Effect Of Isoelectronic And Non-isoelectronic Doping On Thermoelectric Properties Of ZrNiSn Alloys

Thermoelectric materials have long been an attractive solution in pursuing sustainable energy generation by recovering the waste heat to electric power.Today,with the increasing energy crisis,research on thermoelectric materials has become more practical.Among all of the state-of-the-art TE materials,the 18-electron Half-Heusler compounds with semiconducting properties are promising candidates for moderate temperature applications,which are attributed to the excellent power factors,mechanical robustness,thermal stability,tunable properties,and reasonable cost.In this thesis,ZrNiSn based Half-heusler alloy thermoelectric materials are selected as research objects.The arc smelting alloy samples are obtained with a direct current induced hot press.The isoelectronic and non-isoelectronic atoms substitute strategies were used to optimize the thermoelectric properties of the materials.The main results are listed as below:(1)Si partially replaced the Sn of ZrNiSn samples.The experimental results show that the Si atom substitute Sn can effectively adjust the carrier concentration,which leads to improved electrical properties.The best power factor of materials is optimized to 4.35×10^-3W·m^-1K^-2.Si atom substitute Sn raise the disorder of the crystal and enhance the strong point defect phonon scattering,leading to the reduction of the thermal conductivity.As a result,the sample of ZrNiSn_0.98Si_0.02 reached the ZT value for 0.81.PF_eng and ZT_eng reached 1.591W·m^-1K^-1,0.317 respectively.(2)Te partially replaced the Sn of ZrNiSn samples.This replacement introduces additional electrons,which increase conductivity.The Te ionic radius is larger than the ionic radius of Sn.Therefore,the doping also brings additional lattice defects and local distortion of the crystal lattice,which the decrease in the lattice thermal conductivity of the material caused by the introduction of mass fluctuations and stress fluctuations.When the optimal doping amount of Te to the material is 4%,the power factor and lattice thermal conductivity are4.14×10^-3 W·m^-1K^-2,2.8 W·m^-1K^-1 respectively,the ZT value of the material reached 0.84.PF_eng and ZT_eng reached 1.461 W·m^-1K^-1,0.309 respectively.(3)Compering with the isoelectronic and non-isoelectronic doped samples,ZrNiSn_0.96Te_0.04 is selected as based alloy.The Ti atom isoelectronic replace of the Zr site of ZrNiSn can aggravate the mass fluctuation and gravitational fluctuation effect and reduce the thermal conductivity.As a result,the metallic phase of Zr Ni₂Sn is produced by the simultaneous introduction of Ti and Te ions.The Samples obtained excellent electrical conductivity,but the Seebeck coefficient is sharply reduced.At 300 K,The Seebeck coefficient of sample Zr_0.98Ti_0.02Ni Sn_0.96Te_0.04 reduced 50%compared to the ZrNiSn alloy.The thermal conductivity of all samples slightly increased.When the optimal doping amount of Ti to the material is 1%,the ZT value of the material reached 0.68 at 873 K.