Original ArticleSynergistic effects of zirconium- and aluminum co-doping on the thermoelectric performance of zinc oxide
Introduction
Thermoelectric (TE) conversion of waste or solar heat into electricity represents a promising solution to meet growing needs in low-carbon and energy-efficient technologies [[1], [2], [3]]. The efficiency of thermoelectric generation is limited by the Carnot efficiency and characterized by the figure of merit ZT=α2T/ρκ, combining Seebeck coefficient (α), electrical resistivity (ρ), thermal conductivity(κ) of the candidate materials, and working temperature (T). Prospective applications require the thermoelectric materials with high thermal and chemical stability, the absence of toxicity and high natural abundance of the constituent elements. These represent the main advantages of oxide-based TE materials over traditional, Bi2Te3, Bi2Se3, PbTe –based thermoelectrics. Yet, ZT values obtained for the best-known oxide thermoelectrics are much lower than those required by most potential applications [4].
TE oxides arrived at a turning point when good TE properties were reported for NaCo2O4 in 1997 [5]. In the last two decades, more than an order of magnitude enhancement in ZT of oxides was achieved [6,7]. While being rather known for promising optoelectronic, catalytic and photochemical properties [8,9], donor-doped zinc oxide (ZnO) was also considered as a potential high-temperature thermoelectric material [10,11]. Doping with elements capable to possess the oxidation states higher than 2+ is a known straightforward approach to tune TE performance of ZnO. Representative examples include aluminum [[10], [11], [12], [13]], indium [[14], [15], [16]], iron [17], nickel [18], bismuth [15,19], etc. From those, aluminum can be considered as a most used and common dopant. The co-doping strategy was also found fairly effective [[20], [21], [22], [23]]. In particular, the ZT values of Al-Ga-, and Al-Ni- co-doped ZnO materials reach up to 0.47–0.65 at 1173–1243 K, being among the highest observed so far in oxide-based thermoelectrics [20,21]. This behavior was attributed to the microstructural evolution in co-doped ceramics, leading to a decrease in the thermal conductivity while maintaining an appropriate electrical performance. Another interesting effect of co-doping, leading to an enhancement of aluminum solubility due to the presence of nickel cations and a corresponding increase in the charge carrier concentration was demonstrated for Zn(Al,Ni)O [22,23].
Although fourfold coordinated Zr4+ has an ionic radius comparable with that of Zn2+ and is expected to be an efficient electron donor due to high charge, to our best knowledge, zirconium was not yet assessed as a dopant to enhance the thermoelectric performance of bulk ZnO-based materials. Structural and electrical properties of Zr-doped zinc oxides were studied aiming mainly transparent conducting oxide thin films with improved optical and electrical properties [[24], [25], [26]]. Thus, the present work aims to assess the prospects for boosting TE performance of ZnO-based materials through zirconia addition, both within single-doping and co-doping concepts. For the latter, aluminum was selected as a second dopant, based on relatively well-studied thermoelectric properties and related effects in Zn(Al)O system. Particular attention was given to the assessment of the stability at high temperatures, where degradation of the electrical properties represents one of the main obstacles towards the potential application of ZnO-based thermoelectrics [27,28].
Section snippets
Experimental
The set of nominal sample compositions prepared in the present work included single-doped Zn0.997Al0.003O, Zn0.993Al0.007O, Zn0.997Zr0.003O, Zn0.993Zr0.007O, mixed-doped Zn0.993Al0.002Zr0.005O, Zn0.994Al0.003Zr0.003O and Zn0.993Al0.005Zr0.002O, and ZnO reference sample. The materials were prepared using a conventional solid state route starting from ZnO (Alfa-Aesar, 99.99%), Al2O3 (Sigma-Aldrich, 99.7%) and Tosoh grade ZrO2 powders. Multiple annealing steps at 1173–1373 K for 5–15 h with
Results and discussion
Representative room-temperature XRD patterns of the prepared materials are shown in Fig. 1.
All indexed reflections belong to a hexagonal wurtzite structure, indexed in accordance with ICDD reference pattern 04-009-7657. Based on the XRD results, all prepared materials are apparently single-phase, except for the Zn0.993Zr0.007O sample containing a detectable amount of the monoclinic ZrO2 phase (ICDD reference pattern 04-008-7682). The corresponding peak at 2Θ–28.2° is hardly visible for Zn0.993Al
Conclusions
In order to demonstrate the effects of zirconium doping on the thermoelectric performance of bulk ZnO-based thermoelectrics, a set of single-doped and mixed-doped samples with nominal composition Zn1-x-yAlxZryO (x = 0–0.007, y = 0–0.007) was prepared via conventional solid state route. Electrical studies revealed significantly lower resistivity of Zn0.993Al0.005Zr0.002O, Zn0.994Al0.003Zr0.003O and Zn0.993Al0.002Zr0.005O samples as compared to single Al-doped and Zr-doped materials with similar
Acknowledgements
This work was supported by the FCT, including individual grant IF/00302/2012, project CICECO-Aveiro Institute of Materials (ref. UID/CTM/50011/2013), project of bilateral cooperation between FCT and DAAD (Germany) and the project POCI-01-0145-FEDER-031875, financed by COMPETE 2020 Program and operational Program POCI in its FEDER/FNR component, and the Foundation for Science and Technology, in its State Budget component (OE), and when applicable co-financed by FEDER under the PT2020 Partnership
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