| As core foundation of electrical industry, electrical contact materials and components are responsible for making and breaking a current, and thus their performance directly determine switching capacity, service life and reliability of equipment. Ag/CdO electrical contact material is known as a "universal contact" and has been widely used due to its excellent performance such as low contact resistance, welding resistance, arc erosion resistance, etc. In recent years, people more and more pay attention to the harm of Cd element to human body as well as environment, so the development of environmentally friendly replacement materials attract wildly attentions from academia and industries. To date, Ag/conductive ceramic composite material system has become an important research direction of the environmental friendly electrical contact materials.In this thesis, La0.5Sr0.5CoO3-δ (LSCO) micronparticles and nanoparticles were prepared by solid-phase method and the sol-gel method respectively, and the potential advantages of LSCO nanoparticles used as a reinforcement of electrical contact material were also comparatively analysed. LSCO microspheres and fibers were prepared by hydrothermal method and electrospinning method respectively. In order to further improve the interfacial bonding between LSCO and Ag, the above LSCO powders with different morphologies were modified by silver loading through mechanical milling technology, hydrothermal method and electrospinning method, respectively. On this basis, the obtained LSCO powders were applied as reinforced phases to prepare Ag/LSCO electrical contact materials, and the influence of different morphologies of LSCO reinforced phase and silver-loading modification on the properties of Ag/LSCO electrical contact material were investigated. Finally, the arcing erosive behaviors and corresponding mechanisms of Ag/LSCO electrical contact material erosion were studied. The main contents and conclusions are as follows:(1) LSCO ceramic powders were prepared by solid-phase method, sol-gel method, hydrothermal method and electrostatic spinning method, respectively. In order to realize the controllable preparation of LSCO ceramic powders with different morphologies, the influences of varieties and amount of the complexing agent, pH value, concentration of precursor, reaction time, reaction temperature and heat treatment process on the structure and morphology of LSCO ceramic powders were investigated. The results show that:(i) For the preparation of LSCO particles, the solid phase method and sol-gel method can be used to prepare La1-xSrxCoO3_s (x=0.1-0.7) particles. Compared with solid-phase method, the average size of LSCO particles prepared by sol-gel method is about50nm. The LSCO nanoparticles prepared by sol-gel method can decompose and release O2at the temperature ranged from700℃to950℃(close to the melting point of silver of960℃). This feature makes the LSCO nanoparticles more suitable for being used as a reinforced phase of electrical contact material.(ii) For the preparation of LSCO microspheres by hydrothermal method, the high-purity and well-dispersed LSCO microspheres with average particle diameter of5-10μm can be prepared under the conditions that the molar ratio of citric acid to the total amount of metal ions is2:1, the reaction temperature is180℃and the reaction time is30h.(iii) For the preparation of LSCO fibers by electrostatic spinning method, the optimum preparation conditions are that the spinning forming agent is prepared by3.5wt%PVP with the average molecular weight of1300000, and the concentration of metal ions is0.125mol/L. The spinning speed can reach9ml/h under these conditions. After treating the LSCO precursor fiber at800℃, the LSCO ceramic fibers with diameters ranged from0.5μm to2μm can be prepared.(2) Silver-loading modification of LSCO powders with different morphologies is carried out. Silver-loaded LSCO composite particles (LSCOmp), silver-loaded LSCO microspheres (LSCOms) and silver-loaded LSCO fibers (LSCOmf) were prepared by mechanical milling technology, hydrothermal method and electrospinning method, respectively. The results show that:(i) For the preparation of LSCOmp by mechanical milling technology, the optimum conditions are that the mass ratio of Ag powder and LSCO ceramic is80:20, the content of PEG6000is3wt%, and the powders are milled for40h in the air. Under these conditions, LSCOp can be well inserted into silver, leading to the formation of globular LSCOmp.(ii) For the preparation of LSCOms by hydrothermal method, the purity of LSCOms can reach a high level when using glucose as a reducing agent. Although precursor microspheres experiences LSCO ceramic crystallization sinter at800℃, the silver particles can still fully loaded on the surface of LSCO microsphere.(iii) For the preparation of LSCOmf by electrospinning method, silver mirror reaction is introduced in the electrospinning process. The surface of obtained precursor fibers is rough and coated with small particles. After thermal treatment, LSCOmf with the diameter of0.5-2μm can be obtained and are loaded by silver uniformly.(3) Ag/LSCO electrical contact materials were prepared by powder metallurgy method combined with repressing and resintering process, and the influence of LSCO morphology and silver-loading modification on their properties. The results show that:(i) For the preparation of Ag/LSCO electrical contact materials by powder metallurgy method, the optimum repressing and resintering process is that the powders are first suppressed at600MPa and sintered at880℃, and then suppressed at800MPa and sintered at880℃. Under these conditions, the highest density, hardness and minimum resistivity can be achieved in Ag/LSCOmp electrical contact materials.