Thermoelectric Properties And Nanostructuring Of Skutterudites

Thermoelectric materials are semiconducting functional materials,which can interconvert heat and electricity directly.Thermoelectricity is of great interest which offers a reliable,fully solid-state means of power generation and cooling.Among the most promising novel materials are skutterudite-based compounds.Filled skutterudites are suggested as the "phonon-glass/electron-crystal" materials with the good electrical transport properties of a crystal but the poor heat conduction characteristics of a glass.The thermoelectric performance can be significantly improved by nanostructuring the materials,since the nanostructure can lead to a larger reduction in the thermal conductivity than that of the electrical conductivity.In the present work,a novel solvothermal method has been developed to prepare the nanostructured skutterudites.The sintering techniques of hot pressing(HP)and spark plasma sintering(SPS)have been used to prepare the bulk materials.The microstructures and morphologies of the products were investigated.The thermoelectric properties,especially the thermal conductivities of the nanostructured skutterudite compounds,together with the doping,filling atoms and forming solid solutions were studied.The quaternary solid solution of Fe_xNi_yCo_1-x-ySb₃(x=y=0,0.125,0.25,033,0.375)and Yb filled skutterudites were prepared and their thermoelectric properties were studied.It is suggested that the solid solution can be formed between CoSb₃ and Fe_0.5Ni_0.5Sb₃.The results show that the thermal conductivity is greatly reduced by both forming solid solution and filled skutterudites.A ZT value of 0.85 is obtained for Yb_0.15Co₄Sb₁₂at 670 K.Nanostructured CoSb₃,Fe_0.5Ni_0.5Sb₃ and Fe_0.25Ni_0.25Co_0.5Sb₃ were synthesized via a solvothermal method using chlorides as the precursors and NaBH₄ as the reductant in an ethanol solution.It is found that pure skutterudite phase can be obtained at the reaction temperature of about 250℃for 48-72 h.The as-prepared powders were sintered into bulk materials by HP or SPS.Microstructure analysis shows that the bulk materials are composed of fine grains.Taking the CoSb₃ sintered by SPS as example,the average grain size is about 150 nm.Thermoelectric properties measurements indicate that low thermal conductivities are achieved for the bulk materials,hence in increasing the thermoelectric figure of merit(ZT). The ZT values of 0.51,0.61 are obtained for the CoSb₃ sintered by HP and SPS,respectively.The preparation of nanostructured Te-doped CoSb₃ and Yb-filled CoSb₃ were studied. Te-doped CoSb₃ nanopowders were successfully synthesized by solvothermal synthesis.The thermoelectric properties were performed on the hot-pressed samples.The figure of merit is further improved by Te doping.A ZT value of 0.66 is obtained for CoSb_2.8Te_0.2.It is found that a single skutterudite phase can be achieved by annealing the solvothermally synthesized powders at 600℃for 2 h under reductive atmosphere.Incorporating nanoscale constituents into bulk materials to form nanocomposites is a novel approach to obtain attractive thermoelectric properties.A simple bottom-up method was suggested to prepare the nanocomposites of the CoSb₃-based compounds.A bottom-up method is to mix the nanopowders into the matrix powders and sinter the mixture into bulk materials by HP or SPS.N-type CoSb₃ nanocomposites were prepared and their thermoelectric properties were studied.It is found that the Seebeck coefficient increases with increasing content of the nanoinclusions,and the nanocomposite structures are more effective in reducing thermal conductivity than electrical conductivity.The thermoelectric properties of the CoSb₃ nanocomposites are improved.A dimensionless figure of merit of 0.57 is obtained for both the nanocomposites with 5 wt%and 40 wt%nanopowders inclusions,which is about 26%increase of that without nanopowders inclusions.An n-type CoSb₃ nanocomposite consisting nano-and micro-sized grains was also prepared by in-situ solvothermal synthesis and hot pressing.A dimensionless figure of merit reaches 0.5 at 600 K.In the present study, nanocomposites of Yb filled cobalt antimonide,Yb_0.15Co₄Sb₁₂,with different content of CoSb₃ nano-inclusion were prepared by SPS.The nanocrystals in the bulk composites are less than 100 nm.The Seebeck coefficient can be enhanced by incorporating a small amount of CoSb₃ nanoparticles into the bulk materials.Though the nano-inclusion is the unfilled CoSb₃, it is still effective in reducing thermal conductivity.The highest figure of merit reaches 0.9 for the nanocomposites,about 10%increase compared with the Yb_0.15Co₄Sb₁₂matrix.The initial study of the stability of the nanocomposites was also performed in this work.The nanocrystal in the composites maintained nanostructure after annealing below the sintering temperature. The nanocomposites still exhibited attractive thermoelectric properties after annealing with a ZT value of 0.95.