Novel time- and energy-efficient synthesis methods, especially those adaptable to large-scale industrial processing, are of vital importance for broader applications of thermoelectrics. We herein reported a case study of layer-structured oxychalcogenides Bi1–x Pb x CuSeO (x = 0–10%) with emphases on the reaction mechanism of self-propagating high-temperature synthesis (SHS) and the impact of SHS conditions on the thermoelectric properties. The combined results of X-ray powder diffraction, differential scanning calorimetry, and quenching experiments corroborated that the SHS process of BiCuSeO consisted two fast binary SHS reactions (2 Bi+3 Se → Bi2Se3 and 2 Cu+Se → Cu2Se) intimately coupled with two relatively slow solid-state diffusion reactions (2 Bi2Se3+B2O3 → 3 Bi2SeO2 and then Bi2SeO2+Cu2Se → 2 BiCuSeO). The formation rate of the reaction intermediate Bi2SeO2 was the bottleneck in the SHS process of BiCuSeO. Importantly, we found that adding PbO in the starting materials has (i) facilitated the formation of Bi2SeO2 and thus significantly reduced the SHS reaction time; (ii) improved the phase purity and sample homogeneity; (iii) increased the power factor via increasing both carrier concentration and effective mass; and (iv) reduced the lattice thermal conductivity via more point defect phonon scattering. As a result, a state-of-the-art ZT value ∼1.2 has been attained at 923 K for Bi0.94Pb0.06CuSeO. These results not only open a new avenue for mass production of single phased multinary thermoelectric materials but also inspire more investigation into the SHS mechanisms of multinary materials in diverse fields of material science and engineering.

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The ZrNiSn alloy, a member of the half-Heusler family of thermoelectric materials, shows great potential for mid-to-high-temperature power generation applications due to its excellent thermoelectric properties, robust mechanical properties, and good thermal stability. The existing synthesis processes of half-Heusler alloys are, however, rather time and energy intensive. In this study, single-phase ZrNiSn bulk materials were prepared by self-propagating high-temperature synthesis (SHS) combined with spark plasma sintering (SPS) for the first time. The analysis of thermodynamic and kinetic processes shows that the SHS reaction in the ternary ZrNiSn alloy is different from the more usual binary systems. It consists of a series of SHS reactions and mass transfers triggered by the SHS fusion of the binary Ni-Sn system that eventually culminates in the formation of single-phase ternary ZrNiSn in a very short time, which reduced the synthesis period from few days to less than an hour. Moreover, the nonequilibrium feature induces Ni interstitials in the structure, which simultaneously enhances the electrical conductivity and decreases the thermal conductivity, which is favorable for thermoelectric properties. The maximum thermoelectric figure of merit ZT of the SHS + SPS-processed ZrNiSnSb alloy reached 0.7 at 870 K. This study opens a new avenue for the fast and low-cost fabrication of half-Heusler thermoelectric materials.

Pristine CrSe is a narrow-band gap semiconductor but with an inferior ZT value of 0.22 obtained at 623 K. In this paper, we improve the thermoelectric performance of the CrSe material by optimizing carrier concentration, suppressing the bipolar thermal conductivity, and reducing the lattice thermal conductivity simultaneously. First, the effect of different dopants (Nb, Ni, and Mn) on the phase composition and thermoelectric transport properties of MCrSe (M = Nb, Ni, and Mn; x = 0-0.02) compounds are systematically investigated. The roles of those dopants are distinct. Mn-doped samples show superior thermoelectric properties in comparison with those of other-element-doped samples. Doping with Mn significantly increases the carrier concentration, accompanied with a suppression of the intrinsic excitation and a reduction of both the bipolar thermal conductivity and the lattice thermal conductivity of CrSe. To further reduce the thermal conductivity, we have synthesized Mn and S codoped MnCrSeS ( x = 0-0.1) samples. Alloying with S significantly decreases the lattice thermal conductivity and enlarges the band gap, boosting the Seebeck coefficient. The maximum ZT value of MnCrSeS reaches 0.33 at 823 K. Compared with the pristine CrSe sample, the maximum ZT value is increased by 50% and the temperature corresponding to the peak value shifts toward higher temperatures by 200 K.

Despite the achievable high thermoelectric properties in SnSe single crystals, the poor mechanical properties and the relatively high cost of synthesis restrict the large scale commercial application of SnSe. Herein, we reported that co-doping with Na and Ag effectively improves the thermoelectric properties of polycrystalline SnSe. Temperature-dependent carrier mobility indicates that the grain boundary scattering is the dominant scattering mechanism near room temperature, giving rise to low electrical conductivity for the polycrystalline SnSe in comparison with that of the single crystal. Co-doping with Na and Ag improves the electrical conductivity of polycrystalline SnSe with a maximum value of 90.1 S cm À1 at 323 K in Na 0.005 Ag 0.015 Sn 0.98 Se, and the electrical conductivity of the (Na, Ag) co-doped samples is higher than that of the single doped samples over the whole temperature range (300-773 K). Considering the relatively high Seebeck coefficient of 335 mV K À1 at 673 K and the minimum thermal conductivity of 0.48 W m À1 K À1 at 773 K, Na 0.005 Ag 0.015 Sn 0.98 Se is observed to have the highest PF and ZT among the series of samples, with values of 0.50 mW cm À1 K À2 and 0.81 at 773 K, respectively. Its average PF and ZT are 0.43 mW cm À1 K À2 and 0.37, which is 92% and 68% higher than that of Na 0.02 Sn 0.98 Se, 40% and 43% higher than that of Ag 0.02 Sn 0.98 Se, and 304% and 277% higher than that of the previously reported SnSe, respectively.