Research On The Preparation And Properties Of Silver Tin Oxide Composite Material

Silver tin oxide (AgSnO₂) is a composite material, in which SnO₂ particles are dispersed homogeneously in Ag matrix. Nowadays AgSnO₂ is the most promising substitute for toxic silver cadmium oxide(AgCdO) material for electrical contacts. Lots of work has been done on the preparation, processing, use of additives and effect of arc on the microstructure of AgSnO₂ material. But the drawbacks of inhomogeneous microstructure, bad processability and high resistivity are not well solved, which makes the study on the preparation and properties of AgSnO₂ material be practical and theoretical work.The fine and nanoAgSnO₂ powders with core/shell microstructure were prepared by electroless silver plating method assisted by ultrasonic. The control of composition and coating homogeneity were investigated, and SEM, EDS, XPS were used to characterize the Ag coating. AgSnO₂ composite material were prepared by powder metallurgy, and the factors influencing the sintering properties and microstructure were investigated. The mechanism of the change of calcined composite powder morphology and its effect on sintering process was investigated. The formation mechanism of special microstructures in sintered nanoAgSnO₂ powders was analysized, and a good sintering process was given. The followings are the conclusions in detail.The composition of the AgSnO₂ composite powders was controlled by the ratio of reduced silver from the solution, and the ratio was influenced by the plating process. In the solution using formaldehyde (HCHO) as the reductant, the concentration of sodium hydroxide (NaOH) was the decisive factor, while using hydrazine hydrate (N2H4H2O) as the reductant, the ratio of reduced silver was mainly affected by the concentration of N₂H₄-H₂O. The homogeneity of silver coating was influenced by the composition of the plating solution, and using HCHO as the reductant could make better homogeneity than N₂H₄-H₂O, while activated SnO₂ couldn't get more homogeneous coatings than non-activated powders. The mechanism of silver nucleation and growth on surface of SnO₂ caused the difference.AgSnO₂ powers with submicron SnO₂ core were prepared due to the hard agglomeration of nano SnO₂ powders, while AgSnO₂ powders in size of 140nm could be prepared using nano SnO₂ slurry (mean size of 80nm) by electroless plating assisted with ultrasonic.Bubbles occurred on the surface of the compact when the fine AgSnO₂ powders prepared by electroless plating were pressed directly and sintered, which was attributed to the adsorbed volatile gas on the surface of the powders. These bubbles could be removed by calcination of the fine AgSnO₂ composite powders before compaction. The sintering ability of calcined fineAgSnC>2 powders was decreased due to the growth of the powders and the segregation of SnC>2 and Ag. The aggregate of SnCh on the surface of composite powers hindered the sintering process. Density of the sintered compact was more affected by compaction pressure than sintering temperature and soaking time. Defects of black stripe and network were observed in the optical microscope, and these defects couldn't be removed by adjusting sintering process and electroless silver plating method. It was showed that the aggregation of SnO2 powders and pores at the grain boundary composed these defects. The morphology variation of calcined fine AgSnC>2 powders was attributed to the non uniform particle distribution, the connection of silver coating and bad wetting ability between Ag and SnO2. Adding WO3 and Bi2O3+CuO by precipitation could lessen the segregation of SnC>2 from Ag, and in which Bi2O3+CuO was better. The fine AgSnC>2 composite powder using Bi2O3+CuO as the additives could acquire a relative density of 94% by the process of compaction (300MPa ), sintering in air(880°C,lh), repressing (600MPa) and resintering. No defect microstructures could be observed any longer.NanoAgSn02 composite powders could strongly adsorb gases and must be calcined at a temperature over 650 °C .The calcined particles were distributed in size of 200⁵00nm, and most of them had core/shell microstructure. NanoAgSnCh powders had good sintering ability, and relative density of 92% could be achieved by the process of sintering in air, repressing at 700MPa and resintering. No defect microstructures of black stripe and network were observed.As must be pointed out that two special microstructures were observed in the sintered nanoAgSnC>2 compact. The first one was the inner silver aggregates, and the second the pure silver layer on the surface of the compact. They were formed due to the size and microstructure of the composite powders, and could be illustrated with the vacancy diffusion mechanism. The fast densification and pure surface layer could make the compact surface fully dense during sintering, which deterred the leakage of gases in the compact. The densification of surface was determined by compaction pressure and sintering temperature, and the lower the pressure, the higher temperature. A step isothermal sintering process was carried out to not only remove the gases in the compact but also decrease the big pores: compact at low pressure, then sintering to remove the adsorbed gases, and then sintering at higher temperature. This process could achieve a relative density of 94%.The AgSnO2 material prepared by electroless plating and powder metallurgy had good processability, and the extruded wire had high elongation and low resisitivity. The defectsmicrostructure of black stripe and network in the sintered compact were great improved after mechanical processing. NanoAgSnCh composite material with homogenous microstructure was achieved successfully by hot extrusion. The extruded wire had high hardness, tensile strength and elongation., but its processability was decreased due to the second phase dispersion strengthening and the remained pores. Small degree of deformation was needed when the wire was drawn, and repeated annealing was necessary. The resistivity of nanoAgSnO2 wire was much higher than the fine material attributing to the increase of scattering of nanoSnCh particles. The results of arc erosion on the surface of fine AgSnCh rivets indicated that silver rich layer and SnC>2 aggregates were formed, and they were greatly improved by the use of additives due to the increase of wettability of liquid silver on Sn(>2 particles. For nano Ag SnC>2 material, silver rich layer was formed as well after arc ersion, but no SnO2 aggregates were observed, which was attributed to the decomposition of nano SnCh particles under the high temperature of the arc. This character could improve the electrical properties of the material.