Ce:YAG transparent ceramics for applications of high powerLEDs: Thickness effects and high temperature performance

G.H. Liu a,n, Z.Z. Zhou a, Y. Shi b, Q. Liu a, J.Q. Wan a, Y.B. Pan bQ1

a State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences,Shanghai 200050, Chinab Key Laboratory of Transparent Opto-Functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China

a r t i c l e i n f o

Article history:Received 5 August 2014Accepted 11 October 2014

Keywords:Ce:YAG transparent ceramicsLEDsThermal propertiesThickness effectsOptical materials and properties

a b s t r a c t

Ce doped Y3Al5O12 (Ce:YAG) transparent ceramics could be combined with blue chips of high poweroutput, for applications of high power white light emitting diodes (LEDs). In this paper, we found thatthrough the thickness variation of Ce:YAG transparent ceramics, emission spectrum and the colorcoordinates of the assembled LEDs could be adjusted from cold white to warm white light region.Ce:YAG transparent ceramics showed extraordinary high temperature quenching properties, they evenkept almost 90% of initial luminescence intensity (room temperature) at temperature of 200 1C, whichare superior to both the commercial yellow phosphors and white LEDs. By a far infrared temperaturedetecting system, the surface temperature of the assembled LEDs was recorded and compared tocommercial ones, Ce:YAG transparent ceramics with higher thickness showed lower surface tempera-ture, and Ce:YAG transparent ceramics also showed lower surface temperature than commercial whiteLEDs. Ce:YAG transparent ceramics combined to blue chips seems a good candidate for next generationhigh power light sources.

& 2014 Published by Elsevier B.V.

1. Introduction

Typically, yellow powder phosphors (mainly Ce:YAG) combinedwith blue chips (InGaN, with peak emission wavelength of460 nm) can realize white light emitting diodes (LEDs), whichnow is the most conventional method and with high conversionefficiency [1–5]. Although the white LEDs own extraordinaryperformance than traditional light sources (incandescent andfluorescent lamps), some troublesome problems such as packageissues caused by organic materials (resin or silicone) [2,5,6], whichalways have low thermal conductivities (1 W/m K or less), makethe LEDs degrade hugely at high temperatures after the lightsworking for a long time. It is obviously that, some new light designforms and new package materials [7–11] should be adopted torealize higher power light output, to overcome the LEDs devices’efficiency decrease, color shift and long time stability issues.

Solid and sintering ceramic phosphors, especially transparent/transluscent luminescent ceramics such as Ce:YAG [4] owns highthermal conductivity (about 15 W/m K), and also good mechanicaland optical properties, make it one kind of candidate materials tomeet the next generation lighting concept, in which higher power

output and lower light degradation are requested. However,research on the high temperature performance of Ce:YAG trans-parent ceramics is still lack, especially the performance when it ison service.

In this paper, the high temperature quenching of the Ce:YAGtransparent ceramics was discussed and compared to commercialpowder Ce:YAG phosphors and commercial white LEDs. Then,Ce:YAG transparent ceramics with different thicknesses werecombined to blue chips (20 W), after being lighted up, the surfacetemperature of the lamps were recorded by a far infrared tem-perature detecting system (SC-350 from FLIR), and comparisonwas made to commercial white LEDs with power output of 20 W.The comprehensive results of Ce:YAG transparent ceramicsoriented LEDs revealed that it might a good candidate for highpower output lighting system in the near future.

2. Experiments

The Ce:YAG transparent ceramics (with 0.3 at% Ce) were pre-pared by a solid state reaction method, according to our previouswork [7], samples with different thicknesses (0.2 mm, 0.3 mm,0.5 mm, 0.8 mm, 1.5 mm) were designed and double sidespolished to fulfill the final package request to commercial bluechips (20 W, respectively). The emission spectrums at different

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Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/matlet

Materials Letters

http://dx.doi.org/10.1016/j.matlet.2014.10.1140167-577X/& 2014 Published by Elsevier B.V.

n Corresponding author. Tel.:/fax: þ86 021 52412404.E-mail address: liugh@mail.sic.ac.cn (G.H. Liu).

