Preparation And Properties Of Ag/ZnO Electrical Contact Material

Silver-based electrical contact materials are multiphase system or composite materials and have been well developed and extensively applied in the electronic industry, such as control systems, electric instruments and so forth. Among them, metallic oxide-reinforced silver multiphase system with good electrical and thermal conductivity, welding resistance and oxidation resistance properties are mainly used as upgraded electrical contact materials. Silver cadmium oxide (Ag/CdO) composites has been called as "Universal Contact" for decades because of its low and stable contact resistance, strong resistance to corrosion and other excellent properties. However, Ag/CdO causes environmental and health hazards due to toxic CdO vapours and strict environmental regulation, such as the Restriction of Hazardous Substances of the European Union and other series of treaties limit its usage. Silver-zinc oxide (Ag/ZnO) composites, as a pollution-free contact material, has a wide range of applications in relays, contactors, circuit breakers, switches due to its low and stable contact resistance, good anti welding performance, excellent arc resistance corrosion resistance. However, ZnO can easily aggregate to the interface between Ag and ZnO and the mixture of Ag and ZnO can deposit from the silver melt on the contact surface during arcing. Ag/ZnO exhibits worse high temperature behavior than that of Ag/CdO.In this thesis, attempts have been made to study the preparation and properties of modified ZnO reinforced phase and Ag/ZnO electrical contact material. Firstly, ZnO powders with different morphologies were prepared by co precipitation method and hydrothermal method, and the modified ZnO was achieved by the method of redox method and sol-gel method. Secondly, Ag/ZnO electrical contact materials was prepared by powder metallurgy method using ZnO before and after the modification as reinforced phase respectively in order to study the preparation method, morphology, surface modification of ZnO reinforced phase and additives on the mechanical and electrical properties of the Ag/ZnO composites. Finally, Ag/ZnO wires and rivets were prepared through hot pressing and hot extrusion, swaging and drawing and upsetting technology. The influences of process and parameters on wire mechanical and electrical properties were studied in-depth and it also probed into the anti arc erosion mechanism of Ag/ZnO wire. The main contents and results are as follows:(1) ZnO particles, microspheres and micron rods can be prepared by hydrothermal method, precipitation method and Ag nanoparticles coated ZnO, oxide coated ZnO and modified ZnO solid solution can be achieved via the redox method, sol-gel method. The modified ZnO powders have a stable performance. (Ⅰ) ZnO powders average size of 2 μm～5/μm with rod-like shape, triangle cone shape and spheres shape can be achieved by controlling the content of TEA thorugh hydrothermal method process, reacting in 160℃ for 2 h. After calcineing in 800℃, ZnO powders can still remain stable morphology. (Ⅱ) When pH=9, water bath reaction at 60℃ and using sodium dodecyl benzene sulfonate as dispersant, Ag modified spherical ZnO powders can be produced by redox method. (Ⅲ) NiO and CuO coated spherical ZnO powders can be successfully prepared by sol-gel method. A solid solution Zn0.2Ni0.8O formed when NiO modified spherical ZnO. The relative concentrations of ZnO and Zn0.2Ni0.8O were 69.85(0.44)% and 30.15(0.28)% respectively. The relative concentrations of ZnO and CuO were 75.20(0.40) wt% and 24.80(0.26) wt% respectively. (Ⅳ) When there was 5 wt% Ni or Co, calcineing in 500 ℃, ZnO solid solution with good dispersion can be prepared by sol-gel method.(2) Ag/ZnO electrical contact materials were prepared by powder metallurgy method (PM) using particles ZnO, microspheres ZnO, micron rods ZnO, CuO@ZnO, NiO@ZnO and ZnO solid solution as reinforcing phase respectively. The effects of preparation methods, morphology, modified materials, additives, sintering temperature and sintering time on the relative density, hardness, electrical conductivity and electrical life of Ag/ZnO composites were investigated. (Ⅰ) Firstly, the mechanical and electrical properties of Ag/ZnO using ZnO prepared through co-precipitation method as a reinforced phase (Ag/ZnO(c)) were significantly superior to that of Ag/ZnO using ZnO prepared through ammonization method as a reinforced phase (Ag/ZnO(a)). Ag/ZnO(c) had