Study On The Mg₃?Sb,Bi?₂ Based Thermoelectric Materials And Modules

Thermoelectric effect can directly realize conversions between heat and electricity,and no additional substances are generated during the conversion process,which is an environment-friendly form of technology.Among them,thermoelectric refrigeration is one of the most important applications of the thermoelectrics.Compared with the traditional cooling method of compressors,thermoelectric refrigeration has the advantages of adjustable volume,less environmental pollution,and accurate temperature control.However,the only commercial thermoelectric refrigeration material being used in nowadays is bismuth telluride,which has limited the development of thermoelectric refrigeration industry to a certain extent.Magnesium antimonide,as a new thermoelectric material with high z T values in the middle to low temperature ranges,has received intensive attention in recent years owing to its advantages in terms of the material cost,thermoelectric performance and potentials in the modules.Furthermore,the Mg₃?Sb,Bi?₂ alloy systems have been well recognized as one of the most potential thermoelectric materials to replace bismuth telluride for refrigeration applications in the future.Considering the practical values in real applications and high feasibility of the Mg₃?Sb,Bi?₂ material systems,we have prepared series of Mg₃Sb_2-xBi_x materials with high z Ts at room temperature,and explored the specifically-suitable electrode material for Mg₃Sb_2-xBi_x.We also optimized the electrode preparing processes and fabricated the cooling devices made by Mg₃Sb_2-xBi_x.The room-temperature stability and performance-repeatability of the cooling devices have been studied.The content of the thesis can be summarized as follows:1. Firstly,based on the Mn doped Mg₃Sb_2-xBi_x compositions,through adjusting the sintering parameters and modulating excessive ratio of Mg,we have successfully prepared series of N-type samples with z T of 0.7 at room temperature by using combined high energy ball milling and spark plasma sintering?SPS?procedures.2. We have preliminarily explored the electrode material and suitable fabrication techniques for the Mg₃Sb_2-xBi_x systems,and found that Mg₂Cu could be the barrier layer and Cu serve as the electrode layer.Electrode layers with high interfacial bonding strength and low contact resistance to the Mg₃Sb_2-xBi_x materials can be realized by hot press within the temperature range of 400-550 ^oC.3. According to the proposed model of thermoelectric devices,we have designed a practical performance simulation program for thermoelectric cooling devices.The simulation program can fit the data within the temperature range of-100?100 ^oC,according to the temperature dependences of the Seebeck coefficient,resistivity and thermal conductivity.Based on the simulation program,we will be able to calculate the applied electrical current dependence of the temperature difference for the thermoelectric devices at the hot end temperature of?25 ^oC,as well as the optimal height and cross-sectional area of the thermoelectric legs,for which we used Mg₃Sb_2-xBi_x as the N-type and commercial Bi Sb Te for P-type materials.Through optimization of the size of the thermoelectric legs,the maximum cooling temperature difference it can produce reaches 67K.4. We have tested the interfacial bonding strength between the electrode and the Mg₃Sb_2-xBi_x materials,and the tensile force reaches more than 5Mpa,which has met the common requirements of the commercial specification for Bi₂Te₃ based thermoelectric refrigeration devices.5. We have been focusing on the maximization of the temperature difference of Mg₃Sb_2-xBi_x-based refrigeration devices,and basically the experimental results are consistent with the theoretical values.Among them,the device with a cooling temperature difference of about 55K has the highest repeatability,and for a few devices it can reach above 60K.However,the devices still face challenges in terms of the uniforms of Mg₃Sb_2-xBi_x materials and the contact resistance at the interfaces still needs to be improved.We have analyzed the correlations between the repeatability of the temperature difference and the uniformity of the electrode,as well as the electrical performance of Mg₃Sb_2-xBi_x material,and finally stabilized the good TE performance of Mg₃Sb_2-xBi_x legs in the modules.This paves the way for further improvement of the TE performance for Mg₃Sb_2-xBi_x-based devices.