Please cite this article as: Liu GH, et al. Ce:YAG transparent ceramics for applications of high power LEDs: Thickness effects and hightemperature performance. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.10.114i

Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎

temperatures (from room temperature to 230 1C) of the Ce:YAGceramics were recorded by a Hitachi F-4600 fluorescence spectro-photometer equipped with a heating system. White LED moduleswere realized by combining the ceramics with the blue chips, andan optical fiber fluorescence spectrophotometer (USB400, fromOcean Optics Corporation, USA) was also equipped for measuringthe optical spectrum of the corresponding packaged LED modules.An infrared camera system named SC-350 from FLIR corporationwas used to measure the surface temperature rise and down of thepackaged LED modules. The infrared imaging system has an errordeviation of less than 2% from 0 to 300 1C.

3. Results and discussion

In the inner of Fig. 1(a), it could be found that the doublepolished Ce:YAG ceramics were clear and transparent to visiblelight, the transmission spectrum could be found in Ref. [7]. Doublepolished Ce:YAG ceramics with different thicknesses (0.2 mm,0.3 mm, 0.5 mm, 0.8 mm,1.5 mm) were put onto commercial20 W blue chips, After being lightened up, the photoluminescencespectrum of the samples were recorded and shown in Fig. 1(a), as1 #, 2 #, 3 #, 4 #, 5 #, 0 #, respectively. Sample 0 # indicatedcommercial LED source, peak around 460 nm indicated a typicalemission of InGaN LED chip, and peaks at around 535 nm wereattributed to 5d to 4f7/2 and 4f 5/2 transition of Ce 3þ ions. It can befound that, as the thickness of the transparent Ce:YAG ceramicswas increased, the relative intensity of emission peak at around460 nm decreased, while the emission peak at around 535 nmincreased. The thickness effects on the color coordinates of thepackaging LEDs was discussed as followings.

The color coordinates of the samples according to the CIE 1931chromaticity diagramwere summarized in Table 1, and were drawnin Fig. 1(b). Sample with the thinnest thickness (1 #, 0.2 mm)showed a white light color accompanied by a cold blue appea-rance, as recorded by a digital camera in the inner of Fig. 1(b).Samples 2 # and 3 # with thickness of 0.3 mm and 0.5 mm, showednormal white color, as the middle picture shown in the inner of

Fig. 1(b), they were similar to sample 0 # (commercial LEDs). Whenthe thickness was increased to 0.8 mm and above, the color of thepackaging LEDs shifted to light yellow region, with a warm whiteappearance.

Whether solid sintering ceramic phosphors are superior tocommercial powder or not, high temperature stability is one of themost important criteria. In this paper, both the temperaturequenching and temperature rise and down process (when theLEDs were turned on and off) were recorded and discussed, towholly comprehend the practical application process ofwhite LEDs.

Fig. 2 showed the high temperature quenching performance ofCe:YAG transparent ceramics, as well as the commercial Ce:YAGpowder phosphors (YAG-04 from Intematix) and commercialwhite LEDs. The Ce:YAG transparent ceramics showed superiorperformance (with 85% intensity at 230 1C of that at roomtemperature) than the other two kinds, with intensity of 80%and 76% at 230 1C, respectively.

Heat dissipation of a white LED device indicates the capacity torestrain temperature rise, which is mainly related to the thermalconductivity, the surface and inner situation of the phosphormaterials, etc. The temperature rise and down of the assembledLEDs and commercial ones were recorded by an infrared imagingsystem to measure the heat dissipation performance, the resultswere shown in Fig. 3. It could be seen that, the surface tempera-ture of a commercial white LED of 20 W increased to as high as165 1C in 12 min, after it was lightened up. Comparatively, acombination of a 0.5 mm thickness Ce:YAG transparent ceramicswith a blue chip (20 W) showed a slightly better condition, withthe highest surface temperature around 150 1C. As the thickness ofthe Ce:YAG transparent ceramics was increased to 1.5 mm, itpresented an obvious improvement of the surface condition,which only increased to 135 1C in 12 min, shown in the inner ofFig. 3. This phenomena should be interpreted as followings: whenbodies (the same kinds of materials) with the same surface area todissipate heat to the surroundings, they transfer almost the samequantity of heat along their upper surface (length by width, L�W),but the thicker one owns additional larger cross-section area

