X. Chen, Shi, L., Zhou, J., and Goodenough, J. B., “Effects of ball milling on microstructures and thermoelectric properties of higher manganese silicides,” Journal of Alloys and Compounds, vol. 641, pp. 30 - 36, 2015.
Abstract Bulk nanostructured higher manganese silicide (HMS) samples with different grain size are prepared by melting, subsequent ball milling (BM), and followed by spark plasma sintering (SPS). The effects of \BM\ time on the microstructures and thermoelectric properties of these samples are investigated. It is found that \BM\ effectively reduces the grain size to about 90 nm in the sample after SPS, which leads to a decrease in both the thermal conductivity and electrical conductivity. By prolonging the \BM\ time, MnSi and tungsten/carbon-rich impurity phases are formed due to the impact-induced decomposition of \HMS\ and contamination from the tungsten carbide jar and balls during the BM, respectively. These impurities result in a reduced Seebeck coefficient and increased thermal conductivity above room temperature. The measured size-dependent lattice thermal conductivities agree qualitatively with the reported calculation results based on a combined phonon and diffuson model. The size effects are found to be increasingly significant as temperature decreases. Because of the formation of the impurity phases and a relatively large grain size, the ŻT\} values are not improved in the ball-milled \HMS\ samples. These findings suggest the need of alternative approaches for the synthesis of pure \HMS\ with further reduced grain size and controlled impurity doping in order to enhance the thermoelectric figure-of-merit of \HMS\ via nanostructuring.