Effect Of Additive On The Morphology And Performance Of AgSnO₂ Electrical Contact Material

Silver Cadmium Oxide is considered as universal electrical contact for its excellent anti arc erosion properties. Current developmental activities are focused on the innocuous and high performance AgSnO₂ contact materials to replace the hazardous substance AgCdO. But AgSnO₂ contact materials have the problem of low workability, high temperature rise and high welding tendency. Based on the analyse of disabled AgSnO₂ contact materials, The thesis take the means of surface modification on SnO2 to improve the wettability of silver and tin-oxide, finally get the high performance AgSnO₂ contact materials.The former research is mainly about the electrical performance of electrical contact, so need to establish the obligatory theory for developing new electrical contact. Firstly, the thesis summarize the relationship between material factor and electrical performance. The material factor include matrix, second phase, additive and material defect of the material. By and large, the relationship between performance and component, morphology, structure has been established in this thesis. Secondly, the appearance and surface structure of disabled electrical contact, especially the ejection around the contact are investigated by means of stereomicroscope, optical microscopy, scanning electronic microscopy (SEM) and energy spectrum. Thirdly, AgSnO₂ composite powders with additives CuO, Bi₂O₃ were prepared by electroless silver plating. The powders were then sintered by the powder metallurgy of press-sinter process. The metallurgical structure of sintered samples and the morphology of AgSnO₂ composite powders were investigated by means of optical and scanning electronic microscopy. To further the study, AgSnO₂ composite powders were prepared by the method of surface modification on SnO₂ which was modified with WO₃ and CuO+Bi₂O₃ additives by precipitation or evaporation. The AgSnO₂ composite powders were then sintered into samples. By hot extrusion, wire drawing and subsequent treatment, finally get the AgSnO₂ wire and the rivet. The effect of SnO₂ surface modification on the sintering character, metallurgical structure, mechanical property and electrical performance of AgSnO₂ electrical contact is discussed.The analyse of disabled electrical contact indicate that the luster of ejection is related to the mechanism of arc erosion. The grey ejection of AgCdO is mainly caused by splash while black ejection of AgSnO₂ is mainly caused by evaporation. The black ejection of AgSnO₂ is related to the beard shape morphology deposited by evaporant. the characteristic of surface structure of disabled electrical contact is that silver rich layer is on the upper layer while theoxide enrichment zone is under the silver rich layer. The bad wettability of silver and tin-oxide is the main reason of the separation between silver and tin-oxide.The experiment of the effect of additive on the sintering character and the morphology of sintered AgSnC>2 composite powders indicated that the metallurgical structure of AgSnC>2 material without additives exists black net like morphology. The reason is relate to the segregation between tin-oxide and plated silver layer when the composite powders is calcined before sintering. Additives both CuO and Bi2O3 have the effects of improving the sintering density of AgSnC>2, the effect of CuO is much more remarkable. Additive Bi2O3 has obvious effect on eliminating the net like morphology. By adding both CuO and Bi2O3 into electroless silver plated on SnO2 powder, AgSnO2 material's density is improved and the fine oxides distribute more homogeneous in the silver matrix. Besides, the metallurgical structure of the sintered samples by the method of the additives add to the plated powder is more homogeneous than that of additives add to the powder before plating.Using the modifying agent WO3 and CuO+Bi2O3 by surface modification, the density of sintered samples is improved and the net like morphology is eliminated. After surface modification on the tin-oxide, the wettability between silver and tin-oxide is improved, so the segregation between silver and tin-oxide during calcining is weakened. The effect of CuO+Bi2O3 is better than WO3 and modifying method of precipitation is superior to that of evaporation. The AgSnO2WO3 wire with density 9.92g/cm3, hardness Hv90, tensile strength 285Mpa, elongation percentage 26% and the AgSnO2/CuO/Bi2O3 wire with density 9.95g/cm3, hardness Hv91, tensile strength 302Mpa, elongation percentage 22% are finally prepared. It shows that the mechanical property of AgSnO2 wire has been remarkably improved by surface modification. Furthermore, the surface structure after arc erosion have been improved by surface modification on the tin-oxide particles. The effect of WO3 is better than that of CUO+B12O3. The impurity and the aggregation of SnO2 may appear on the surface of the wire during wire drawing. This phenomenon is bad to the quality of the wire surface. Impurity has main effect on the electrical performance, as it not only makes the melt area easier to splash, but also the rivet surface getting coarse after cooling.