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Fig. 1. (a) EmiQ2 ssion spectrum of LEDs with Ce:YAG transparent ceramics combined to blue chips, (b) color coordinate of the LEDs with Ce:YAG transparent ceramics ofdifferent thickness. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Table 1Color coordinate of the samples.

Samples 1 # 2 # 3 # 4 # 5 # 0 #

Thickness (mm) 0.2 0.3 0.5 0.8 1.5 –

Color coordinate (X.Y) (0.29, 0.31) (0.29, 0.34) (0.35, 0.38) (0.36, 0.43) (0.39, 0.49) (0.32, 0.33)

G.H. Liu et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎2

Please cite this article as: Liu GH, et al. Ce:YAG transparent ceramics for applications of high power LEDs: Thickness effects and hightemperature performance. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.10.114i

(width by thickness,W� T, and length by thickness, L� T) than thethinner ones, which is benefit to the heat transportation, so amuch more lower surface temperature is generated.

It seems that thicker ceramics are good for heat dissipation of awhite LED device, but as thickness increases, problems will be in

vision. For example, one should optimize the doping concentra-tions and the thickness, to realize equilibrium between good heatdissipation and chromaticity properties. Furthermore, as thethickness increases, it might be much more difficult to obtain anenough transparent and clear ceramics. So, furthermore researchwork is needed to design and optimize the properties of theCe:YAG transparent ceramics to fulfill the needs of white LEDs.

4. Conclusions

The optical properties of Ce:YAG transparent ceramics withspecific doping concentration and different thicknesses werediscussed. Through thickness adjusting of the ceramics, to realizethe ratio adjusting of blue (from commercial InGaN chips) toyellow (Ce3þ from the ceramics) light, LEDs device with bluishcold white color, normal white color and yellowish white color (orwarm white) could be obtained. Ce:YAG transparent ceramics alsoshowed excellent high temperature quenching stability, and afterbeing packaged, the LEDs module showed lower surface tempera-ture when they were lightened up, which was benefit from thehigher thermal conductivity of the ceramics, compared to resinand other packing materials. It also supplies a clue that thicker Ce:YAG transparent ceramics is good for the heat to diffuse, and therelationship between thickness and doping concentration isnecessary to be discussed in the future work, to realize a muchmore reliable design of LEDs, where heat dissipation and chroma-ticity properties are both satisfied.

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2008;14:1387–91.[7] Shi Y, Pan YB, Feng XQ, Li J, Guo JK. J Inorg Mater 2010;25:125–8.[8] Nishiuraand S, Tanabe S. J Ceram Soc Jpn 2008;116:1096–9.[9] Haiyu Huang. J Funct Mater 2012;43:1288–91.[10] Huaidong Jin, Weidong Xiang. Chin J Inorg Chem 2011;27 (1285–129).[11] D.Q. Chen, Y. Chen, Int Ceram, http://dx.doi.org/10.1016/j.ceramint.2014.06.073.

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Fig. 2. High temperature quenching properties of Ce:YAG transparent ceramics,and compared to commercial phosphors and white LEDs.

Fig. 3. Temperature rise and down of LEDs with Ce:YAG transparent ceramicscombined to blue chips, and compared to commercial white LEDs. (For interpreta-tion of the references to color in this figure legend, the reader is referred to the webversion of this article.)

G.H. Liu et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎ 3

Please cite this article as: Liu GH, et al. Ce:YAG transparent ceramics for applications of high power LEDs: Thickness effects and hightemperature performance. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.10.114i