Jinze Zhai

Enhancement of the thermoelectric performance of bulk SnTe alloys via the synergistic effect of band structure modification and chemical bond softening

SnTe alloys, which have the same crystal structure as PbTe, have attracted increasing attention. Here, we demonstrate that the synergistic effect of band structure modification and chemical bond softening can be realized simultaneously in In & Mn doped SnTe bulk alloys. The Seebeck coefficient and power factor are synergistically improved by co-doping of In and Mn. In doping is known to introduce a resonance level. Mn doping reduces the separation of light- and heavy-valence bands. The combination of these effects significantly enhances the Seebeck coefficient at room temperature owing to around a factor of five times increase in the band effective mass. The reduction of thermal conductivity is from the decrease of both the electronic and phononic parts. The electronic thermal conductivity is decreased by the increase in defect scattering, as can be confirmed by the carrier mobility. The force constant of the bonds around the Te site is decreased due to the co-doping of In & Mn, which indicates that the chemical bonds are softened, which leads to low sound velocity and lower lattice thermal conductivity. As a result, the peak thermoelectric figure of merit, zT = 1.03 has been achieved for Sn0.89In0.01Mn0.1Te at 923 K. This strategy of using the synergistic effect of band structure modification and chemical bond softening could be applicable to other thermoelectric materials.

Right Heterogeneous Microstructure for Achieving Excellent Thermoelectric Performance in Ca0.9R0.1MnO3−δ (R = Dy, Yb) Ceramics

Perovskite manganite Ca0.9R0.1MnO3-δ (R = Dy, Yb) ceramics have been synthesized by a traditional solid-state reaction with multicalcination processes. A heterogeneous microstructure including large and small micrometer-sized grains, coherent interfaces, and oxygen defects has been formed with optimized calcination time. The carrier concentration of the third-calcined samples is enhanced approximately 3 times compared with those synthesized through conventional methods. Thus, the electrical resistivity of the third-calcined Ca0.9R0.1MnO3-δ (R = Dy, Yb) ceramic samples obviously decreases, leading to a higher power factor. Additionally, the thermal conductivity is also reduced by multiscale scattering of the heterogeneous structure. The lowest lattice thermal conductivities of Dy- or Yb-doped samples are 1.24 and 1.22 W m-1 K-1, respectively. Thus, the high thermoelectric performance for Ca0.9R0.1MnO3-δ (R = Dy, Yb) has been achieved by the multicalcination process. The highest figure of merit is almost 30% higher than that of the first-calcined samples. Therefore, a heterogeneous microstructure formed by optimized multicalcination can effectively optimize the thermoelectric performance of oxides.

Low thermal conductivity and high figure of merit for rapidly synthesized n-type Pb1-xBixTe alloys

High figures of merit of n-type Pb1-xBixTe alloys have been achieved by rapid synthesis at low temperature. The effects of Bi dopant and microwave hydrothermal technology on microstructure and thermoelectric performance have been studied. The solid solubility limit of Bi in PbTe is between x = 0.02 and 0.03. Homogenous nanopowders of about 70 nm have been synthesized by the microwave hydrothermal method. When followed by hot pressing, sub-microscale grain sizes are also formed for Pb1-xBixTe alloys. With increase in Bi, the carrier concentration is improved within the solubility limit. This leads to low electrical resistivity and higher power factor at high temperature. A higher power factor of 8.5 μW cm-1 K-2 is obtained for x = 0.02 sample at 623 K. In addition, the introduction of Bi effectively prohibits the p-n transition and bipolar thermal conductivity of pristine PbTe. Thus, a low lattice thermal conductivity of 0.68 W m-1 K-1 is achieved at 673 K, combining scattering of alloys, grain boundaries, dislocations and defects. As a result, the highest peak figure of merit, i.e., zT = 0.62 at 673 K is achieved for Pb0.98Bi0.02Te sample, which is comparable with that of Bi-doped PbTe alloys synthesized by the conventional melting method. Thus, the right synthesis conditions of the microwave hydrothermal method can rapidly result in thermoelectric materials with comparable figures of merit.