Right sizes of nano- and microstructures for high-performance and rigid bulk thermoelectrics

Abstract

In this paper, we systematically investigate three different routes of synthesizing 2% Na-doped PbTe after melting the elements: (i) quenching followed by hot-pressing (QH), (ii) annealing followed by hot-pressing, and (iii) quenching and annealing followed by hot-pressing. We found that the thermoelectric figure of merit, zT, strongly depends on the synthesis condition and that its value can be enhanced to ∼2.0 at 773 K by optimizing the size distribution of the nanostructures in the material. Based on our theoretical analysis on both electron and thermal transport, this zT enhancement is attributed to the reduction of both the lattice and electronic thermal conductivities; the smallest sizes (2∼6 nm) of nanostructures in the QH sample are responsible for effectively scattering the wide range of phonon wavelengths to minimize the lattice thermal conductivity to ∼0.5 W/m K. The reduced electronic thermal conductivity associated with the suppressed electrical conductivity by nanostructures also helped reduce the total thermal conductivity. In addition to the high zT of the QH sample, the mechanical hardness is higher than the other samples by a factor of around 2 due to the smaller grain sizes. Overall, this paper suggests a guideline on how to achieve high zT and mechanical strength of a thermoelectric material by controlling nano-and microstructures of the material.waste heat recovery | energy harvesting A thermoelectric (TE) device is a solid-state device that converts heat directly into electricity and vice versa (1-5). As there are no moving parts involved and the device configuration is simple, TE devices have demonstrated long-term reliability in various space missions, usually running for tens of years without maintenance (6). However, they are not yet widely used in many other energy conversion applications on earth mainly due to their low conversion efficiencies. The conversion efficiency of a TE device largely depends on the material properties, i.e., the figure of merit (1, 3), zT = [S 2 /ρ(κ L + κ e )]T, where T is the absolute temperature, S is the Seebeck coefficient, ρ is the electrical resistivity, and κ L and κ e are, respectively, the lattice (or phonon) and electronic thermal conductivities. Increasing the zT has proven challenging because the constituent TE properties are interdependent; for example, decreasing the electrical resistivity results in decreasing the Seebeck coefficient and increasing the electronic thermal conductivity.

Ultrahigh power factor and thermoelectric performance in hole-doped single-crystal SnSe

“…The Lorenz number L tot was calculated based on a multi band model (46,47): (13) Where L 1,2 and σ 1,2 are the Lorenz number and electrical conductivity of band 1 and band 2, respectively. (14) and , where E g is the band gap and…”

Inelastic X-Ray Scattering Studies of Phonon Dispersion in PbTe and (Pb,Cd)Te Solid Solution

“…On the other (1251) hand, a modification of the heat transport in this material by high doping or mixing/alloying with a variety of compounds seems to be a hot topic for the last few years. The search for mechanically stable PbTe-based thermoelectric materials by the crystal doping with Sn, I, or Na [4], improved mechanical properties of PbTe containing Ge [5], Na [6] and Ca [7] are just a few examples of papers related to the problem mentioned above. In particular, the properties of (Pb,Cd)Te solid solution formed by PbTe alloying with CdTe -well-known and widely investigated compound of II-VI type -were the subject of several reports in the past.…”

Thermoelectric properties of p-type PbTe/Ag2Te bulk composites by extrinsic phase mixing

“…[5][6][7][8] Notably, the ZT values have been significantly improved through the following approaches; Fermi level tuning at the resonant level in Na-doped PbTe (1.5 at 773 K), 3 convergence of the band structure in PbTe 1−x Se x (1.8 at 850 K), 4 and hierarchical microstructure control in SrTe-and Na-doped PbTe system (2.2 at 915 K). 6 Exceptionally high ZT materials (ZT ≥ 2.0) were followed by the nano-and micro-structure modulation.…”