pubs.acs.org/NanoLett
In Situ Nanostructure Generation and Evolution within a Bulk Thermoelectric Material to Reduce Lattice Thermal Conductivity Steven N. Girard,† Jiaqing He,†,‡ Changpeng Li,§ Steven Moses,§ Guoyu Wang,§ Ctirad Uher,§ Vinayak P. Dravid,‡ and Mercouri G. Kanatzidis*,†,| †
Department of Chemistry, Northwestern University, Evanston, Illinois 60208, ‡ Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, § Department of Physics, University of Michigan, Ann Arbor, Michigan 48109, and | Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439 ABSTRACT We show experimentally the direct reduction in lattice thermal conductivity as a result of in situ nanostructure generation within a thermoelectric material. Solid solution alloys of the high-performance thermoelectric PbTe-PbS 8% can be synthesized through rapid cooling and subsequent high-temperature activation that induces a spontaneous nucleation and growth of PbS nanocrystals. The emergence of coherent PbS nanostructures reduces the lattice thermal conductivity from ∼1 to ∼0.4 W/mK between 400 and 500 K. KEYWORDS Thermoelectric materials, thermal conductivity, nucleation and growth, semiconductors
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nanopowder of Bi2Te3 exhibited a ZT of 1.4 at 370 K.6 For each of these materials, the ZT is enhanced almost entirely by a drastic reduction in the lattice thermal conductivity presumably as a result of phonon scattering at the interfaces of nanoscale inclusions.7,8 Much of the recent work in thermoelectrics has been aimed at creating nanostructured composites of existing materials to retard the lattice thermal conductivity while largely unaffecting electrical transport.
enewable energy initiatives have increased interest in thermoelectric materials as an option for inexpensive and environmentally friendly heat-to-power generation. The potential of a thermoelectric material is determined by its figure of merit ZT, defined as ZT ) S2σT/ κtot, where S is the thermopower or Seebeck coefficient, σ is the electrical conductivity, T is the operating temperature, and κtot is the total thermal conductivity (a sum of the electronic κelec and lattice κlat vibrations). The conventional bulk thermoelectric material PbTe has seen limited use in heat-to-power devices as a result of low maximum ZT values of between 0.8 and 1, corresponding to an average efficiency of around 5-6%.1,2 Enhancing the ZT of existing material systems to >1 increases the potential applications of thermoelectric technology for heat-to-power generation. One approach toward enhancing ZT has been to incorporate appropriate nanostructures within existing bulk materials, therefore reducing the lattice thermal conductivity and elevating ZT.3 Recently, nanostructuring of thermoelectric materials has led to significant increases in ZT: Venkatasubramanian et al. showed that a layered superlattice of Bi2Te3/Sb2Te3 prepared by chemical vapor deposition (CVD) produced a material with a ZT of 2.4 at 300 K,4 Hsu et al. showed the bulk nanostructured material AgPbmSbTem+2 (LAST) to have a ZT of 1.7 at 500 K,5 and Poudel et al. showed that a pressed
To date, nearly all high figure of merit thermoelectrics (ZT > 1.5) are nanostructured and exhibit lower lattice thermal conductivities than would be expected from homogeneous solid solution theory. Therefore, it is widely believed that nanostructuring is responsible for such low lattice thermal conductivities although direct evidence of this is lacking. We recently reported a number of bulk PbTe-based nanostructured materials, including Ag(Pb1-ySny)mSbTe2+m (LASTT),9 Na1-xPbmSbyTem+2 (SALT),10 PbTe-Sb-Pb,11 and Pb1-xSnxTe-PbS,12 all of which exhibit values of ZT above 1.4 from 650 to 700 K. Naturally formed nanostructured phases (i.e., without milling or secondary processing) in these systems are elegantly formed within the parent material according to precise thermal treatment. While these bulk materials exhibit enhanced ZT, none exist as a purely solid solution analogue. Because of this, we can only compare phonon scattering in these material systems indirectly with homogeneous bulk PbTe. In order to make a direct link between nanostructuring and phonon scattering, one
* To whom correspondence should be addressed,
[email protected]. Received for review: 03/2/2010 Published on Web: 07/26/2010 © 2010 American Chemical Society
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DOI: 10.1021/nl100743q | Nano Lett. 2010, 10, 2825–2831
needs a material which comes in two forms: a homogeneous (i.e., solid solution) crystal and nanostructured crystal. To address this, some of the most convincing reports to date have presented materials that have been crushed into nanosized crystals through milling and subsequently hot pressed, resulting in reduced lattice thermal conductivity as compared to their solid solution parent materials.6,13,14 However, milling is a highly energetic process that may introduce chemically reacted surfaces, material amorphization, and increased likelihood of contamination,15,16 and pressed nanoscale powders of thermoelectric materials have exhibited incoherent interfaces, crystal defects, grain composition inhomogeneities, and nonuniform stresses that likely alter thermoelectric transport when compared directly to the homogeneous parent bulk material.14,17,18 A reduction of the lattice thermal conductivity below the alloy limit has also been observed in vapor grown artificial thin superlattice films and nanowires of Si/Ge.19-21 In this report, we chose to control phase immiscibility in the PbTe-PbS thermoelectric system to selectively create solid solution alloys that can be thermally manipulated to generate a nanostructured material. For the system PbTePbS, nanostructures are formed by a precipitation process dependent on its composition and temperature. The PbTePbS system exhibits a miscibility gap that is defined by the plot of chemical potential (Gibbs free energy G) for a given isotherm across the composition range x.22-24 If a sample of PbTe-PbS is rapidly cooled from the melt to a lower temperature, the initial composition is expected to be completely homogeneous, i.e., a solid solution. Where the free energy plot has positive curvature (∂2G/∂x2 > 0) the free energy of the metastable solid solution alloy can only be decreased by precipitation of distinct nuclei of a second phase, characterized as nucleation and growth.25,26 For this reason, it can be expected that a solid solution alloy of PbTe1-xSx within the nucleation and growth region (x > 0.85 and x < 0.30 at 500 °C)27 will only exhibit precipitation once the requisite nucleation energy barrier is overcome.28 Further heating will cause the nucleated precipitates to coarsen into larger structures. The phase relations in this thermoelectric material system present a unique opportunity to study and better understand the effects of nanoscale particle incorporation on thermoelectric material properties. Because it is an entirely natural solid-state transformation, there is no chance of the aforementioned limitations in our investigation: we can directly compare the same sample, as both a solid solution and a nanostructured material. The PbTe-PbS 8% composition was chosen specifically due to its phase location fully within the nucleation and growth region of the phase diagram at temperatures
