Effect Of Element Doping On The Thermoelectric Properties Of NbFeSb And TiCoSb Half-Hassler Alloys

With the continuous development of science and technology in today's world,people's demand for energy is increasing year by year.On the one hand,the consumption of energy gradually reduces the reserves of traditional fossil energy.On the other hand,the environmental pollution caused by oil energy consumption is becoming more and more serious.Thermoelectric devices have attracted much attention due to their flexible size,light weight,no noise,no mechanical movement,no pollution,easy control and unique advantage in the use of thermal energy.In this paper,high temperature Half-Heusler alloy thermoelectric materials were taken as the research object to study the effects of Ti,Hf and Ta co-doping p-type NbFeSb and V,Nb,Ta,Zr and Hf single-doping n-type TiCoSb on the thermoelectric properties of the materials.Nb_{1-2 x}Hf_xTi_xFeSb,Nb_0.84-_yTa_yTi_0.08Hf_0.08FeSb and Ti_1-xA_xCo Sb(A=V,Nb,Ta,Zr and Hf,x=0,0.025,0.05,0.075,0.1,0.125,0.15)were prepared by Arc+levitation two-step melting,long time annealing at 900?,mechanical milling,spark plasma sintering process.The phase,microstructure and thermoelectric properties of all samples were studied in detail and the following conclusions were drawn:(1)The HH alloy phase was obtained in Nb_{1-2 x}Hf_xTi_xFeSb(x=0,0.02,0.04,0.06,0.08,0.1)samples.The co-doping of Ti and Hf instead of Nb provides holes,thus increasing the carrier concentration of p-type NbFeSb,The conductivity of the sample increased with the increase of doping content,and the seebeck coefficient remained above 150?V/K at 873 K,when the doping amount of Ti and Hf is 8 at.%,the power factor is optimized to the maximum with the carrier concentration is around 2.8×10²¹ cm^-3,which is close to the optimal carrier concentration of NbFeSb system,the power factor of sample x=0.08 and sample x=0.1 both over 5000?W/m^-1K^-2 at 873 K,the sample x=0.08 obtained 5600?W/m^-1K^-2 At 600 K.At the same time,the co-doping caused a large lattice distortion and reduced the thermal conductivity,the lattice thermal conductivity decreased from 8 W/m^-1K^-1at x=0 to 3 W/m^-1K^-1 at x=0.08,a decrease of 62.5%,and the total thermal conductivity of the sample was reduced from 9 W/m^-1K^-1 of x=0 to 5.8 W/m^-1k^-1 of x=0.08 at 873 K,a reduction of 35.5%.At last the ZT of Nb_0.84Ti_0.08Hf_0.08FeSb reached 0.87 at 873 K.(2)In Nb_0.84Ti_0.08Hf_0.08FeSb continue to dope with Ta,and Nb_0.84-yTa_yTi_0.08Hf_0.08FeSb is still HH phase.The doping of Ta has no significant effect on carrier concentration of the system,which remained at about 2.9×10²¹ cm^-3,and the electrical transport performance of the samples can not change significantly.However,due to the significant difference in the atomic mass and size of Ti,Nb and Hf in the matrix of Nb_0.84Ti_0.08Hf_0.08FeSb,lead to the fluctuation scattering and mass fluctuation scattering of stress field,the resulting strong lattice distortion and point defect scattering centers effectively scatter phonons.At 873 K,lattice thermal conductivity dropped from 3.6 Wm^-1K^-1 of sample y=0 to 2 Wm^-1K^-1 of sample y=0.24,a decrease of 55.5%,and the total thermal conductivity decreased from 6Wm^-1K^-1 of sample y=0 to 4 Wm^-1K^-1 of sample y=0.24,a decrease of about 33%.A thermoelectric optimization value as high as 1.1 was obtained.(3)Pure TiCoSb phase was obtained in TiCoSb system with single doping of V,Nb,Ta,Zr and Hf.The carrier concentrations of five single-doped samples were calculated through PPMS test.Among them,Zr and Hf,as electronic dopants,tended to decrease the carrier concentration.V,Nb and Ta as donor dopants,increases the carrier concentration.The electrical transport performance test showed that Ta could improve the conductivity of the sample most obviously,and at room temperature,it increased from 23.23 S/m of sample x=0 to 59159 S/m of sample x=0.025.At 973 K,the 3543 S/m of sample x=0 increased to29702 S/m of sample x=0.025.Under the condition of equal doping amount,the capacity to increase carrier concentration and conductivity is:Ta>Nb>V.The results of reducing the Seebeck coefficient were:V>Nb>Ta.The optimization effect of the power factor was:Nb>Ta>V was found.The thermal properties of the samples were tested and it was found that the single doping of the five elements could reduce the thermal conductivity by introducing the point defect as the scattering center.V doping in the low temperature interval,Ta doping in the high temperature interval has a better effect on the reduction of lattice thermal conductivity.At 973 K,Ta doping reduced the lattice thermal conductivity from 6.51Wm^-1K^-1 of sample x=0 to 2.38 Wm^-1K^-1 of sample x=0.15,a reduction of 63%,and reduced the total thermal conductivity from 6.53 Wm^-1K^-1 of sample x=0 to 2.73 Wm^-1K^-1 of sample x=0.15,a reduction of 58%.Under the condition of the same doping amount,The effect of five single doping on the thermal conductivity was compared and can be expressed as:Ta>Hf>V>Nb>Zr.