Noble metals Pt, Au, and Ag as nucleating agents in BaO/SrO/ZnO/SiO2 glasses: formation of alloys and core–shell structures

Abstract Noble metals such as Ag can be used as nucleation agents in glass ceramics. In glasses, it is incorporated predominantly as Ag I . At temperatures slightly above the glass transition temperature, T g , Ag I reacts with Sb III to Sb V and metallic Ag. Usually, face-centered cubic Ag particles are nearly spherical and get facetted during crystal growth. By contrast, in the case of BaO/SrO/ZnO/SiO 2 glasses, silver has, in comparison to other noble metals, another significant, yet different effect. It forms metallic particles (hexagonal phase) with plate-like morphology during thermal treatment at 675 °C. In the second step of thermal treatment at 760 °C, this phase most probably expels some metallic Sb, which is oxidized by Sb V (present in the surrounding glass phase) to Sb III . As a result, the plate-like morphology is maintained and a crystalline shell around the metallic core is formed, mainly consisting of ZnO with some SiO 2 and antimony oxide, as proved by scanning transmission electron microscopy combined with energy-dispersive X-ray spectroscopy. This shell triggers the volume crystallization of Ba 0.5 Sr 0.5 Zn 2 Si 2 O 7 , a phase with low thermal expansion. By comparison, alloying of Au with Sb does not occur according to the phase diagram. Instead, a thermal treatment at temperatures slightly above T g leads to nanocrystalline, spherical Au particles. Hence, alloying and subsequent decomposition of the alloy is a prerequisite for the formation of plate-like noble metal particles.