Strong enhancement of phonon scattering through nanoscale grains in lead sulfide thermoelectrics

Abstract

We present nanocrystalline PbS, which was prepared using a solvothermal method followed by spark plasma sintering, as a promising thermoelectric material. The effects of grains with different length scales on phonon scattering of PbS samples, and therefore on the thermal conductivity of these samples, were studied using transmission electron microscopy and theoretical calculations. We found that a high density of nanoscale grain boundaries dramatically lowered the thermal conductivity by effectively scattering long-wavelength phonons. The thermal conductivity at room temperature was reduced from 2.5 W m−1 K−1 for ingot-PbS (grain size >200 μm) to 2.3 W m−1 K−1 for micro-PbS (grain size >0.4 μm); remarkably, thermal conductivity was reduced to 0.85 W m−1 K−1 for nano-PbS (grain size ∼30 nm). Considering the full phonon spectrum of the material, a theoretical model based on a combination of first-principles calculations and semiempirical phonon scattering rates was proposed to explain this effective enhancement. The results show that the high density of nanoscale grains could cause effective phonon scattering of almost 61%. These findings shed light on developing high-performance thermoelectrics via nanograins at the intermediate temperature range. Thermoelectric materials that transform waste heat generated by equipment or buildings into electricity are emerging as an important green energy technology. Currently, researchers are trying to improve thermoelectric substances by embedding within them nanoscale precipitates that allow these materials to capture more heat. An international team led by Jiaqing He from the South University of Science and Technology of China has now discovered a way to improve this process by systematically introducing nanoscale crystal structure defects, or ‘nanograins’, into lead sulfide (PbS) particles. Their approach tripled the thermoelectric performance of this low-cost mineral from its bulk state without introducing charge-disrupting centers commonly associated with nanoscale precipitates. Detailed analysis revealed that the densely packed nanograins trap heat by scattering solid-state vibrations, or phonons, while simultaneously suppressing ‘bipolar’ interactions between charge carriers that can diminish thermoelectric power. We report here on the effects of grains of PbS with different length scales on thermal conductivity reduction and bipolar effect ‘suppression’ through macro-properties/microstructure analysis. We found that nanograins can achieve the above goals simultaneously. Combining experimental results and theoretical calculations, we found that the effect of nanograins on the reduction in lattice thermal conductivity can surpass that of nanoprecipitates. Improved properties corresponding to the lowest lattice thermal conductivity in a PbQ (Q=Te, Se, S) system (0.5 W m K−1 at 923 K) and the highest ZT value in PbQ nanocrystalline materials were achieved by the nanograin method.