Photoinduced crystallisation of CaF2 from a Na2O/K2O/CaO/CaF2/Al2O3/ZnO/SiO2 glass
Glasses in the system Na2O/K2O/CaO/CaF2/Al2O3/ZnO/SiO2 show the ability to precipitate nanocrystalline CaF2 in the glass matrix via a surface-controlled crystallisation accompanied by a phase separation. The Al2O3 concentration affects the phase separation in the glass and with decreasing Al2O3 concentration the opacity of the glass decreases and the optical quality is increased. For the development of a new type of photo-thermo-refractive (PTR) glass, the adaptation of the photo-thermo-refractive process was adapted to this glass system by doping with CeO2, Ag2O, SnO2, Sb2O3, KBr and the perfomance of UV exposure and two-stage temperature treatment. The cerium sensitises the glass to UV light due to its electron donor capability. The decomposition of the absorption bands of the core on the basis of a physical model using Gaussian functions results in an envelope of 5 + 2 bands. The low optical basicity of the base glass and additionally strongly reducing melting conditions could not completely shift the Ce3+/Ce4+ redox equilibrium to Ce3+. Antimony has a crucial role as an electron transfer agent in the reduction of the Ag+ to Ag0 in the first heat treatment step. The formation of elemental Ag is photosensitive only in the presence of Sb5+ ions in the glass. In the first heat treatment step, the Ag0 agglomerates in the previously exposed areas. A characteristic yellow coloration occurs due to the silver plasmon resonance. Computer simulations of the Ag-plasmon resonance peak positions based on the Mie theory prove the formation of a complete Ag-AgBr core/shell structure of the particles. The Ag/AgBr particle formation depends on the exposure time and the temperature and act as nucleating centers for the heterogeneous CaF2 crystallisation, which is photosensitive only in the presence of bromide. The volume density of the CaF2 crystalls increases with the exposure time. The CaF2 nanocrystals, which are up to a size of 300 nm in diameter, have an inhomogeneous polycrystalline microstructure which suggests an outward growth from a center. Refractive index changes were generated in the glass on the order of n = 10-5 to 10-4.
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