Synthesis and characterization of Mg2Si/Si nanocomposites prepared from MgH2 and silicon, and their thermoelectric properties

Silicon (Si) nanoparticles embedded in a Mg2Si matrix (Mg2Si/xSi) have been successfully synthesized at 623 K from MgH2 and Bi containing Si nanoparticle powders. The use of MgH2 in this synthetic route avoids the formation of oxides through the generation of hydrogen and provides a route to homogeneously mixed Si nanoparticles within a doped Mg2Si matrix. The samples were characterized by powder X-ray diffraction, thermogravimetry/differential scanning calorimetry (TG/DSC), electron microprobe analysis (EMPA), and scanning transmission electron microscopy (STEM). The final crystallite size of Mg2Si obtained from the XRD patterns is about 50 nm for all the samples and the crystallite size of Si inclusions is approximately 17 nm. Theoretical calculations indicate that similar to 5 mol% concentrations of Si nanoparticles with diameters in the 5-50 nm range could decrease the lattice thermal conductivity of Mg2Si by about 1-10% below the matrix value. Reduction in thermal conductivity was observed with the smallest amount of Si, 2.5 mol%. Larger amounts, x = 10 mol%, did not provide any further reduction in thermal conductivity. Analysis of the microstructure of the Bi doped Mg2Si/xSi nanocomposites showed that the Bi dopant has a higher concentration at grain boundaries than within the grains and Bi preferentially substitutes the Mg site at the boundaries. The nanocomposite carrier concentration and mobility depend on the amount of Bi and Si inclusions in a complex fashion. Agglomerations of Si start to show up clearly in the Bi doped 5 mol% nanocomposite. While the inclusions result in a lower thermal conductivity, electrical resistivity and Seebeck are negatively affected as the presence of Si inclusions influences the amount of Bi dopant and therefore the carrier concentration. The x = 2.5 mol% nanocomposite shows a consistently higher zT throughout the measured temperature range until the highest temperatures where a dimensionless figure of merit zT similar to 0.7 was obtained at 775 K for Mg2Si/xSi with x = 0 and 2.5 mol%. With optimization of the electronic states of the matrix and nanoparticle, further enhancement of the figure of merit may be achieved.