Heavy element doping for enhancing thermoelectric properties of nanostructured zinc oxide

Abstract

ZnO is a high melting point high charge carrier mobility semiconductor with potential as a thermoelectric material, but its high thermal conductivity is the limiting factor for increasing the thermoelectric figure of merit ZT. Here, we demonstrate that doping ZnO with heavy elements can significantly enhance ZT. Indium doping leads to ultralow κ~3 Wm , yielding ZT 1000K ~ 0.45 that is ~80% higher than nonnanostructured In-Zn-O alloys. Although Bi doping also yields high Seebeck coefficient of α 300K~5 00 μVK -1 , Bi segregation, grain growth and defect complexing are unfavorable for increasing ZT. Thus, besides increased impurity scattering of phonons, the concurrence of nanostructuring and charge carrier concentration control is key to ZT enhancement. Our results open up a new means to realize high ZT thermoelectric nanomaterials based on ZnO.Keywords: Nanobulk thermoelectrics, indium doping, bismuth doping, zinc oxide, first principle transport calculations, high figure of merit ZT This is an author-produced, peer-reviewed version of this article. The final, definitive version of this document can be found online at RCS Advances, published by The Royal Society of Chemistry: RSC Publishing. Copyright restrictions may apply. DOI: 10.1039/c3ra46813e 2 Table of Contents FigureHigh efficiency thermoelectric devices offer great potential for harvesting waste heat into electricity 1 , but require materials with thermoelectric figure of merit ZT>1 above 600 K. We recapitulate that ZT is defined as  2 /  is the Seebeck coefficient,  is the electrical conductivity and  is the thermal conductivity, and the numerator  2  is also known as the power factor. Zinc oxide 2 and its alloys are attractive candidates because of their high thermal stability, corrosion resistance, low-cost, and non-toxicity. But, unlike low bandgap semiconductor thermoelectrics with complex crystal structures and heavy elements, ZnO is composed of light elements and has a 3.3 eV bandgap and a relatively simple wurtzite structure. In contrast to conventional thermoelectric materials, which typically have heavy band features, n-type ZnO has a singly degenerate s-electron conduction band, with a low effective mass, m * =0.24m e , and high mobility. Thus, even though ZnO can exhibit a high Seebeck coefficient 3,4 it does so at low carrier concentrations. In general, the Wiedemann Franz relation can be used to write ZT= r 2 /L, where L is the effective Lorentz number and r= e /, where κ is the sum of a lattice part κ L and an electronic part κ e . Good thermoelectrics have sizeable values of the ratio r at carrier concentrations where  is also high. In ZnO, the lattice thermal conductivity is generally too high (e.g.,  L~~5 Wm -1 K -1 at 1000 K) 5 to achieve high values of r. Lowering  L by nanostructuring combined with tapping into local strain effects, e.g., arising from the anisotropic thermal expansion 6 , will be essential to realize high ZT in ZnO-based materials. The challenge with decreasing  L by nanostructuring alone ...

Hot probe measurements of n-type conduction in Sb-doped ZnO microwires

“…Different Seebeck coefficient values ranging from À0.2 to À0.45 mV/K were reported for ZnO depending on the doping elements. 37,38 Similar measurements were done on other representative ZnO:Sb wires, with different lengths and the results showed similar values for the Seebeck coefficient. In Fig.…”

Enhanced Thermoelectric Performance of Bi2O2Se with Ag Addition

“…In view of the low cost of raw materials and the high stability, thermoelectric oxides have been considered as promising candidates for high temperature applications. Typical oxides [1,6,7,8,9] (such as Ca 3 Co 4 O 9 , CaMnO 3 , and ZnO) have been extensively investigated over the past 20 years. However, their ZT values are still too low to be used in commercial applications due to the mediocre electrical conductivity and high thermal conductivity.…”

Segmentation of low‐cost high efficiency oxide‐based thermoelectric materials

“…Under such high‐temperature conditions, oxide TE materials are promising candidates due to their natural durability, robustness to the environments in additional with their low‐cost and abundant source compared with conventional TE intermetallic compounds . They have therefore been widely studied with the aim to improve the value of zT using for example heavy element doping , nano‐structuring , nanowire and optimized morphology , and nano‐inclusion . During recent years, remarkable results have been reported on Al and Ga dually‐doped ZnO and Ca 3 Co 4 O 9 nanocomposite with the peak zT reached the values of 0.65 at 1200 K and 0.61 at 1140 K, respectively.…”