The sol-gel synthesis of anti-Stokes luminescent glass materials
The materials commonly used and the synthetic problems that arise in the field of transparent glass materials with anti-Stokes luminescent properties are described. The literature has shown that rare earth doped oxide glasses, whilst chemically durable and relatively easy to manufacture, have weak anti-Stokes emissions, whereas rare earth doped fluoride materials have considerably improved anti-Stokes luminescence but are less chemically stable and highly reactive. Recent interest in glass ceramics has proposed alternative matrices with the advantages of both fluoride and oxide host materials. These materials consist of rare earth doped fluoride crystallites in an oxide glass matrix giving the emission characteristics of a fluoride crystal protected in the stable surroundings of an oxide matrix. The work in this thesis initially set out to build on these lattices but broadened in the light of the findings outlined below. The synthesis of silica based glasses using sol-gel techniques is covered thoroughly in this work and the incorporation of various cations into these matrices has been investigated. Amongst the cations added to silica sol-gel reactions are titania, alumina and yttrium. Their reactions and the factors that promote transparent and crystallite-free samples have been reported and discussed. Several advances in this field have been made including a detailed investigation into the synthesis and properties of yttrium based sol-gel reactions which have not been previously reported. The incorporation of fluorine into these materials has also been described and the various problems encountered have been examined. Several novel methods of incorporating fluoride ions into silica lattices are proposed and examined. It was found that in the presence of fluoride ions, the rare earth dopants form crystallites within the glass matrix. In a similar way, it was found that glass ceramics also formed when a titanium alkoxide is incorporated with fluoride ions during the sol-gel synthesis of these lattices. In these cases it was shown that TiO2 in the form of anatase or rutile crystallites can be grown in the glass matrix. Moreover the phase of TiCb grown can be easily controlled by choice of annealing temperature and firing times. Distinct anti-Stokes emissions are produced depending upon the form of titania present. These new materials are reported and discussed.