Effect Of Multi-principal High Entropy Doping On Structure And Properties Of TiCoSb Based Half-Hasler Thermoelectric Materials

Along with the rapid development of the national economy,China's demand for energy is also growing rapidly.The non-renewable fossil energy sources such as oil and natural gas that are currently mainly relied on will be exhausted in recent decades.Therefore,it is particularly important to develop renewable energy.As one of the renewable energy technologies,thermoelectric materials have the advantages of safety,reliability and environmental friendliness,and have received increasing attention.Typical medium-high temperature half-Heusler thermoelectric materials such as MCoSb,MNiSn and MFeSb have great application potential.In this paper,TiCoSb thermoelectric materials are used as the matrix to investigate the effects of several different elements on the thermoelectric properties of materials when they are dissolved in the Ti site,and several elements are selected to perform high entropy doping at the Ti site in equal amounts.The concept of highentropy effect is adopted to introduce thermoelectric materials with its ability to produce large lattice distortion,and it can achieve the goal of reducing the thermal conductivity of the lattice.On this basis,the doping element content is adjusted and the carrier concentration is optimized to further optimize the thermoelectric properties of the material.Through systematic experimental research,the following main conclusions are drawn:?1?The five elements of Zr,Hf,V,Nb and Ta are doped separately at the Ti site of TiCoSb at a content of 10 at.%,and the thermoelectric figure of the material is improved to some extent.The resistivity of the material is rapidly reduced due to the incorporation of V,Nb or Ta,the power factor of the material is increased,and the electrical properties are greatly optimized.Since there are large differences between the five doping elements and the Ti atoms in terms of atomic radius and relative atomic mass,the crystal lattice will be greatly distorted,and the scattering of phonons will be enhanced and reduced the material lattice thermal conductivity.The lattice thermal conductivity of samples doped with Hf and Ta decreased to 2.4W/m/K and2.6W/m/K,respectively;in the case of the same doping amount,five elements were used to reduce lattice heat.The conductivity is:Hf>Ta>V>Zr>Nb;the ability to increase the power factor is:Nb>Ta>V>Zr>Hf;the ability to increase the ZT value of the sample is:Ta>Nb>V>Hf>Zr.When the final Ti sites are doped with 10 at.%Nb and Ta,respectively,the material achieves an optimum ZT value of0.3 and0.32 at a high temperature of 873K.?2?Selecting the four elements Zr,Hf,V,Nb to dope in the Ti position of TiCoSb,a simple high-entropy half-Hasler Ti_1-x(Zr_0.25Hf_0.25V_0.25Nb_0.25)_x is formed.Due to the lattice distortion of the high entropy effect,it reduces the lattice thermal conductivity of the material.When the doping amount is 0.55,the thermal conductivity of the material lattice is as low as 2.38 W/m/K,and the optimum power factor and ZT of the sample with a doping amount of 0.5 at 873 K are1880?W m^-1 K^-2 and 0.54respectively.?3?In the Ti_0.5Zr_0.125Hf_0.125V_0.125-yNb_0.125+yCoSb system,by adjusting the relative contents of V and Nb which increases Nbcontent,the sample unit cell parameters increases.The carrier concentration and the electrical properties of the sample are optimized,and the power factor is improved.At the same time,the increase in the amount of Nb doping further reduces the lattice thermal conductivity of the sample.Finally,when the doping amount is y=0.085,the lowest lattice thermal conductivity of the material at 873 K is 1.79 W/m/K;when the doping amount is y=0.085,the maximum power factor at the high temperature of 873 K is 2060?W m^-1 K^-2,ZT achieved a maximum value of 0.71 at y=0.065.Compared with the unadjusted V,the Nb content of the sample ZT increased by nearly 31%,greatly improving the thermoelectric properties of the material.?4?The two elements of Nb and Ta are most effective in improving the thermoelectric properties of the matrix.When the two elements are doped at the Ti site,since the covalent radius of Nb and Ta are larger than Ti,the unit cell parameters of Ti_1-z(Nb_0.5Ta_0.5)_zCoSb samples gradually increased with the increase of Nb and Ta content.When Nb and Ta content increase,the resistivity of the sample decreased greatly,but the Seebeck coefficient decreased by a small amount,so the sample maintained a high power factor,and the material obtained3940?W m^-1 K^-2 at z=0.35.The lattice thermal conductivity is also significantly reduced.The lattice thermal conductivity of the sample decreased to a minimum of 1.4 W/m/K at 873 K at z=0.4.The sample with z=0.35 achieved a maximum ZT value of 0.94 at 873 K,greatly improving the thermoelectric figure of merit of the material.