Wetting of ZnO-ceramic with alloys of the silver-copper system in vacuum

Zinc oxide is a wide-gap semiconductor with unique properties, used for the manufacture of catalysts, electrodes, transistors, etc. In these applications, there is a need for metallization and joining of ZnO-based materials, using of molten metal filler is an effective method. Contact of zinc oxide and liquid metals is almost not studied in comparison to other oxide materials. In this work the wetting of zinc oxide based ceramic with metal melts of the silver-copper system in high vacuum was studied. Alloys with concentration of copper (% (at.)) 0 (pure silver), 5, 10, 20, 30, 39, 100 (pure copper) were used. Increasing the concentration of copper in the silver-copper melts significantly improves the wetting of ZnO-ceramic with these liquids, the contact angles decrease from 137° for the pure silver to 28° for the pure copper. Investigations of the microstructure show presence of relatively thin transition layers on the interfaces for silver-copper alloys, for the pure copper case the interface has complex “island-like” microstructure. Also a ruination of ZnO-ceramic substrates due to an interaction with metal melt was noted, the higher copper concentration, the more intensive ruination. Also some signs of the presence of zinc in the metal drops solidified on the zinc oxide surface were revealed, such as, intensive oxidation of the drops in air at room temperature. The oxidation shows complex multiphase microstructure of the solidified drops. After wetting of the zinc oxide with pure copper the solidified metal has microstructure of brass (Cu—Zn alloy) with low zinc concentration. The effects observed can be explained by sublimation of the zinc oxide under experimental conditions (high temperatures, vacuum) with formation of gaseous products (oxygen and vapor of zinc), which subsequently dissolve in silver-copper liquid drops. In particular, the dissolution of oxygen in the Ag—Cu melt improves its wetting of surfaces of solid oxides and presence of zinc in solidified drops provides its oxidization in air. Keywords: zinc oxide, wetting with metals, contact interaction, microstructure, transition layer.