Fabrication And Thermoelectric Properties Of Electronegativity Guests Filled CoSb₃-based Skutterudites

Thermoelectric(TE)technology can convert thermal energy into electricity and vice versa directly based on Seebeck and Peltier effect.It provides a clean and reliable technology in thermoelectric cooling and thermoelectric generation.CoSb3-based filled-skutterudites possess a great potential in TE power generation application in intermediate temperatures since they have excellent TE properties and robust mechanical properties.Skutterudite is considered as an ideal'Phonon-Glass-Electron-Crystal(PGEC)' TE material,and one can introduce appropriate guest atoms into the relatively large icosahedral voids at the 2a sites of the unit cell in skutterudites to form so called 'filled-skutterudites'.In the past two decades,many studies have focused on the effects of electropositive guest fillers on the electrical and thermal transport properties in CoSb3-based skutterudites.Recently some electronegative elements such as S,Se,Cl and Br have been filled into the voids in CoSb3 with a small amount of n-type dopant Te on the Sb sites.Thus to conduct researches on these electronegative guests filled skutterudites may obtain excellent thermoelectric materials that possess high zT values,while it may explore the new applications of skutterudites.The thesis is concentrated on thermoelectric properties of electronegative guests filled skutterudites,including electronegative guests S and Se filled skutterudites.A systematic research on the indepth mechanism of the electronegative guests filled skutterudites has been explored by designing different filling fractions and doping contents,followed by analysing of phase compositions,microstrucutre as well as the thermoelectric transport properties.The method to improve the thermoelectric properties of the electronegative guests filled skutterudites has also been discussed.The main achievements are described as follow:1.The filling fraction limits of electronegative guest S filled skutterudites with Ni doping on Co sites as well as Te on Sb sites have been studied.The filling fraction limits for electronegative guest S filled skutterudites under different doping contents of Ni or Te have been studied by combining the analysis of XRD,SEM and thermoelectric properties measurements.S could be introduced into the void sites with the self-charge compensation of Ni and Te,while via the effect of self-charge compensation of S,the doping limit of Ni at the Co sites have also increased.Under the same doping content of compensation element,Te compensation can obtain higher S filling fraction than Ni compensation system;while under the same carrier concentration,carrier mobility of Ni compensation system is lower than Te compensation system.2.Designed and synthesised S filling and Pd doped skutterudites.Carrier concentrations will decrease when increasing the filling fraction of S,while the Hall mobilities are nearly unchanged.By increasing the doping contents of Pd on Co sites,Hall mobilities will be deteriorated although the carrier concentrations will increase at the same time.Both by increasing the filling fractions and the doping contents of Pd will decrease the lattice thermal conductivity,while for S filling will have a more remarkable effect on lattice thermal conductivity reduction.By introducing appropriate filling fractions of S and doping contents of Pd can improve the thermoelectric properties of these S filling and Pd doped skutterudites.Finally,a maximum zT value was obtained at 700 K for sample with nominal composition S0.05Pd0.25Co3.75Sb12.3.Designed and synthesised self-charge compensated skutterudites SeyCo4Sb12-xSex.These Se-containing samples tend to form a fully charge-compensated skutterudite compounds with the doping content of Se at the Sb sites being twice the filling fraction of Se at the void sites.This can be proved by designing different ratio of Se at the void sites and the Sb sites as well as the analysis of phase compositions,microstructure and the thermoelectric transport properties.All samples have low carrier concentrations as well as relatively large Seebeck coefficients,leading to similar semiconducting electrical transport properties.All samples possess low lattice thermal conductivity.The thermal transport properties of these compounds are comparable with Ba-filled skutterudites and a lowest KL of 2.1 Wm-1K-1 is obtained for Se0.2Co4Sb11.6Se0.4 at room temperature.4.Double filled skutterudites,including at least one electronegative guest,have been synthesis.Double filled skutterudites with one electropositive guest and one electronegative guest,such as Ba and S,Ba and Se,In and S have been synthesised and the thermoelectric properties have been studied.Due to the electronegativity difference between electropositive and electronegative guest,secondary phases with strong ionic bonds were found in these compounds.For electronegative guest S and Se double filled skutterudites,they have a similar thermoelectric properties with the S or Se single filled skutterudites.Although the synthesis of the double filled skutterudites,including at least one electronegative guest,have been synthesis unsuccessful,it has broaden the knowledge of electronegative guest filled skutterudites.