Pure lead-free SnTe has limited thermoelectric potentials because of the low Seebeck coefficients and the relatively large thermal conductivity. In this study, we provide experimental evidence and theoretical understanding that alloying SnTe with Ca greatly improves the transport properties leading to ZT of 1.35 at 873 K, the highest ZT value so far reported for singly doped SnTe materials. The introduction of Ca (0–9%) in SnTe induces multiple effects: (1) Ca replaces Sn and reduces the hole concentration due to Sn vacancies, (2) the energy gap increases, limiting the bipolar transport, (3) several bands with larger effective masses become active in transport, and (4) the lattice thermal conductivity is reduced by about 70% due to the contribution of concomitant scattering terms associated with the alloy disorder and the presence of nanoscale precipitates. An efficiency of ∼10% (for ΔT = 400 K) was predicted for high-temperature thermoelectric power generation using SnTe-based p- and n-type materials.

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.

In this paper, we propose a heterogeneous material for bulk thermoelectrics. By varying the quenching time of Na doped PbTe, followed by hot pressing, we synthesized heterogeneous nanocomposites, a mixture of nanodot nanocomposites and nanograined nanocomposites. It is well-known that by putting excess amounts of Na (i.e., exceeding the solubility limit) into PbTe, nanodots with sizes as small as a few nanometers can be formed. Nanograined regions with an average grain size of ca. 10 nm are observed only in materials synthesized with an extremely low quenching rate, which was achieved by using a quenching media of iced salt water and cold water. Dimensionless thermoelectric figures of merit, zT, of those heterogeneous nanocomposites exhibited a zT around 2.0 at 773 K, which is a 25% increase compared to zT of a homogeneous nanodot nanocomposite with the largest quenching time in our experiment, i.e. furnace cooled. The power factor increase is 5%, and the thermal conductivity reduction is 15%; thus, zT increase mainly comes from the thermal conductivity reduction.

In thermoelectric energy conversions, thermal conductivity reduction is essential for enhancing thermoelectric performance while maintaining a high power factor. Herein, we propose an approach based on coated-grain structures to effectively reduce the thermal conductivity to a much greater degree when compared to that done by conventional nanodot nanocomposite. By incorporating CdTe coated layers on the surface of SnTe grains, the thermal conductivity is as low as 1.16 W/m-K at 929 K, resulting in a thermoelectric figure of merit, i.e., zT, of 1.90. According to our developed theory, phonons scatter coherently due to the phase lag between phonons passing through and around the coated grain. Such scattering is induced by the acoustic impedance mismatch between the coated layer and the grain, resulting in a gigantic phonon-scattering cross section. The phonon-scattering cross section of the coated grains is several orders of magnitude larger than that of the nanodots with the same impurity concentration. The power factor was also slightly increased by the energy filtering effect at the coated surface and additional minority carrier blocking by the heterointerfaces. This scheme can be utilized for various bulk crystals, meaning a broad range of materials can be considered for thermoelectric applications.