Multi-scale Microstructures And Thermoelectric Performance Optimization Of（Bi,Sb）₂（Te,Se）₃ Alloys

(Bi,Sb)₂（Te,Se）₃ alloys are the the best and so far are the most widely used commercial thermoelectric （TE） materials near room temperature. The traditional unidirectional grown （Bi,Sb）₂（Te,Se）₃ alloys show maximum zT～1.0 near 300 K, and are mainly applied for semiconductor refrigeration. Further zT values optimization and effective adjustment of peak zT temperature, making it suitable for low/mid-temperature power generations, are the main objective of （Bi,Sb）₂（Te,Se）₃ alloys in the past few years. This work adopts hot deformation technique, and optimizes chemical compositions based on intrinsic point defects. We study the formation and change rule of intrinsic point defects, nanostructures, oriented textures, and their effects on the TE properties of （Bi,Sb）₂（Te,Se）₃ materials. The main results are listed as below:1. We develope hot deformation-induced multi-scale microstructures technique. The micro scale textures, nanostructures, and atomic scale line and point defects are introduced into the hot pressed （Bi,Sb）₂（Te,Se）₃ alloys, and improve the TE transport properties. The study indicates that the carrier mobility is improved by micro scale texture enhancement and grain growth; the carrier concentration is increased by atomic scale donor-like effect caused by non-basal slip; the lattice thermal conductivity is reduced by dynamic recrystallization induced local nanostructures and high-density lattice line defects. The peak zT value of repetitive hot deformed Bi₂Te₂Se₁ alloys reaches 1.0 at 513 K.2. Direct hot deforming zone melted ingots could introduce multi-scale microstructures into materials and obtain high-performance n-type Bi₂Te₃-xSe_x alloys that matching well with p-type counterparts. The study indicates that the weak donor-like effect induced by direct hot deformation offsets the partial texture loss, which maintains the high power factor of zone melted ingots. Second, the lattice thermal conductivity is substantially reduced by broad wavelength phonon scattering through tuning the multi-scale microstructures, which includes grain size reduction and texture loss at micro scale, nanoscale distorted regions, and atomic scale lattice distortions and point defects. The integration of these effects in the n-type Bi₂Te₂.79Se₀.21 alloy results in high zT～1.2 at 357 K, which is the best n-type Bi₂Te₃ based materials near room temperature.3. The formation, factor of intrinsic point defects and their effect on TE properties were studied in detail. We find the point defects play the important impacts on the TE transport of of （Bi,Sb）₂（Te,Se）₃ alloys, and propose the atomic scale intrinsic point defect engineering as a new strategy to simultaneously optimize the electrical and thermal properties. The formation energy of point defects and the donor-like effect are respectively engineered by changing alloy composition and hot deformation. A peak zT-1.2 at 445 K is obtained for of hot deformed n-type Bi₂Te₂.3Se₀.7 alloys; a peak zT～1.3 at 380 K is obtained for of hot deformed p-type Bi₀.3Sb₁.7Te₃ alloys.4. Some antisite defects were introduced into lattice by making Bi₂Te₂.3Se₀.7 slightly Se-poor via controlling the point defect growth enviorenment, which not only maintain the high zT values, but also simply the preparation program. These antisite defects not only reduce the lattice thermal conductivity by scattering the phonons, but also increase the carrier concentration by promoting the donor-like effect during the hot deformation process. As a result, the peak zT value of once hot deformed Bi₂Te₂.3Se₀.69 alloys reaches 1.2 at 450 K, which is identical to three-time hot deformed Bi₂Te₂.3Se₀.7alloys.5. The adverse effect of intrinsic conduction on the TE properties was suppressed via raising the materials’band gap and majority carrier concentration, and realizes the effectivity adjustment of peak zT temperature. High-performance p-type Sb₂Te₃ alloy for mid-temperature power generations is fabricated by In-Ag co-alloying. In alloying can simultaneously improve the Seebeck coefficient by raising the formation energy of antisite defects, reduce the lattice thermal conductivity via the lattice distortions, and suppress the ambipolar diffusion by band gap widening. Ag accepotor doping optimize the hole concentration and futher shifte the peak zT to high temperature. Similarly, In alloying is also effective for n-type Bi₂Se₃ alloys.