Optimierung und Charakterisierung von lumineszierenden Lithium-Aluminium-Boratgläsern und -glaskeramiken

Luminescent borate glasses and glass ceramics offer a promising alternative as temperature-stable light converters. This work follows the research approach to improve the efficiency of the luminescent system by means of specific network changes and scattering centres. To increase the light output, the glasses are processed into glass ceramics to increase the optical path length by scattering on the grown crystallites in the glass. For this purpose, glass systems are produced on the basis of lithium borate and lithium-aluminium borate with the lanthanide ions Eu3+, Tb3+, and Dy3+ as optical activators. The glass systems are analyzed by Raman spectroscopy and by differential scanning calorimetry (DSC) as well as optical spectroscopy. The crystallization process in the glass is monitored by in situ X-ray diffraction and differential calorimetry, both isothermic and non-isothermic. The results show that the lithium concentration has an influence on the near-field structure, the glass transition, the crystallization temperature, and the photoluminescence properties of the glass. The amount of boroxole groups in the near-field structure decreases with increasing lithium concentration. In addition, the glass transition temperature and thus the upper limit of temperature stability shows a maximum at a lithium-to-boron ratio between 1 : 3 and 1 : 4. Additional doping with aluminium oxide leads to lower glass transition temperatures. The crystallization point for both glass systems shifts to lower temperatures as the lithium concentration increases. The influence of the doped lanthanide ions on glass transition and crystallization temperature is almost negligible. The Eu3+-doped lithium borate glasses show a quantum efficiency of almost 90% (at 396 nm), the Tb3+-doped glasses of about 60 % (at 486 nm), and the Dy3+-doped glasses of about 30 % (388 nm). These values are obtained for a lithium-to-boron ratio of 1 : 6. If the lithium concentration is increased, the quantum efficiency decreases. The addition of aluminium oxide at the expense of boron oxide facilitates glass production and increases glass stability, but reduces quantum efficiency by up to 10 % compared to the aluminium-free lithium borate glasses. A comparison of the glass stability of the glass systems studied shows that a lithium aluminium borate glass with a lithium-to-boron ratio of 1 : 2 is best suited for the production of glass ceramics. Thermal processing leads to the formation of Li2B4O7 and Li2AlB5O10 crystallites in the glasses. The size and amount of crystallites can be controlled over annealing time and annealing temperature as well as the heating rate. Optical studies on the glass ceramics show that the scattering properties increase monotonously upon prolonged thermal processing.