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In this research, p-type BiSbTe/ZrO2 nanocomposite powders were fabricated by high-energy ball milling. Different weight percentages of ZrO2 (2, 4, and 6 wt. %) nanoparticles were incorporated into the bulk (BiSbTe) matrix by consolidation of as-synthesized nanocomposites (NCs) powder by spark plasma sintering at 673 K. The phase and existence of ZrO2 nano-inclusions was confirmed by X-ray diffraction and transmission electron microscopy-selected area electron diffraction analysis. The Seebeck coefficient of the BiSbTe/ZrO2 NCs was significantly improved (∼36% for 4 wt. % added NCs) by a decrease in the carrier concentration and energy filtering effect, whereas the thermal conductivity was much reduced via strong scattering of carriers/phonons. The peak thermoelectric figure-of-merit (1.34 ± 0.06) was obtained for BiSbTe into which 2 wt. % ZrO2 was dispersed, which was approximately 20% greater than that of the undispersed sample. The hardness of the nanocomposites was significantly improved (∼27%) due to grain-boundary hardening and a dispersion strengthening mechanism.

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Developing inexpensive and rapid fabrication methods for high efficiency thermoelectric alloys is a crucial challenge for the thermoelectric industry, especially for energy conversion applications. Here, we fabricated large amounts of p-type Cu0.07Bi0.5Sb1.5Te3 alloys, using water atomization to control its microstructure and improve thermoelectric performance by optimizing its initial powder size. All the water atomized powders were sieved with different aperture sizes, of 32–75 μm, 75–125 μm, 125–200 μm, and <200 μm, and subsequently consolidated using hot pressing at 490 °C. The grain sizes were found to increase with increasing powder particle size, which also increased carrier mobility due to improved carrier transport. The maximum electrical conductivity of 1457.33 Ω−1 cm−1 was obtained for the 125–200 μm samples due to their large grain sizes and subsequent high mobility. The Seebeck coefficient slightly increased with decreasing particle size due to scattering of carriers at fine grain boundaries. The higher power factor values of 4.20, 4.22 × 10−3 W/mk2 were, respectively, obtained for large powder specimens, such as 125–200 μm and 75–125 μm, due to their higher electrical conductivity. In addition, thermal conductivity increased with increasing particle size due to the improvement in carriers and phonons transport. The 75–125 μm powder specimen exhibited a relatively high thermoelectric figure of merit, ZT of 1.257 due to this higher electric conductivity.

In this research work, p-type BiSbTe/ZnO (2 wt%) nanocomposite powders were fabricated by high-energy ball milling at different milling times, and subsequently, powders were consolidated by spark plasma sintering at 673 K temperature. The existence of nanoinclusions was confirmed by SEM-EDS mapping. Vickers hardness values greatly improved due to reduction in grain size which prevents the crack propagation and dispersion strengthening mechanism. The decrease in carrier density, which plays a critical role in thermoelectrics, dramatically increases the Seebeck coefficient, and subsequently, decreases the electrical conductivity upon the dispersion of ZnO nanorods into the BiSbTe matrix. The thermal conductivity was noticeably reduced by~13% in BiSbTe/ZnO composites for 5-minute samples due to blocking of carriers/phonons at interfaces, and/or grain boundaries. The peak ZT of 0.92 was obtained for BiSbTe matrix, and 0.91 for BiSbTe/ZnO (5 minutes) composites at room temperatures. K E Y W O R D Sball milling, Bi 2 Te 3 alloys, nanocomposites, spark plasma sintering, thermoelectric materials 1 | INTRODUCTION Thermoelectric (TE) materials have been crucially used in renewable energy conversion technologies to overcome the energy crisis in the present world. The efficiency of TE materials can be defined by the dimensionless figure of merit, ZT = (a 2 r/K) T, where a, r, K, and T are the Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature, respectively. 1 In general, the temperature gradient develops an electrostatic potential, which is defined as the Seebeck coefficient (a = DV/DT for small DT). The Seebeck coefficient gradually decreases with carrier density, while the electrical conductivity increases. The thermal conductivity is proportional to the carrier density, which is contributed by both electrons and phonons. The ZT should be high, and at least unity (ZT ≥ 1) to maintain the device efficiency over 10%. 2,3 For ambient room temperature (RT) applications, bismuth telluride (Bi 2 Te 3 )-based alloys are much attracted due to their high-energy conversion thermoelectric efficient at RT. 3 So far, many researchers have studied TE properties and reported improved ZT values by forming composite structures and dispersion of nanoinclusions in bulk matrix thereby controlling the Seebeck coefficient and/or electrical conductivity via introduction of energy filtering effect that can filter the low energy electrons and allows only high energy (hot) electrons. 4,5 In addition, thermal conductivity is also reduced by phonon scattering at grain boundaries and/or interfaces between nanoinclusion and matrix. Hsu et al 6 reported enhanced TE properties in the AgPb m SbTe m+2 system with nanoscale inhomogeneous of Ag and Sb embedded in PbTe matrix that can dramatically decrease the thermal conductivity by increasing phonon scattering without affecting the electrical properties. Li et al 7 reported BiSbTe-based nanocomposites with high ZT values through dispersion of SiC nanopartic...

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