| In the illumination field, human has experienced the three stages of incandescent lamps, neon lights, and gas discharge lamp. In recent years, due to the blue and ultraviolet LED chips to be rapidly developing and commercialized, the LED development has a breakthrough in the lighting industry. White LED are promising lighting devices to replace traditional incandescent lamps and fluorescent and become the fourth generation of solid-state lighting light, and will be applied largely in real life, because of its energy efficiency, low voltage drive, small size, long life, no pollution, and can provide controllable spectral characteristics and color temperature, and many other advantages,To date, the most common commercial white LED is composed of a blue LED chip and yellow phosphor Ce3+:YAG, the blue LED chips and the yellow fluorescent from the Ce3+ doped yttrium aluminum garnet phosphor under the excitation of LED are mixed into the pure white light. But this white light is lack of red and uncomfortable to the human’s eyes, moreover single excitation wavelength and correlated color temperature (higher than 6000K) are not benefit to the wide application in high-quality lighting.Based on the above considerations, the Eu2O3, Tb4O7 single doped and Eu2O3/Tb4O7 co-doped yttrium aluminium garnet glass ceramics have been prepared by heat-treating lithium-yttrium-aluminium-silicate (LYAS) precursor glasses. The luminescence properties, fluorescence emissions and energy transfer mechanism of europium and terbium ions have been studied in the rare-earth doped YAG glass ceramics. The color coordinates and corresponding color temperature are calculated by investigating the fluorescence excitation spectra of the rare-earth doped YAG glass ceramics under the different excitation conditions. The relationship of color temperature and excitation power is exposed, which is a promising way to obtain effective warm white light emission by adjusting the power of pumping source and providing a technical reference to development the display materials. The followings are results this work achieved:I. Eu/Tb doped YAG glass ceramics have been prepared by heat-treating the procurer LYAS glasses, and the phase structure and microstructure of the YAG glass ceramics are characterized. From the XRD diffraction patterns and SEM pictures, the single phase of YAG micro-crystals growing with excellent orientation (111) in the matrix presents the primary crystallite dimension of～46nm and the secondary grain size of ～8μm, indicating that the primary nanocrystals with high energy would assemble with their interfaces matching coherently to minimize the surface energies during the heat treatment process.2. Spectral characteristic of Eu doped YAG glass ceramics. Eu doped YAG glass ceramics present bright violet, and a broad blue emission located at 423nm and a group of red emission peaks are observed under the excitation of 365nm. According to the spectra properties, the effect of the silicon in the matrix is not the responsible for the broadband blue emission, indicating that Eu2+ and Eu3+ coexisted in Eu doped YAG glass ceramics. The Eu doped YAG glass ceramics emitting prominent red fluorescence under the 254nm excitation, and the blue emission from the Eu2+ is weaker obviously, and the red fluorescent of Eu3+ plays an important role in the YAG glass ceramic host. Various fluorescence emissions have been obtained in the Eu doped YAG glass ceramics under different excitation wavelengths though the analysis of relative spectral power, and the trajectory of wavelength-dependent is represented in the chromaticity diagram.3. Radiative transition emissions of Tb3+ in YAG glass ceramics. The conspicuous green fluorescent emissions are observed in Tb doped YAG glass ceramics under the excitation of 365nm, and the color ordinate (0.311,0.443) is located at the green area in the chromaticity diagram, which provide a reference to obtain the warm white light once introduce the green fluorescence of Tb3+ ions into the YAG crystal formed in the glass ceramics.4. Dynamic-colour white fluorescences in Eu/Tb codoped YAG glass ceramics. Orange red emission has been observed in the Eu/Tb codoped YAG glass ceramics under the 254nm excitation, the emissions of Tb3+ and Eu3+ occupy the important position in the emission spectrum, but the fluorescence of Eu2+ is weaker. However, the characteristic emissions of Eu2+ have been enhanced and the attractive white light has obtained in the Eu/Tb codoped YAG glass ceramics, indicating that long wavelength excitation is premise to obtain the warm white light. Under 365nm excitation, the Fluorescence close to the white point, and color coordinates is (0.360,0.308), color temperature is 4137K. |
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