Thermoelectric Properties Of NbFeSb-based High Entropy Half-heusler Alloys

Entropy reflects the disorder of the system,the greater the entropy,the greater the disorder.The greater the disorder of the system,the greater the scattering of phonons will be.Reducing the thermal conductivity of thermoelectric materials is an effective way to improve the thermoelectric performance of the materials.In order to research the effects of high entropy effect the performance of the thermoelectric material,multiple of elements were doping in the Nb site of half-Heusler thermoelectric materials Nb Fe Sb to obtain high configuration entropy from the relationship between entropy and thermal conduction,and integrating the design thought of high entropy alloys.Furthermore,the influences of Co doping in Fe and Sn doping in Sb on the structure and properties was further studied.The samples was prepared by arc melting combined with high temperature smelting,and then sintered by Spark Plasma Sintering method to form dense bulk samples.The structure,organization and thermoelectric properties of the samples were studied in detail and the following results were obtained.?1?The influence of each doped element on the thermoelectric properties of NbFeSb was studied by adding 10 at.%of Ti,V,Zr and Mo respectively in Nb site.The results show that Zr or Ti doped keeped the p-type conduction behavior of NbFeSb.Among them,doping of Zr has the most obvious optimization effect of power factor,but the effect on the reduction of the lattice thermal conductivity is poor especaily in high temperature.Doping of Ti optimized power factor ation and reduced the lattice thermal conductivity of NbFeSb.Both Mo or V doping transformed NbFeSb into n-type conduction behavior,and the power factor has been greatly reduced.But V doping can effectively reduce the lattice thermal conductivity,which is4.8w/m K at 873K.The increase of power factor is attributed to the increase of carrier concentration,and the decrease of lattice thermal conductivity is due to the difference of size and mass between the doped element and Nb.?2?The M_xNb_1-xFeSb?x=0,0.15,0.20,0.25,0.30,0.40?series samples were prepared by doping the equal proportion four elements M(M=Zr_0.25Mo_0.25V_0.25Ti_0.25).The result analysis showed that M doping increased the carrier concentration of the system,and optimized the power factor to 5040?W/mK²?x=0.25,473K?.High configuration entropy effect and mass fluctuation effect reduce the thermal conductivity from 16.1 W/mK to 8.4W/mK at room temperature,and from 6.2 W/mK to 4.1 W/mK at 800 K,lattice thermal conductivity were reduced about 50%to 3 W/m/K.A maximum figure of merit of 0.78 is obtained for the sample of x=0.3 at 873 K.?3?A series of samples of T_xNb_1-xFeSb?x=0,0.1,0.15,0.2,0.25,0.30,0.40?were prepared by adding the Hf element to form five elements T doping(T=Zr_0.2Mo_0.2V_0.2Ti_0.2Hf_0.2).The results showed that the addition of Hf further reduced the resistivity,and the maximum power factor could reach 6500?W/mK²?x=0.1,473 K?.At the same time,the lattice thermal conductivity was further reduced,the lowest value reached2.5W/mK?x=0.4,873K?,and the maximum ZT value of 0.88 was obtained at 873K for the sample of x=0.2.?4?Co doping on Fe site of T_0.25Nb_0.75Fe_1-xCo_xSb increased the resistivity and lattice thermal conductivity,while the Seebeck coefficient also decreases.The thermoelectric performance were entirely deteriorated.Sn doping on Sb site increased the carrier concentration and optimizes the power factor of T_0.25Nb_0.75FeSb_1-xSn_x,which increases the power factor to 5540?W/mK²,futher more the total thermal conductivity and lattice thermal conductivity were decreased.A high ZT value of 0.85 was obtained at 873 K for the sample of x=0.05,which is about 19.7%increase than the sample of T_0.25Nb_0.75FeSb.?5?There are positively correlated between the configuration entropy and the reduction of thermal conductivity.The reduction effect to the lattice thermal conductivity is more and more weak,which may be related to change of lattice vibration entropy at the sample time.In the NbFeSb system,the mass fluctuation effect caused by doping element has obvious effect on the reduction of thermal conductivity.The multi-element doping can also reduce the carrier mobility,resulting the bad effect to the electrical conductive.Therefore,in order to improve the thermoelectric performance of materials it is not enough to just increase the configuration entropy.The doping content of multi-elements is far lower than that when the mixture reaches the maximum configuration entropy.