Synthesis And Thermoelectric Properties Of Nano-Skutterudite Compounds By Chemical Methods

Skutterudite compounds with a general formula MX₃ (M=Co, Rh, Ir and X=P, As, Sb) became one of the most promising thermoelectric (TE) because of the high carrier mobility and high Seebeck coefficient. However, the figure-of-merit ZT to evaluate to the performance of TE materials and conversion of thermo-electricity of binary skutterudite are not famous owing to its relatively large thermal conductivity. Presently, the main approaches to improve ZT value are doping including filling and substitution and engineering the materials on a nanometer scale. The former have been investigated widely, and ZT value was improved largely, but which is not enough to satisfy commercial application. In this paper, we exported new chemical methods to synthesize nano-powder of CoSb₃ base compounds, and prepared the nanocrystalline bulk of CoSb₃ base compounds by SPS technology, and expect improving the electric transport properties, decreasing the lattice thermal conductivity and consequently enhancing the thermoelectric figure of merit ZT value.The binary CoSb₃ compound powder was synthesized by cross-coprecipitation. The starting reagents were CoCl₂, SbCl₃ and both precipitators of sodium hydroxide and ammonia. After dispersed, filtrated, washed and dried, a precursor composed of antimony oxide and cobalt hydrate was obtained at room temperature and between pH=5～10. The CoSb₃ powder was obtained after the precursor has been heat treated and the CoSb₃ bulk was sintered by SPS technology using the CoSb₃ powder obtained. The results indicate that the pH value, reduction atmosphere, temperature, remaining time and molar ratio of Co/Sb can affect the phase constituent and microstructure of the powder. The CoSb₃ powder with about 200 nm and uniform distribution size can be obtained in the proper conditions. The grain size of bulk CoSb₃ increases with raising sintering temperature and extending sintering time. The thermoelectric properties, especially thermal conductivity can be affected by grain size. The smaller of the grain size of CoSb₃ bulk, the lower of thermal conductivity, especially the lattice thermal conductivity.In the course of synthesizing skutterudite powder by copricipitation with precipitator of carbonate, precipitator, pH value, temperature, doping, procedure and the content of precipitator can produce effects on the phase constituent and microstructure. If the precipitator is excessive NaHCO₃ and without adjusting pH value, the average particle size of skutterudite powder is about 100 nm in diameter. The size of powder is 60～70 nm if the experimental procedure is changed, that is, the soluble salt in the precursor was removed by washing after thermal process. However, because of impurities in the skutterudite powder by this method, the thermoelectric properties of Fe_xCo_4xSb₁₂ and In_yCo₄Sb₁₂ compounds are not satisfying, and their maximal power factor are 0.9×10^-3 and 0.68×10^-3 WK^-2m^-1 (600 K) respectively.The synthesis of nanostructure CoSb₃ by ethanol Sol-Gel method was investigated. It can be concluded that the phase constituent of samples with same value of C₆H₈O₇/Co (Sb/Co) change from Co-rich phases to Sb-rich phases with Sb/Co (C₆H₈O₇/Co) increasing. When stoichiometric proportion is appropriate, the single-phase compound CoSb₃ can be obtained that composed of spherical about 50 nm in diameter and nano-rod about 30 nm in diameter and 100～200 nm in length. It is be proved that nanorods grow along the [110] direction. The rod-like CoSb₃ compound is ultimately produced due to the differences in growth rates between the crystal faces. The formation of a carbonyl coordination compound CO₂(CO)₈ between the Co atoms reduced by H₂ and the CO formed by citric acid decomposition causes the differences in the growth rate. In the local spaces without CO, the CoSb₃ nuclei become spherical.We developed a new method to synthesize nano-powder of skutterudite, namely, nonaqueous coprecipitation. The characteristics of this method are good repeatability, fine particle and without impurity. Ethanol and sodium hydroxide were solvent and precipitant respectively. The greysh-green precursor composed of NaCl, Co(OH)₂ and Sb₂O₃, and both of the latter are amorphous. After dried, grinded, heat treated washed and vacuum dried, the precursor became the nanometer powder of CoSb₃ base compounds. Temperature, the content of precipitator and doping can influence the particle size of CoSb₃ base compounds. The lower temperature is, the more precursor content is and the more species of element is, the smaller particle size is. The smallest average particle size of CoSb₃ base compounds obtained by nonaqueous coprecipition is 20～30 nm.