Effects of using MgO, CaO additives as sintering aid in pressureless sintering of M2Si5N8:Eu2+ (M = Ba, Sr) phosphor ceramics for amber LED automotive applications
Introduction
White light-emitting diodes (LEDs) (WLEDs) are currently used in a wide variety of applications due to their high power efficiency and long lifetime. Recently, the LED market has expanded into areas such as automotive headlights, lighting, and displays, requiring high power efficiency and highly stable packaging. WLEDs typically use YAG:Ce3+ yellow phosphors mixed with organic binders or silicone resins in InGaN-based blue LED chips. However, silicone resins are vulnerable to heat, and chemical instability, degradation, and yellowing characteristics reduce the reliability of the LED packaging [[1], [2], [3], [4], [5]].
A study reported packaging material technology in which a plate-shaped phosphor, such as phosphor in glass and phosphor ceramic (PC), that do not use silicone resin, is mounted on a blue LED chip to overcome the issues related to silicone resin [6]. Phosphor in glass (PiG) is a sintered mixture of phosphor powder in a transparent glass frit The fabrication process is simple as it requires a small amount of phosphor and glass frit powder that can be sintered at a low temperature—below 800 °C [[7], [8], [9], [10]]. However, in the case of automotive LEDs, excellent thermal conductivity is required to maintain high reliability and durability as the output power increases. PiG has reduced heat dissipation due to the low thermal conductivity of glass and reduced efficiency due to light scattering caused by the glass binder. To address these issues, single phosphor powder is sintered to fabricate reliable PC plates [[11], [12], [13], [14]].
However, in order to sinter phosphor, gas pressure sintering, hot pressing, and hot isostatic pressing methods are used in the heat treatment process that involve high temperature and high pressure, which makes mass production difficult [[15], [16], [17], [18]]. In particular, in the case of the amber light of the turn signal lamp in automobiles, amber-colored PiG, yttrium aluminium garnet (YAG), and CASN phosphors (using Ca-α-SiAlON) mounted on a reliable GaN-based chip with blue wavelength are used due to excellent reliability resulting from the use of glass, and it can be sintered at approximately 700 °C. However, amber-colored PiG has low phosphor efficiency, which makes highly efficient amber LED production challenging [[19], [20], [21]].
To solve these problems, in this study, we synthesized Eu2+ activated M2Si5N8:Eu2+ (M = Ba, Sr) phosphors and added an oxide-based sintering aid mixture to lower the heat treatment temperature and allow pressureless sintering [[22], [23], [24], [25], [26]]. The Ba and Sr compositions were varied to synthesize amber-colored phosphors and a sintering aid mixture was added – MgO and CaO – to improve density [27,28]. To synthesize amber-colored phosphor, the addition amount of Eu was fixed at 0.03 mol and after the mole ratio between Ba and Sr was adjusted, the synthesized phosphor was added with MgO and CaO to fabricate a molded product. After that, under the 95% N2 and 5% H2 mixed gas atmosphere, the temperature was maintained at 1650 °C for 4 h to prepare a high density sintered product. Optimum sintering temperature and additive content conditions were derived through density measurements and SEM analysis. XRD analysis confirmed the crystallinity of phosphor powder and sintered sample and TEM-EDS analysis confirmed the distribution of MgO and CaO used as the sintering aid. The sintered sample was processed at 80 μm thickness and mounted on Blue LED to evaluate its optical properties.
Section snippets
Synthesis of phosphor
To synthesize M1.97Si55N8:Eu2+0.03, (M = Ba, Sr) phosphors, Ba3N2 (High Purity Chemicals Co. Ltd, 99%), Sr3N2 (High Purity Chemicals Co. Ltd, 99%), Eu2O3 (Sigma-Aldrich, 99%) and Si3N4 (High Purity Chemicals Co. Ltd, 99%) were weighed according to determined composition ratios. The chemical equivalence ratio was (Ba, Sr):Si:Eu = 2-x:5:x, (x = 0.03), and the Ba:Sr ratio was varied from 10:0 to 0:10, in increments of 2, to prepare six powder compositions. To prevent oxidation of Ba3N2 and Sr3N2,
Analysis of phosphors
Fig. 2 shows the XRD pattern analysis results of the (Ba1-x,Srx)1.97Si5N8:Eu2+0.03 phosphors with varying values of x. When x = 0, Ba2Si5N8 (JCPDS # 85–0102) shows the diffraction peaks, and when x = 1, the Sr2Si5N8 (JCPDS # 85–0101) pattern is displayed. As x increases, the pattern changes from Ba2Si5N8 (JCPDS # 85–0102) to Sr2Si5N8 (JCPDS # 85–0101), confirming the synthesis of (Ba1-x,Srx)1.97Si5N8:Eu2+0.03 phosphors with excellent crystal phases.
Structure parameters calculated from the
Conclusion
High density 607 nm Amber Phosphor Ceramic for high power automotive LEDs was prepared by pressureless sintering with the addition of MgO and CaO sintering aids. (Ba0.8,Sr0.2)1.97Si5N8:Eu2+0.03 phosphor was synthesized from a solid-state reaction at 1600 °C for 8 h under a mixed gas atmosphere while the Ba/Sr composition was varied. The XRD analysis showed excellent crystal phase of the synthesized phosphor and the shape and size of phosphor particles were confirmed by SEM images. According to
CRediT authorship contribution statement
Jeong Woo Lee: designed the experiments, wrote the manuscript, carried out the fabrication process. Jae Min Cha: carried out the fabrication process. Byeong Hoon Bae: carried out the fabrication process. Sung-Woo Choi: carried out the characterization analysis. All authors analyzed the data, wrote the manuscript. Hyun-Do Jung: developed the concept, designed the experiments, wrote the manuscript. Chang-Bun Yoon: developed the concept, wrote the manuscript.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study was funded by The Catholic University of Korea, Research Fund, 2020 and the Basic Science Research Program [No. 2018R1C1B6001003] through the National Research Foundation of Korea funded by the Korean government (MSIT) and the Ministry of Trade, Industry and Energy (MOTIE) of Korea and conducted under the “Competency Development Program for Industry Specialists,” undertaken by the Korean Institute for Advancement of Technology (KIAT) (No. P0002007, HRD program for 3D Printing based
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