Preparation And Properties Of Lanthanide-doped Fluorosilicate Transparent Glass-ceramics With Luminescence Behaviors

Recently, oxyfluoride transparent glass ceramics(TGCs) doped with lanthanide ions(Ln³⁺) attract researchers great interests, since they exhibit high luminescent efficiency, Vis-NIR transparency, chemical stability and excellent up-conversion behaviors. Such glass ceramics are also characterized with their adjustable structure, controllable single-phase fluoride crystallizing and heavy Ln³⁺-doping level, which assure their potential application in the field such as solid lasers, optoelectronic communication devices and color displays.This thesis introduces the lanthanide spectroscopy theories as well as the glass nucleation and crystallization thermodynamics, gives an overview of the recent progress and problems on Ln³⁺-doped oxyfluoride TGCs, and puts forwards their future research directions. We studied glass stabilities and crystallization behaviors of fuorosilicate glass system, summarized the designing rules and methods in order to obtain fuorosilicate glass with controllable single-phase fluoride crystallization behaviors. Based such aspects, several new types of fluorosilicate luminescent TGCs have been developed successfully, which containing Ln³⁺-doped fluoride nano-crystallites. Using thermal analysis (DTA), X-ray diffracition (XRD), high resolved transmission electronic microscope (HRTEM), photoluminescence spectra (PL) and Judd-Ofelt theory, we studied the correlation among heat treating techniques, precipitated crystal size, crystallinity, transmittance and luminescence behaviors. A series of important conclusions and innovative results with practical significance were obtained.The glass stabilities and crystallization behaviors of MF₂-Al₂O₃-SiO₂(M=Mg, Ca, Sr, Ba) glass have been studied. It is found that the glass forming ability droped with the increasing field strength of alkaline earth ions(M²⁺); introducing some AlF₃, ZnF₂, NaF and Na₂O to partially replace MF₂ and Al₂O₃, the glass stability and the single-phase crystallization ability can be improved simultaneously. Then, the compositon of 50SiO₂-20Al₂O₃ -10NaF-20MF₂ was considered as more suitable than others for preparing Ln³⁺-doped oxyfluoride TGCs. In MF₂-Al₂O₃-SiO₂(M=Mg, Ca, Sr, Ba) glass system, their stability decreased with substituting BaF₂for Al₂O₃, but reverse for substituting Na₂O for BaF₂;when substituting AlF₃ for Al₂O₃, the glass stability decreased initially and then increased, which suggested O^2- and F^- had different effects on the glass matrix.For the MF₂-ZnF₂-NaF-Al₂O₃-SiO₂ glass system, how the composition affecting fluoride single-phase crystallization and how the heat treating techniques affecting the glass ceramic microstructure were revealed. With Mg→Ca→Sr→Ba, crystallization activated energy of MF₂ increased, and single phase crystallization of MF₂ need higher F/O ratio, which can be assigned to the effect of alkaline earth cation field strength. With the heat treating time prolonged, the crystal size and crystallinity of MF₂ increased quickly at first, then slow down, but the maximum values of them are lower than 40% and 30nm, respectively. Accordingly,Vis-NIR transparent glass ceramics can be easily obtained at large technique range. For example, after heat treating for 16hr, the Vis-NIR transmittance of GCCaEr16h still remain higher than 85%.Er³⁺ ions doping behavior has been investigated. The results of HRTEM and EDXS revealed that Er³⁺ ions have dissolved into the precipitated fluoride crystal lattice. XRD results indicated the quantity of Er³⁺ in the fluoride crystals increased with the heat treating time or temperature increasing. This is agree well with the PL spectra. Judd-Ofelt calculation confirmed that Er³⁺ ions entering into the fluoride crystals led to their higher ligand symmetry and weaker covalence properties than in the glass matrix, which made Ω₂ and Ω₄ increased significantly.According to the dependence of up-conversion intensity on the pumping power, Er³⁺ up-conversion luminescence mechanisms have been revealed. It is found that the red and green up-conversion are two photon processes, and the blue upconversion are three photon process, which are all based the primary ESA and ETU mechanisms. By analyzing 1.5μm luminescence decay spectra of Er³⁺, it was found that 1.5μm luminescence followed the double exponential decay law, where the slow decay is corresponding to Er³⁺ in the precipitated fluoride nano-crystals, and the quick decay is corresponding to Er³⁺ in the glassy host. With the heat treating temperature increasing, the ratio of slow decay became higher, which suggested more Er³⁺ ions had been in the fluoride lattice.Nonlead luminescence TGCs containing CaF₂:Er³⁺. SrF₂:Er³⁺and BaF₂:Er³⁺ have been prepared successfully. PL spectra measurements and calculation using Judd-Ofelt, McCumber theory and F-L formula revealed: the upconversion and 1.5μm NIR luminescence properties of three type of alkaline earth fluorosilicate TGCs are all better than that of lead fluorosilicate TGCs. Take GCCaEr as an example, its quantum efficiency(η) and FOM for gain (σ_e^peak×τ_mea) are about 3 times than that of lead fluorosilicate TGCs, and its FOM for bandwidth (σ_e^peak×△λ_eff ) is 14% larger than lead fluorosilicate, so they have good potentials in laser and optical communication field. In summary, the above studying reslut provide theoretical guidance and experimental method for developing a kind of nonlead luminescence TGC.Luminescence TGCs containing CaF₂:Er³⁺/Yb³⁺, SrF₂:Er³⁺/Yb³⁺ and BaF₂:Er³⁺/Yb³⁺ have been prepared successfully and investigated. PL spectra measurements and Judd-Ofelt calculation reveal Er³⁺/Yb³⁺ codoped TGCs have much broader 1.5μm band width, much stronger 1.5μm luminescence, red and green up-conversion luminescence than Er³⁺ single-doped TGCs. For Er³⁺/Yb³⁺ codoped TGCs, both of their 1.5μm FOM for bandwidth(σ_e^peak×△λ_eff and for gain(σ_e^{peak ×τ_mea} are better than ZBLAN. Especially for GCCaErYb, FOM for bandwidth and for gain are 1.2 and 1.5 times than that of ZBLAN, respectively. Furthermore, these TGCs have better chemical stabilities, mechanical strength and are easily prepared, so they can be candidates for substituting fluoride glass and crystal in the field of laser and optical communication.Luminescence TGCs containing Ba₂LaF₇:Er³⁺, Sr₂LaF₇:Er³⁺, Sr₂GdF₇:Er³⁺ and Sr₂YF₇:Er³⁺ have been prepared successfully, using cheap raw materials(alkaline earth fluoride and rare earth oxide). It is found for such system that glass stability andcrystallization phase composition are mainly affected by F/O ratio and MF₂/R₂O₃ ratio. In the broad heat treatment temperature ranges, single-phase M₂RF₇:Er³⁺ can crystallize in these glass system. Therefore, it is easily to obtained the TGCs containing these nano-crystals, which possess high Vis-NIR transmittance, high luminescence efficiency, long 1.5μm-luminescence lifetime and broad 1.5μm-luminescence bandwidth. This provide an new idea and means to develop the Ln³⁺-doped fluorosilicate luminescence TGCs with high performance and low cost.