Scientists fabricate novel ternary ceramic phosphor for warm white-LEDs

By efficiently converting the blue light emitted from InGaN chips into yellow light and mixing them into white light, classic yellow Y₃Al₅O₁₂: Ce (YAG) phosphor has proved itself to be the most prominent phosphor in white light emitting diodes (w-LEDs). However, the color rendering index (CRI) of YAG: Ce-based w-LEDs is usually limited to < 70 and its correlated color temperature (CCT) is often higher than 4000 K for the lack of red component in the emission spectrum. In modern indoor lighting, a higher CRI and a lower (or warmer) CCT are required.

To improve the light quality and enhance the luminous efficiency (LE) of YAG: Ce phosphor, a research team from the Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Science has fabricated a novel ternary ZrO₂-Al₂O₃-YAG: Ce composite ceramic phosphors recently.

After packaged with blue chips, the w-LED based on ZA-YAG: Ce ceramic shows an enhanced CRI of 64 compared to Al₂O₃-YAG: Ce and a high LE of 92.36 lm/W, which is greatly promoted compared to the previous report of ZrO₂-YAG: Ce. The result was published in Journal of the European Ceramic Society.

In their experiment, commercial Y₂O₃, α-Al₂O₃, CeO₂, ZrO₂ powders were used as raw materials. The powders were weighted according to the formula and fully mixed. After ball milling in ethanol for 24 hours, the slurry was dried at 75 degrees C and granulated with a 200-mesh sieve.

After pressed into disks under uniaxial pressure of 10 MPa, the disks were processed under cold isostatic pressing at 200 MPa. The pressed disks were then heated in a muffle oven at 700 degrees C for three hours to remove the organic ingredients. Finally, the phosphor ceramic samples were obtained after sintered at the temperature of 1,650-1,700 degrees C for 12 hours, respectively.

From the microstructure evolution, the researchers found that the addition of ZrO₂ can remarkably promote the densification process of samples. From the EDS analysis result, they discovered that the three phases of ZrO₂, Al₂O₃, and YAG can co-exist with each other independently after sintering although Y in YAG and Zr in ZrO₂ can exchange to a certain extent.