High thermoelectric performance of Na-doped β-Zn4Sb3 prepared by NaCl-flux method

Abstract

In this study, β-Zn4Sb3 was prepared by using the NaCl-flux method according to the stoichiometric ratio of Zn4+xSb3(NaCl)7 (x = 0.1, 0.2, 0.3, 0.4). An effective carrier concentration regulation is realized by changing the content of Zn and the doping of Na. X-ray powder diffraction results indicate that the crystal samples prepared via the NaCl-flux method were all hexagonal rhombic crystal structures, and the space group was R3¯c. Electron probe microanalysis results showed that Na atoms were introduced into the Zn4Sb3 structure successfully. The transmission electron microscope and the corresponding selected area electron diffraction confirmed that the prepared samples had good crystal quality. X-ray photoelectron spectroscopy results show that the valence states of Zn, Sb, and Na were +2, −2 and −3, and +1, respectively. All samples exhibit P-type conduction behavior, and the carrier concentration of the material increases because of the increment in the Zn content of the raw material. The carrier concentration ranges from 1.86 × 1020cm−3 to 2.45 × 1020cm−3. Moreover, the metal Zn phase increases the carrier concentration. Thus, that the sample has higher electrical conductivity, and the power factor of the material is optimized. The thermal conductivity of the sample increases because of the appearance of metal Zn phase. The sample with the final nominal composition of β-Zn4.1Sb3(NaCl)7 exhibited the best thermoelectric performance. The maximum ZT value of 1.4 is obtained at 673 K because of its high power factor and low thermal conductivity. This value is higher than those of β-Zn4Sb3 prepared by ball milling (∼1.21 at 673 K) and hot pressing (∼1.09 at 673 K).