| Thermoelectric materials can realize the mutual conversion of thermal energy and electrical energy through the Seebeck effect and Peltier effect,and typical applications are thermocouples,isotope temperature difference power generation,semiconductor refrigeration and other fields.However,the conversion efficiency of samples is low and the cost of raw materials is high,and it has not been widely used in the field of temperature difference power generation.The conversion efficiency of thermoelectric materials is characterized by the thermoelectric optimal value ZT(ZT=(S2σ)T/κ),and increasing the ZT value requires increasing the Seebeck coefficient and conductivity,and reducing the thermal conductivity.Zintl phase material has the characteristics of complex chemical bond structure and band degeneracy,in line with the material design concept of "phonon glass-electron crystal",is a large class of thermoelectric materials with development potential,among which Mg3(Sb,Bi)2 based materials have excellent performance in the low temperature zone at room temperature,rich in component elements,low price,light weight,and multi-Bi component materials show excellent performance at room temperature,which is expected to replace expensive Bi2Te3 materials.According to the literature research,Mg3.2Bi1.5Sb0.49Te0.01 was selected as the matrix for the preparation and performance modification study,and the geometric configuration of the thermoelectric module was simulated and analyzed on this basis.The main conclusions of this paper are as follows:Firstly,the effects of sintering temperature on the microstructure,electrical properties and thermal properties of Mg3.2Bi1.5Sb0.49Te0.01 materials were studied.The results show that increasing the sintering temperature can reduce grain boundary scattering,improve carrier mobility,and improve the thermoelectric properties of the material.The carrier mobility of the samples prepared at sintering temperatures of 793 K,893 K and 943 K increased by 270%from 12.34 cm2V-1s-1 to 46.10 cm2V-1s-1 with the increase of sintering temperature.However,too high a temperature will cause a change in the composition of the material,and the sintering temperature of 993 K makes the sample become a p-type material.Therefore,943 K is the optimal sintering temperature for the Mg3.2Bi1.5Sb0.49Te0.01 component.The room temperature carrier mobility of the 793 K sintered sample was low,and with the increase of the test temperature,the conductance decreased after the first increase,the absolute Seebeck coefficient gradually increased,and the thermal conductivity increased after the first decrease.With the increase of sintering temperature,the conductivity in the low temperature region of the sample increased significantly,and the whole temperature region decreased with the increase of the measurement temperature,but the Seebeck coefficient did not change much,and the total thermal conductivity was slightly increased by the increased electron thermal conductivity.Both the PF and ZT values of the material reach a maximum at the optimal sintering temperature of 943 K.Mg3(Sb,Bi)2 material has a small band gap,showing obvious bipolar effect in the high temperature region,the Seebeck coefficient tends to be saturated,the thermal conductivity in the high temperature region increases,and the thermoelectric properties in the high temperature region decrease.Second,by replacing the element doping of the Mn position at the Mg position,the thermoelectric properties of the material were optimized,and a series of samples with different Mn doping amounts Mg3.2-xMnxBi1.5Sb0.49Te0.01(x=0,0.01,0.03,0.05,0.1)were obtained.After Mn doping,the carrier concentration of the material decreases,indicating that Mn ions do not exist as good electron donors.However,with the increase of doping amount,the mobility of the sample is greatly improved,and the effective mass decreases,which indicates that the Mn doping changes the band structure of the material.The effective quality decreases and the conductivity of the material increases with the increase of doping,and the absolute Seebeck coefficient decreases,which increases the power factor of the material,and reaches the maximum power factor when the Mn doping amount is 0.03.Due to the significant increase in electrical conductivity,the thermal conductivity of electrons increases,and the total thermal conductivity of the material increases after Mn doping,but the sum of lattice thermal conductivity and bipolar thermal conductivity decreases.Mn doping increased the ZT value of the sample,where the ZT value reached 1.02 at 500K for a sample with a doping amount of 0.03,and ZT～1.07 at 500K for a 0.1 sample with a second phase.Thirdly,the effect of doping of transition metal elements such as Fe,Co,Ni and Cu on the thermoelectric properties of materials was studied,and a fixed doping amount was 0.03,and a series of doped samples Mg3.17Z0.03Bi1.5Sb0.49Te0.01(Z=Mn,Fe,Co,Ni,Cu)were prepared.Except for Ni doping,the conductivity of other doped samples was improved compared with the parent material.Further analysis combined with Hall effect shows that Cu and Mn doping can change the band structure of the material,reduce the effective mass of state density,and greatly improve carrier mobility.The doping of Fe and Co can increase the carrier concentration of the material and increase the effective mass of the state density.Except for Ni doping,the absolute Seebeck of the doped materials was lower than that of the parent material,and the power factor was increased,among which the Doping of Mn and Co had the most obvious improvement of the power factor.The total thermal conductivity of the material increased by elemental doping,but the increase in total thermal conductivity by Fe and Co doping was weaker than that of Mn doping.In addition to Ni doping,the doping of other elements improved the thermoelectric properties of the material to varying degrees,among which the effect of Mn,Fe and Co doping was obvious.Co-doped samples have the best thermoelectric performance,with ZT values up to 1.1.Fourth,based on the best performance of the material after elemental doping,the influence of different configurations on conversion efficiency and output power under ideal conditions is calculated by the thermoelectric effect module of COMSOL.The results show that the longer the side length of the thermoelectric arm,the higher the output power and the lower the conversion efficiency.The higher the height of the thermoelectric arm,the higher the conversion efficiency and the lower the output power;The higher the temperature,the higher the conversion efficiency and output power.Taking into account various factors,the optimal geometry of the thermoelectric module is 3mm×3mm×4mm.At a high temperature of 523 K,the module output power of the eight pairs of thermoelectric arms is 0.94 W and the conversion efficiency is 6.5%. |
