Thermoelectric power factor in nano- to microscale porous composites
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Institute of Nanoscience and Nanotechnology, National Center for Scientific Research “Demokritos”, Athens 15310, Greece; and Institute of Radiophysics & Electronics, National Academy of Sciences of Armenia, Ashtarak 0203, Armenia
Dimitris G. Niarchos
Institute of Nanoscience and Nanotechnology, National Center for Scientific Research “Demokritos”, Athens 15310, Greece (Received 23 November 2014; accepted 5 May 2015)
The peculiarities of the Seebeck coefficient and power factor are studied in porous thermoelectric materials with spherical hollow pores of varying diameter from nanometer to micrometer length scales. The pores are assumed to be randomly dispersed throughout the matrix material. The influence of trap centers situated at pore interfaces on the power factor is investigated. Using the model based on gamma distribution of the pore sizes, the analytical expression is obtained for the power factor at the arbitrary level of the Fermi energy. Limiting cases of nondegenerate and degenerately doped porous semiconductors are examined as well. The results are compared with calculations for a multilayer composite in which each layer contains pores of a single length-scale. It is shown that the presence of hollow pores with multiscale hierarchical disorder leads to more considerable enhancement in the thermopower over its value in the bulk. Necessary conditions for the enhancement of the power factor are found.
I. INTRODUCTION
In recent years, granular nanocomposites and porous materials have attracted extensive interest because of their potential as candidates for increasing the dimensionless thermoelectric figure-of-merit ZT 5 rS2T/j, where r is the electrical conductivity, S is the Seebeck coefficient, j is the thermal conductivity, and T is the average absolute temperature.1–6 For most of the commercially available thermoelectric materials, ZT is well below one. This fact limits the efficiency of energy conversion between electricity and heat, and the application of thermoelectric devices for power generation and refrigeration. In 2002, Harman et al. reported ZT 5 1.6 at T 5 300 K for PbSeTe/PbTe nanodot superlattices. 7 Recently, a dramatic enhancement in ZT has been achieved in bulk thermoelectric nanostructures with all-scale hierarchical architecturing for lead chalcogenides PbTe, PbSe, and PbS.8,9,10 A recorded high ZT value of ;2.2 at 915 K has been demonstrated by the introduction of 1–17 nm SrTe nanoscale precipitations and 0.1–1 lm micrograins in the Na-doped PbTe matrix.9 The increase in ZT value has come either from the Contributing Editor: Terry M. Tritt a) Address all correspondence to this author. e-mail: [email protected]; [email protected] DOI: 10.1557/jmr.2015.151 2618
J. Mater. Res., Vol. 30, No. 17, Sep 14, 2015
http://journals.cambridge.org
Downloaded: 26 Jul 2016
reduction in lattice thermal conductivity or improvement in thermoelectric power factor P 5 rS2, or both of them. Note that there are many theoretical and experimental studies on the enhancement of the Seebeck c
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