The Electronic Structure,Material Fabrication And Thermal Electrical Properties Of CoSb₃Compound Materials

Thermoelectric materials can realize the conversion between heat energy and electric energyunder different conditions. As a kind of energy saving materials, they nowdays have immenseapplication in our society under the severe energy crisis. CoSb₃compound has been identified as acandidate for good thermoelectric materials because it has a large Seebeck coefficient and goodelectrical conductivity. The thermal conductivity of CoSb₃, however, is still high for making it anefficient thermoelectric part. For improving thermoelectric properties of CoSb₃compound, it is crucialto lower the thermal conductivity and further to improve its Seebeck coefficient and electricalconductivity.In the present work, CoSb₃thermoelectric material is chosen as studying object. The effects ofthe electric structure change on electrical transport properties and the means to reduce its thermalconductivity were investigated. On the one hand, the electronic structures of doped and undopedCoSb₃compounds were calculated using the first principle plane-wave pseudo-potential based on thedensity function theory. On the other hand, doped and nanostructured CoSb₃compounds wereprepared by ways of mechanical alloying （MA）, cold isostatic pressing,sintering, iron liquid assistedmicrowave radiation etc. The phase structure and thermoelectric properties were measured XRdiffraction, scanning electronic microscope, transmitting electronic microscope （TEM）,4-point probemethods and laser thermal conductivity instrument etc. The main achievements obtained are asfollowings:1. The mechanical alloying （MA） process of Co and Sb materials was systematically studiedand it was found that that single CoSb₃phase could not prepared merely by this method. But singleCoSb₃compounds with homogeneous grain size were synthesized by solid state reaction （SSR） andMA methods respectively. The sample sysnthesized by SSR shows the p-type conductivity, while thelatter appears transformation from n to p-type. The electrical resistivity of the two kinds of samplesboth decrease with increasing temperature, indicating a semiconductor behavior. The electricalproperties of CoSb₃samples synthesized by SSR under different temperature and pressure conditionswere investigated. Experimental results show that the electrical resistivity decreases with increasingpreasure and the Seebeck coefficient increases first and then decreases with increasing temperature.The maximum power factor value of526.76μW/m·K²was obtained under the condition of650℃and300MPa. The dynamics changing about the process of mechanical alloying was analyzed. It is found that the activating energy for CoSb₃is lower than for CoSb2and CoSb₃will decompose to CoSb2phase with prolonging milling time. Thermodynamic data of all possible reactions were also analyzed.It can be found that the maximum negative value of formation enthalpy and free energy were obtainedin the reaction of CoSb₃formation. So CoSb₃is easily obtained comparing with other phases, which isconsistent with the experimental result.2. The energy band structures and density of states of doped and undoped CoSb₃compoundswere calculated using the first principle plane-wave pseudo-potential method based on the densityfunction theory. The band structure of all doped compounds become complex and dense and Fermisurfaces move to conduction and valence band respectively for n and p-type compounds. The densityof states near the Fermi surface of doped compounds increases comparing with undoped compound.So the number of charge carries increases after doping and electrical conductivity becomes improvedconsumedly, which guided our experimental.3. Intrinsic, n-type and p-type CoSb₃compounds were prepared by mechanical alloying（MA）-cold isostatic pressing （CIP）-sintering methods respectively. Their phase composition andthermoelectric properties were investigated. The value of soluble limit x of Te in Co4Sb12-xTexcompounds is between0.5and0.7. Co4Sb12-xTexcompounds show n-type conductivity. The maximumvalue2665.80μW/m·K²of power factor for Co₄Sb_11.7Te_0.3is obtained at600K, which is about14times as much as that of CoSb₃. Ni-doped compounds also exhibit n-type conductivity and themaximum value2292.92μW/m·K²of power factor for Co_3.5Ni_0.5Sb₁₂is obtained at550K, which isabout12times as much as that of CoSb₃. The value of solution limit x of Fe in Co4-xFexSb12wasbetween0.3and0.5. Co4-xFexSb12compounds show p-type conductivity. The maximum value1406.3μW/m·K²of power factor for Co_3.7Fe_0.3Sb₁₂is obtained at600K, which is about7.4times asmuch as that of CoSb₃. The electrical resistivity of doped compounds all increase with increasingtemperature, so they exhibit degenerated semiconductor characteristics.4. Co₄Sb_11.7Te_0.3composites with different nano-TiO2content （x%, x=0,0.1,0.3,0.6） wereprepared by MA-CIP-sintering methods. XRD patterns show that all samples well correspond toCoSb₃skutterudite diffraction plane, with no TiO2and other imputy phase appearing. The SEMphotographs about fracture surface of compounds show that the samples have high relative density.Nano TiO2particles agglomerate into irregular clusters of different sizes and they locate at the grainboundaries and some are distributed on the surface of Co₄Sb_11.7Te_0.3particles. Thermoelectricproperties measurement results show that they all have no obvious difference in Seebeck coefficient,and the electrical conductivity of dispersed compounds all decrease comparing with nondispersedcompound. Both dispersed samples with x=0.6and1.0have much lower thermal conductivity values than that of non-dispersed sample. The reduction in thermal conductivity is mainly due tophonon-large defect scattering due to the presence of TiO2clusters between the crystal grains ofCo₄Sb_11.7Te_0.3. The maximum value of ZT is0.79for sample with0.6wt%TiO2at700K, which is11%higher than that of non-dispersed sample. It indicates that thermoelectric properties will befurther improved if we disperse nano particles on doped compound.5. Using CoCl2and SbCl₃as precursors and NaBH4as reducing agent, we obtained nano CoSb₃particles of about10nm with the assistance of microwave radiation. It is difficult to obtain pure CoSb₃compound if we mix SbCl₃and CoCl2.6H2O in according with its stoichiometry ratio exactly and onlyCoSb2phase was obtained because of the volatilization of SbCl₃. Pure CoSb₃could be synthesizedwhen the Sb/Co ratio reaches5:1, indicating that the content of SbCl₃should be excessive. Theinfluence of microwave radiation time on the crystallization of CoSb₃was investigated. The XRDpatterns of the samples became sharper and narrower with increasing radiation time when it is shorterthan15min, showing that these atoms were in better order and the crystalline degree was improved.However, the patterns become weaker when the radiation time was prolonged to20min. The reason isdue to the volatilization of a great deal of solvent and the synthesized CoSb₃powders will be exposedin air and oxidated quickly. The average particle size of synthesized CoSb₃is about10nm and themorphology is isotropic. Microstructured and nanostructured bulk samples were prepared byMA-CIP-sintering methods and their thermoelectric properties were measured and compared. Thenanostructured sample exhibits n-type conduction in the whole temperature measurement range. Theelectrical resistivity of nanostructured sample is much lower than the microstructure sample for theimpurity existing in chemical precursors CoCl2·6H2O and SbCl₃. The nanostructured sample haslower thermal conductivity because it has more grain boundaries than sample with microstructure.The maximum ZT value of nanostructured sample was found to be0.11at650K, which is about11times as much as that of sample with microstructure.