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Fabrication Of A Newly Developed AgC Electrical Contact Material And Research Of Its Properties

Posted on:2006-06-04Degree:DoctorType:Dissertation
Country:ChinaCandidate:H F YuFull Text:PDF
GTID:1101360155960340Subject:Materials science
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In view of the distinguished feature and application prospect of nanomaterials, the nanotechnology was first applied in the fabrication of silver/graphite electrical contact materials, and newly developed AgC electrical contacts were prepared in the thesis. And also, their physical and mechanical properties and arc erosion resistance were systematically researched.To improve the properties of traditional blending AgC electrical contact material, electroless plating technique was employed in powder preparation to improve their component segregation and microstructure nonuniformity. Nanocrystalline AgC coating powders were then prepared under the combination of the reducer liquid spraying-electroless plating method and nanosized graphite, which came from the high-energy milling and worked as the heterogeneous cores of Ag atoms nucleating. Because of its well sintering densification, the properties of block contacts were totally improved.The graphite powders with over 99.5% content of C and 200 mesh granularity were used as raw material and milled for the best ten hours on the QM-1SP planetary mill and one-dimension nanosized graphite was then fabricated, with the average thickness of 50-60nm. The X-ray diffraction test showed that the average grain size of Ag in the as-prepared electroless plating Ag-5%C powders was about 50nm.In the thesis, the sintering properties of fabricated nanocrystalline AgC electroless plating powders and the physical and mechanical properties of their block contacts were researched, along with the influence of milling time on their properties and microstructure and the sintering time on the properties. At the same time, the AgC contacts fabricated by three different techniques were compared on their microstructure and properties and accordingly a brief mechanism model was established. At last, the influence of nanocrystalline electroless plating powders on the properties of conventional blending AgC contacts was researched.The research showed that with milling time going, graphite orientation structure appeared in the AgC block contacts. At that time, the electrical conductivity went lower, which was highest when uniformly microstructured, but it rose again as the orientation structure growing, with the decline of its hardness and density ratio at thesame time. While with the sintering temperature growing, the density ratio, hardness and electrical conductivity of the contacts increased and arrived their highlight at about 840 °C. The Ag-5%C material fabricated by milled graphite spraying-electroless plating technique had superior physical and mechanical properties and uniform microstructure to those made by blending and dropping-electroless plating techniques. With the reducer liquid spraying-electroless plating method, the contact area between reaction solution and reducer in unit time and the ratio of graphite powders separated in the reaction solution working as heterogeneous cores of Ag atoms nucleating were greatly increased. At the same time, the local concentration of reducer in reaction solution was largely reduced, and then the growth of Ag atoms was suppressed. Because of the factors mentioned above, the refinement of electroless plating powders and their grains and the improvement of electroless plating effect were achieved, and the component segregation of graphite in the Ag matrix was well eliminated. Mixed with various content of the as-prepared nanocrystalline electroless plating powders, the micropores in blending AgC material were filled by the growth of nanocrystalline grains. As a result, the mechanical and physical properties of the fabricated contacts were improved.The uninterrupted experiment for erosion behavior of the prepared new type AgC contact and its traditional blending counterpart by breaking arcs were done by using an ASTM Contact Material Automatic Measuring Device. In the meantime, the experiment for erosion behavior of 6 group contacts, the two contacts mentioned and four blending contacts mixed with nanocrystalline electroless plating powders, by breaking arcs were done. The arc erosion resistance properties and characteristics of the as-prepared new type AgC contact material were tested and studied, and also the improvement mechanism of the former was analyzed and discussed.As was shown in the uninterrupted experiment for erosion behavior, compared with coarse graphite blending material, the new type AgC electrical contact had less average weight loss of breaking arcs and 40% higher arc erosion resistance and better resistance against welding. In the experiment for erosion behavior by stages, the samples had similar weight loss at the beginning, but with breaking time growing, the fabricated new type AgC contact showed better arc erosion resistance at every stage than blending contacts. For blending materials, the relationship between their loss and breaking times took the shape of an exponent function curve, whose exponent was larger than 1. That meant in the anaphase of breaking, the arc erosion got aggravatedand the properties of contacts got worse and even noneffective because of the existence of electric vortex erosion, which was caused by the aggravation of the potholes of contact surface. And for milling-electroless plating Ag-5%C contact, the relationship between its loss and breaking times presented the shape like a linearity function curve, which meant the weight loss tended to be stable at every stage and the situation mentioned above won't happen. After arc erosion, such morphology characteristics were formed on the contact surface of AgC materials as loose structure, Ag enrichment structure, graphite sediment structure, arc impact crater structure, gas pore and hole structure and crack structure. Under the impact of arcs, the new type electrical contact were superior to its traditional blending counterpart to prevent the melted Ag beads to spray and get away from the matrix and keep up the formation and development of surface cracks.The new type AgC contact material, employed on the miniature circuit breakers from Schneider Shanghai low voltage terminal Apparatus Co., Ltd. and ABB Beijing low voltage Apparatus Co., Ltd., had respectively passed the short circuit circulation test by China National Centre for Quality Supervision and Test of Low Voltage Apparatus. It had been supplied to several manufacturing corporations like SSLVTA and ABB Beijing by small batch and good economic income had been achieved.Prepared by the combination of high-energy milling, electroless plating and powder metallurgy technique, with carbon nanotubes as the fiber reinforcer, the new type carbon nanotubes-reinforced AgC electrical contact material was fabricated and its National Invention Patent was applied. The carbon nanotubes aggregate employed had the size of tens of microns, which was comprised of carbon nanotubes sized 30-60nm. And the average grain size of Ag in the as-prepared carbon nanotubes-reinforced Ag-5%C electroless plating powders was about 50nm. In electroless plating powders, graphite and carbon nanotubes were coated by microsized Ag granules with flocculent structure and the floccules had small and uniform internal micropores, which was helpful to the further densification in sintering. Thus, the prepared carbon nanotubes-reinforced Ag-5%C contact showed better physical and mechanical properties compared with blending contacts and better hardness even with other milling-electroless plating contact.Although they had similar weight loss at the original stages, the new Ag-5%C electrical contact prepared by milling-electroless plating technique and nanotubes-reinforced milling-electroless plating technique showed better arc erosion...
Keywords/Search Tags:electrical contact, silver/graphite, nanotechnology, milling, reducer liquid spraying-electroless plating, arc erosion, carbon nanotubes
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