Fabrication of ultrafine powder using processing control agent, and investigation of its effect on microstructure and thermoelectric properties of p-type (Bi, Sb)2Te3 alloys

Abstract

Low-dimensional materials can significantly enhance the efficiency of thermoelectric devices for power generation and cooling applications. In the present work, ultra-fine powders of p-type (Bi, Sb)2Te3 alloys are fabricated through high energy ball milling using stearic acid as a process control agent (PCA). The influence of the PCA addition on powder characteristics, microstructure and thermoelectric transport properties are studied. Further, the ultra-fine powder is subjected to calcination (Cal-PCA) and subsequently consolidated all powders using spark plasma sintering (SPS). The PCA, Cal-PCA, and non-PCA powder morphological effects on the microstructure and thermoelectric properties are systematically investigated and elucidated thoroughly. The electron beam scattering diffraction (EBSD) results confirmed that the PCA sample exhibited very fine grains (average grain size of ∼800 nm) compared to the non-PCA (average grain size of about 2.6 µm), while the grains were distributed randomly for all samples. Formation of fine grains and partial existence of the stearic acid (carbon and oxygen phases) in the matrix were strongly inhibiting the transport of the carriers that severely decreased the carrier mobility, reflecting the severe reduction in electrical conductivity for PCA sample compared to Cal-PCA and non-PCA. The lowest thermal conductivity (κ) of 0.745 W/mK was achieved for the PCA sample, which is 19%, 12% lower than that of non-PCA, and Cal-PCA samples. The strong reduction in κ was mainly attributed to the dramatic decrease in the phonon thermal conductivity owing to phonon scattering at numerous grain boundaries and oxide phases. The obtained high electrical conductivity with balanced thermal conductivity in Cal-PCA sample is attributed to the significant improvement in ZT of 1.1, which is 27%, and 47% higher than that of the Non-PCA sample at room temperature, and 350 K, respectively.