Research On Thermoelectric Properties Of Skutterudites

Thermoelectric(TE) materials are semiconducting functional materials,which can convert heat energy directly to electricity or reversely.TE power generators and cooling devices have been extensively applied in waste heat harvesting,space flight, microeleetronic cooling with the merit of pollution-free,noiseless,easy-maintenance and reliability.Skutterudite-based compounds considered to be promising novel TE materials in middle temperature range.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 preparation of multicomponent alloy such as filled skutterudites can optimize material compositions and improve thermoelectric properties.In this work,skutterudites are prepared by vacuum melting and annealing from pure Co,Sb,Ni,Yb,La,Te and other alloying or doping elements,and also by vacuum hot pressing from the alloy powders.The compositions and microstructures of the powders and the hot pressed bulk skutterudites were investigated by x-ray diffraction(XRD) and scanning electron microscopy(SEM).Transport properties of the hot pressed bulk samples were measured.The main results of the present work are listed as follows.1.The single filled(La,Yb,Ce) and double filled(La-Ca,La-Yb) skutterudites were prepared and their thermoelectric properties were studied.It was shown that the thermal conductivity is greatly reduced by forming single filled and double filled cobalt antimonide,but the thermoelectric properties for the La-Yb filled skutterudites, La_xYb_0.2Co₄Sb₁₂,were not so good.The highest ZT values for the La_0.5Co₄Sb₁₂, Yb_0.2Co₄Sb₁₂,Ce_0.2Co₄Sb₁₂,La_0.4Ca_0.1Co₄Sb₁₂ were 0.69(773K),1.24(773K),0.55 (723K),and 0.74(735K) respectively.It was found that the heavy RE(rare earth) element Yb was better than the light ones,La and Ce,in both electrical conductivity and thermal conductivity,and similiarly RE-AE was better than their separate element.2.Thermoelectric(TE) materials La_0.3Co_4-xNi_xSb₁₂(x=0,0.1,0.5,0.75,1) were prepared by melt-annealing method.The solid solubility limit of Ni in La-filled CoSb₃ is between 0.5 and 0.75 when the filling rate of La is 0.3.Transport properties measurements indicate that all these La_0.3Co_4-xNi_xSb₁₂ samples are n-type.The absolute Seebeck coefficient values decrease with the increasing of the value of x, however,the values of electrical conductivity and thermal conductivity increase with the addition of Ni.As a result,the La_0.3Co_3.9Ni_0.1Sb₁₂ Skutterudites exhibit a maximum power factor of 3.97×10^-3Wm^-1K^-2 at 520K and a maximum ZT value of 0.64 at 773K.3.Three samples La_0.5Co₄Sb_11.9Te_0.1:Sb₂Te₃:Bi₂Te₃=1:0.05:0.04(mall rate), La_0.2Yb_0.2Co₄Sb_11.8Te_0.2 and La_0.1Yb_0.2Ca_0.1Co_3.9Ni_0.1Sb_11.9Te_0.1 were designed and prepared in this work.It was found that a small quantity of Te and Bi could help to deduce the temperature at the highest ZT value in the composite La_0.5Co₄Sb₁₂.Due to Te subsititution for Sb,a ZT value～1 at 773K for La_0.2Yb_0.2Co₄Sb_11.8Te_0.2,which is 233%higher than that of La_0.2Yb_0.2Co₄Sb₁₂.The thermoelectric properties could be further improved by Te doping.Combination double RE filling with a small quantity of Te doping could effectively increase the scattering of phonons to greatly decrease the thermal conductivity.The highest ZT value of 0.6 is obtained for La_0.1Yb_0.2Ca_0.1 Co_3.9Ni_0.1Sb_11.9Te_0.1 at 708K.