The Effective Doping And Thermoelectric Performance Optimization Of N-type Ti₂FeNiSb₂-based Double Half-heusler

Thermoelectric technology uses the special physical properties of thermoelectric materials to realize the conversion between heat and electricity.It is environmentally friendly,reliable and has no mechanical parts.The larger the temperature difference and the higher the average ZT value,the higher the thermoelectric conversion efficiency.Double half-Heusler is a new type of thermoelectric materials with lower intrinsic lattice thermal conductivity,and the same substrate can be synchronously optimized to N/P type thermoelectric materials.Ti₂Fe Ni Sb₂ is one of the double half-Heuslers which has been proved to be stable by theoretical calculations and experiments.Some researchers have followed up and studied the effect of regulating of Fe/Ni ratio and p-type doping on the Sb site.However,the performance is not satisfied.In this work,we prepared series samples with different doping and alloying.The cmpostions were designed,the samples were prepared,and the thermal and electrical properties were tested and analysized.The thermoelectric properties of the samples with different doping on the same site and different doping on the different sites were compared and studied.Firstly,Ti₂Fe_1-xCo_xNi Sb₂ samples with n-type properties were obtained by doping Co on the Fe site of Ti₂Fe Ni Sb₂.The power factor of the sample with Co content x=0.4 reaches about 1.8 m W m^-1 K^-2,while its thermal conductivity decreases to?3.6 W m^-1 K^-1,which makes the maximum ZT of Ti₂Fe_0.6Co_0.4Ni Sb₂reach?0.5 at 973 K.On the basis of optimized Co content,Ti_2-yHf_yFe_0.6Co_0.4Ni Sb₂was prepared by Hf alloying.The lattice thermal conductivity of the samples with y=0.2 decreases by 38%compared with the substrate.The thermal conductivity of the samples with Hf content y=0.4 decreases to?1.5 W m^-1 K^-1 at 970 K,which enables the maximum ZT of Ti_1.6Hf_0.4Fe_0.6Co_0.4Ni Sb₂ reach?0.7 at 970 K.(Ti_1.8Hf_0.2)_2-x(Ta,V)_xFe Ni Sb₂ was obtained by Ta,V doping and Hf alloying on the Ti site.The sample with Ta doping content x=0.3 has the lowest thermal conductivity of about 2.2 W m^-1 K^-1 at 773 K,which makes the ZT reach?0.35 at873 K.The sample with V doping content x=0.4 has the highest power factor of about 0.9 m W m^-1 K^-2 at 883 K and the maximum ZT value of about 0.32 at 873 K.In terms of thermal properties,the room temperature thermal conductivity and the room temperature lattice thermal conductivity of Ta-doped samples are lower than those of V-doped samples,indicating that the large mass difference or the use of the heavy elements are beneficial to reduce the lattice thermal conductivity.Te and Se were doped on the Sb site.The results show that the electrical conductivity of Ti_1.8Hf_0.2Fe Ni Sb_1.6Te_0.4sample reaches?3.2×10⁴ S m^-1 at 773 K.The thermal conductivity of the optimized sample decreases by more than 77%at 293 K,and the lowest thermal conductivity of Ti_1.8Hf_0.2Fe Ni Sb_1.6Te_0.4 sample reaches?1.5 W m^-1K^-1 at 293 K.For the optimized Se-doped samples,the highest electrical conductivity of Ti_1.8Hf_0.2Fe Ni Sb_1.6Se_0.4 reaches?6×10⁴ S m^-1,the power factor of Ti_1.8Hf_0.2Fe Ni Sb_1.7Se_0.3 reaches?0.8 m W m^-1 K^-2 at 973 K,and the maximum ZT value of Ti_1.8Hf_0.2Fe Ni Sb_1.8Se_0.2 reaches?0.25 at 873 K.Comparing the thermoelectric properties of Te-doped and Se-doped samples,Se-doped samples have higher electrical conductivity and higher power factor.In terms of the ZT value,Se-doped samples are higher.Finally,by comparing the thermal and electrical properties of the samples doped with different elements at different sites,it can be found that the power factor of the samples doped with Co on the Fe site is much higher.In terms of the thermal conductivity,the Ti-site and Sb-site optimized samples show bipolar diffusion effect.For the ZT value,the Co-doped sample is the highest.This study demonstrates a more systematic understanding of the n-type Ti₂Fe Ni Sb₂-based thermoelectric materials,provides a reference direction for other researchers who is about to investigate this material,and promotes the application process of Ti₂Fe Ni Sb₂-based thermoelectric materials.