Hybrid-structured ZnO thermoelectric materials with high carrier mobility and reduced thermal conductivity

Abstract

Nanostructure engineering has been extensively applied to ZnO in an effort to improve its performance in thermoelectric material, solar cell, and nanogenerator applications. Nano-structured ZnO bulks are limited by their inherently low mobility caused by the high density of grain boundaries and interfaces. In this study, a hybrid micro/nano structure composed of nearly coherent grain boundaries with a low misorientation degree among the nanograins was successfully fabricated in Zn 1Àx Al x O (x ¼ 0, 0.01, 0.02, 0.03, 0.04 mol) bulks via hydrothermal synthesis and spark plasma sintering. Despite the large amount of nanograin boundaries and interfaces in the resulting material, a high carrier mobility value (50.7 cm 2 V À1 s À1 ) was obtained in the x ¼ 0.2 sampleclose to the level shown by ZnO single crystals and far higher than that of its ordinary nano-structured counterparts (<15 cm 2 V À1 s À1 ). A reduced thermal conductivity value of 2.1 W m À1 K À1 at 1073 K was also obtained in the micro/nano-structured x ¼ 0.02 bulk due to extremely effective scattering at boundaries and interfaces also present in the nano-structured counterparts. After the simultaneous optimization of both electrical and thermal transport properties, the micro/nanostructured x ¼ 0.02 sample showed a high ZT value (up to 0.36) at 1073 K. The proposed micro/nanostructure may also be applicable to other thermoelectric materials for further ZT enhancement.

A self-forming nanocomposite concept for ZnO-based thermoelectrics

“…Although the highest observed performance is still lower than attained for some ZnObased thermoelectrics (e.g., those sintered from nanopowders using SPS technique 22,24,25 ), the results provide a clear insight on how electrical and thermal transport can be simultaneously improved in these materials. Thus, the concept of a self-forming composite with controlled interactions between the components may open new pathways for decoupling the electrical and thermal transport parameters in oxide thermoelectrics.…”

“…3) involves a partial replacement of Al 3+ by Zr 4+ cations as a driving force for the spinel formation, while the scheme (Eq. 4) represents a known decomposition reaction, often responsible for affecting the thermal transport and electronic conduction in ZnO-based thermoelectrics 3,21,24,25,35 . One may also account for the partial substitution of zirconium in ZrO2 by zinc 51 , accompanied by the segregation of ZnAl2O4 to compensate for the cation stoichiometry change in the matrix phase: Thus, the reactions (Eqs.…”

Synergistic effects of zirconium- and aluminum co-doping on the thermoelectric performance of zinc oxide

“…4A, while the variations of the Seebeck coefficient and thermal conductivity on co-doping present rather minor contribution. It is believed that further significant enhancement of the thermoelectric performance of mixed-doped compositions is possible only by applying nanostructuring approaches similar to those implemented in recent works (e.g., [27,52,53]), having a major objective to suppress extremely high thermal conductivity of zinc oxide.…”

Influence of processing parameters on the densification and the microstructure of pure zinc oxide ceramics prepared by spark plasma sintering

“…Sintering above 750 °C resulted ceramic grain sizes of about 10 μm due to high surface area of the starting powder. Zhang et al [25] sintered micro nanostructured (hybrid) powders using SPS in which they got less dense ceramic of 84% at 900 °C.…”