Sodium/silver ion exchange between a non-bridging oxygen-free boroaluminosilicate glass and nitrate melts

Abstract

A sodium boroaluminosilicate glass, in which no non-bridging oxygen could be detected, is contacted with AgNO3—NaNO3 melts at 375 °C. A diffusion-controlled Na+/Ag+ exchange has been observed in the bulk of the glass up to a degree of exchange of 70 % (Fickian behaviour). For a higher degree of exchange network-structure relaxation processes appear as a result of the viscosity-lowering effect of silver, causing a change of the diffusion conditions during the ion exchange in the glass (non-Fickian behaviour) . The time-independent equilibrium concentrations at the glass surface refer to a remarkable affinity of the glass for the highly polarisable Ag+ cations (degree of exchange: 98 %). This result can be explained by coordinative binding of silver on the basic bridging oxygen of the AlO4/2 tetrahedrons into the glass network.