Strong Reduction of Thermal Conductivity in Nanostructured PbTe Prepared by Matrix Encapsulation

Abstract

Developing advanced thermoelectric (TE) materials can impact conversion technologies of waste heat to electrical power. It is well expected that by fabricating TE materials in a nanostructured form their properties can be significantly enhanced. 1 Efficient TE materials must exhibit a large TE figure of merit, ZT, defined as ZT ) σS 2 /κ, where S is the TE power (absolute Seebeck coefficient), σ is the electrical conductivity, and κ is the thermal conductivity. The Seebeck coefficient is generally inversely related to the electrical conductivity and as a result maximization of ZT is difficult.One modality in achieving high performance is through the minimization of the thermal conductivity using nanostructures. Superlattice thin film structures grown by molecular beam epitaxy (MBE) PbSe 0.98 Te 0.02 /PbTe 2-4 have achieved ZT values > 3. 5 These MBE-grown materials contain pyramidal-shaped "nanodots" of PbSe with uniform size (∼20 nm) embedded inside a matrix of PbTe. This assembly possesses record low values of thermal conductivity (∼0.3-0.4 W/(m‚K)) while at the same time retains a high power factor. Therefore, it is of significant interest to devise general, convenient, low cost synthetic methodologies for incorporating similar nanometer scale inclusions into bulk semiconductor materials in an effort to mimic the high ZT superlattice structures. Examples of materials with naturally occurring nanostructuring and enhanced TE properties have been reported. 6,7 Here we describe the intentional preparation of nanometer sized inclusions of Sb, Bi, and InSb in bulk PbTe using a general liquid matrix encapsulation technique. We observe that nanocrystals with large contrast in average mass (e.g., Sb and InSb) with that of the PbTe medium achieve stronger scattering of acoustic phonons than nanocrystals with minimal contrast. We also find that the reduction in lattice thermal conductivity is not monotonic with increasing concentration of nanoparticles, but there is an optimum concentration beyond which the lattice thermal conductivity actually increases. These results are in agreement with theoretical expectations and recent reports that embedded nanocrystals of ErAs promote strong scattering of acoustic phonons in a InGaAs matrix. 8 There is a plethora of published work on the preparation of stable free-standing semiconductor nanocrystals capped with surfactants, 9 embedded in polymers 10 or glasses. 11 However, there is relatively little effort devoted to preparing nanocrystals inside solid matrices or bulk crystals. 12-14 In general, bulk crystals with nanocrystals embedded in them represent a fascinating set of nanostructured materials whose scope extends beyond the field of thermoelectrics especially when the properties of guest/matrix are chosen for specific functions.To achieve nanoscale matrix encapsulation of a minority phase A inside a majority phase B, we choose the former to have very low or no solubility in the solid state but to be completely soluble in the liquid state. We choose the major phase B to ha...

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“…There are, moreover, studies showing that introducing barriers like grain boundaries in polycrystalline samples may scatter long-wavelength acoustic phonons, thus reducing κ even further [19,24,[26][27][28][29][30][31], in particular for nanostructured bulk samples [32][33][34].…”

Coherent and incoherent phase stabilities of thermoelectric rocksalt IV-VI semiconductor alloys

“…[1][2][3][4][5][6][7][8][9][10][11][12] The prototypical example is PbTe, with a approaching 1.0. 13 It has recently been shown that alloying PbTe with other semiconductor systems can increase the figure of merit through the formation of nanoscale precipitates, which reduce the lattice thermal conductivity, , of the system without significantly reducing the electronic conductivity, .…”

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“…Thus, increasing such conversion efficiency requires both high ZT and a large temperature gradient across the materials. One effective way to achieve high ZT is to reduce lattice thermal conductivity through embedding micro/nanostructures, which provide multiple scattering mechanisms for phonons 1,2,[4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] . Other strategies address the challenge of maximizing a power factor from existing semiconducting materials by band alignment 5,6,19,20 , modifying their electronic structures close to the Fermi level 21,22 , and increasing band degeneracy by convergence of bands 23 .…”