Preparation, Characterization, And Luminescence Of Eu³⁺-doped Gadolinium Tungstate, Y₃Al₅O₁₂:Ce³⁺Phosphor, And Their Mixtures

The white light emitting diodes (LEDs) have been paid increasing attention in recent years due to their excellent properties such as low power consumption, environmental benefit and environmental protection as a new type of solid-state lighting sources.Nowadays, white light LEDs that are made of a blue LED chip coated with a yellow phosphor of YAG:Ce occupied the main market. The yellow phosphor of Ce3+-doped YAG can efficiently absorb blue light emitted by GaN-based diodes and emit a broad band of yellow emission. The white light is achieved by the combination of residual blue light and the yellow luminescence. However the main disadvantage of this kind of white LED is that it lacks red component in the emission spectrum, resulting in this kind of white LED suffers from the drawbacks of poor color rendering and high color temperature. For the purpose of overcoming the aforementioned disadvantages, it is indispensable to explore how to improve the color rendering index of the yellow phosphor. If we can blend some other red-emitting phosphors also excited by GaN-based diodes with the YAG:Ce yellwo phosphor, it can be expected to expand the YAG:Ce emission band, i e inserting red band into the tradidtional YAG:Ce phosphor. This is the aim of the thesis.A series of stable Eu3+-doped gadolinium tungstates and the optimal photoluminescence properties of Y3Al5O12:Ce fluorescent powders were prepared by a co-precipitation method and their photoluminescence properties were studied. After that, the red fluorescent powder and yellow fluorescent phosphor were mixed together at different weight ratios. It is found that the emission spectra of the blended phosphors excited with blue light at465nm is the superposition of the two phosphors, which means that the mixture’s emission can be tuned by controlling the weight ratios.Under the same preparation conditions, different doping concentration leads to different crystal structures; Gd6WO12:Eu3+with a tetragonal lattice structure tends to be synthesized at low doping concentration (10-40mol%) yet Gd2WO6:Eu3+with a monoclinic lattice structure easily to grow at high doping concentration (60-80mol%). Also different doping concentration leads to different intensity of the red emission. The profile of emission spectra exited either by395nm or465nm light for each sample is almost the same except for the emission intensity. The main emission peaks attributed to the characteristic transition from5D2to7F2of Eu3+. Specially, the powders at40mol%Eu3+doping concentration shows excellent red emission, Although different doping concentration leads to different emission intensity. This kind of powders might find their promising application to compensate the white LEDs for the lack of red light component and to improve rendering index. We also studied the blended phosphors mixed YAG:Ce with Gd6WO12:Eu3+without quenching or shifting the YAG:Ce emission, It is found that the blended phosphors not only have bright white light but also show high rendering index. And the color temperatures of the blended phosphors gradually decrease as the ratios change from5:1to1:3. When the ratios of Y:R is1:3, the color temperature is4600K, it can meet the demand of people with low color temperature in lighting applications. The blended phosphors could find potential application for white GaN-based light-emitting diodes.