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<p><italic>P</italic>-type ternary Bi_0.5Sb_1.5Te₃ alloys are fabricated via mechanical alloying (MA) and spark plasma sintering (SPS). Different ball sizes are used in the MA process, and their effect on the microstructure; hardness, and thermoelectric properties of the <italic>p</italic>-type BiSbTe alloys are investigated. The phases of milled powders and bulks are identified using an X-ray diffraction technique. The morphology of milled powders and fracture surface of compacted samples are examined using scanning electron microscopy. The morphology, phase, and grain structures of the samples are not altered by the use of different ball sizes in the MA process. Measurements of the thermoelectric (TE) transport properties including the electrical conductivity, Seebeck coefficient, and power factor are measured at temperatures of 300- 400 K for samples treated by SPS. The TE properties do not depend on the ball size used in the MA process.</p>

Summary In this study, we developed hierarchical nanostructures by incorporating bismuth‐doped and undoped rare earth oxide (Gd1.98Bi0.02O3 and Gd2O3) nanoparticles (NPs) into Bi0.5Sb1.5Te3 (BST) matrix. The influence of Gd2O3 and Gd1.98Bi0.02O3 nano‐inclusions on microstructure and thermoelectric properties of BST composites have been investigated. The results reveal that the addition of 2 wt% Gd1.98Bi0.02O3 (bismuth‐doped RE‐oxide) into the matrix forms hierarchical structures that cause an enhanced electrical conductivity attributed to increasing carrier concentration as the substitution of bismuth preserves the comprehensive structure type and charge carrier concentration. Simultaneously, the Seebeck coefficient increases for the composite samples compared with a bare sample, due to an increase of effective mass by carrier energy filtering, while the total thermal conductivity decreased significantly by an enhanced phonon scattering at the nanoparticles, which is about 9% lower than that of BST matrix. Consequently, the ZT reached 0.87 at 300 K for BST/Gd1.98Bi0.02O3 composite, which is 30% higher than BST matrix. The result indicates that the Bi‐doped rare earth oxide (Gd1.98Bi0.02O3) incorporation into BST could improve the thermoelectric properties of the BST‐based materials. Furthermore, this work presumes that the doping effect of bismuth into the gadolinium oxide can reduce the initial defects in the structure of the synthesized Gd2O3.

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