By Shih-Han Lo, Northwestern University, Professor Vinayak P. Dravid Group
Overview - Thermoelectric Materials
Thermoelectric materials reversibly convert (waste) heat into electricity. In recent decades, thermoelectricity has evolved into a rapidly growing field for renewable and clean 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.1,2 ZT represents the ratio of power factor (σ2S; where σ = electrical conductivity and S is the Seebeck coefficient) and thermal conductivity (κ). Having been stuck at a nominal value of 1 for decades, innovations in the field have rapidly enhanced ZT in recent years. The generally accepted threshold value of 2 or better is desired for ZT for wider practical and commercial deployment. ZT can be enhanced by increasing the power factor and/or reducing the (lattice) thermal conductivity.
More in Depth - Panoscopic Approach toward High-Performance Thermoelectric Materials
The reduction of thermal conductivity is highly related to the material’s structure, from atomic-level features to meso- and microscale structures. Thus, we have shown in recent years that efficient reduction in thermal conductivity of thermoelectrics can be achieved by invoking all-length scales in bulk thermoelectrics. This “panoscopic” tailoring of materials’ microstructures enhances phonon scattering across different wavelengths yet preserves their electronic transport, leading to high-performance thermoelectric materials. Heat-carrying phonons with short mean free paths can be scattered by nanoscale precipitates embedded in the matrix, and ones with long mean free paths can be scattered by controlling and fine-tuning the mesoscale architecture of the nanostructured thermoelectric materials.
By taking such a panoscopic approach to the scattering of heat-carrying phonons across integrated length scales, we go beyond nanostructuring and demonstrated a ZT value of ~2.2 at 915 kelvin in p-type PbTe endotaxially nanostructured with SrTe.3,4 This approach has also been proven with various material systems, primarily chalcogenides,5,6 and future innovations are targeted towards other materials. So stay tuned!
All images courtesy Shih-Han Lo. [Top] Hierarchical approach to improving ZT [Bottom] High-resolution TEM micrograph at left showing nanoscale precipitates in the matrix and low-resolution at right showing mesoscale grain size
Approachable review from 2008 on the development of the field of thermoelectrics, progress to that point, and current and future applications
1. L. E. Bell "Cooling, Heating, Generating Power, and Recovering Waste Heat with Thermoelectric Systems," Science 321, 1457 –1461 (2008)
Review article from 2009 with background on the progress of thermoelectric materials and emphasis on the approaches being undertaken to improve ZT through new material development and microstructural control
2. J. R. Sootsman, et al., "New and Old Concepts in Thermoelectric Materials," Angew. Chem. Int. Ed. 48, 8616–8639 (2009)
Work from the author's group demonstrating ZT improvement by nanoscale tailoring via nanostructuring of PbTe with endotaxial SrTe
3. K. Biswas, et al., "Strained endotaxial nanostructures with high thermoelectric figure of merit," Nat Chem 3, 160–166 (2011)
Work from the author's group detailing the approach to improving ZT by controlling phonon scattering at multiple length scales and demonstrating high ZT in mesostructured PbTe nanostructured with SrTe
4. K. Biswas, et al., "High-performance bulk thermoelectrics with all-scale hierarchical architectures," Nature 489, 414–418 (2012)
Paper from the author demonstrating high ZT in tellurium-free thermoelectrics by simultaneous multiple-scale tailoring of phonon scattering and electronic band tuning through compositional alloying of Cd-containing phase
5. L.-D. Zhao et al., "High Thermoelectric Performance via Hierarchical Compositionally Alloyed Nanostructures," J. Am. Chem. Soc. 135, 7364–7370 (2013)
Paper from the author emphasizing microscopic characterization and performance of a range of high-ZT tellurium-free materials with multiple-length scale meso- and nano-structuring
6. Y. Lee et al., "High-Performance Tellurium-Free Thermoelectrics: All-Scale Hierarchical Structuring of p-Type PbSe–MSe Systems (M = Ca, Sr, Ba)," J. Am. Chem. Soc. 135, 5152–5160 (2013).