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Partial crystallization of the erbium/ytterbium co-doped oxyfluoride glass DOI:10.15199/28.2018.6.2

  1. INTRODUCTION The commercially available optical fibers (single mode or multimode) consist of a fused silica core (refraction index 1.46 < n < 1.49) and a glass cladding with lower refraction index [1, 2]. REE doped, optically active glass fibers (or rather their cores) have various matrices (fused silica [3], fluoride glasses (for instance ZBLAN) [4, 5] or silica-based oxyfluoride glasses [6]). Rare earth element doped fibers become optically active media, converting for instance NIR radiation into red light (Er3+ [7, 8]), into green light (Ho3+ [9], Tm3+ [10]) or into blue light (Pr3+ [11]). They can also amplify the NIR radiation (Er3+ 1550 nm in EDFA [12]). An apparent requirement for EDFA amplifiers is a high intensity of erbium emission. It is always higher in erbium doped single crystals (homogeneous band broadening) than that in erbium doped glasses (inhomogeneous band broadening). The best example is an erbium doped laser crystal GdVO4:Er, where the emission cross-section (related to the luminescence intensity) value achieves 1.6·10-20 cm2 [16] whereas it equals only 0.8·10-20 cm2 in fused silica glass. The consequence of a single crystal predominance as a host for REE doping (and emission) is the new material substitution (glass-ceramics), which combines technological properties of glass and optical properties of single crystal [13]. Ytterbium co-doping appeared as a next invention enhancing material (silica glass) absorption at 980 nm [4]. With respect to the higher absorption of the Er/Yb system at 980 nm in comparison to materials containing only erbium and cross-energy transfer from the ytterbium ions to the erbium ions followed by higher luminescence at 1550 nm excited by 980 nm pump, the ytterbium is sometimes called a sensitizer. To fabricate such a composite material of space-homogeneously distributed, REE enriched nanocrystals embedded in a glassy host, the glass partial crystallization (controll[...]

The improvement of moisture stability of yellow phosphor encapsulated with SiO2

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During the past decades the phosphor materials have attracted much attention because of interesting optical and structure properties. They have been studied and applied in many fields such as cathoderay tubes (CRTs), flat panel displays (FPDs), fluorescent lamps, X-ray detectors, light emitting diodes (LEDs) and more. However due to the environmental factors, unmodified phosphors are sensitive to environmental conditions such as moisture, temperature and charging [1÷5]. In addition, unmodified phosphor particles will conglomerate because of the surface electricity charging. This sensitivity limits the utility of the phosphors. Thus an effective protection method of each of such inorganic material from moisture and environmental conditions would be highly desirable. Numerous attempts have been made to enhance the chemical stability of phosphor particles, by changing synthesis parameters or by mixing the phosphor with another compound. Also a surface coating using nanoparticles has been frequently applied to enhance the chemical stability of several phosphors. The qualified coating must meet basic requirements: complete coverage of the particles with thin and transparent layer and should protect core material from environmental factors and working conditions without loss of optical properties. A number of techniques have been used to coat particles namely mechanical milling, atomic layer deposition (ALD) [6], pulsed laser ablation (PLD) [7], sol-gel [8÷10] etc. In these techniques, sol-gel is the most favourable one because this technique puts forward excellent control of stoichiometry, density and microstructure. There have already been some reports on protective coating of phosphors so far. Due to reports from literature, organic coating [11, 12] and inorganic coating [4, 13] was proposed to improve the phosphor. Whereas organic encapsulation can fulfil the moisture resistance it fails when higher temperature is applied hence [...]

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