Influence of sample thickness and concentration of Ce dopant on the optical properties of YAG:Ce ceramic phosphors for white LEDs

Translucent, polycrystalline YAG:Ce disks with 0.1, 0.5, and 1 mol% concentrations of cerium as well as different resulting thicknesses of 0.5 and 0.8 mm were prepared by reaction sintering of yttria, alumina, and cerium oxide under vacuum at a temperature of 1800 degrees C. The obtained samples displayed a microstructure with a Y3Al5O12 main phase interrupted by an intended scattering phase of alumina. Furthermore, the total forward transmittance was between 75 and 78% with the typical absorption bands of cerium at wavelengths of 330 and 460 nm. Phosphor conversion light-emitting diodes (LEDs) with 0.88 W power dissipation have been prepared from these YAG: Ce ceramic disks as converters and blue 455 nm LED chips for excitation. Their color coordinates in the CIE (Commission Internationale de l'Eclairage) diagram determined by spectral photometry depend on the concentration of the Ce dopant and the ceramic converter thickness. The highest luminous flux of 276 lm with an efficiency of 76.6 lm/W was measured for a converter with 0.5 mol% dopant concentration and 0.8 mm thickness emitting a yellowish white light.