Desenvolvimento de vidros e vitrocerâmicas oxifluoretos com propriedades luminescentes

Abstract

The glass systems GeO2-K2O-LaF3, GeO2-La2O3-NaF, GeO2-K2O-NaF-LaF3 and GeO2-Na2O-LaF3 were studied, with the aim of obtaining oxyfluoride glasses and glass-ceramics with luminescent properties. The lanthanum fluoride (LaF3) used was synthesized in the laboratory. From the XRD (X-ray diffraction) of the synthesized LaF3, the efficiency of the synthesis route used to obtain it from the reaction of solutions of La2O3 dissolved in nitric acid and NaF in water was proven. After the synthesis of glasses using the melting-cooling method under established conditions, the samples were thermally analyzed by DSC (Differential Scanning Calorimetry or Differential Thermal Analysis), treated at Tx1 (first temperature at which crystallization begins) to obtain a glass-ceramic and subsequently crystalline phase was identified. It was possible to observe, from XRD, the formation of KLaF4 crystals in the glass sample with composition 55GeO2-15K2O-30LaF3 and of NaLaF4 in the glass sample 55GeO2- 15NaF-40LaF3. As it is easier to synthesize, the sample 55GeO2-15K2O-30LaF3 was chosen for incorporation of rare earth ions. A sample with composition 55GeO2- 15Na2O-30LaF3 was also synthesized to evaluate the influence of the change in alkali metal oxide in the glass network. The glasses of these compositions were doped with europium (Eu3+) and co-doped with erbium and ytterbium (Er3+/Yb3+), remaining transparent, thus demonstrating good hosts for these elements. Subsequently, these obtained samples were heat treated to obtain glass-ceramics. From the heat treatment it was possible to obtain transparent luminescent glass-ceramics with the phases KLaF4 in samples with potassium and NaLaF4 in samples with sodium. By studying luminescence spectroscopy, a reduction in symmetry around the europium ion in doped glass-ceramics was observed, due to the increase in the emission intensity of the band at 594nm. An increase in the emission intensity related to the transition at 539nm in the glass ceramics of the 15K30LaErYb precursor glass was also noted. It was observed in the energy upconversion spectrum of the sample co-doped in the system with K2O, splittings of the Stark levels in the glass ceramics suggesting that the Er3+ ion is in a crystalline environment. Furthermore, an increase in the emission band in green compared to red in glass-ceramics was also observed, also indicating lower phonon energy associated with the presence of crystals in the glass matrix. The lifetime of glass-ceramics doped with Eu3+ was reduced in relation to glass samples and it is believed that this may be related to the higher asymmetry ratio of these samples. The 15K30LaErYbTT4h40 sample presented a long lifetime, compatible with that found in the literature. Finally, it was possible to obtain luminescent and transparent glasses and vitroceramics with a high molar percentage of LaF3, achieving the main objectives of the work.