Preparation And Properties Of Ag/MAX Composite Electrical Contact Materials

Ag-based electrical contacts are the key parts of low-voltage switches,which determine the lifetime of switchgears and the reliability of circuits.With excellent contacting performance,the universal contact Ag/CdO has been favored by various industries since the beginning of the last century.However,the toxic Cd vapor,generated in the application,endangers the human health and the natural environment.In the new century,with the increasingly stringent global environmental policies,some developed countries and regions,such as United States,Japan and Europe,began to restrict the use of Ag/CdO contacts;only minimal use is allowed in the field of aerospace industries.Some non-toxic materials,such as Ag/SnO₂,Ag/ZnO,Ag/C,etc.,were developed to replace Ag/CdO.However,the drawbacks of these contacts,such as poor machinability,larger contact resistance,high temperature-rise,huge material transfer,etc.,are still unsolved.Until now,Ag/CdO has not been completely replaced by these contact materials.In developing countries,in particular,Ag/CdO still dominates in low voltage switch materials.Therefore,it is urgent to seek new eco-friendly contact materials with comparable performance to Ag/CdO.In this work,novel non-toxic Ag/MAX and Ag/MXene composite electrical contact materials were prepared by selecting MAX and its derivative 2-D MXene as the reinforcements.Wettability between MAX(MXene)and Ag,pressureless preparation of MAX powders,influence of composition and temperature on Ag/MAX composites,arc erosion performance and mechanisms of Ag/MAX(MXene)were systematically studied.Main contents and achievements are as follows.Wettability between Ag and MAX(MXene)was studied by using the sessile drop method.Ag and MAX were found to show reactive wettability.As temperature increases,the stability of MAX structure decreases.When M-A bond breaks,and the escaped A interdiffuses with Ag,leading to a decrease in the interfacial tension and hence an increase in wettability.With further increasing temperature,the interdiffusion between the Ag and A deepens,and the contact angle significantly reduces(<17°),finally resulting in the formation of dense Ag-A interface layer.For the case of MXene and Ag,however,reactive wetting does not occur due to the lack of A atomic layer,leading to high interfacial tension and hence the poor wettability(contact angle of¹00°).Removal of the-O and-F functional groups from the MXene surfaces further deteriates the wettability of Ag-MXene(contact angle?130°).High purity MAX(Ti₂SnC,Ti₂AlC)powders were prepared.The Ti₂SnC powder shows three unique morphologies(flower-like,bowknot-like,and triangle-like).The reaction path of the Ti₂SnC powder from the Ti/Sn/C raw materials were confirmed by combining the DSC,XRD,SEM,EDS analysis,and the formation mechanism of Ti₂SnC powder with different morphologies was proposed.The synthesis of reinforcment powders provides a foundation for the the subsequent investigation of Ag/MAX composites.Ag/MAX composite electrical contact materials with uniform microstructure and excellent performance were prepared by a wet mixing process followed by pressureless sintering.Too low content of MAX(<10 wt%)damages the formability of the composite.Too high content of MAX(?20 wt%)not only leads to the aggregation of MAX,but also the decreases of density,hardness and resistivity.Higher heating temperature damaged the structure of MAX,resulting in its decomposition,diffusion with Ag and even oxidation.In order to obtain the Ag/MAX composite with uniform microstructure,slight interface reaction,moderate hardness,high electrical conductivity,the MAX content should be controlled within 10²0 wt%with low heating temperature(<800?).Data on the basic properties and preparation process of Ag/MAX composites provided the base for the subsequent research on its arc erosion properties.Ag/MXene(Ti₃C₂)composite,with uniform microstructure and stripe-like Ti₃C₂,was prepared by powder metallurgy.The Ag/Ti₃C₂ composite possesses moderate hardness(64 HV)and excellent processability,and its resistivity was 29%lower than that of Ag/Ti₃AlC₂.The excellent electrical conductivity of Ti₃C₂ itself,the existence of surface hydrophilic function group and its the small electron scattering area improved the electrical conductivity of the Ag/Ti₃C₂ composite.However,the gap between Ti₃C₂ layers may act as the starting point of arc erosion,leading to inferior arc erosion resistance of Ag/Ti₃C₂ to that of Ag/Ti₃AlC₂.The Ag/x wt%MAX(x=10,12)electric contact materials developed in this work exhibit comparable arc erosion resistance to Ag/CdO.The satisfactory performance is attributed to the good wettability of Ag-MAX,and the high thermal conductivity of the Ag/MAX composites.With increasing MAX content(?15 wt%),the thermal conductivity of Ag/MAX significantly decreases,and the thermal accumulation accelerates the decomposition of MAX,which rapidly deteriorates the arc erosion performance of the Ag/MAX composites.Therefore,in order to obtain the Ag/MAX composites with excellent arc erosion resistance,the MAX content should be controlled within 15 wt%.The changes of microstructure and composition of Ag and MAX(Ti₃AlC₂,Ti₂SnC,Ti₂AlC)during electric arc erosion were studied by dynamic electric arc discharging experiment,and the arc erosion mechanism of Ag/MAX composites was clarified.The destruction process of MAX can be regarded as the rapid"decomposition-oxidation"process.Electric arc discharge increases the temperature-rise of the contact surface,leading to the breakage of the M-A bond,and the decomposition of MAX into TiC_x and A.Part of A diffuses into the Ag matrix,and the other part of A evaporates into the surrounding air.The TiC_x is oxidized into Ti_xO_y instantaneously at high temperatures.In the early stage of arc erosion,the decomposition and oxidation is relatively minor.The chemistry and structure of the MAX were well maintained,playing the role of limiting material loss.In the intermediate stage,the increasing temperature accelerates the decomposition and oxidation of MAX,and the amount of Ti_xO_y increases.In the late stage,the structure of MAX is seriously damaged,and a large number of Ti_xO_y aggregate into blocks,which reduces the restriction to the flow and splash of liquid Ag,and the material loss of contact starts to increase.When Ti_xO_y blocks are destroyed by electrical arc,new Ag/MAX was exposed to the surface and continues to undergo arc erosion,which leads to the degradation of composite in the aspect of component,structure and function.In conclusion,this paper aims to prepare the new eco-friendly Ag/MAX composite electrical contact materials and study the relationship between its structure and properties.The preparation of MAX powders provides a theoretical basis for the synthesis and morphology regulation of reinforcement.Wettability provides theoretical basis for the preparation and performance analysis of Ag/MAX from the aspect of interface reaction.The investigation on the Ag/MAX preparation process confirmed the effects of temperature and composition on its structure and properties.The arc erosion performance of Ag/MAX with different components provide theoretical reference for the design and application of Ag based electrical contact materials.The changes in microstructure and phase composition of Ag and MAX during dynamic discharge suggest the failure mechanism of MAX and its influence on the surface structure and electrical contact performance of Ag/MAX,which paves the way for the design,structure optimization,performance improvement,and failure analysis of Ag/MAX in the future.