Effect of Y2O3 addition and milling time on the synthesis of nanocrystalline Ag-ZnO composite powder via mechanical alloying

The combined effect of Y2O3 addition and milling time on the synthesis and characterization of Ag-ZnO electrical contact material was investigated via mechanical alloying technique. For this purpose, a planetary ball mill including tungsten carbide grinding medium, was used to synthesize composite powder containing elemental silver (Ag), zinc oxide (ZnO) and yttrium oxide (Y2O3) powders. Meanwhile, stearic acid was also added to the powder mixture to limit excessive cold welding and agglomeration of the powder particles. The average particle sizes (APS) and morphological change of powders before and after milling were characterized by laser diffraction analysis (Mastersizer) and scanning electron microscopy (SEM), respectively. The powder mixture was exhibited minor fluctuations in APS during the early stages of milling. Further, the tendency of fracturing event predominates over cold welding. Since fine-grained (or nanocrystalline) materials are superior in properties and performance to those of conventional ones, it is essential to design new materials having fine and homogeneous dispersion of constituent powder particles via mechanical alloying technique. However, it is also important to reduce powder contamination originated from excessive milling, which has detrimental effects for electrical contact applications. Hence, the optimal milling time to achieve nanocrystalline grain size was determined. Accordingly, APS was decreased from 18.646 mu m to 1.077 mu m after the ball -milling time of 25 h, and equiaxed nanostructured material was synthesized. The use of 1 wt% of Y2O3 ceramic particles was found effective to reduce particle size to nanoscale. Therefore, 25 h of milling duration was found to be sufficient to obtain nanocrystalline Ag-ZnO-Y2O3 composite powder having fine and homogeneously dispersed Y2O3 dopant by virtue of particle size reduction.