Thermoelectric Nanomaterials

Utilizing the decomposition of metastable Pb2Sb6Te11 into PbTe (light) and Sb2Te3 (dark), a layered (lamellar) microstructure of PbTe and Sb2Te3 is produced where the interlamellar spacing can be controlled by the temperature and time of the decomposition process. Adjacent PbTe and Sb2Te3 lamellae are crystallographically oriented indicating high quality epitaxial interfaces. Average lamellar spacings as small as 180 nm are observed, corresponding to a PbTe layer thickness of 40 nm. These nanoscale multilayers, formed by bulk processing, resemble thin-film superlattice thermoelectric materials which have shown exceptionally high thermoelectric efficiency. Chemistry of Materials 19(4) pp 763 - 767 (2007).

Motivation

Recently zT > 2 has been observed in thin film superlattices or “quantum well” materials with feature sizes of tens of nanometers [1-3]. The first signficant result has been that of Venkatasubramaniam (2001) who demonstrated zT = 2.4 using Bi2Te3-Sb2Te3 quantum well superlattices with 6 nm periodicity. The highest value for bulk alloys of the same composition is about 1.1. Harman and coworkers prepared quantum dot superlattices in the PbTe-PbSeTe system (described as PbSe nanodots embedded in a PbTe matrix) and demonstrated zT values of 1.6, which are significantly higher than the bulk alloy value of 0.34. While this work was inspired by the prediction that quantum confinement would lead to increased Seebeck coefficient and therefore higher zT [4], it has been shown theoretically as well as experimentally [5] that a reduction in thermal conductivity, not an increase in the Seebeck coefficient, is mainly responsible for the enhanced figure of merit. Furthermore, significant thermal conductivity reduction (which may itself be due to quantum size effects) has been found in both the in-plane [3] and cross-plane [2] directions [5]. Thus, the enhancement in thermoelectric properties occurs regardless of the orientation of the superlattices or nanostructures relative to the measurement direction.

In our work at Caltech, we have pursued controlled transformation and rapid solidification as a means of obtaining ‘self-organized’ nanostructured thermoelectrics in bulk volumes as an alternative to the layer-by-layer top-down approach.

Nanostructured thermoelectrics by controlled transformation

A solid-solid transformation (one solid to two solids) can lead to very regular structures. Nanometer size structures are routinely produced in the formation of GP zones in Aluminum alloys. We have found that the Sb2Te3 – PbTe pseudobinary is not a simple eutectic but with rapid cooling, a third phase with approximate stoichiometry Pb2Sb6Te11 is formed. The Pb2Sb6Te11 phase is not stable above 300C and transforms into Sb2Te3 and PbTe upon annealing. As both Sb2Te3 and PbTe are both commercial thermoelectric materials with well understood methods of doping, these composites are ideal for applications.

Figure: Microstructure of Pb2Sb6Te11 solid solution transformed into Bi2Te3 and PbTe rich regions by annealing. The lighter regions are PbTe rich, the darker regions Bi2Te3 rich.

Adjacent PbTe and Sb2Te3 lamellae are crystallographically oriented indicating high quality epitaxial interfaces which may be a key aspect of why thin film superlattices reduce thermal conductivity without similar reduction in electron mobility. The size of the microstructure (interlamellar spacing) can be controlled by both the time and temperature of anneals. Chemistry of Materials 19(4) pp 763 - 767 (2007)

Nanostructured thermoelectrics by Rapid Solidification

Rapid solidification is a well-developed tool for the control of alloy microstructure. It is most successfully employed upon cooling a melt through a eutectic or slightly off-eutectic composition. Feature sizes (interlamellar spacing, secondary dendrite arm spacing, etc.) typically exhibit a power law dependence on an experimental parameter such as cooling rate, solidification time, or growth rate of the solid-liquid interface, with higher cooling rates producing finer microstructures. A wide variety of phase morphologies can be produced, from 2-dimensional lamella to 1-dimensional rods (e.g. [7, 8]), as well as complex dendritic features, which may or may not display preferred orientation between neighboring grains.
Investigations into the Bi2Te3 – PbTe and Sb2Te3 – PbTe pseudobinary systems indicate that a rich variety of microstructures that can be systematically controlled by cooling rate are possible with this method [6]. Both dendritic and layered structures have been found with a variety of feature sizes from hundreds of microns to less than one micron.Acta Materialia 55, p 1227-1239 (2007)

Figure: Microstructure of Pb-Sb-Te alloys solidified by water quenching showing acicular (a), eutectic (b) and dendritic (c) microstructures. The compositions are 21.7 mol% (PbTe) (a), 40 mol% (PbTe) (b) and 62.5 mol% (PbTe) (c), respectively.

Figure: Control of the size of the microstructure in rapidly cooled composites of PbTe-Sb2Te3. Shown are the 22 mol% PbTe compositions.

 

1. Caylor, J. C., et al., "Enhanced thermoelectric performance in PbTe-based superlattice structures from reduction of lattice thermal conductivity," Applied Physics Letters, vol. 87, pp. 23105, 2005.
2. Venkatasubramanian, R., et al., "Thin-film thermoelectric devices with high room-temperature figures of merit," Nature, vol. 413, pp. 597-602, 2001.
3. Harman, T. C., et al., "Quantum dot superlattice thermoelectric materials and devices," Science, vol. 297, pp. 2229-2232, 2003.
4. Hicks, L. D., M. S. Dresselhaus, "Effect of Quantum-Well Structures on the Thermoelectric Figure of Merit," Physical Review B, vol. 47, pp. 12727-12731, 1993.
5. Chen, G., "Nanoscale Heat Transfer and Nanostructured Thermoelectrics," The Ninth Intersociety Conference on Thermal and Thermomechanical Phenomena In Electronic Systems (IEEE Cat. No.04CH37543), pp. 8, 2004.
6. Ikeda, T., et al., "Solidification Processing of Te–Sb–Pb Alloys For Thermoelectric Applications," presented at 24th International Conference on Thermoelectrics. Proceedings, ICT'05, 2005.
7. Aguiar, M. R., et al., "Growth and microstructural characterization of SnSe-SnSe2 composite," Journal Of Materials Science, vol. 34, pp. 4607-4612, 1999.
8. Wang, Y., H. Jones, P. V. Evans, "Eutectic solidification characteristics of Bridgman grown Al-3Fe-0.1V alloy," Journal Of Materials Science, vol. 33, pp. 5205-5220, 1998.
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