PbSe-MnSe纳米复合热电材料的微结构和电热输运性能

Microstructures and thermoelectric transports in Pb Se-Mn Se nano-composites

相比于常见的热电材料PbTe,另一种硫族铅化合物PbSe具有熔点高、Se储量更丰富等优势,从而越来越受到科学界的关注.本文采用熔融淬火结合快速热压烧结工艺制备了Pb_(0.98-x)Mn_xNa_(0.02)Se(0 ≤x≤0.12)纳米复合热电材料,系统地研究了不同Mn含量对材料微纳结构、机械性能和热电性能的影响规律.发现纳米复合样品中有面心立方结构的Mn Se球状和薄层状析出物,显微硬度得到显著增强.少量固溶的Mn增加了能带简并度,使功率因子提高,球状析出物使声子散射增强、热导率降低,体系的热电优值ZT得到优化;但是当Mn含量更高时,赛贝克系数趋于饱和,连续析出物使晶格热导率反常增大,ZT没有得到进一步改善.通过进一步调节Na含量优化了载流子浓度,获得了ZT=0.65的PbSe-Mn Se纳米复合热电材料.

Thermoelectric materials can generate electricity by harnessing the temperature gradient and lowering the temperature through applying electromotive force.Lead chalcogenides based materials,especially Pb Te-based ones,have shown extremely high thermoelectric performance.Pb Se has a similar crystal structure and band structure to Pb Te.Compared with the commonly-used Pb Te,Pb Se possesses a high melting point and has an abundant reserve of Se,making it attractive to high temperature thermoelectric applications.It has been theoretically proposed that Mn-doping in lead chalcogenide should be able to lower the temperature of band degeneracy,and experimental evidences have been represented in Mn-Pb Te.However,such an experimental study as well as the investigations of influences of Mn on microstructure,mechanical,electrical and thermal properties has not been conducted in Mn-Pb Se.In this work,Pb-0.98-xMnxNa-0.02）Se（0≤x≤0.12）materials are prepared by the melting-quenching techniques combined with rapid hot-press sintering.Effects of Mn doping on the microstructures,mechanical and thermoelectric properties of Pb Se samples are systematically studied.The refined lattice parameters from X-ray powder diffraction patterns show that the solubility of Mn in the matrix is in a range from 0 to 0.04.The back-scattered electron images and elemental maps reveal that the Mn Se-rich impurity phases exist in the Pb Se matrix,which makes the Pb Se-Mn Se system a nano-composite system.Pb-0.96Mn-0.02Na-0.02 Se has also such microstructures,implying that the solubility of Mn should be below 0.02.Cubic-phase Mn Se-rich precipitates have the sizes ranging from 50 nanometers to 1–5 micrometers.They are well dispersed in the Pb Se-rich matrix,as round or layered microstructures.The mechanical properties of the nanocomposites can be determined by micro-hardness measurements.Interestingly,the average Vickers hardness values of the Pb SeMn Se nanocomposites are significantly improved,which are 16.6% and 51.6% harder respectively in x = 0.02 and 0.06 samples than those of pristine Pb Se.Smaller Mn content can optimize the figure of merit ZT due to the band convergence and additional phonon scattering by precipitates,while higher Mn content has little influence on ZT because of the saturated Seebeck coefficient and anomalous increase in lattice thermal conductivity.As a result,the highest figure of merit is 0.52 at 712 K,which is achieved in the Pb-0.96Mn-0.02Na-0.02 Se sample.By further adjusting the Na content from 2% to 0.7%,the carrier concentration is optimized.Thus,the Seebeck coefficient and power factor become higher.A figure of merit of 0.65 is achieved at 710 K in the Pb Se-Mn Se nano-composite with a nominal composition of Pb-0.973Mn-0.02Na-0.007 Se.We suggest that further optimizing the electrical properties may achieve a higher thermoelectric performance in the PbSe-MnSe system.