Synthesis And Thermoelectric Properties Of N-type In And RE/AE Double-filled Skutterudite Compounds

As a representative of PGEC thermoelectric (TE) materials, the filled-skutterudite is one of the most promising thermoelectric materials. Recently the indium-doped CoSb3 reportedly showed high thermoelectric performance, but there is an argument on whether the indium can be filled into the CoSb3. A high ZT value of 1.43 was achieved in the InxCeyCo4Sb12 skutterudites for its nanocomposite structure with the in-situ nano-InSb particles in CoSb3 crystal boundaries. However, it was unknown whether the in-situ nano-InSb particles could form in all the skutterudites which were double-filled by indium and rare-earth (RE) element or indium and alkaline-earth (AE) element, or whether the thermoelectric performance could rise by the nano-InSb structure in the In and RE/AE double-filled skutterudites. The study on these urgent problems has not been reported yet.In this thesis, a series of In-doped n-type CoSb3 samples were produced by the method of melting-quenching-annealing and spark plasma sintering technique. Their structure and thermoelectric performance were investigated systematically, such as XRD, ICP, DSC, EDX, TEM, FESEM, Hall, electrical conductivity, thermal conductivity and so on. The existence forms of indium in skutterudites were investigated. And we also studied the influence of the five rare-earth elements (RE=La, Yb, Ce, Pr and Eu) on thermoelectric properties of RE single-filled skutterudites RE0.1Co4Sb12 and In-RE double-filled skutterudites In0.1RE0.1Co4Sb12. We found that the in-situ nano-InSb composite structure could be a general form in the In-RE double-filled skutterudites and explored its influence on TE properties. We investigated the nano-InSb generated conditions, and enhanced the TE performance by optimizing doping amount. The TE properties of In-AE double-filled skutterudites were also studied, and the in-situ nano-InSb composite structure was also found in the In-AE double-filled skutterudites by controlling the chemical components while the TE performance was improved.We investigated the existence forms of indium in skutterudites. The indium could be filled into the icosahedral structure of skutterudites. When the doping amount of In was below its filling fraction limit, all the indium could be filled into the icosahedral structure with the valence state+1. When the doping amount of In was above its filling fraction limit, the excess part of indium formed the in-situ nano-InSb particles which were dispersed in the skutterudites crystal boundaries as the second phase. The filling fraction limit of In of the In and RE/AE double-filled skutterudites, was decided by In and RE/AE filling fraction limit in single-filled skutterdutes, doping amount of RE/AE and relative filling competitive ability of In and RE.When all the doping In was filled into the icosahedral structure, we studied the influence of five rare-earth elements on thermoelectric properties of RE single-filled skutterudites RE0.1Co4Sb12 (RE=La, Yb, Ce, Pr and Eu) and In-RE double-filled skutterudites In0.1RE0.1Co4Sb12. For the RE0.1Co4Sb12 and In0.1RE0.1Co4Sb12 samples, the electrical and thermal transport properties were obviously influenced by valence state of the filling-ion and filling fraction. The Seebeck coefficient of all the skutterudites filled by Yb, Pr or Ce was incteased remarkably, which might be caused by highly enhanced effective mass of electrons and high density of states at Fermi energy. With the high electrical property and decreased thermal conductivity, the Pr-filled skutterudites of RE0.1Co4Sb12 samples got a good ZT value of 0.9 at 750 K. Among the In0.1RE0.1Co4Sb12 samples, the In0.1Yb0.1Co4Sb12 sample obtained the highest power factor and the lowest thermal conductivity, so it got the highest ZT value of 1.22 at 800 K.When the doping amount of In was above its filling fraction limit, we studied the structure and properties of In0.3RE0.1Co4Sb12 samples, and the in-situ nano-InSb composite structure was found a general form in In0.3RE0.1Co4Sb12 samples, and its influence on TE properties was also explored. With excess indium-doping amount, the in-situ nano-InSb composite structure was a general form in the In0.3RE0.1Co4Sb12 samples, and these nano InSb particles with the average diameter below 100 nm were dispersed in skutterudites crystal boundaries. The formation mechanism of the in-situ nano-InSb structure was as follows:the InSb was molten and coated on the grain boundaries of skutterudites evenly during the process of SPS, and the nano-InSb structure formed in the rapid cooling process in which the growth of InSb crystal was restrained. The in-situ nano-InSb structure made these In0.3RE0.1Co4Sb12 samples possessed of low thermal conductivity with high power factor, and the In0.3Pr0.1Co4Sb12 and In0.3Eu0.1Co4Sb12 reached high ZT values of 1.26 and 1.21 at 800 K respectively.Based on the above results, the generated conditions of the in-situ nano-InSb structure in In-RE double-filled skutterudites were investigated by optimizing doping amount while the TE performance was improved. For In and Eu double-filled skutterudites, Eu had an ultrahigh filling fraction limit and a stronger filling ability than In, and there was only InSb phase except the skutterudites phase. With proper filling fraction and nano-InSb amount by optimizing of chemical component, the In0.20Eu0.25Co4Sb12 sample got a state-of-the-art ZT value of 1.31 at 800 K. We found that the praseodymium could also be filled into the skutterudites, and its filling fraction limit was as high as 0.21, and a high ZT value of 1.03 was attained for the Pr0.2Co4Sb12 sample at 800 K. Due to the weaker filling ability of Pr than In, there were impurities of Co3Pr, PrSb and CoSb2 in the In0.20PrxCo4Sb12 samples when x≥0.20. The skutterudites In0.20Pr0.10Co4Sb12 got a good ZT value of 1.26 at 800 K. Above all, the in-situ nano-InSb structure in In-RE double-filled skutterudites was decided by the following factors:praparation process, In and RE doping amount, In and RE filling fraction limit and the relative filling competitive ability of In and RE.The TE properties of In-AE double-filled skutterudites were studied, and the in-situ nano-InSb structure was also found in the In0.2CaxCo4Sb12 samples of which the TE performance was improved. With the same filling fraction, the In and AE (AE=Ca, Sr and Ba) double-filled skutterudites have lower thermal conductivity than In-RE filled skutterudites, and all the ZT values of In0.2AE0.2Co4Sb12(AE=Ca, Sr and Ba) samples reached above 1.2 at 800 K. The in-situ nano-InSb composite structure was also found in the In0.2CaxCo4Sb12 samples, and the amount of nano-InSb increased with x increasing, and all these samples had high power factors and low lattice thermal conductivity simultaneously. The In0.2Ca0.25Co4Sb12 sample got a very high power factor 4.9 mWm-1K-2 and a maximum ZT value of 1.34 at 800 K.