Department of Chemistry, Northwestern University, Evanston, Illinois 60208, USA.
Nature. 2012 Sep 20;489(7416):414-8. doi: 10.1038/nature11439.
With about two-thirds of all used energy being lost as waste heat, there is a compelling need for high-performance thermoelectric materials that can directly and reversibly convert heat to electrical energy. However, the practical realization of thermoelectric materials is limited by their hitherto low figure of merit, ZT, which governs the Carnot efficiency according to the second law of thermodynamics. The recent successful strategy of nanostructuring to reduce thermal conductivity has achieved record-high ZT values in the range 1.5-1.8 at 750-900 kelvin, but still falls short of the generally desired threshold value of 2. Nanostructures in bulk thermoelectrics allow effective phonon scattering of a significant portion of the phonon spectrum, but phonons with long mean free paths remain largely unaffected. Here we show that heat-carrying phonons with long mean free paths can be scattered by controlling and fine-tuning the mesoscale architecture of nanostructured thermoelectric materials. Thus, by considering sources of scattering on all relevant length scales in a hierarchical fashion--from atomic-scale lattice disorder and nanoscale endotaxial precipitates to mesoscale grain boundaries--we achieve the maximum reduction in lattice thermal conductivity and a large enhancement in the thermoelectric performance of PbTe. By taking such a panoscopic approach to the scattering of heat-carrying phonons across integrated length scales, we go beyond nanostructuring and demonstrate a ZT value of ∼2.2 at 915 kelvin in p-type PbTe endotaxially nanostructured with SrTe at a concentration of 4 mole per cent and mesostructured with powder processing and spark plasma sintering. This increase in ZT beyond the threshold of 2 highlights the role of, and need for, multiscale hierarchical architecture in controlling phonon scattering in bulk thermoelectrics, and offers a realistic prospect of the recovery of a significant portion of waste heat.
大约三分之二的已用能源以废热的形式损失,因此迫切需要高性能的热电材料,这些材料可以直接且可逆地将热能转换为电能。然而,由于迄今为止热电材料的品质因数 ZT 较低,其实际应用受到限制,而 ZT 根据热力学第二定律控制着卡诺效率。最近通过纳米结构化来降低热导率的成功策略,在 750-900 开尔文的范围内实现了创纪录的 1.5-1.8 的 ZT 值,但仍低于普遍期望的 2 的阈值。体热电材料中的纳米结构允许有效散射相当一部分声子谱中的声子,但长平均自由程的声子基本不受影响。在这里,我们表明,通过控制和微调纳米结构热电材料的介观结构,可以散射具有长平均自由程的热载声子。因此,通过在分层方式中考虑所有相关长度尺度上的散射源——从原子尺度的晶格无序和纳米尺度的内延沉淀物到介观尺度的晶界——我们实现了晶格热导率的最大降低,并大大提高了 PbTe 的热电性能。通过在集成长度尺度上对热载声子的散射采取这种全景方法,我们超越了纳米结构化,并在浓度为 4 摩尔%的 SrTe 内延纳米结构化和粉末加工与火花等离子烧结的介观结构化的 p 型 PbTe 中,在 915 开尔文下实现了约 2.2 的 ZT 值。ZT 值超过 2 的这一增加突出了在控制体热电材料中的声子散射方面,多层次的层次结构的作用和必要性,并为回收大量废热提供了现实的前景。