Microstructure And Thermoelectric Properties Of ZnSb Based Materials

Thermoelectric material, which could directly and reversibly convert heat energy into electrical power, has been applied in the field of the deep exploration, industry waste heat and automobile, etc. Zn Sb is a kind of material with abundant resource and low cost, which exhibits low thermal conductivity and high Seebeck coefficient, displaying the potential of high thermoelectric properties. However, low electrical conductivity and relative high lattice thermal conductivity leads to the low ZT value, which restricts its practical application. In this thesis, Na or Ag acceptor doping and Cd isoelectrically substitution was adopted to increase the electrical conductivity and reduce the lattice thermal conductivity, respectively, consequently leading to improve the ZT value. Zn Sb based thermoelectric materials were prepared combining the vacuum melting, ball milling and hot press. The effect of acceptor doping and isoelectric substitution on the microstructure and thermoelectric properties was investigated using XRD, SEM, ZEM-3, Laser flasher. The related mechanism was also clarified.The experiment shows that Zn Sb based thermoelectric materials prepared by the vacuum melting, ball milling and hot press is pure phase with the crystal size of ~ 1mm. Na doping increases obviously the carrier concentration and the electrical conductivity, thereby the power factor(PF) is remarkably improved and the maximum PF is 16.1μ W/cm K2 at 400℃. The electronic thermal conductivity and total thermal conductivity are increased due to the increased the electrical conductivity. According to the equation ZT=(S2σ/κ)T, the maximum ZT value of 0.994 at 350℃ is obtained when the Na doping is 0.5 at.%, which is much higher than that of the undoped Zn Sb(0.42). Based on the result of Na doped Zn Sb, Cd is used to replace Zn to further reduce the lattice thermal conductivity. It is found that Cd substitution not only obviously decreases the lattice thermal conductivity, but also slightly improves the power factor, consequently leading to the ZT value is much enhanced. The maximum ZT value of 1.22 at 300℃ is obtained when the Cd content is 8 at.%.Ag doping displays the same regularity with Na doping regarding to the influence on thermoelectric transport behavior. Ag doping remarkably increases the electrical conductivity and thermal conductivity and decreases Seebeck coefficient, consequently leading to the improved power factor, the maximum power factor is 22.6μW/cm K2 at 150℃.As a result, the ZT value is increased from 0.63 for undoped Zn Sb to 0.95 for Ag doped sample at 400℃. At the basis of the Ag doped Zn Sb, the Cd is chosen to replace Zn to reduce the lattice thermal conductivity. With the increase of Cd content, the lattice and total thermal conductivity are reduced, while the power factor is slightly reduced. Finally, the ZT value is further increased from 0.95 to 1.1 for the sample of Ag0.01Cd0.09Zn0.9Sb.