We have developed a Ce:YAG (Y<sub>3</sub>Al<sub>5</sub>O<sub>12</sub>) glass-ceramic phosphor for the white LED. The glass-ceramic phosphor was obtained by a heat treatment of a Ce-doped SiO<sub>2</sub>-Al<sub>2</sub>O<sub>3</sub>-Y<sub>2</sub>O<sub>3</sub> mother glass between 1200°C and 1500°C for the prescribed time of period. We confirmed that, by XRD measurements, only YAG crystal precipitated in the mother glass after the heat treatment. It was shown from SEM observation that the YAG crystals with a grain size of approximately 20μm were uniformly dispersed in the glass matrix. The yellow emission, around 540nm in wavelength, was observed from the glass-ceramic phosphor, when it was excited by a blue LED (465nm). The white light due to the mix of yellow and blue light was observed from the glass-ceramic plate with a thickness of 0.5mm. The YAG glass-ceramic phosphor showed a high-temperature resistance and a good performance in a damp heat test. Moreover, a higher thermal conductivity of 2.18 Wm<sup>-1</sup>K<sup>-1</sup> and bending strength of 125MPa were observed compared with a conventional soda-lime glass or an epoxy resin. In addition, since the YAG glass-ceramic phosphor can be formed in a plate-like shape, there is no need to be sealed in resins for the fabrication of the LED devices. Therefore, it is expected that this newly developed glass-ceramic phosphor is a promising candidate for the realization of resin-free, high-temperature and high-humidity resistant, long-life white LED devices.
Optical properties of the Ce:YAG glass-ceramic (GC) phosphor for the white LED were investigated. Concentration dependence of fluorescence intensity of Ce<sup>3+</sup>:5d→4f transition in the GC showed a maximum at 0.5mol%Ce<sub>2</sub>O<sub>3</sub>. Quantum efficiency (QE) of Ce<sup>3+</sup> fluorescence in the GC materials, the color coordinate and luminous flux of electroluminescence of LED composite were evaluated with an integrating sphere. QE increased with increasing ceramming temperature of the as-made glass. The color coordinates (x,y) of the composite were increased with increasing thickness of the GC mounted on a blue LED chip. The effect of Gd<sub>2</sub>O<sub>3</sub> substitution on the optical properties of the GC materials was also investigated. The excitation and emission wavelength shifted to longer side up to Gd/(Y+Gd)=0.40 in molar composition. As a result, the color coordinate locus of the LED with various thickness of the GdYAG-GC shifted to closer to the Planckian locus for the blackbody radiation. These results were explained by partial substitution of Gd<sup>3+</sup> ions in the precipitated YAG micro-crystals, leading to the increase of lattice constant of unit cell, which was confirmed by X-ray diffraction.