Hanus Riley, Agne Matthias T, Rettie Alexander J E, Chen Zhiwei, Tan Gangjian, Chung Duck Young, Kanatzidis Mercouri G, Pei Yanzhong, Voorhees Peter W, Snyder G Jeffrey
Materials Science and Engineering, Northwestern University, Evanston, IL, 60208, USA.
Materials Science Division, Argonne National Laboratory, Argonne, IL, 60439, USA.
Adv Mater. 2019 May;31(21):e1900108. doi: 10.1002/adma.201900108. Epub 2019 Apr 10.
The influence of micro/nanostructure on thermal conductivity is a topic of great scientific interest, particularly to thermoelectrics. The current understanding is that structural defects decrease thermal conductivity through phonon scattering where the phonon dispersion and speed of sound are assumed to remain constant. Experimental work on a PbTe model system is presented, which shows that the speed of sound linearly decreases with increased internal strain. This softening of the materials lattice completely accounts for the reduction in lattice thermal conductivity, without the introduction of additional phonon scattering mechanisms. Additionally, it is shown that a major contribution to the improvement in the thermoelectric figure of merit (zT > 2) of high-efficiency Na-doped PbTe can be attributed to lattice softening. While inhomogeneous internal strain fields are known to introduce phonon scattering centers, this study demonstrates that internal strain can modify phonon propagation speed as well. This presents new avenues to control lattice thermal conductivity, beyond phonon scattering. In practice, many engineering materials will exhibit both softening and scattering effects, as is shown in silicon. This work shines new light on studies of thermal conductivity in fields of energy materials, microelectronics, and nanoscale heat transfer.
微/纳米结构对热导率的影响是一个具有重大科学意义的课题，尤其对于热电学而言。目前的认识是，结构缺陷通过声子散射降低热导率，其中假定声子色散和音速保持不变。本文展示了在PbTe模型系统上的实验工作，该工作表明音速随着内部应变的增加而线性降低。材料晶格的这种软化完全解释了晶格热导率的降低，而无需引入额外的声子散射机制。此外，研究表明，高效钠掺杂PbTe的热电品质因数（zT > 2）的提高主要归因于晶格软化。虽然已知不均匀的内部应变场会引入声子散射中心，但本研究表明内部应变也可以改变声子传播速度。这为控制晶格热导率提供了新途径，超越了声子散射。实际上，许多工程材料都会同时表现出软化和散射效应，如硅中所示。这项工作为能源材料、微电子学和纳米尺度热传递领域的热导率研究带来了新的启示。
