Researches On Preparation And Thermoelectric Performance Of PbTe-based Bulk Nano-materials

With the ’Big Three’ energy source(oil, coal and natural gas) decreasing and the electric energy increasing, there are more and more attentions are drawn by the thermoelectric materials, which could convert thermal energy into electric energy directly. Besides, the serious environment problems on the gas pollution(such as CO2) emitted by the traditional energy source put the thermoelectric materials on the stage. There are two main methods to enhance the figure-of-merit(ZT) of the thermoelectric materials: doping and nanostructuring. The electrical conductivity of the thermoelectric materials can be increased by doping, while the Seebeck coefficient increased and the thermal conductivity deduced can be both achieved by nanostructuring.Lead telluride(Pb Te), with a narrow band gap(0.32 e V at 300 K) and high carrier mobility, is one of the best thermoelectric materials can be obtained at medium temperature(450-800 K) currently. Strontium doped Pb Te nanopowders were prepared via hydrothermal method, then the powder was densified into bulk sample by hot pressing method. The doping concentrations and hydrothermal reaction time of Pb Te samples on the thermoelectric properties are investigated systematically. The main results are as follows:1. Sr-doped Pb Te nanopowders were prepared via the hydrothermal method employing Pb(NO3)2 and Na2 Te O3 as the precursors. Na OH and Na BH4 is used as the p H regulator and reducing agent, respectively. Then as-prepared powders were compacted into pellets by hot-pressing method. The hydrothermal reaction time on the thermoelectric properties of Pb Te samples is investigated. The spectra of XRD shows that the peak of Te is observed when the reaction time is 8 h, while only the peak of Pb Te is observed when prolonging the reaction time to 10 h and 12 h. so it is proved that the reaction is not sufficient under 8 h. The electrical conductivity increases with the hydrothermal reaction time increased at room temperature, while the electrical conductivity decreases with the reaction time increased under the temperature of 450-700 K. The maximum of the Seebeck coefficient is ~ 429 μV/K at about 358 K with the reaction time of 12 h, which is increased by 22 % than that of the samples of 8 h. When the reaction time is 10 h, the thermal conductivity reaches the minimum value. At 373 K, the ZT value of the Pb Te samples with the reaction time of 12 h is 10 times than that of the samples of 8 h。2. The Sr-doped Pb Te bulk samples were prepared by hydrothermal and hot-pressing method as previous chapter, and Sr(NO3)2 was used as Sr source. The doping concentrations on the thermoelectric properties were investigated. The results manifest that the electrical conductivity increases with the doping concentration, and the electrical conductivity of the Sr-doped Pb Te samples with the doping concentration of 4 mol% is 4 times than that of undoped Pb Te samples. The optimal doping concentrations for the Seebeck coefficient is 2 mol%, the maximum of the Seebeck coefficient is 462 μV/K at about 450 K, which is increased by 42 % by that of undoped samples. The optimal doping concentrations for the power factor is 4 mol%, the maximum is ~ 6.5 μWcm-1K-2 at 500 K, which is 5 times than that of undoped samples. When the doping concentration is 4 mol%, the thermal conductivity is minimal. When the doping concentration is 4 mol%, the ZT value reaches its maximum value, about 0.41 at 673 K, which is more than 30 % higher than that of undoped samples.3. With the doping concentrations identical, the hydrothermal reaction time on the thermoelectric properties of Sr-doped Pb Te samples is investigated. For the electrical conductivity of the Sr-doped Pb Te samples with doping concentrations of 2 mol% and 4 mol%, both of them increases first and then decreased with the hydrothermal reaction time increased, but the reaction time for maximal electrical conductivity is 12 h and 10 h, respectively. The optimal reaction time is 10 h for the Seebeck coefficient of the samples of 2 mol% Sr-doping. For the samples of doping 4 mol% Sr, the values of the Seebeck coefficient changes from positive values to negative values for the samples with reaction time of 12 h and 24 h. For the samples with doping concentrations of 2 mol% and 4 mol%, the reaction time for optical power factor is 12 h and 10 h, while the reaction time for lowest thermal conductivity is 8 h and 10 h, respectively. For the samples with doping concentrations of 2 mol%, the maximum ZT value is obtained at about 473 K with the reaction time of 12 h, which is almost 4 times than that of samples with reaction time of 8 h. For the samples with doping concentrations of 4 mol%, the maximum ZT value is achieved at about 673 K with the reaction time of 10 h, which is 20 % higher than that of samples with reaction time of 8 h.