Preparation of YAG:Ce3+ phosphor by sol-gel low temperature combustion method and its luminescent properties

Synthesis and characterization of YAG:Ce phosphors for white LEDs

“…For YAG:Ce and YAG:Ce, Re powder, manufacturing the sol− −gel combustion method follows these general steps: dis− solving raw materials, stirring, adding the chelating agents, stirring, aging and thermal steps. Dissolving and mixing the raw materials for hydrolysis and condensation of molecular precursors and sol formation followed by gelation: the gel is prepared using oxides [23,28,[45][46][47][48][49][50][51]. In these steps, the most important parameters are: temperature, time of stirring, pH, the ratio between the complexing agent and the metal cations, and the presence of additives (glycerin, ethanol, polyethylene glycol, citric acid, etc.)…”

“…After that the gel can be transformed into a xerogel by slow evaporation at temperatures of about 100-150°C, during a time which can be from 10 hours to 10 days. In the drying step elimination of toxic gases is possible due to the decomposition of nitrates [29,48]. As in the solid state process, the thermal treatment is a decisive stage.…”

“…After an inter− mediate grinding the YAG:Ce, Re powders were sintered at 1000-1300°C for 10 to 18 h. The thermal treatment can be performed in different types of atmosphere (static, inert, oxidative or reductive), but in order to prevent the oxidation of Ce 3+ a weak reduction atmosphere is recommended. Bel− low some possible reaction underlying the sol−gel process for YAG:Ce and YAG:Ce, Re synthesis is presented [48,52] 2 The citrate sol−gel process is based on the formation of polychelates between metal ions and C=O from citric acid, and they may be followed by their polyesterification with a polyalchool (i.e. ethylene glycol).…”

“…Average crystallite size of the YAG:Ce and YAG:Ce, Re powders, obtained by the methods presented above, was reported to be 20-100 nm (estimated using Scherrer's equa− tion, Dhkl=kl/bcosq, where D is the average grain size, k is the dimensionless shape factor -Scherrer's constant, l is the X−ray wavelength, b is the full width at half maximum intensity (FWHM) -expressed in radians, q is the Bragg dif− fraction angle) [23,27,36,48].…”

“…At lower temper− atures the Ce 4+ ® Ce 3+ transition may be incomplete, or Ce 3+ cannot enter totally in the YAG lattice -in this case, Ce 3+ ions can be present on the surface. Higher temperatures of synthesis increase the solubility of dopant ions in the YAG crystal lattice, thus leading to a larger number of ions active in the luminescence process [26,48,57]. Thus, the emission intensity may improve by increasing the homogeneous distri− bution of Ce 3+ ions in YAG.…”