Interfacial reactions between tellurite melts and silica during the production of microstructured optical devices

Interfacial reactions between silica glass and tellurite melts were studied under confined conditions in the temperature regime of 400–700 °C, applying two different sampling techniques: isothermal heat-treatment of a several micrometer thick tellurite film, confined in a silica/tellurite/silica sandwich, and capillary filling of tellurite melts into silica microcapillaries. The sandwich technique provides detailed ex situ insights on the interface chemistry, microstructure and diffusion after given treatment times and temperatures. Data on dynamic viscosity, surface tension, wetting behaviour and eventual scaling effects was obtained from the capillary filling technique. For temperatures > 500 °C, silica is completely wet by the considered tellurite melts. At T > 600 °C and for a treatment time of 20 min or longer, cationic diffusion of Na+ and Te4+ into the silica substrate occurs to a depth of several micrometers. At the same time, the tellurite melt attacks the silica surface, leading to the formation of a stationary silica–tellurite reaction layer and silica dissolution. Dissolved silica was observed to re-precipitate from the tellurite melt by liquid–liquid phase separation. In the early reaction stages, as a result of alkali diffusion into the silica substrate, β-quartz crystallizes at the interface (what can be avoided by using alkali-free filling glasses). Obtained data set the boundary conditions for the generation of tellurite–silica all-solid fiber waveguides by melt infiltration of silica photonic crystal fibers or microcapillaries.