Paper: Rough Silicon Nanowires as High Performance Thermoelectric Materials [View All]

[OKIsItJustMe]

http://www.cchem.berkeley.edu/pdygrp/pubpdf/155.pdf

Enhanced thermoelectric performance of rough silicon nanowires

Allon I. Hochbaum^1*, Renkun Chen^2*, Raul Diaz Delgado1, Wenjie Liang¹, Erik C. Garnett¹, Mark Najarian³, Arun Majumdar^2,3,4 & Peidong Yang^1,3,4

Approximately 90 per cent of the world’s power is generated by heat engines that use fossil fuel combustion as a heat source and typically operate at 30–40 per cent efficiency, such that roughly 15 terawatts of heat is lost to the environment. Thermoelectric modules could potentially convert part of this low-grade waste heat to electricity. Their efficiency depends on the thermoelectric figure of merit ZT of their material components, which is a function of the Seebeck coefficient, electrical resistivity, thermal conductivity and absolute temperature. Over the past five decades it has been challenging to increase ZT . 1, since the parameters of ZT are generally interdependent¹. While nanostructured thermoelectric materials can increase ZT . 1 (refs 2–4), the materials (Bi, Te, Pb, Sb, and Ag) and processes used are not often easy to scale to practically useful dimensions. Here we report the electrochemical synthesis of large-area, wafer-scale arrays of rough Si nanowires that are 20–300 nm in diameter. These nanowires have Seebeck coefficient and electrical resistivity values that are the same as doped bulk Si, but those with diameters of about 50 nm exhibit 100-fold reduction in thermal conductivity, yielding ZT 5 0.6 at room temperature. For such nanowires, the lattice contribution to thermal conductivity approaches the amorphous limit for Si, which cannot be explained by current theories. Although bulk Si is a poor thermoelectric material, by greatly reducing thermal conductivity without much affecting the Seebeck coefficient and electrical resistivity, Si nanowire arrays show promise as high-performance, scalable thermoelectric materials.

...

In conclusion, we have shown that it is possible to achieve ZT = 0.6 at room temperature in rough Si nanowires of ~50 nm diameter that were processed by a wafer-scale manufacturing technique. With optimized doping, diameter reduction, and roughness control, the ZT is likely to rise even higher. This ZT enhancement can be attributed to efficient scattering throughout the phonon spectrum by the introduction of nanostructures at different length scales (diameter, roughness and point defects). The significant reduction in thermal conductivity observed in this study may be a result of changes in the fundamental physics of heat transport in these quasi-one-dimensional materials. By achieving broadband impedance of phonon transport, we have demonstrated that the EE Si nanowire system is capable of approaching the limits of minimum lattice thermal conductivity in Si. Modules with the performance reported here, and manufactured from such a ubiquitous material as Si, may find wide-ranging applications in waste heat salvaging, power generation, and solid-state refrigeration. Moreover, the phonon scattering techniques developed in this study could significantly augment ZT even further in other materials to produce highly efficient solid-state thermoelectric devices.

...

---

¹Department of Chemistry, ²Department of Mechanical Engineering, ³Department of Materials Science and Engineering, University of California, Berkeley, California 94720, USA. ⁴Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.

^*These authors contributed equally to this work.

Entire paper available at the link