(ii) The performances of Ag/LSCO electrical contact materials prepared by LSCO reinforced phase with three different morphologies have their own advantages each other. The electrical contact material prepared with LSCOP reinforced phase has advantages in improving the arcing time, arcing energy, bouncing times and material loss, but leads to a higher resistivity, a higher contact resistance and a poor welding resistance performance. For the LSCOS reinforced phase, the ability of lowering resistivity and the contact resistance is between that of the two other reinforced phases. Electrical contact materials prepared with LSCOs reinforced phase have a good welding resistance performance, but leads to a longer arcing time, a higher arcing energy, a more bouncing time and a worse lost of materials. LSCOf reinforced phase has advantages in improving the physical performance and contact resistant of the materials. Electrical contact materials prepared with LSCOf reinforced phase have a good welding resistance.(iii) From the aspects of physical properties, for Ag/LSCOmp electrical contact materials, the density is increased from9.57g/cm3to9.72g/cm3, the Vickers hardness is increased from80.07to102.53, the resistivity is reduced from3.73μΩ·cm cm to3.10μΩ·cm; For Ag/LSCOms electrical contact materials, the density is increased from9.51g/cm3to9.75g/cm3, the Vickers hardness is increased from85.65to94.97, the resistivity is reduced from2.96μΩ·cm to2.17μΩ·cm; For Ag/LSCOmf electrical contact materials, the density is increased from9.72g/cm3to9.78g/cm3, the Vickers hardness is increased from88.09to102.70, the resistivity is reduced from2.81μΩ·cm to2.03μΩ·cm;(iv) From the aspects of electrical properties, for Ag/LSCOmp electrical contact materials, the average contact resistant is reduced from9.31mΩ to6.29mΩ, the arcing time is reduced from6.74ms to5.61ms, the arcing energy is reduced from557.40mJ to439.96mJ, the average bouncing time is reduced from1.18to1.12, the lost percent of material transfer is reduced from7.63%to0.07%, the welding resistance is improved; For Ag/LSCOms electrical contact materials, the average contact resistant is reduced from5.52mQ to5.42mQ, the arcing time is reduced from18.22ms to14.78ms, the arcing energy is reduced from3021.80mJ to2045.46mJ, the average bouncing time is reduced from1.97to1.75, the lost percent of material transfer is reduced from13.27%to0.77%, the welding resistance performance maintains well, the combination property is improved; For Ag/LSCOmf electrical contact materials, the average contact resistant is reduced from4.41mQ to4.27mΩ, the arcing time is reduced from15.39ms to14.01ms, the arcing energy is reduced from2511.85mJ to2369.02mJ, the average bouncing time is reduced from1.80to1.60, the lost percent of material transfer is reduced from12.42%to1.05%, the welding resistance maintains well, the combination property was improved.(V) Ag/LSCOmp electrical contact materials have a high contact resistant, whereas other properties are better than that of Ag/CdO and Ag/SnO2prepared at the same conditions. Ag/LSCOmp is therefore expected to replace Ag/CdO in the contact resistance of less demanding areas. Ag/LSCOms electrical contact materials have a high contact resistant and its arcing time as well as bouncing time is superior to Ag/CdO and inferior to Ag/SnO2, Whereas other properties are better than that of Ag/CdO and Ag/SnO2. Ag/LSCOms is expected to replace Ag/CdO in the contact resistance of less demanding areas. The combination property of Ag/LSCOmf is better than Ag/SnO2prepared at the same condition. Though the material transfer and electrical life are inferior to Ag/CdO, its other properties are superior to Ag/CdO. Ag/LSCOmf electrical contact materials are expected to replace Ag/CdO in the electrical life of less demanding areas.(4) The arcing erosive behaviors of Ag-based electrical contact materials reinforced by LSCOP powders before and after Ag-loading modification. The results shows that:(i) There are five kinds of arcing erosive morphologies on the Ag/LSCOp electrical contact after arc erosion, such as undulating hill structure, globular structure, skeletal structure, pore or hole, and crack. These characteristic morphologies might be associated with a weak binding force between LSCO reinforced phase and silver, and the composition segregation after arc erosion. Once these erosion morphologies appear, the performance of the material will decline sharply.(ii) There are two kinds of arcing erosive morphologies on the Ag/LSCOmp electrical contact, such as spongeous structure and corrugate structure. These characteristic morphologies might be associated with the strong binding force between LSCO reinforced phase and silver matrix, improved wettability, increased uniformity and the improved recombination viscosity during arcing erode. These indicate that Ag/LSCOmp have high arc erosion resistant capacity, stable performance, and long electrical life.(iii) The arc erosion behaviors of Ag/LSCO electrical contact material are related to their structure and preparation process. By means of mechanical milling technology, the Ag-loading modification of LSCO can efficiently improve interfacial bonding between LSCO and silver matrix, thereby increasing the arc erosion resistance property of Ag/LSCO electrical contact material. |
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