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Nanostructured silicon germanium thermoelectric materials prepared by mechanical alloying and sintering method have recently shown large enhancement in figure‐of‐merit, ZT. The fabrication of these structures often involves many parameters whose understanding and precise control is required to attain large ZT. In order to find the optimum parameters for further enhancing the ZT of this material, we have grown and studied both experimentally and theoretically different nanostructured p‐type SiGe alloys. The effect of various parameters of milling process and sintering conditions on the thermoelectric properties of the grown samples were studied. The electrical and thermal properties were calculated using Boltzmann transport equation and were compared with the data of nanostructured and crystalline SiGe. It was found that the thermal conductivity not only depends on the average crystallite size in the bulk material, but also it is a strong function of alloying, porosity, and doping concentration. The Seebeck coefficient showed weak dependency on average crystallite size. The electrical conductivity changed strongly with synthesis parameters. Therefore, depending on the synthesis parameters the figure‐of‐merit reduced or increased by ∼60% compared with that of the crystalline SiGe. The model calculation showed that the lattice part of thermal conductivity in the nanostructured sample makes ∼80% of the total thermal conductivity. In addition, the model calculation showed that while the room temperature hole mean free path (MFP) in the nanostructured sample is dominated by the crystallite boundary scattering, at high temperature the MFP is dominated by acoustic phonon scattering. Therefore, the thermal conductivity can be further reduced by smaller crystallite size without significantly affecting the electrical conductivity in order to further enhance ZT.

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Higher manganese silicide (HMS) alloys have a complex band structure with multiple valleys close to the conduction and valence band edges, which complicates the analysis of their electronic transport properties. We present a semi-classical two-band model that can describe the charge carrier and phonon transport properties of p-type HMS in crystalline and bulk nanostructured forms. The effect of grain boundaries is modeled with an interface potential scattering for charge carriers and diffusive and refractive scattering for phonons. A unique set of effective masses and acoustic phonon deformation potentials are introduced that can explain both electrical and thermal transport properties versus temperature. The acoustic phonon and ionized impurity scatterings for charge carriers and phonon-phonon, point defect, and electronphonon scattering mechanisms for phonons are included in the model. The simplicity of the presented model would be valuable especially for practical purposes. The thermoelectric transport properties of nanostructured HMS were calculated versus grain size and it was shown that even though bulk nanostructuring of HMS enhances thermoelectric performance, it is not sufficient to enhance considerably the figure-of-merit.

P-type nanostructured bulk Si0.8Ge0.2 and Si0.8Ge0.2 composites with CrSi2 nano-crystallite inclusions were synthesized via sintering approach. The composite structure showed power factor enhancement compared with nanostructured Si0.8Ge0.2 alloy. The experimental data for both structures were modeled with solving the multiband Boltzmann transport equation in the relaxation time approximation for charge carriers and phonons. The Si0.8Ge0.2 crystallite boundary scattering was modeled by a cylindrical potential barrier at the interfaces and the effects of CrSi2 nano-inclusions were modeled by spherical potential barriers in the Si0.8Ge0.2 lattice. The model calculations revealed that the enhancement in power factor is not an effect of hot carrier energy filtering, but it is due to the enhancement in charge carrier mobility in the composite structure. The analysis of charge carrier mobility components showed that while in nanostructured Si0.8Ge0.2 the ionize impurities and acoustic phonons are dominant scatterers, in the composite structure the scattering by CrSi2 nano-inclusions and acoustic phonons are dominant. The optimum size of the CrSi2 nano-inclusions for enhancing ZT was predicted with the characteristic that ZT drops rapidly when the crystallite size decreases, but it changes slowly as it is increased above its optimum value.