research for power generation from waste heat has been explosively renewed due to increasing global concern for energy and environmental issues. The waste heat can be directly converted into electric power by the thermoelectric generation and its efficiency is mainly governed by the properties of thermoelectric materials. ZnO has attracted considerable attention as n-type oxide thermoelectric material due to its abundance, non-toxicity, and low cost. The performance of a thermoelectric material is determined by dimensionless figure of merit, ZT = $S^2\sigma T \kappa^{-1}$, where S, $\sigma$, T and $\kappa$ are Seebeck coefficient, electrical conductivity, absolute temperature and thermal conductivity, respectively. Therefore, the reduction of the thermal conductivity without significant loss of power factor ($S^2\sigma$) is strongly demanded for the enhancement of the thermoelectric performance. In this respect, we investigated the effect of nanostructures and interface control using carbon nanomaterials (CNMs) on the charge transport and thermoelectric properties of ZnO. Because thermoelectric phenomenon is governed by the transport mechanisms of both electron and phonon, understanding the temperature-dependent charge transport mechanism is of great importance for further improvement of ZnO thermoelectric materials First, 2 mol% Al-doped ZnO (AZO) nanoparticles were consolidated into AZO nanocomposite with $ZnAl_2O_4$ nanoprecipitates by spark plasma sintering and its high-temperature charge transport and thermoelectric properties were investigated up to 1073 K. The carrier concentration in the nanocomposite was not dependent on the temperature, while the Hall mobility showed positive temperature-dependence due to grain boundary scattering. The negative Seebeck coefficient of the nanocomposite was linearly proportional to the temperature, and the density of the state effective mass ($m_d^*$) was evaluated to be $0.33 m_e$ by using the Pisarenko relation. Drastic reduction of thermal conductivity ($\kappa < 6.5 W m^-1 K^-1$) was achieved in the nanocomposite, and the maximum ZT of 0.088 was obtained at 1073 K. Second, we reported extraordinary charge transport behavior and thermoelectric properties in AZO-reduced graphene oxide (RGO) nanocomposites. Although the most challenging issue in semiconductor nanocomposites is their low mobilities, the AZO-RGO nanocomposites exhibit single crystal-like Hall mobility despite the large quantity of nanograin boundaries, which hinder the electron transport by the scattering with trapped charges. Due to the significantly weakened grain boundary barrier and the proper band alignment between the AZO and RGO, freely conducting electrons across the nanograin boundaries can be realized in the nanocomposites. Furthermore, reduction of lattice thermal conductivities in the AZO-RGO nanocomposites was achieved by the enhanced phonon scattering. The maximum ZT of 0.098 and 0.088 at 1073 K were achieved in the AZO-1 wt% RGO nanocomposite. We achieved further enhancement of ZT in AZO nanocomposite system through interface control using graphene. Third, the effects of the interface control using multiwalled carbon nanotube (MWCNT) on the charge transport and thermoelectric properties in the AZO nanocomposite were investigated. As compared with the AZO nanocomposite which prepared without the CNMs, the AZO-MWCNT nanocomposite exhibited the enhanced mobility with increased carrier concentration. Grain boundary scattering could be sufficiently suppressed due to reduction of Schottky barrier by the existence of the MWCNT at the grain boundary, leading to the single crystalline charge transport behavior in the AZO-MWCNT nanocomposite below 873 K. However, the AZO-MWCNT nanocomposite exhibited quite different temperature dependence in its mobility, and its origin was the different charge distribution at the grain boundary depending on the type of the CNMs. This result will improve our understanding of the interface control for the enhanced charge transport in relevant application fields of CNM-hybrid nanocomposites.