Vitrocerámicos transparentes del sistema SiO2-Al2O3-Na2O-K2O-LaF3/YF3: Mecanismos de cristalización y propiedades ópticas

Abstract

Rare-earth (RE) doped oxyfluoride glass-ceramics possess interesting optical properties with applications in telecommunications and optoelectronics. These materials combine the transparency and mechanical and chemical resistance of aluminosilicate glasses with the low phonon energy and facile incorporation of RE ions in the fluoride crystals. The incorporation of RE ions in the crystalline phases enhances the optical emission intensity, a major property of these materials. Nanocrystallization in four different oxyfluoride glasses in the system SiO2- Al2O3-Na2O-K2O-LaF3/YF3 has been studied. Heat treatments above the glass transition temperature give rise to the devitrification of different crystalline phases, depending on the composition: LaF3, NaLaF4, KLaF4 and NaYF4. Crystalline fraction and crystal size are dependent on the time and temperature of thermal treatment. Thermal and structural characterisation has been studied using several techniques, including viscosity, dilatometry, X-ray and neutron diffraction, quantitative Rietveld refinement, transmission electron microscopy techniques (EELS, EFTEM, Simulations), differential scanning calorimetry (DSC), 27Al, 19F and 23Na NMR, and EXAFS. The crystallization mechanism is shown to occur via regions of La- and Siphase separation in the glass, from which the fluoride crystals develop during heat treatment. The interface between the glass matrix and the crystals in the demixed ranges is enriched in network formers, mainly SiO2, creating a viscous barrier, which inhibits further crystal growth and limits the crystal size to the nanometric range. The influence of the addition of RE ions (Tm3+, Eu3+, Yb3+, Er3+) on nanocrystallisation has been analysed. The optical characterisation of the RE-doped glasses and glass-ceramics has been performed, mainly focussed on confirming the distribution of RE ions between glassy matrix and crystals, and on the upconversion (UC) emission processes (doping with Tm3+ provides an up UC 2 emission in the blue region (450-480 nm) from excitation in the NIR (790 nm)), and the differences between glasses and glass-ceramics. Finally, preliminary results of the production of waveguides through ionic exchange and their characterisation (optical modes, attenuation of light during propagation along the guide, stress profile) will be shown