Study On The Improvement Of Electrical Contact And Mechanics Properties Of Ag/SnO₂ Materials And Related Mechanisms

The environmental protection Ag/SnO₂ electrical contact material has a wide range of application prospects in the fields of relay,protector,circuit breaker,air switch and other low-voltage electrical appliances.The main role is to undertake the current connection,conduction and disconnection of the current.The performance of Ag/SnO₂ material has direct impacts on the reliability,stability and even service life of electrical system.However,the Ag/SnO₂ material has poor interfacial wettability and low bonding strength between two phases,which is easy to produce serious stress concentration during deformation,leading to machining difficulties.By optimizing the forming pressure,changing the loading mode of additives,adjusting the temperature and pressure of high temperature repressing,this thesis systematically studied the electrical contact and mechanical properties and related mechanisms of Ag/SnO₂ materials.The main contents are as follows:（1）The Ag/SnO₂,Ag/ZnO,Ag/Ni and Ag/W materials were prepared by powder metallurgy method.The effects of forming pressure on the physical and electrical contact properties of different types of silver-based contact materials were systematically studied.The results showed that the relative densities of the Ag/SnO₂and Ag/ZnO materials significantly increased with the increase of forming pressure.While,the densities of Ag/Ni and Ag/W alloys obviously decreased.When pressure increased from 260 MPa to 600 MPa,the conductivity of the Ag/SnO₂ material increased from 35.2%IACS to 52.6%IACS.When the pressure reached 800 MPa,however,the conductivity did not increase significantly.The electrical contact properties of the Ag based contact materials were greatly affected by forming pressure.The Ag/SnO₂ and Ag/ZnO materials prepared under 600 MPa showed the lower arc energy and contact resistance.Especially,the Ag/ZnO material exhibited the best arc erosion resistance,and the contact resistance was less affected by forming pressure.Although the conductivity of Ag/W alloy was up to 93%IACS,the mass loss of anode was larger after arc erosion.The surface of cathode was rough and the obvious cracks were generated.In addition,the contact resistance was greatly affected by forming pressure.In contrast,the mass loss of the Ag/Ni alloy was lower and mainly occurred at cathode,resulting in a good arc erosion resistance.The Ag/SnO₂ material prepared at 600 MPa had the best physical and electrical contact performance.（2）The thermal expansion coefficient between SnO₂ particles and Ag matrix in Ag/SnO₂ electrical contact material was very different,which is easy to cause interfacial thermal mismatch stress in the process of service,resulting in the initiation and propagation of microcracks during service.In this study,the in situ formation of CuO nanoparticle additive on SnO₂ reinforcements was used to relieve interfacial thermal stress in Ag/SnO₂ materials.The results showed that the electrical conductivity of the Ag/CuO@SnO₂ materials increased by 41%and the contact resistance was quite stable.The CuO nanoparticles formed at the interface between SnO₂ and the Ag matrix effectively inhibited the initiation and propagation of cracks in the Ag/SnO₂ material during the arc erosion process.The arc erosion resistance of the prepared material was significantly improved,accompanied by a significant decrease in the arc energy.The finite element simulation results showed that the in-situ CuO nanoparticles significantly relieved the stress concentration at the interface,the strain distribution was uniform,and the maximum instantaneous temperature decreased by about 32%.（3）Short-time hot repressing was proposed to densify the Ag/SnO₂ contact material prepared by powder metallurgy.The effect of the repressing temperature on the Ag/SnO₂ material containing CuO additive was systematically investigated under a constant pressure of 200 MPa.And explore the appropriate high temperature repressure.The results showed that the relative density of the material repressed at800℃ for 10 min can be increased to as high as 99.75%.The microstructure of the Ag/SnO₂（CuO）material was obviously refined and homogenized by short-time hot repressing.The dispersive distribution of the SnO₂ particles in the Ag matrix was induced,resulting in the elimination of particle aggregation.It was worth noting that almost all the CuO additive was decomposed into Cu₂O after hot repressing,which was beneficial for improving the electrical contact properties.Furthermore,the arc energy of the repressed material significantly decreased with increasing repressing temperature,which can be attributed to the high density and uniform microstructure.During arc erosion,the mass loss of the Ag/SnO₂（CuO）materials repressed at 800℃ was smallest,and the formation of pores and cracks was inhibited.However,the relative density of the material showed a tendency to decrease when the repressing temperature reached 850℃,the relative density and hardness of the material decreased,the mass loss increased.The Ag/SnO₂ material prepared by 100 MPa high temperature repressing pressure had the best performance,the density and conductivity were 99.85%and 76.93%IACS.In addition,the compressive strength,elongation and electrical contact performance were excellent.（4）The effects of In₂O₃ additive content and size on mechanical and electrical contact properties of fully densify Ag/SnO₂ materials were investigated.The conductivity of the Ag/SnO₂ material increased by 6.2%as content of nano In₂O₃additive reached 1.0%.The grain size was obviously refined,and about 90%grain sizes less than 1μm.The plastic flow of Ag matrix was strengthened,and the pores around SnO₂ particles were filled to increase the density of the material.ABAQUS software was used to simulate the compression process of Ag/SnO₂ material with and without In₂O₃ additive.The results showed that In₂O₃ additive could effectively relieve the stress concentration around SnO₂ particles,the strain distribution inside Ag/SnO₂（nano In₂O₃）material was uniform,and the compressive strength was significantly improved.The addition of In₂O₃ additive effectively inhibited the formation of holes and cracks,consistent with the compression section image.The section had clear deformation zone,but the elongation decreased slightly.However,the conventional In₂O₃ additive can cause serious stress concentration inside the material,leading to the initiation of cracks.After arc erosion,the Ag/SnO₂ material with 1%nano In₂O₃ reduced the arc break arc energy and mass loss,but the contact resistance increased.There was no SnO₂ aggregation on the eroded surface and the eroded area was small,with the addition of conventional In₂O₃ additive,the surface of the material had holes,and the mass loss increased.The research results of this thesis have a certain guiding effect on further optimization of preparation method,performance improvement and wide application of Ag/SnO₂ electrical contact materials.