Materials Today Communications
Effects of tetraethyl orthosilicate content and sintering temperature on the properties of Y3Al5O12:Ce3+ phosphor ceramics for automotive headlamps
Introduction
White light-emitting diodes (WLEDs) find numerous applications owing to their high power efficiency and long service life [1]. Currently, the expansion of the LED market into other areas such as automotive headlamps, lighting, and displays has created a demand for high-output-power and high-stability LED packages. Typically, WLEDs are fabricated by mixing Y3Al5O12 :Ce3+ (YAG, yellow phosphor) with an organic binder or silicon resin in an InGaN-based blue LED chip. However, the low thermal and chemical stabilities of silicon resins make them prone to degradation and yellowing, thus affecting the reliability of LED packages [[2], [3], [4], [5], [6], [7]]. These shortcomings may be overcome by avoiding silicon resins, e.g., using methods relying on the mounting of phosphor (phosphor in glass (PiG) and phosphor ceramic (PC)) plates [8]. PiGs are prepared by sintering transparent glass frit–phosphor powder mixtures. As glass frit powder can be sintered below 800 °C, low-temperature thermal treatment can be achieved using small phosphor amounts to simplify the treatment process [[9], [10], [11], [12], [13]]. In automotive LEDs, superior thermal conductivity is required to maintain high and long-term reliability with an increased power output. However, PiGs suffer from insufficient heat dissipation due to the low thermal conductivity of glass and exhibit low efficiency due to light scattering by glass binders.
The above problems can be mitigated by manufacturing high-reliability PCs in plate form through phosphor powder sintering [[14], [15], [16], [17]]. However, this technique is unsuitable for mass production, as it relies on gas pressure sintering, hot pressing, and hot isostatic pressing, i.e., requires high temperatures and pressures [[18], [19], [20], [21]]. In the case of YAG PC, an extremely high sintering temperature (and hence, a carbon-type vacuum furnace or gas pressure sintering) is required, which, together with the fact that sintering is performed in batch mode, results in very low productivity.
Herein, tetraethyl orthosilicate (TEOS) was used as a low-melting-point additive to improve YAG sinterability and thus facilitate the materialization of high-density ceramics. YAG-TEOS mixtures could be sintered at lower temperatures and atmospheric pressure to yield high-density YAG PC, and the effects of TEOS content and sintering temperature on PC performance were examined in detail [[22], [23], [24], [25], [26]] using density measurements and scanning electron microscopy (SEM). The crystallinity of sintered pellets was determined using X-ray diffraction (XRD) analysis, while TEOS distribution was estimated by transmission electron microscopy–energy-dispersive X-ray spectroscopy (TEM-EDS) analysis. Optical characteristics were examined for a 350-μm-thick sintered pellet mounted atop a blue LED. The best-performing sample (550 nm, density =4.727 g/cm3, relative density = 98.5 %) was obtained at a TEOS content of 1 wt.% and a sintering temperature of 1500 °C, showing the peak wavelength and color coordinate area of YAG.
Section snippets
YAG PC fabrication
Commercial nanoYAG powder (YAG:Ce, Baikowski, France) and TEOS (Sigma Aldrich, USA) were used. TEOS (loading = 0, 0.25, 1, 2, or 4 wt.%; additionally, TEOS contents of 0.5 and 1.5 wt.% were tested in some cases) was thoroughly mixed with YAG powder, and the mixture was weighed, subjected to 12-h wet ball milling with alumina balls at 120 rpm, and sieved through a 200-mesh sieve. The obtained phosphor powder (0.25 g) was uniaxially pressed (1 ton, 2 min; Standard Press, DongJin Institute, Korea)
Results and discussion
Fig. 2 shows the effects of TEOS content and sintering temperature on sample density, revealing that the density of samples sintered at 1450 °C was low (4.02 g/cm3) at TEOS contents of <1 wt.%, increasing to a maximum of 4.52 g/cm3 at a TEOS content of 2 wt.%. Given that the theoretical density of YAG equals 4.80 g/cm3, this behavior suggests that complete YAG sintering requires higher temperatures. At sintering temperatures above 1500 °C and TEOS contents of >1 wt.%, density ranged from 4.65
Conclusion
YAG PC for automotive headlamps was manufactured by 5-h sintering (at 1450, 1500, 1550, or 1600 °C) in N2:H2 (95:5, v/v) in the presence of TEOS (0, 0.25, 1, 2, or 4 wt.%) as a sintering agent. The sintered samples were characterized in terms of (fracture) surface morphology, density, crystallinity, and optical properties. The sample obtained under optimal conditions (1 wt.% TEOS and 1500 °C) featured the expected XRD pattern of YAG, a density of 4.727 g/cm3, a relative density of 98.5 %, and a
Author contributions
Ji Na Song: Conceptualization, Methodology, Software. Byeong Hoon Bae: Data curation, Writing – original draft preparation. Kang Hyeck Hu: Visualization, Investigation. Dong Su Lee: Methodology. Hyun-Do Jung: Software, Supervision, Writing – reviewing and editing. Chang-Bun Yoon:Supervision, Writing – reviewing and editing.
Declaration of Competing Interest
The authors do not have any conflicts of interest to declare.
Acknowledgement
This work was supported by the GRRC program of the Gyeonggi Province [Grant No. GRRC-KPU2020-A01, Multi-Material Machining Innovative Technology Research Center] and the Catholic University of Korea, Research Fund, 2020. The funding sources were not involved in the study design; collection, analysis, and interpretation of data; writing of the report; and the decision to submit the article for publication.
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