Synthesis And Thermoelectric Propertities Of ZrCoBi-based Half-Heusler Compounds

Half-Heusler alloy has a wide application prospect in the field of thermoelectric power generation due to its excellent thermoelectric properties,high mechanical strength and good thermal stability.It was found that ZrCoBi-based Half-Heusler compounds possess high ZT value and thermoelectric conversion efficiency due to their high band degeneracy and low phonon thermal conductivity.In this thesis,Sn doped ZrCoBi-based compounds were prepared by induction melting combined with sparking plasma sintering technology,and their thermoelectric properties were investigated.In addition,the effects of annealing treatment on the structures and the thermoelectric properties of ZrCoBi-based compounds were studied.The main conclusions are as follows:(1)ZrCoBi_1-xSn_x(x=0,0.05,0.10,0.15,0.20,0.25)series compounds were prepared by using induction melting combined with sparking plasma sintering by replacing Sn/Bi matrix.The results show that at the same temperature,the conductivities of ZrCoBi_1-xSn_x samples increase with the increase of the doping concentration of Sn.When the Sn doping concentration is fixed,the conductivities of the ZrCoBi_1-xSn_x samples decrease with the increasing temperature due to the enhanced scattering effect.The sign of Seebeck coefficients of undoped ZrCoBi samples is negative,showing N-type transport behavior.Sn doped ZrCoBi is a P-type semiconductor(both Seebeck coefficients are positive).At the same temperature,the Seebeck coefficients of the Sn-doped ZrCoBi_1-xSn_x samples decrease with the increase of the doping concentration.When the Sn doping concentration is fixed,the Seebeck coefficients of the ZrCoBi_1-xSn_x samples increase with increasing temperature.Due to the differences in mass and size between Sn and Bi atoms,the alloy scattering effect is enhanced,and the thermal conductivity of the sample is greatly reduced.The thermal conductivities of the ZrCoBi_1-xSn_x samples decrease with the increase of the Sn doping concentration.Sn doping increases the power factor of the ZrCoBi-based compound and reduces its thermal conductivity.The ZrCoBi_0.80Sn_0.20 sample can obtain a ZT value as high as 1.02 at around 730 K,which is 162%higher than that of the matrix.(2)In order to study the effect of heat treatment on the structure and thermoelectric properties of ZrCoBi-based compounds,we have annealed Sn-doped ZrCoBi_1-xSn_x samples at 700?,750?and 800?,respectively.The XRD result shows that there is an impurity phase Bi in the sample annealed at 700?.In the sample annealed at 750?,in addition to the impurity phase of Bi,there is also CoZr₂ phase.The sample annealed at 800?is standard Half-Heusler phase.Furthermore,the presence of the impurity phase reduces the conductivity and Seebeck coefficient of the Sn-doped ZrCoBi_1-xSn_x samples,but at the same time,the impurity scattering effect caused by the presence of impuritie phases of Bi and CoZr₂ further reduces the thermal conductivity of the materials.In summary,it can be seen that the thermal conductivity of the sample annealed at 750?is the lowest,and the sample annealed at 800?has the highest power factor and ZT value.