a relative density of 95.2%, hardness of 71.4 Hv and electrical conductivity of 2.43μΩ.cm. The corresponding phases of Ag/ZnO composites have changed from well-crystallized states to amorphous states after the life testing. From surface profiles, obvious holes, Ag enrichment zones, ZnO enrichment zones, Ag skeletons, bubbling areas were observed in both two eroded contacts. Vertically aligned ZnO nanorods arrays were observed for the first time on the surfaces of eroded Ag/ZnO(c) composites after the life testing without any other supporting equipment. (Ⅱ) The dispersity, size and shape of ZnO have effects on properties of Ag/ZnO composites. Ag/ZnO composites with amorphous ZnO particle of the smallest size had the best relative density and electrical conductivity, which were 94.4% and 37 MS/m respectively. The one with rod ZnO of the largest size had the least mass loss and the best resistance to arc erosion. (Ⅲ) Sintering temperature had a significant influence on properties of Ag/ZnO composites. The optimum sintering temperature of coated spherical ZnO as reinforced phase for the preparation of Ag/ZnO composites was 850℃. Electrical conductivity, relative density and hardness of Ag/ZnO composites were the best by the addition of NiO solid solution Zn0.2Ni0.8O. They were 34.8 MS/m,95.8% and 52.8 Hv respectively. The addition of CuO made arcing energy and mass loss of Ag/ZnO contacts lower, which were 9.1 mg and 1700 mJ for beraking energy and 800 mJ for making energy. (IV) The mechanical and and electrical properties of the Ag/ZnO composites with different additive depended on the sintering temperatures and sintering time highly. The addition of CuO can improve the performance of Ag/ZnO composites effectively and when ZnO and CuO was at the ration of 1:1 and sintered at 850℃ for 4 h, it had the best mechanical properties, electrical properties, arc corrosion resistance and the least mass loss. Besides the normal morphologies observed in erosion contacts, ZnO nanorods arrays were observed on the surfaces of eroded Ag/ZnO contact with certain contents of CuO after the life testing without any other supporting equipment. When ZnO and CuO was at the ratio of 2:1, the formation of ZnO nanorods can be described using SVLS mechanism. When ZnO and CuO was at the ratio of 1:1, the formation of ZnO nanorods can be described using SVS mechanism. (V) XPS analysis was used as a tool to predict the relative degree of Ag splashing. It has the potential to serve as an index for evaluate the anti-arc property for Ag/ZnO practical applications in low currents apertures in the future.(3) Ag/ZnO wires and rivets were prepared through hot pressing and hot extrusion, swaging and drawing and upsetting technology. The influences of the number of drawing on wire mechanical and electrical properties were studied in-depth. The influence of load current and operating cycles on the electrical life of rivet were studied and the erosion mechanism of Ag/ZnO electrical contact material and devices were discussed. (Ⅰ) The properties of Ag/ZnO electrical contact materials can be improved by the large deformation process such as drawing. Ag/ZnO electrical contact material prepared using the rod like ZnO as the reinforcing phase had the best performance. (Ⅱ) With the increase of the current, the mass loss and welding force of different rivets had gradually rising trends, arcing time and arcing energy reduced to a certain extent. Rivet made from Ag/ZnO composites prepared using the rod like ZnO as the reinforced phase had an relatively lower mass loss, arcing time and arcing energy under the condition of low current (9 A～40 A) and better stability under the condition of high current (50 A～60 A). As the increase of operation cycles under the same operation current, arc erosion regions showed as a center divergent expansion trend and arc region appeared as corrosion pits and protrusions. Rivet made from Ag/ZnO composites prepared using the rod like ZnO as the reinforced phase started to produce mass loss latest and Rivet made from Ag/ZnO composites prepared using the rod like ZnO as the reinforced phase started to produce mass loss earliest. (Ⅲ) Arc energy during the switching process will cause serious corrosion on contact surface. Corrosion morphologies of prepared Ag/ZnO electrical contact materials can be divided into bubble enrichment zone, Ag enrichment zone, ZnO enriched area, pore, skeleton, droplet splash zone, regional fracture and atypical morphology region.