Preparation Of High Performance BiCuSeO-based Oxychalcogenide And Its Thermoelectric Properties

Thermoelectric (TE) materials have attracted much attention due to their potential applications in a variety of practical waste heat recovery systems, air conditioning and refrigeration. Thermoelectric oxides bear great potentials for mediate-to high-temperature thermoelectrical applications, owing to their chemical stability at high temperature, heat resistant, free of pollution, nonpoisonous, and ease manufacture. BiCuSeO is believed to be a promising oxide-based TE material in the moderate temperature range. In this work we employ the solid-state reaction (SS) combined with SPS and mechanical alloying (MA) combined with SPS to prepare the samples. On the basis of Cu site doping, Bi site doping, composite structure, we systematically investigated the influences on thermoelectric performance of BiCuSeO and enhancing mechanisms of properties.The pure BiCuSeO was synthesised by solid-state reaction and the nano-grain powder can be get after ball milling. The sample sinstered at 973 K shows the best performance, its highest ZT value can reach 0.62 at 873 K. BiCuSeO ceramics also can be synthesised by MA combined with SPS. The best performance of BiCuSeO synthesised by MA & SPS method can reach 0.35 at 873 K.ceramics were prepared SS & SPS method. For system, the band gap decreases with the concentration of Ag increasing, which prompts the electrical conductivity increasing. The electrical conductivity increases from 5.89 Scm-1 to 37.36 Scm-1 at room temperature and from 24.30 Scm-1 to 47.74 Scm-1 at 923 K.Bi1-xAgxCuSeO ceramics were synthesised by SS & SPS method. The grain size of the doped samples is about 200 nm, which is much smaller than that of the pristine one (about 500 nm), it indicates that Ag dopant can effectively suppress the grain growth. In addition, much more nanodots of 5-10 nm appear with Ag addition. The thermal conductivity can be suppressed significantly due to grain refinement and nanoinclusions. Due to decreased thermal conductivity and increased electrical property, the ZT value increases 30% at 873 K.The samples doped with Ni2+ or Pb2+ were synthesised by MA & SPS method. The grains show plate shape with size of 3～5 μm and thickness of ～00 nm. The electrical property can be improved by both of Ni2+ and Pb2+ doping, especially, it shows better performance with Pb2+ doping. The power factor of the sample doped with 10% Pb is above 600μW/mK2 during the test temperature, which is 5-6 times than the un-doped sample. The sample doped with 8% Pb shows the best thermoelectric performance, which one shows the highest ZT of 0.95 at 873 K, which is about 3 times than that of the un-doped specimen.Bi1-xLaxCuSeO and Bi1-x-yPbxLayCuSeO ceramics were prepared SS & SPS method. The locations of the heavy and light bands can be adjusted with La3+ doping, which leads to light band much more important to the carriers transport. With the La fractions increasing, the mobility of carriers increases from 4.3 cm2/V/s to 36.3cm2/V/s, which was improved about 8 times. Pb2+/La3+ co-doping can improve the electrical conductivity significantly. Thus, the power factor is enhanced very much, the value of 7.58 μWcm-1K-2 is about 2 times higher than that of the pure BiCuSeO. The ZT value can reach 1.12, which is about 2 times of the pure specimen.The BiCuSeO/Cu2Se and BiCuSeO/La0.8Sr0.2CoO3 composite ceramics were sintered by SPS. When the fraction of Cu2Se reaches the percolation threshold, the mobility of carriers increases quickly. Thus, the electrical conductivity at 823 K enhanced from 23.8 S/cm to 65.4 S/cm, increasing about 2 times. The electrical properties and thermal properties can be optimized at the same time with embedded La0.8Sr0.2CoO3 nanoparticles. La0.8Sr0.2CoO3 was introduced into BiCuSeO, increasing the electrical conductivity from 3.3 Scm-1 to 52.3 Scm-1 at 303 K, from 35.8 Scm-1 to 97.3 Scm-1 at 873 K; respectively. The thermal conductivity increases while La0.8Sr0.2CoO3 insertion, but as the fraction of La0.8Sr0.2CoO3 increases, the thermal decreases due to the enhanced phonon scattering. In short, the enhanced thermoelectric figure of merit of 0.67 at 873 K for the sample with 20 vol % La0.8Sr0.2CoO3 is 26% higher than that of the pure sample.