Basic Study On Preparation And Application Of β-Zn₄Sb₃ Thermoelectric Module

Most of the energy in the world is dissipated in the form of thermal energy.Thermoelectric（TE）materials have attracted much attention because they can directly and reversibly convert heat into electric energy and reuse waste heat in different fields such as thermal power generation,automobile exhaust and industrial manufacturing.At present,in order to accelerate the realization of sustainable energy society and meet the growing power demand of society,the development of promising thermoelectric power generation technology has become one of the main hotspots in the field of renewable energy.At present,Pb Te and Bi₂Te₃ systems are used commercially as high-performance thermoelectric materials.However,these two materials contain toxic elements,expensive raw materials and low thermoelectric conversion efficiency,which are limited in the application of large-scale thermoelectric power generation.Therefore,the thermoelectric materialβ-Zn₄Sb₃ with high crustal richness,low cost and environmental protection has attracted extensive attention of researchers.β-Zn₄Sb₃ is a typical"electronic crystal phonon liquid"thermoelectric material,which has excellent thermoelectric properties in the medium temperature region.However,due to many difficulties in the preparation of thermoelectric modules,its practical application and development are relatively slow.Based on these key problems,this thesis systematically studies the key problems involved in the preparation and application of its thermoelectric module by takingβ-Zn₄Sb₃ as the research object.The main conclusions are as follows:1.For the first time,the solderable metallization treatment was realized on the surface ofβ-Zn₄Sb₃ by electroless plating.The results show that this method has less pollution and lower cost than the traditional electroplating process,and the nickel layer has good quality and high bonding strength.Compared with the hot pressing synthesis method,it has less stress and is not easy to fall off at high temperature.The occurrence of intergranular corrosion was avoided by mechanical coarsening,sensitization activation and alkaline pre plating.Finally,through the optimization of process parameters,the ideal plating conditions of p H 5.0,temperature 85℃and plating time20 min are obtained,which also opens up an overall process route for electroless nickel plating of other thermoelectric materials.2.In view of the high coupling of various parameters of thermoelectric properties and the difficulty of further performance optimization except doping,the effect of lattice strain on its thermoelectric properties is studied from the perspective of first principle.The results show thatβ-Zn₄Sb₃ is an intrinsic p-type slightly degenerate semiconductor,and it is difficult to prepare stable n-type materials in the experiment,The strain can exist in a stable form inβ-Zn₄Sb₃.Among them,the Seebeck coefficient is effectively increased under tensile strain,so as to obtain a higher power factor.Moreover,both tensile and compressive strains can further reduce the lattice thermal conductivity.Thanks to the increase of power factor and the decrease of thermal conductivity,the optimal thermoelectric performance is obtained under the tensile strain of 3 GPa,and its maximum ZT value reaches 1.73 at 700 K,which provides a new idea for the optimization of thermoelectric performance ofβ-Zn₄Sb₃.3.In order to understand the possible Ni diffusion phenomenon of thermoelectric module under high-temperature service and the influence of diffusion on its performance,the change of thermoelectric performance ofβ-Zn₄Sb₃ after Ni diffusion was studied by heat treatment.The results show that Ni will seriously diffuse with Zn at higher temperature,and it is easy to form interface reaction layer,which increases the micro defects at the interface,and Zn will also oxidize and volatilize due to high temperature,aggravating the deterioration of the material.Finally,due to the significant reduction of conductivity,the ZT value also decreased significantly.After Ni diffusion of sample S4（heat treatment=700 K,8h）,the maximum ZT value at 700 K was only0.9,which was significantly reduced by about 30%compared with that before heat treatment.In addition,the first principle analysis shows that the volatilization of a small amount of Zn will not affect the large change of the Seebeck coefficient ofβ-Zn₄Sb₃.If Ni replaces Zn doping,a huge resonance peak will be introduced,which will greatly improve the Seebeck coefficient.Finally,the thermoelectric module prepared by matching with n-type Pb Se has stable output characteristics.When the temperature difference is△T=400 K,its maximum output power is 1.2 W and the maximum conversion efficiency is 4.7%.The research experience is provided for accelerating the development and application ofβ-Zn₄Sb₃ thermoelectric module in the future.