Improvement of thermoelectric properties with the addition of Sb to ZnO
Introduction
Thermoelectric materials with a high enough figure-of-merit so as to ensure high-temperature thermoelectric power generation have been limited to only a few compounds such as SiGe [1], [2], [3] and FeSi2 [4], [5], [6]. However, these compounds must be used in the limited conditions, i.e., at low temperatures or under vacuum, because of oxidation, vaporization, and decomposition. In answer to the aforementioned problems, in 1997, a single-crystal p-type NaCo2O4 was recognized as a good candidate for applications in the areas of thermoelectric generation [7]. The NaCo2O4 has a high figure-of-merit (8.8 × 10−4 K−1) and high thermopower (100 μV K−1) at 300 K [7]. NaCo2O4 consists of an Na cation and CoO2 blocks alternately stacked along the c-axis to form a layered structure [8]. Because oxides have the merits of long-time use at high temperatures, they could possibly be regarded as promising thermoelectric materials owing to their potential for thermoelectric power generation by waste heat. Since then, p-type oxide thermoelectric materials have been investigated extensively, and have been found to exhibit comparable thermoelectric properties to the conventional materials.
Presently, high-performance n-type oxide materials are in demand, and Ba1−xSrxPbO3 (0 ≤ x ≤ 0.6) and ZnO-based oxides have been investigated as potential candidates previously [9], [10], [11], [12], [13]. Ban+1PbnO3n+1 homologous compounds have layered structures consisting of n perovskite layers separated by a rock-salt BaO layer. An increase in n value from 1 to ∞ results in a change in conduction behavior from a semiconducting to a semimetallic one [14], [15]. Ba1−xSrxPbO3 consistently shows a metallic behavior below 670 K and a semiconducting behavior above 670 K [11]. The crystal structure of the layered Ba4Pb3O10 has been identified by profile analysis from neutron powder diffraction data [16]. Zinc oxide (ZnO) is an important II–IV group semiconducting material. Due to its versatile properties, it has been applied in electronic and electro-optic devices. It is an important wide and direct band gap semiconductor with Eg = 3.3 eV at room temperature and has a wurtzite structure (a = 3.249 and c = 5.206 Å) and P63mc (1 8 6) space group [17], [18].
To date, efforts in improving the thermoelectric properties have concentrated on new compound development, alloy design, e.g., dopant addition, or microstructural control of existing materials. Yasukawa and Murayama [11] reported that the incorporation of Sr for Ba in BaPbO3 improved its thermoelectric properties. The highest power factor (∼4.3 × 10−4 Wm−1 K−2) was attained for Ba0.4Sr0.6PbO3 at 773 K. In addition, previously, we reported that the addition of Al and/or Ti to ZnO improved its thermoelectric properties [12], [13]. However, the obtained results are insufficient for practical applications of thermoelectric power generation. In the present study, we attempted to improve the high-temperature thermoelectric properties of ZnO by the addition of Sb2O3. The microstructure and thermoelectric properties of the Zn1−xSbxO (0 ≤ x ≤ 0.05), which was fabricated by a solid-state reaction method, are systematically discussed herein, especially with regard to adding Sb2O3.
Section snippets
Experimental
All Zn1−xSbxO (x = 0, 0.005, 0.01, 0.02, 0.03, 0.04, and 0.05) samples were fabricated by a solid-state reaction starting from high-purity ZnO and Sb2O3 powders. A mixture of the ZnO and Sb2O3 powders and ethyl alcohol was milled for 6 h using a planetary mill (FRITSCH pulverisette 6) and ZrO2 ball as a grinding media. The resulting slurries were dried at 353 K in an oven for 24 h. The mixed powders were calcined in a mullite crucible at 1273 K for 5 h. The calcined powders were milled in the
Results and discussion
The XRD patterns of the as-sintered ZnO, Zn0.99Sb0.01O, Zn0.97Sb0.03O, and Zn0.95Sb0.05O at room temperature are shown in Fig. 1(a–d), respectively. The primary phase of the as-sintered Sb2O3-added Zn1−xSbxO (0.01 ≤ x ≤ 0.05) samples was the solid solution of Zn1−xSbxO with a wurtzite structure, which conforms with previous studies [18], [20], [21], [22]. In addition to the Zn1−xSbxO phase, only a small amount of cubic spinel α-Zn7Sb2O12 (a = 8.589 Å) was detected in the Zn1−xSbxO samples, also, is in
Conclusions
The primary phase of the as-sintered Sb2O3-added Zn1−xSbxO (0.01 ≤ x ≤ 0.05) samples was the solid solution of Zn1−xSbxO with a wurtzite structure. In addition to the Zn1−xSbxO, only a small amount of cubic spinel α-Zn7Sb2O12 (a = 8.589 Å) remained in the Zn1−xSbxO samples. The added Sb2O3 (0.01 ≤ x ≤ 0.05) did not fully dissolve in the ZnO crystal lattice due mainly to its rather low solubility, forming the Zn7Sb2O12 phase. Both the grain size and the density were lowered with Sb2O3 content because the Zn
Acknowledgement
The authors would like to acknowledge the financial support provided for this research by the Korea Energy Management Corporation (KEMCO).
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