Enhanced Thermoelectric Properties of Codoped Cr₂Se₃: The Distinct Roles of Transition Metals and S

Pristine Cr₂Se₃ is a narrow-band gap semiconductor but with an inferior <i>ZT</i> value of 0.22 obtained at 623 K. In this paper, we improve the thermoelectric performance of the Cr₂Se₃ 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 M_2<i>x</i>Cr_{2–2<i>x</i>}Se₃ (M = Nb, Ni, and Mn; <i>x</i> = 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 Cr₂Se₃. To further reduce the thermal conductivity, we have synthesized Mn and S codoped Mn_0.04Cr_1.96Se_{3–3<i>x</i>}S_3<i>x</i> (<i>x</i> = 0–0.1) samples. Alloying with S significantly decreases the lattice thermal conductivity and enlarges the band gap, boosting the Seebeck coefficient. The maximum <i>ZT</i> value of Mn_0.04Cr_1.96Se_2.7S_0.3 reaches 0.33 at 823 K. Compared with the pristine Cr₂Se₃ sample, the maximum <i>ZT</i> value is increased by 50% and the temperature corresponding to the peak value shifts toward higher temperatures by 200 K

Manipulating the Combustion Wave during Self-Propagating Synthesis for High Thermoelectric Performance of Layered Oxychalcogenide Bi_1–<i>x</i>Pb_<i>x</i>CuSeO

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 Bi_1–<i>x</i>Pb_<i>x</i>CuSeO (<i>x</i> = 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 <i>ZT</i> value ∼1.2 has been attained at 923 K for Bi_0.94Pb_0.06CuSeO. 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