| Electrical contacts are the key components in the electronic and electrical equipment and devices,and their performance has a great influence on the breaking capacity,operational reliability and service life of the equipment and devices.Ag-based composites are the most widely-used electrical contact materials due to their excellent thermal and electrical conductivity,good anti-arc erosion performance and anti-welding properties.However,their electrical contact performances and reliability in application are still needs to be further improved.Furthermore,the underlying mechanisms between micro structure and performance of electrical contact materials are still unclear.In this dissertation,the Ag-SnO2 and Ag-Ni contact materials were fabricated,and their microstructural factors,including morphology,size,volume fraction,and composition of the second phase reinforcements were designed and optimized to achieve significantly improved electrical contact properties.The main results of this work are as follows:SnC2O4 precursors with four different morphologies,including particles,tubes,rods and needles,were synthesized by a chemical precipitation method.After calcination at 600 ? for 2 h,the precursors were thermally decomposed into SnO2 powders,retaining their original morphologies.In the SnSO4 solution,part of the Sn2+ ions tend to hydrolyze,forming some Sn3O(OH)2SO4 complex plates.When H2C2O4 solution was added to SnSO4 solution,the Sn3O(OH)2SO4 complex plates tend to absorb onto the active surface of the SnC2O4 precursors,resulting in the formation of particulate precursors.In contrast,when SnSO4 was added to H2C2O4 solution,the amount of Sn3O(OH)2SO4 complex particles is comparatively small due to the dissolution of Sn3O(OH)2SO4 in the oxalate acid solution,and one-dimensional precursors are obtained.In the later stage of reaction,the as-formed one-dimensional SnC2O4 compounds tend to dissolve at the center of end surface,resulting in the formation of tube-like precursors.The dissolution of SnC2O4 can be inhibited by lowering pH value or the molar ratio of Sn2+to C2O42-.Furthermore,PVP was found to have an effect to prevent the lateral attachment of SnC2O4 chains between each other.By reaction at 80 ?for 2 h,SnC2O4 needles with high aspect ratio(about 40:1)can be obtained with the addition of 1 wt%PVP.A particulate reinforced Ag-SnO2 electrical contact material was prepared by a novel citric acid assisted heterogeneous precipitation method.In the heterogeneous precipitation process,the addition of citric acid can give rise to the uniform dispersion of SnO2 particles and the heterogeneous precipitation of Ag particles onto them.By reducing the volume fraction of SnO2 and/or increasing the particle size of SnO2,electrical conductivity of the Ag-SnO2 electrical contact materials can be improved,while the hardness would be reduced.For Ag-SnO2 electrical contact materials reinforced by 18.3 vol%SnO2,the hardness and electrical conductivity can both meet GB-T20235-2006 with the SnO2 size of 1?2 ?m.The strengthening effect of the particulate reinforced Ag-SnO2 electrical contact materials is attributed to both direct and indirect strengthening mechanisms,and indirect strengthening was found to be the major mechanism(mainly by grain refinement strengthening,then by dislocation strengthening,and Orowan strengthening is trivial).The highest tensile strength of the Ag-SnO2 composite is about 219 MPa when the volume fraction of SnO2 is 18.3 vol%.Ag-SnO2 electrical contact materials reinforced by second phase particles with various morphologies were prepared and characterized.Compared to the particulate reinforced Ag-SnO2 electrical contact materials,the Ag-SnO2 electrical contact materials reinforced by one-dimensional SnO2 shows higher hardness and electrical conductivity.Notably,the sample reinforced by SrnO2 rods shows the highest yield strength(199 MPa),tensile strength(234 MPa),hardness(87.8 HV)and electrical conductivity(66.9%IACS).After 100 thousand contacts at 24 V/10 A with a resistance load,the Ag-SnO2 electrical contact materials were mainly eroded by the cathode arc.After arc erosion,cathode lost materials and pits were formed on its surface,while anode gained materials and bulges were formed.The Ag-SnO2 electrical contact material cathode reinforced by SnO2 tubes loss the least mass(12.8 mg).The rivets fabracited from the Ag-SnO2 reinforced by SnO2 needles present a high anti-fusion performance that no fusion occurred in 800 thousand break operations.The mass transfer can be reduced by the addition of In2O3,especially In2O3 with high specific surface area.Moreover,the cathode mass loss of the Ag-SnO2-In2O3 electrical contact materials can be decreased by the addition of nano-sized In2O3.The precipitation-coordination equilibrium of the Ag+-Ni+-C2O42--H2O system was simulated by MATLAB software.Two Ag and Ni oxalate precursors with different morphologies(plates and particles)were synthesized via precipitation of Ag+ and Ni2+ ions by C2O42-at two intervals with maximum precipitation.In the case of high H2C2O4 concentration and low pH value(closed to 1),the precursor was composed of Ag2C2O4 hexagonal plates with NiC2O4·2H2O plates adhered onto the surfaces.In the case of low H2C2O4 concentration and neutral pH(6?8),the precursor was composed of irregular Ag2C2O4 and NiC2O4,2H2O particles.Metallic Ag-Ni powders can be directly obtained by thermal decomposition of the precursors at 400 ? in inert gas.Finally,two Ag-Ni electrical contact materials reinforced by Ni with different morphologies(plates and particles)were prepared using the metallic Ag-Ni powders derived from the two typical precursors,respectively.The sample reinforced by Ni plates shows a better performance of electrical conductivity(74.2%IACS).While the sample reinforced by dispersed Ni particles exhibits a higher hardness(117 HV),higher breakdown strength(2.06×107 V·m-1),and lower material transfer(-0.5 mg gain in anode after 100 thousand contacts).A core-shell structured Ag-Ni powder were prepared by a pilling-coating method,and then the powder was hot-pressed into Ag-Ni specimens with a 3D Ni network structure Annealing of the pelletized powder was found to influence continuity of the Ni networks in the specimens.Without powder annealing,the 3D Ni networks in the specimens is intermittent.While,for the pelletized powder annealed at 400 ?,the obtained Ag-Ni specimens has a continuous 3D Ni network structure.In the process of severe plastic deformation,the Ni network was drawn into Ni fibers.With the increase of real strain,the dispersion of Ni in cross section is more uniform,and length of the Ni fibers in longitudinal section is longer.When the real strain reaches 6.0,the Ni fibers is about 50?100 ?m and 200?1000 ?m for the intermittent Ni networks and continuous Ni networks,respectively.The Ag-Ni electrical contact materials reinforced by long fibers show a high conductivity about 91.5%IACS.After 100 thousand contacts at 24 V/10 A with a resistance load,only 1.8 mg cathode mass loss was found for the Ag-Ni electrical contact materials reinforced by long fibers. |
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