Dissolution and reprecipitation of silver in sintering silver-glass dispersions
Synthesis and Properties of Functional Materials
Final Report Abstract
Size/field strength of alkali ions direct the type of borate units and the proportion of 4-fold coordinated boron N4. Hence, atomic packing density, Tg, and viscosity of the glasses vary in relation to the alkali component present in the sample. Whereas N4, Tg and viscosity only slightly increase with the alkali field strength F, the packing density APD strongly increases. Nevertheless, the diffusion coefficients of silver show only minor variation among the different glasses LZB, NZB, KZB and RZB. Obviously, silver diffusion is governed only by glass viscosity or related structural features, i.e., NBO or N4. Thus, the mixed alkali effect is not relevant to silver diffusion. Diffusion coefficients are very low (10^-14–10^-13 cm2/s) comparable to data attributed to Ag/O2-co-diffusion in previous literature. Conformingly, diffusion seems to be slightly promoted by ambient oxygen. Melt dissolution experiments on NZB show that Ag dissolution is very limited, i.e., < 0.01mol%. Despite this low dissolution limit and the measured small silver diffusion coefficients, essential silver transport could be detected during the metallic silver – glass diffusion pair experiments. In the result of solution and precipitation, various silver species were formed after interaction between glass and silver: Ag+ as isolated ion or part of Ag+-Ag+ pairs or molecular clusters [Agm]n+ (UV-vis and fluorescence spectroscopies); nano-sized spheres (TEM), larger droplets or particles (heating-stage microscopy). In the sintered pastes, silver spheres and dendrites could be distinguished by means of electron microscopy. Despite their different crystallization behavior, all alkali zinc borate glasses under study show a quite similar early sintering, whereas the sintering onset slightly decreases with decreasing alkali field strength, reflecting the decrease in viscosity. This finding is consistent with the revealed structural similarity among these glasses and the weak trends caused by different alkali ions. Sintering atmosphere has minor impact on sintering behavior of silver-glass-paste, that fits the observation regarding the low silver dissolution limit.
