By efficiently converting the blue light emitted from InGaN chips into yellow light and mixing them into white light, classic yellow Y₃Al₅O₁₂: Ce (YAG) 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(2700–3200 K) are required.

To improve the light quality and enhance the luminous efficiency (LE) of YAG: Ce phosphor, a research team from Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, successfully 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 h, the slurry was dried at 75 ^oC 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 ^oC for 3 h to remove the organic ingredients. Finally, the phosphor ceramic samples were obtained after sintered at the temperature of 1650 ~1700 ^oC for 12 h, 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. The emission peak of Ce³⁺ redshifts from 535 nm to 560~ 570 nm with the increasing of ZrO₂, which is argued to be related to the non-radiative transition process between the 5d₁ level of Ce³⁺ and oxygen vacancy levels. The w-LED based on B1650 shows a high CRI of 64 and a high LE of 92.36 lm/W, which is greatly enhanced compared to the previous report. The result has indicated the great application prospect of ZA-YAG: Ce ceramics in warm w-LEDs.

This work was supported by Innovation Program of Science and Technology Commission of Shanghai Municipality and the International Partnership Program of Chinese Academy of Sciences.

Figure (a) The pictures of ZA-YAG:Ce ceramics of A1650-D1650 as sintered, and after air annealing at 1200 ^oC and 1400 ^oC; (b) Normalized PL and PLE spectra of A1650-D1650 as-sintered; (c) Normalized PL spectra of A1650-D1650 after (c) 1200 ^oC and (d) 1400 ^oC air annealing. (Image by SIOM)

Article website:
https://doi.org/10.1016/j.jeurceramsoc.2020.10.027

Contact:
Mr. Wu Xiufeng
General Administrative Office
Shanghai Institute of Optics and Fine Mechanics, CAS
Email: xfwu@siom.ac.cn