Nanomaterials fascinate the world due to their singular physicochemical properties which are essential in various technological applications, such as in biomedicine, chemical sensing, infrared detection, memory storage, photocatalysis, photovoltaics, thermoelectric conversion, etc. Of these, thermoelectricity is a field which deals with the generation of electricity from the differences of temperature across a sample, or vice versa, with the major advantage of having no moving parts, thereby making the fabrication of portable thermoelectric energy conversion modules feasible [1], [2], [3] and [4]. Radioisotope thermoelectric generators (RTGs) have been successfully utilized in various spacecrafts [5] like Voyager, Galileo, Cassini-Huygens, Curiosity, Pluto New Horizons, etc. However, on earth, these materials are not widely in general use due to their low efficiency for targeted applications, apart from in some commercial applications such as vehicle-seat coolers/warmers, low-wattage power generators, laser diode coolers, etc. In order to realize extensive societal applications, it is widely agreed that the materials must possess a relatively high thermoelectric figure of merit, ZT. Hicks and Dressalhaus [6] and [7] predicted that the ZT could be significantly increased by nanoengineering the materials. Low dimensional materials increase the ZT due to the low lattice thermal conductivity compared to their bulk counterparts resulting from large scale phonon scattering. Several researchers experimentally put studied these predictions by nanoengineering materials and achieved a relatively large increment in the ZT with ultralow thermal conductivities and a significant enhancement in Seebeck coefficient. Venkatasubramanian et al. published [8] the enhancement in ZT to ca. 2.4 at 300 K, as observed in their p-type Bi2Te3/Sb2Te3 superlattices.
Lead and silver based telluride materials are widely known for their potential applications in medium-high temperature thermoelectric (TE) energy conversions. Although there are numerous reports concerning the syntheses of lead telluride and a few on silver telluride are available, only a few publications discuss about the lead-silver-telluride based composites. These include self-assembled superlattice structures of PbTe/Ag2Te thin films [9]. The Kanatzidis group have published on a series of quaternary semiconductor thermoelectric materials containing at least two of the elements, Ag, Pb, and Te [10], [11] and [12]. One such material is the n-type Ag1-xPb18SbTe20 (LAST) material that exhibits low thermal conductivities of ca. 1 W m−1 K−1 and a ZT of ca. 2.1 at 800 K. Nevertheless, of these synthetic procedures, to the best of our knowledge, there are currently no reports available on the synthesis of Pb-Ag-Te based mesoporous structures by colloidal technique. It is known that porous material shows much lower thermal conductivity than that of its nonporous counterpart, due to phonon confinement in the nanosized grains and phonon scattering at the pores surface. However, it is necessary to probe the true extent of the effect of this quasi-porosity on electrical and lattice thermal conductivities and their contributions in the enhancement of the thermoelectric figure of merit.
This issues’ cover image shows the false-colored electron micrograph of the quasi-hollow-mesoporous admixtures of lead, silver and tellurium. The present image was captured using a Zeiss Auriga 39-16 high resolution scanning electron microscopy fitted with GEMINI column performed at Laboratorio Avanzado de Nanoscopia Electronica (LANE) – CINVESTAV-IPN, Mexico D.F. [13]. Re-dispersed specimen was drop-casted onto a carbon tape and micrographed using a high efficiency annular In-lens detector at an accelerating voltage of 5 kV, and at a working distance of 3.7 mm.
We synthesized this material by colloidal chemistry using the nitrates of lead and silver as cationic and tellurium shot as anionic precursors. This involves hot injection of a tellurium precursor solution into the cationic complex essentially prepared in an inverse-micellar environment with a certain amount of reaction time. The resultant material was washed and examined for structural, morphological and compositional information. Energy dispersive x-ray spectroscopic analysis (EDAX) indicated near-stoichiometric Ag2Te comprising small amounts of lead. X-ray diffraction patterns demonstrated the crystallinity of the synthesized material. The obtained material may be concluded to be self-assembled into an arbitrary (spherical, zigzag, coral like) hierarchically mesoporous pattern due to the prevailing van der Waal's forces, as well as the employed inverse-micellar media during the synthesis process in addition to the Ostwald ripening. The clusters are in the size range of a few tens of nanometers (nm) to a few hundreds of nm, yet the average cavity size, of ca. 30 nm, confirms the mesoporosity of the formed material. This morphological aspect might be able to reduce the transmission of phonons by scattering at boundaries and thereby decreasing the thermal conductivity of the material.
Further reading
1. C. Wood, Rep. Prog. Phys., 51 (1988) http://dx.doi.org/10.1088/0034-4885/51/4/001
2. L.E. Bell, Science (2008) http://dx.doi.org/10.1126/science.1158899
3. G.J. Snyder, E.S. Toberer, Nat. Mater. (2008) http://dx.doi.org/10.1038/nmat2090
4. A.D. LaLonde, et al., Mater. Today (2011) http://dx.doi.org/10.1016/S1369-7021(11)70278-4
5. http://www.fas.org/nuke/space/index.html
6. L.D. Hicks, M.S. Dresselhaus, Phys. Rev. B (1993) http://dx.doi.org/10.1103/PhysRevB.47.12727
7. L.D. Hicks, M.S. Dresselhaus, Phys. Rev. B (1993) http://dx.doi.org/10.1103/PhysRevB.47.16631
8. R. Venkatasubramanian, et al., Nature (2001) http://dx.doi.org/10.1038/35098012
9. J.J. Urban, et al., Nat. Mater. (2007) http://dx.doi.org/10.1038/nmat1826
10. Hsu, et al., Science (2004) http://dx.doi.org/10.1126/science.1092963
11. B.A. Cook, et al., Adv. Funct. Mater. (2009) http://dx.doi.org/10.1002/adfm.200801284
12. F.P. Poudeu Pierre, et al., Angew. Chem. Int. Ed. (2006) http://dx.doi.org/10.1002/anie.200600865
13. http://lane.cinvestav.mx/Infraestructura/AURIGASEM.aspx