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The diffusion of Ni and Sn is undesirable in thermoelectric (TE) device. However, the quantitative effect on TE properties is unknown. In this work, the effect of Ni/Sn doping on the TE properties of p-type Bi2Te3 based alloys and the corresponding mechanism are investigated to provide beneficial reference for device designers and researchers. The results show that nickel doping in Bi0.4Sb1.6Te3 can make the intrinsic excitation temperature shift to lower temperature region due to the formation of the second phase NiTe, which greatly reduces the carrier concentration and increases the electrical resistivity. Ultimately, the thermoelectric figure of merit (ZT value) deteriorates rapidly with the increase of Ni content, and only 7 wt ‰ Ni results in a 32% drop in ZT value. Tin doping moves the intrinsic excitation temperature to higher temperature region due to the promotion of Sn’Bi to the generation of anti-sites (Bi’Te and Sb’Te), which significantly increases the carrier concentration and deteriorates the Seebeck coefficient. At last, the ZT value also declines sharply with the increase of Sn content, and just 7 wt ‰ Sn doping results in a 52% drop in ZT value.

Thermoelectric properties of BiCuSeO were improved combing Y-doping and grain refinement. The samples were prepared by mechanical alloying and resistance pressing sintering. The results indicate Y can reduce the bandgap, increase the carrier concentration and electrical conductivity, increase the lattice vibration amplitude and thermal conductivity. Grain refinement by increasing ball milling strength can increase the concentration and power factor by enhancing Cu vacancies, and enhance the scattering of low- and intermediate-frequency phonons, so as to reduce the thermal conductivity. Finally, the dimensionless figure of merit(ZT) value is increased to 0.93, which is 98% higher than that of the pure sample.

Although the mechanical strength of n-type polycrystalline Bi2Te3 based alloys prepared by conventional powder metallurgy technology is good, its thermoelectric properties have poor repeatability and contamination is introduced. In this work, n-type polycrystalline Bi2Te3 based alloys with uniform microstructure were fabricated by applying equal channel angular extrusion (ECAE) method to extrude the cast ingot directly for four passes at 783 K. The initial lamellae coarse grains with hundreds of microns were broken down efficiently and refined uniformly to only 10–20 μm. And the Vickers hardness was increased by 2.3 times due to the refined grains. On the other hand, it is noteworthy that the ECAE process is also an ideal method to improve the thermoelectric properties of cast ingot by boosting electrical transport properties due to the donor like effect induced by heavy deformation and reducing thermal transport properties due to the enhanced phonon scattering. In order to further optimize the thermoelectric performance, BiCl3 doping was used to precisely tune the carrier concentration to the best range. The intrinsic excitation temperature of Bi2Te3 based alloys gradually shifted from low temperature (below 303 K) to high temperature (463 K) owing to the increased carrier concentration. Finally, the extruded Bi2Te3 based alloy doped with 0.064 wt % BiCl3 obtained the maximum dimensionless figure of merit (ZT) of 0.68 at 423 K, which was far higher than the nearly zero of the initial cast ingot.

Thermoelectric materials have great application prospects in the fields of micro-refrigeration, waste heat utilization and catalysis, and BiCuSeO is one of the oxide thermoelectric materials with the best comprehensive thermoelectric properties at present. Herein, the dispersion of 20-nm SiC in BiCuSeO matrix has greatly increases the thermoelectric and mechanical properties of the system. SiC nanoparticles dispersed in BiCuSeO matrix can act as energy filters to increase the Seebeck coefficient, on the other hand, as phonon scattering centers to scatter the low- and mid-frequency phonons effectively to reduce the lattice thermal conductivity, combined with point defects such as Pb Bi + scattering the high-frequency phonons, thus achieving the scattering enhancement of full-frequency phonons. In addition, the mechanical strength and machinability of BiCuSeO have been improved significantly due to the dispersion strengthening effect of SiC nanoparticles.

Introduction of porous structure is a feasible and promising method for improving thermoelectric performances for thermoelectric materials. Meanwhile, the uniformity of pores distribution plays a decisive role in the objective evaluation of thermoelectric properties for porous structures. In this work, extremely uniform porous structures in p-type Bi0.4Sb1.6Te3 bulks were obtained by pre-mixing with NH4HCO3 combined with resistance pressing sintering (RPS) process. The thermoelectric performance was obviously affected by the porous structure. The increasing porosity resulted in a significant decrease in electrical and thermal conductivity. It was inspiring that the reduction of thermal conductivity could compensate for the deterioration of electrical conductivity, leading to the enhancement in ZT value. The maximum ZT value of 1.11 was obtained for the sample prepared by pre-mixing with 5 ​wt % NH4HCO3 at 343 ​K, which was 20% higher than that of the fully dense sample. Finally, the advantages on the homogeneity of pores, thermoelectric properties and mechanical properties of the sample prepared by this method are confirmed through comparing with the sample prepared without NH4HCO3, when their relative densities both are 90%.