The Fast Fabrication And Thermoelectric Performance Of Bismuth Telluride Based Materials By Combustion Synthesis

Nowadays,the energy crisis and environmental pollution are becoming more and more serious,which seriously restricts the social development and human survival.As a consequence,the development of new energy sources and the use of new energy materials are of great importance.Thermoelectric material has become one of the focuses of international society for the use of the Seebeck effect and the Peltier effect to realize the direct conversion of heat energy and electric energy.Bismuth telluride based compounds are the best thermoelectric materials near room temperature.So far the commercial products are mainly prepared by zone melting?ZM?method,and the highest ZT value is about unit.However,the ZM material is susceptible to cleavage due to high orientation,leading to poor mechanical properties and machinability,severely limiting the yield and stability of devices.Therefore,it is very important for the commercial application to prepare bulk materials with excellent thermoelectric and mechanical properties in large-scale.The target of this study is bismuth telluride based compounds.Aimed at solving the problem that the thermoelectric and mechanical properties of commercial zone melting materials are poor,combustion synthesis?CS?combined with plasma activated sintering?PAS?technology is developed to the ultra fast and low cost preparation of bismuth telluride based materials with excellent thermoelectric and mechanical properties.The phase transformation during combustion synthesis process and reaction mechanism are studied,as well as the effects of the process parameters and components on the thermoelectric properties.Self propagating high temperature synthesis?SHS?and thermal explosion synthesis?TE?are the two modes of CS,which is simple,energy conservation and environmental protection,suitable for large-scale industrial production.The main contents and results of this study are as follows.For the binary compound A₂B₃?A=Bi,Sb,B=Te,Se?,the possibility of synthesis by combustion synthesis was explored,and the results show that Bi₂Te₃,Bi₂Se₃ and Sb₂Se₃ can be synthesized by SHS.But Sb₂Te₃ can not be synthesized by SHS,and can be synthesized by TE for external energy supply.Study on the thermodynamics and kinetics parameters of CS process were investiagated.The adiabatic combustion temperature,combustion temperature,propagation velocity of combustion wave in the SHS process of Bi₂Te₃ are 860 K,816 K and 6 mm s^-1,respectively.The adiabatic combustion temperature,combustion temperature,propagation velocity of combustion wave in the SHS process of Bi₂Se₃ are 995 K,946 K and 12 mm s^-1,respectively.The adiabatic combustion temperature,combustion temperature,propagation velocity of combustion wave in the SHS process of Sb₂Se₃ are 888 K,867 K and 1 mm s^-1,respectively.While the calculated adiabatic combustion temperature for Sb₂Te₃ is only703 K,and no enough energy to sustain the reaction in the SHS process.In addition,the phase transformation,microstructure formation process and the reaction mechanism were studied by DSC simulation combined with combustion wave quenching method.The results show the appearance and disappearance of liquid phase and intermediate phase in the CS process of binary compound A₂B₃,indicating the dissolution reaction precipitation mechanism.In the study of n type ternary compound Bi₂Te_3-xSe_x?x=0¹?,the thermodynamics and kinetics parameters of SHS process were investiagated.The combustion temperature,propagation velocity of combustion wave in the SHS process of Bi₂Te_2.4Se_0.6 are 872 K and 8 mm s^-1,and the preparation of Bi₂Te_3-xSe_x by SHS is a comprehensive embodiment of Bi₂Te₃ and Bi₂Se₃.DSC simulation combined with combustion wave quenching method show that the reaction mechanism is dissolution reaction precipitation mechanism.Moreover,n type Bi₂Te_3-xSe_x bulk thermoelectric materials can be obtained by SHS-PAS technology within 20 min,and the thermoelectric and mechanical properties are much higher than that of commercial ZM material.The sample Bi₂Te_2.4Se_0.6 obtians a maximum ZT value of 0.95 at 423 K,and the average ZT value between 298 K and 523 K is 0.81,21.7%enhancement compared with the ZM sample.In addition,compressive strength and bending strength of the sample Bi₂Te_2.4Se_0.6 are 104 MPa and 70 MPa,improved by 6⁷ times compared with the ZM sample.In view of p type ternary compound Bi_xSb_2-xTe₃?x=0.2⁰.6?,the sample Bi_0.5Sb_1.5Te₃ can be synthesized only by TE due to insufficient heat to sustain SHS reaction.The temperature and time of TE have an important influence on the synthesis of single phase for the compound Bi_0.5Sb_1.5Te₃.Single phase can not be obtained by TE at 773 K,while single phase is obtained by TE at 823 K for more than 140 s,and the maximum reaction temperature is about 834 K.Sigle phase can also be obtaned by TE at 873 K for only 60 s,and the highest reaction temperature is about 851 K.DSC simulation combined with characteristic point analysis show that the reaction mechanism is dissolution reaction precipitation mechanism.Moreover,p type Bi_xSb_2-xTe₃ bulk thermoelectric materials can be obtained by TE-PAS technology within 20 min,and the thermoelectric and mechanical properties are much higher than that of commercial ZM material.The sample Bi_0.5Sb_1.5Te₃ obtains a maximum ZT value of 1.1 at 348 K,and the average ZT value between 298 K and 523 K is 0.86,about 46%enhancement compared with the ZM sample.In addition,compressive strength and bending strength of the sample Bi_0.5Sb_1.5Te₃ are 80 MPa and 64 MPa,about 5 times improvement compared with the ZM sample.ANSYS finite element analysis shows that the temperature in the centre of the sample with the size?30 x 12 mm³ is higher than that in the edge during the sintering process,and experimental studies indicate the uniformity of large size sample can be improved with the increase of sintering time.N type Bi₂Te_2.4Se_0.6 and p type Bi_0.5Sb_1.5Te₃ large size bulk material with excellent uniformity were prepared by CS-PAS technology,and thermoelectric devices were prepared with the large size samples.When the temperature difference between the heat end and the cold is about250 K,the open circuit voltage and maximum output power of the thermoelectric device are 5.7 V and 3.40 W,and the thermoelectric conversion efficiency is 5.2%,about 42%enhancement compared with the commercial ZM device.