Preparation And Properties Of TiCoSb-Based Halfheusler Thermoelectric Materials

Nowadays,with the increasingly severe energy crisis and environmental pollution,the ability of thermoelectric materials that can directly convert electricity and heat make them have broad application prospects.Half-Heusler thermoelectric materials are becoming a new research hotspot in the field of thermoelectric due to their good thermal stability,high mechanical strength,abundant constituent elements in the earth,and their high intrinsic Seebeck coefficient and electrical conductivity.However,their further development is limited by their high intrinsic thermal conductivity.The performance of thermoelectric materials can be improved effectively by adding some impurity elements similar to the constituent elements into the matrix.In addition,more phonon scattering mechanisms can be introduced to reduce the lattice thermal conductivity through the increase of configuration entropy.In this study,TiCoSb-based Half-Heusler thermoelectric materials were prepared by microwave synthesis combined with rapid hot-pressing sintering.The phase composition and microstructure of the materials were analyzed and characterized.The effects of Ti-Nb substitution,Sb-Sn substitution,Sb-Te substitution and Co-multielement substitution of TiCoSb on its thermoelectric transport properties were studied respectively.The main contents of this study are as follows:1.Ti_1-xNb_xCo Sb（x=0.01～0.07）samples,TiCoSb_1-xTe_x（x=0.01～0.20）samples and TiCoSb_1-xSn_x（x=0.01～0.20）samples with high density were successfully prepared by microwave synthesis,ball milling combined with rapid hot-pressing sintering.The substitution of Te and Nb element was beneficial to the n-type conduction of TiCoSb,and the electrical conductivity and Seebeck coefficient were enhanced greatly.The maximum power factor was increased from0.07μWcm^-1K^-2 of TiCoSb to 5.58μWcm^-1K^-2 of Ti_0.93Nb_0.07Co Sb and 3.83μWcm^-1K^-2 of TiCoSb_0.90Te_0.10 at 725K,respectively.With the incorporation of Sn element,the Seebeck coefficient and power factor were increased.Due to the point defects,mass/stress field fluctuations and interfacial scattering introduced by element doping,the thermal conductivity of doped-samples with different elements was decreased significantly.The ZT of Ti_1-xNb_xCo Sb,TiCoSb_1-xTe and TiCoSb_1-xSn_x samples were optimized obviously.The maximum ZT value was0.098 of Ti_0.93Nb_0.07Co Sb at 725K,which was 2 orders of magnitude higher than that of TiCoSb.2.The lattice distortion caused by the increase of configuration entropy can scatter phonons strongly and reduce the lattice thermal conductivity of materials.By means of the proportional substitution of Fe,Co and Ni at Co sites of the traditional ternary Half-Heusler TiCoSb,the Ti_1-xNb_x（Fe Co Ni）Sb pseudo-ternary Half-Heusler alloys were successfully prepared based on the optimization of the electrical properties of Nb-doping,and the phase composition and thermoelectric properties were analyzed and characterized.Benefiting from the increased power factor and greatly reduced lattice thermal conductivity,the ZT of Ti（Fe Co Ni）Sb sample was significantly higher than that of TiCoSb pure phase,and the ZT values of Nb-doped samples were significantly higher than that of Ti（Fe Co Ni）Sb sample.3.In order to further increase the configurational entropy of TiCoSb,Cu and Mn were added on the basis of replacing Co in equal proportion with Fe,Co and Ni,and the multi-principal elements high-entropy Ti_1-xNb_xMSb（M=Mn,Fe,Co,Ni,Cu,x=0.01～0.10）Half-Heusler thermoelectric materials were prepared by the same process.Multielement substitution at Co sites changed the carrier type from n-type semiconductor to p-type semiconductor.And secondary phases such as Fe and Ti Sb appeared in the matrix,which significantly increased the electrical conductivity compared with TiCoSb.The power factor was significantly optimized.The increase of entropy and the incorporation of Nb introduced more phonon scattering mechanisms,which greatly reduced the lattice thermal conductivity,the minimum lattice thermal conductivity was decreased from 5.51Wm^-1K^-1 of TiCoSb to 2.37Wm^-1K^-1 of Ti_0.99Nb_0.01MSb at 725K,which was decreased by 57%.Due to the optimization of electrical and thermal properties,the ZT of all high-entropy samples was larger than that of TiCoSb pure phase.At 725K the maximum ZT of Ti_0.99Nb_0.01MSb was 6.8 times of Ti MSb,which was much higher than that of TiCoSb.