Changes in the microstructure and optical properties of doped sodium-calcium silicate glass upon secondary heat treatment

The phase separation in Na₂O-CaO-SiO₂ system containing four different microcomponents (Fe₂O₃, P₂O₅, Ag, Au) is studied. Two glass samples of close composition have been synthesized with the same set of the microcomponents and different content of gold and silver. Both glass samples have been melted in an open platinum crucible in an electric furnace at 1400°C for three hours and then poured and quenched on a cast iron plate and annealed at 520°C for one hour. Formation of the nanoparticles occurred upon secondary heat treatment at 550 – 600°C. The phase separation in the glass sample with a composition corresponding to the immiscibility limit in the NCS system occurs through formation of the core – shell type gold-silver nanoparticles, which serve as nucleation centers for the droplets of the second glass phase enriched in silica. Eventually, quartz crystallizes inside the droplets. The impact of these processes on the optical properties of the material including dichroism is discussed. The phase separation and quartz formation are confirmed by SEM and X-ray analysis data, respectively. The size of the nanoparticles was calculated from electron spectra using computer simulation. Iron in the glass samples thus obtained is in oxidation states +2 and +3, which determines the color of the glass before and after the secondary heat treatment in reflected light. The glass color in the transmitted light after the secondary heat treatment is attributed to the light absorption by nanoparticles. The dependence of the nanoparticle shape on the gold-silver ratio in glass is presented. The shape of nanoparticle is close to oblong or oblate ellipsoid within the range of the gold – silver ratios considered in the study.

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