Study On The Mg₃（Sb,Bi）₂-based New Thermoelectric Materials And Devices

With the rapid development of science and technology,people enjoy a more comfortable and abundant social life.On the other hand,more and more new problems emerged from the new science and technology,the most serious of which is environmental pollution caused by fossil energy.Currently,the most feasible solution is to develop new green energy to replace fossil energy.Thermoelectric cooling is a method to control heat flow,which has the advantage of fast response,no emissions,and low maintenance cost.However,most current research on thermoelectric materials focuses on increasing thermoelectric figure of merit（z T）and neglects some key factors such as material cost,thermodynamic and chemical stability,and mechanical property.Although researchers have discovered and synthesized many high-performance thermoelectric materials,only bismuth telluride（Bi₂Te₃）has been used in practice for thermoelectric cooling.However,after more than half a century of development,Bi₂Te₃materials face significant bottlenecks.First,telluride is a rare precious metal with limited reserves and some environmental risks,which limits further development and application of thermoelectric refrigeration technology.Second,Bi₂Te₃ materials lack thermoelectric properties in the near-space temperature range（250-600 K）,and the corresponding thermoelectric conversion efficiency is low,which hinders the promotion and application of Bi₂Te₃ in the development of thermoelectric power generation technology.Therefore,it is an important goal in this field to find new thermoelectric materials that can replace the Bi₂Te₃ material system and develop high-performance thermoelectric devices.Currently,n-type Mg₃（Sb,Bi）₂ material is considered as one of the thermoelectric materials that can replace Bi₂Te₃ because of its excellent thermoelectric performance in wide temperature range and its low cost.However,it is challenging to build a practical commercial cooling module based on n-type Mg₃（Sb,Bi）₂ material for the following three reasons.First,n-type Mg₃（Sb,Bi）₂ material has poor thermoelectric stability and tends to decompose in water.Second,the most common Fe and Ni electrodes for n-type Mg₃（Sb,Bi）₂ material can achieve good interfacial properties,but there are still problems such as poor stability,poor repeatability,and poor controllability.Therefore,we still need to find more reliable electrode materials and develop better electrode layers.Third,there are many problems in the process of building the n-type Mg₃（Sb,Bi）₂-based cooling module,such as cutting the Mg₃（Sb,Bi）₂ material,welding the thermoelectric legs,characterizing the performance of thermoelectric cooling modules and optimizing the modules.Therefore,we start from the following three aspects to conduct relevant research.（1）The room temperature z T value of n-type Mg₃（Sb,Bi）₂ sintered by conventional method can reach about 0.7,which can meet the room temperature cooling requirement.However,the thermoelectric stability of the material needs further improvement.First,based on the single band（SPB）model,we determined the optimum Sb:Bi component ratio and carrier concentration of the n-type Mg₃（Sb,Bi）₂ material to achieve the optimum thermoelectric performance at room temperature.Then,based on the principle of defect engineering,we introduced the process of thermal deformation to synthesize n-type Mg₃（Sb,Bi）₂ samples,which greatly improved the thermoelectric stability of n-type Mg₃（Sb,Bi）₂ materials.At the same time,we propose a dimensionless index Q to describe the reliability of thermoelectric materials.The value Q takes into account not only the material z T,but also other important parameters.（2）Considering the drawbacks of conventional Fe and Ni electrodes,we have found a transition layer material Mg₂Cu suitable for Mg₃（Sb,Bi）₂ based on thermodynamic phase diagram analysis and designed the electrode structure of Mg₃（Sb,Bi）₂/Mg₂Cu/Cu.This electrode structure has the following three advantages:the degree of diffusion reaction is low and Mg₂Cu does not reduce the thermoelectric properties of n-type Mg₃（Sb,Bi）₂ matrix;The contact resistivity is less than 12μΩ·cm².The electrode interface has high mechanical strength and good thermal matching.Based on the above,we have successfully built a practical high performance thermoelectric cooling module with a maximum temperature difference of 59 K at 300 K and good service performance based on n-type Mg₃（Sb,Bi）₂ and p-type Bi₂Te₃.（3）N-type Mg₃（Sb,Bi）₂ material undergoes strong modulation of grain boundary scattering at room temperature.Reducing grain boundary scattering is an important way to improve the performance of n-type Mg₃（Sb,Bi）₂ material.Inspired by the experimental results of Mg₂Cu and Ni doping,we attempted to combine Mg₂Ni with n-type Mg₃（Sb,Bi）₂.By using the defect technique,the grain boundary density was reduced to 1/3 of the original value and the scattering of electrons at the grain boundaries was greatly reduced.In addition,the carrier concentration is further increased,and the power factor is increased by 25%.At the same time,the local vibration mode of interstitial Ni atoms is introduced to promote the resonance absorption of phonons,and the lattice thermal conductivity is significantly reduced.Finally,achieve the average z T of Mg_3.2Bi_1.4975Sb_0.5Te_0.0025 increased by 15%.