Silver dissolution and precipitation in an Na2O–ZnO–B2O3 metallization paste glass

Thermally stimulated interactions between silver and glass, that is, silver dissolution as Ag⁺ and precipitation as Ag⁰ were studied in two glass series of molar target composition xAg₂O–(19 − x)Na₂O–28ZnO–53B₂O₃ with x = 0, 0.1, 0.5, 5 and (19Na₂O–28ZnO–53B₂O₃)+yAg₂O with y = 0.01, 0.05. These act as model for low-melting borate glasses being part of metallization pastes. The occurrence of metallic silver precipitates in melt-quenched glass ingots demonstrated that silver dissolved only in traces (< 0.01 mol%) in the glasses. The dissolved silver was detected by means of Raman spectroscopy and energy-dispersive X-ray spectroscopy. Increasing x in the batch could not lead to a significant increase of the silver ion fraction in the glass as possible in binary silver borate glasses. In situ observation of heated AgNO₃ mixed with the base glass frit in a hot stage microscope showed that Ag⁰ precipitation occurs already at the solid state. At higher temperatures, small droplets of liquid silver were found to move freely within the melt, whereas coalescence caused a stepwise increase of their size. These results contribute to the understanding of formation of silver precipitates in metallization pastes described in the literature.