Thermoelectric materials can be used for solid state power generation and heating/cooling applications. The figure of merit of thermoelectric materials, ZT, which determines their efficiency in a thermoelectric device, remains low for most conventional bulk materials. Nanoscale and nanostructured thermoelectric materials are promising for increasing ZT relative to the bulk. This review introduces the theory behind thermoelectric materials and details the predicted and demonstrated enhancements of ZT in nanoscale and nanostructured thermoelectric materials. We discuss thin films and superlattices, nanowires and nanotubes, and nanocomposites, providing a ZT comparison among various families of nanocomposite materials. We provide some perspectives regarding the origin of enhanced ZT in nanoscale and nanostructured materials and suggest some promising and fruitful research directions for achieving high ZT materials for practical applications.

Enhancement of the thermoelectric properties in nanoscale and nanostructured materials

Magnesium silicide (Mg2Si)-based alloys are promising candidates for thermoelectric (TE) energy conversion for the middle to high range of temperature. These materials are very attractive for TE research because of the abundance of their constituent elements in the Earth’s crust. Mg2Si could replace lead-based TE materials, due to its low cost, nontoxicity, and low density. In this work, the role of aluminum doping (Mg2Si:Al = 1:x for x = 0.005, 0.01, 0.02, and 0.04 molar ratio) in dense Mg2Si materials was investigated. The synthesis process was performed by planetary milling under inert atmosphere starting from commercial Mg2Si pieces and Al powder. After ball milling, the samples were sintered by means of spark plasma sintering to density >95%. The morphology, composition, and crystal structure of the samples were characterized by field-emission scanning electron microscopy, energy-dispersive spectroscopy, and x-ray diffraction analyses. Moreover, Seebeck coefficient analyses, as well as electrical and thermal conductivity measurements were performed for all samples up to 600°C. The resultant estimated ZT values are comparable to those reported in the literature for these materials. In particular, the maximum ZT achieved was 0.50 for the x = 0.01 Al-doped sample at 600°C.

Synthesis and Characterization of Al-Doped Mg2Si Thermoelectric Materials

Synthesis and characterization of Bi-doped Mg2Si thermoelectric materials

Magnesium-based thermoelectric materials (Mg2X, X = Si, Ge, Sn) are considered one of the most attractive groups for large-scale application, due to their materials' high availability, low cost, low mass density, and reasonably high efficiency. In this work, we present an overview of the recent developments relating to magnesium-based thermoelectric materials and review the current approaches towards high thermoelectric efficiency.

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Recent progress in magnesium-based thermoelectric materials

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 ∼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 ∼ 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.

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

Nanocomposites and heavy doping both are regarded as effective way to improve materials’ thermoelectric properties. 0.7at% Bi-doped Mg2Si nanocomposites were prepared by spark plasma sintering. Results of thermoelectric properties tests show that the doping of Bi atom effectively improves the electrical conductivity of Mg2Si，and the nanocomposite structures are helpful to reduce thermal conductivity and increase Seebeck coefficient, hence improving the thermoelectric performance. A maximum dimensionless figure of merit of 0.8 is obtained for the Bi-doped Mg2Si nanocomposite with 50 wt % nanopowder inclusions at 823K, about 63% higher than that of Bi-doped Mg2Si sample without nanopowder inclusions and 119% higher than that of microsized Mg2Si sample without Bi-doped, respectively.

Preparation and Thermoelectric Properties of Bi-Doped Mg2Si Nanocomposites

The single phase of Bi-doped Mg2Si0.5Sn0.5 compounds have been successfully fabricated by solid state reaction-spark plasma sintering (SPS). The effect of Bi doping concentration on the thermoelectric properties of Mg2Si0.5Sn0.5 is mainly investigated. The doping of Bi atom introduces impurity energy to Mg2Si0.5Sn0.5 compounds, which results in the increase of carrier concentration ( ), meanwhile it causes the increase of crystal distortion, enhancing the scatter of phonon. The results show that with the increasing of Bi doping content, the electrical conductivity (σ) increase, the absolute Seebeck coefficient ( ) and thermal conductivity ( ) decrease slightly in the measuring temperature range between 300 K and 800K. When the doping concentration of Bi is up to 2.5at% (nominal molar percent), the sample shows a maximum value of the figure of merit, ZT, is 0.78 at 800K.

Wei Jun Luo

Papers

Preparation and Thermoelectric Properties of Bi-Doped Mg2Si Nanocomposites

Effect of Bi Doping on the Thermoelectric Properties of Mg2Si0.5Sn0.5 Compound