Thermoelectric Properties Of N-type CoSb-based Half-Heusler Alloys

Due to the energy crisis and environmental pollution problems,exploring green renewable energy and improving energy efficiency have become a more and more hot topic.Thermoelectric?TE?materials are semiconducting functional materials that can directly convert thermal energy into electricity.The half-Heusler material is considered a promising cadidate due to its good thermal stability,mechanical properties,and particularly the non-toxicity.It is an environmentally friendly material that meets the current standards for new green energy materials.In this thesis,n-type CoSb-based half-Heusler alloys have been prepared by arc melting,high energy ball milling,followed by hot pressing.The basically physical properties and thermoelectric properties of the alloys are studied.Optimize the TE property of?Ti,Hf?CoSb based alloys with eighteen valence electrons by electronic doping at the?Ti,Hf?site and Co site,respectively.We obtained an n-type half-Heusler TE material by Ta doping at the Hf site.By isoelectronic substituting Hf with different contents of Ti,the lattice thermal conductivity is reduced because of the greater differences in mass and size between Hf and Ti atoms.We got the highest ZT⁰.63 at 900 K for Ti_0.45Hf_0.45Ta_0.1CoSb.We also obtained an n-type half-Heusler TE material by Ni doping at Co site.The highest ZT⁰.33 at 900 K was obtained for Ti_0.9V_0.1Co_0.8Ni_0.2Sb.The TE properties of the n-type?Nb,V?CoSb based alloys with nineteen valence electrons are mainly investigated by V-vacancy in the VCoSb alloy,Ti doping at the V site,solid solution with V at Nb site of the NbCoSb alloy,and solid solution with V at Nb site of the Nb_0.8CoSb,etc.The results showed that the V vacancy improved the purity and the TE performance of VCoSb.We got the highest ZT⁰.53 at 900 K for V_0.9CoSb.We can obtain an n-type half-Heusler TE material by Ti doping at the V site of VCoSb.After annealing,the ZT value was increased by 25%.A highest ZT⁰.5 at 900 K was obtained for annealed V_0.9Ti_0.1CoSb.The TE properties of VCoSb-NbCoSb alloy solid solution with and without V vacancy was investigatived.We got the highest ZT⁰.46 at 900 K for both Nb_0.5V_0.5CoSb and Nb_0.3V_0.5CoSb.Through the study of the phase equilibria of the V-Co-Sb ternary system at600°C,it is found that the ternary single-phase region is really a phase with certain solid solubility rather than just a stoichiometric compound,which is consistent with the reported results that impurities are always detected in VCoSb alloy.This finding may shed light on the follow-up study on VCoSb based half-Heusler alloy with19valence electrons.