Optimization Of Thermoelectric Properties Of Mg₃Sb_2-xBi_x Compounds Via Electronic Structure Calculation Combined With Pb Doping

Under the dual background of energy crisis and environmental degradation,thermoelectric materials as a kind of energy conversion materials have attracted the attention from large quantities of researchers worldwide.Among thermoelectric materials systems,the Zintl-phase Mg₃Sb₂ compound show the merits of high Seebeck coefficient,low thermal conductivity,low cost,non-toxic and eco-friendly constituents,making it potential for the future application.In this work,the effects and regulation mechanisms of Bi solid solution and Pb doping on the electrical and thermal transport properties of Mg₃Sb₂ compounds are systematically explored via theoretical calculations combined with experimental studies.The main results are as follows:(1)The electronic structures of the Mg₃Sb_2-xBi_x(x=0,1)compounds were calculated based on density general function theory(DFT).The solid solution of Bi in Mg₃Sb₂ can reduce the band gap,which is conducive to the increase of carrier concentration.Both Mg₃Sb₂and Mg₃Sb Bi compounds present a valence band maximum structure with a single energy pocket.Bi element induces an increase trend of the density of states on the valence band maximum.Compared with the partial density of states of Mg atom,the partial density of states of Sb atom contributes more to the total density of states on the top of the valence band of Mg₃Sb₂compound.(2)Mg_3(1+0.04)Sb_2-xBi_x(0?x?0.8)compounds were prepared using mechanical alloying method combined with spark plasma sintering(SPS)technology.The carrier concentration,conductivity and power factor of the samples increase with the Bi content rising.In the low temperature range,the solid solution of Bi significantly reduces the total thermal conductivity,with the lowest value of 0.65 Wm^-1K^-1 obtained at around 400 K for the Mg_3(1+0.04)Sb_1.2Bi_0.8sample.However,in the higher temperature range,the total thermal conductivity increases with the increasing of Bi content.The maximum ZT value of 0.077 is obtained in the Mg_3(1+0.04)Sb_1.8Bi_0.2 sample at 800 K,which is 93%higher than that of the undoped sample at the same temperature.And meanwhile,in the temperature range of 450-800 K,the average ZT value of this sample reaches 0.051,which is 219%higher than that of the undoped sample.(3)Taking the composition of Mg_3(1+0.04)Sb_1.8Bi_0.2 sample as the matrix,Pb-doped Mg_3(1+0.04)Sb_1.8-yBi_0.2Pb_y(y=0,0.005,0.01,0.02)compounds were synthesized by mechanical alloying method followed by SPS.The XRD results indicate that the doping limit of Pb in Mg_3(1+0.04)Sb_1.8Bi_0.2 is located in the range between 0.01 and 0.02.Based on the testing results of thermoelectric properties,the doping of Pb significantly increases the carrier concentration up to over 10¹⁹ cm^-3,and meanwhile,Seebeck coefficient decreases.The Pb doping significantly improves the thermoelectric figure of merit in the low temperature stage.In the temperature range of 400-700 K,Pb doping significantly reduces the thermal conductivity of the matrix material.Pb doping improves the thermoelectric figure of merit in the low temperature stage.At about 500 K,the ZT value of y=0.005 sample reaches to 0.039,which is77%higher than that of the undoped sample.The average ZT of y=0.005 sample in the temperature range of 300-600 K is 0.025,which is 56%higher than that of the undoped sample.