23 Low thermal conductivity and high figure of merit for rapidly synthesized n-type Pb1−xBixTe alloys
This paper, written by researchers from Shandong University and others, discusses Low thermal conductivity and high figure of merit for rapidly synthesized n-type Pb1−xBixTe alloys. The paper is published in an important journal < Dalton Transactions >. IF：4.099
In recent years, the research work of microwave chemical instrument used in the synthesis of materials has become a hot direction of scientific research, which has been paid great attention to by many scholars!
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.
N-type Pb1−xBixTe alloys with x = 0.00, 0.01, 0.02, 0.03 and 0.04 have been rapidly synthesized by the microwave hydrothermal method followed by hot pressing. Single cubic crystal structures, compact microstructures and homogenous element distribution have been formed in all samples. The solid solubility limit of Bi in PbTe has been confirmed to be between x = 0.02 and 0.03. Most grain sizes of Bi-doped samples are at sub-microscale and are smaller than those of pristine PbTe. With increase in Bi content, the carrier concentration is improved until the solubility limit, and the lowest electrical resistivity is found at x = 0.02 and 0.03. The values of Seebeck coefficients are negative with Bi doping, and the change in behavior with Bi content is consistent with carrier concentration. Following that, the maximum power factor value of about 8.5 μW cm−1 K−2 has been achieved at 623 K for x = 0.02. Since bipolar thermal conductivity is prohibited with the introduction of Bi, the lattice thermal conductivity is still low with increase in temperature. In addition, the scattering of alloys, grain boundaries, dislocations, and defects are introduced and strengthened. Thus, the lowest lattice thermal conductivity of 0.68 W m−1 K−1 is obtained at 673 K for Pb0.98Bi0.02Te alloy. As a result, the improved power factor at high temperature and lowest lattice thermal conductivity lead to the highest figure of merit of zT = 0.62 at 673 K for Pb0.98Bi0.02Te. This figure of merit is comparable to that of Bi-doped PbTe alloys synthesized by the conventional melting method. Therefore, the microwave hydrothermal method at optimized reaction conditions can rapidly result in n-type Pb1−xBixTe alloys with comparable thermoelectric performance.
In a typical synthesis of PbTe nanopowders, 2 g of NaOH, 1.9788 g PbN2O6, 1.3646 g Na2TeO3 and 0.6 g NaBH4 were successively added to 50 mL deionized water (proportionate BiCl3 was added for Bi-doped PbTe samples with decreasing quantity of PbN2O6). The solution was transferred to an 80 mL Teflon autoclave and was heated to 140 °C for 30 min at the heating rate of 10 °C min−1 in the microwave hydrothermal synthesizer (XH-800S, Beijing XiangHu Science and Technology Development Co., Ltd, China). The products were cooled to room temperature naturally and then centrifuged and washed several times with deionized water and ethanol. After that, these washed products were dried to powder in a vacuum oven. The obtained PbTe nanopowders were sintered by hot pressing at 773 K for 1 h under 20 MPa. Finally, Pb1−xBixTe samples with high relative densities >97% were synthesized.