Rapid migration of silver on roughened glass surfaces in a methanol/air mixture near 500 °C

Abstract

Experimental results presented here show that at elevated temperatures (about 500 °C) silver can be mobile on tin oxide, as well as other metal oxide surfaces. Because the migrating species seems to be Ag+ both diffusion and electrically assisted migration occurs. This movement is made visible in the form of tiny silver droplets on the glass surface by the presence of methanol vapor mixed in air. Without oxygen no migration occurs at all. It is clear that the movement requires a continuous or dynamic conversion of silver ions to silver oxide and back again, through the presence of both oxidizing and reducing agents. We show how it is possible to measure the diffusion constant and the electrical mobility constant by an appropriate analysis of the time evolution of a circular front of silver droplets measured with a video camera connected to a microscope zoom lens. Typical values on a roughened glass surface at 500 °C are for mobility, 42±13×10−6 cm2/V s, and for diffusion 12±6×10−6 cm2/s. Similar values for the electrical mobility on a smooth tin oxide surface have been measured. At this temperature diffusion on pure silica or even Pyrex glass powder is very slow, but the diffusion constant for MgO or CaO powder, annealed onto a sandblasted glass surface, is about 10−4 cm2/s. These oxides may be responsible for the movement seen on soda glass surfaces. The activation energy is about 4 eV and seems related to the work function of silver metal.