We present our achievement which is a ceramic plate phosphorable to produce white light when directly combined with commercially available blue light emitting diodes. The ceramic phase structure is that the Al2O3 particle is uniformly distributed in the Ce:YAG matrix. The Al2O3-Ce:YAG ceramic phosphor has a better luminous efficacy than the transparent Ce:YAG ceramic phosphor under the same test condition. The Al2O3 particle plays an important role in promoting the luminous efficacy. The Al2O3 particle changes the propagation of the light in ceramic, and it reduces the total internal reflection. That is why the composite phase ceramic phosphor improves extraction efficiency of light.
© 2015 Optical Society of America
Nowadays, white light emitting diodes (WLEDs), a new type of luminescent source, have received considerable attention because of their advantage in energy conservation and broad application potential over conventional illumination devices. The current leading commercial WLED normally combines an InGaN blue-emitting chip with Ce:YAG yellow-emitting powder phosphor, and the powder phosphor is packed on the chip surface by using epoxy resin [1–7]. However, the deterioration of the organic resin matrix, because of its poor heat-resistance, becomes a serious problem under the increasing temperature and output power of LED chips. This raises several issues in WLED practical application, such as degradation of luminous efficacy and long-term reliability, shifts in emission color, and reduction of lifetime.
In order to solve the problem, researchers have investigated novel durable phosphors without resins. All-inorganic phosphors are on track to replace the “phosphor powder”-“organic matrix” combination, such as transparent ceramic phosphor [8, 9] and glass ceramic phosphor as practical alternatives to solve the deterioration of the organic resin matrix. Sakata et al. invented the ceramic composite which has an irregularly twisted lamellar structure with Al2O3 and Ce:YAG . However, it’s reported that the luminous efficacy is still not excellent . In a thin transparent Ce:YAG ceramic-based WLED, a maximum luminous efficacy reaching 93 lm/W at a low correlated color temperature (CCT) of 4600 K was realized . However, the Ce:YAG nanopowders need to be synthesized by co-precipitation, so the increasing cost of the complex fabrication process is an unavoidable challenge for industrial production. Notably, transparent Ce:YAG phosphor-in-glass also shows a good performance . Zhang et al. reported that transparent Ce:YAG phosphor-in-glass yielded a luminous efficacy of 124 lm/W, a correlated color temperature of 6674K and a color rendering index (CRI) of 70, at an operating current of 350mA. However, the thermal conductivity of glass is far inferior to ceramic. This may limit its application in some cases, for instance, when laser is adopted as the excitation light source.
In the present study, we have made two-phase Al2O3-Ce:YAG ceramic phosphor with Ce3+-correlated yellow emission. The ceramic phosphor uniformly mixes hexagonal α- Al2O3 with cubic YAG. We also investigate its resulting luminescent properties. The luminous efficacy of the WLED based on the composite phase ceramic phosphor reaches 95 lm/W, superior to that of the single-phase transparent Ce:YAG ceramic.
2. Experimental methods
The ceramic samples with chemical formula a(Ce0.001Y0.999)3Al5O12:bAl2O3 were prepared by solid-state reaction at high temperature and high purity powders Y2O3 (99.999%), Al2O3(99.999%) and CeO2 (99.99%) were used as raw materials. 0.1 wt% MgO and 0.4wt% TEOS were used as the additives, 1wt% PEG as the dispersant [13, 14]. In the weighting process, the mole ratio of Al2O3/Y2O3 was controlled in 2~8, and CeO2 is the dopant at a mole ratio of Ce/(Y + Ce) = 0.1%. The final ceramic component proportion is Al2O3/YAG = 0~10.35 in mole, 0~1.78 in weight, and 0~2.08 in volume, calculated according to their molecular weight and density.
Homogeneous slurry of the mixed raw materials was obtained by wet ball milling with ethanol for 12h, and then dried in oven at 70°C. The obtained powders were uniaxially pressed into disks at 10MPa and then isostatically cold pressed at 210 MPa. The compacted disks were sintered at 1700°C for 10 h under a vacuum of 10−3 Pa. Al2O3-Ce:YAGceramics withφ16 mm in diameter was obtained. We cut and polished the ceramics to the appropriate thickness for producing white light on the blue LED chip.
The crystalline phases of the samples were identified by XRD measurement with a Cu Kα radiation source (Ultima IV Diffractometer, Rigaku, Japan) in the angular range of 2θ = 10–90°at a scan step width of 0.02°. Elemental analysis of the Al2O3-Ce:YAGceramic was carried out by energy dispersive X-ray spectroscopy (EDS) under the SEM. Fracture surface microstructure was observed by the SEM (JSM-6510, JEOL, Japan). Luminous efficacy was evaluated with an integrating sphere (Everfine PMS-50 system) in Shanghai Semiconductor Lighting Engineering Research Center.
3. Results and discussion
The obtained samples of the ceramic phosphor are shown in Fig. 1. In the system which is Al2O3 and Y2O3 as raw materials, the final phases depend on the sintering temperature . In our case (Fig. 2), the XRD reveals the characteristic peaks of α-Al2O3 and cubic-YAG, and no peaks from other crystalline phases. The samples with different molar ratios of Al2O3/Y2O3 have different relative peak intensities fromα-Al2O3 and cubic-YAG, and with increasing of the molar ratio of Al2O3/Y2O3, the intensity ofα-Al2O3 increases correspondingly. Furthermore, the SEM-EDS mapping of the ceramic phosphor also confirms only Al2O3 and cubic-YAG existed, as demonstrated in Fig. 3. The region marked by “Spectrum 12” is YAG phase and the area of “Spectrum 14” is Al2O3 phase according to EDS analyses (Table 1), which is in accordance with the XRD results mentioned above. Through the observation of the fracture surface morphology, someα- Al2O3 particles are embedded in the YAG matrix and the others have been extracted. Therefore, we find some convex shapes and concave impressions. Supplementary note, in the XRD and EDS there is no information to reflect the state of Ce ion because of its low concentration.
SEM observations on the fracture surface of the samples were carried out to investigate variation of the microstructure with the mole ratio of Al2O3 /YAG, as show in Fig. 4. Obviously, the YAG crystal is mainly with transgranular fracture mode, the Al2O3partical with intergranular fracture mode, and meanwhile most ofAl2O3particalshave been extracted from the YAG matrix forming concave impressions. The Al2O3 particles sized 2-3μm are homogeneously dispersed in the YAG matrix. And with the decreasing of the mole ratio of Al2O3 /YAG, the amount of Al2O3 particles is decreasing and the transparency is changed from translucent to transparent, as seen in the inset of Fig. 4. The thicknesses of the samples in Figs. 4(a)-4(c) are 0.44mm, 0.44mm and 0.64mm, respectively. Notably, though the sample in Fig. 4(c) is much thicker than others, it exhibits the best transparency. The Al2O3 particles change the linear propagation of light in ceramics, therefore the in-line transmittance deceases with increasing Al2O3 content. We also use the thermal etching to make the YAG grain boundary exposed, and the SEM observation reflects an interesting information of YAG grain. As shown in Fig. 4(d), the grain size of YAG is 5-15μm. Comparing the result with the report by Liu  under the same sintering temperature, we find that the existence of Al2O3 don’t influence the grain size of YAG.
Figure 5 exhibits WLED emission spectra with the ceramic phosphoroftheAl2O3/YAG mole ratio 0.65 when put on the blue chip. The CRI of the sample is 63.4 and the CCT is 5585K.The blue LED chip emits blue light with the dominant wavelength of 452.6 nm under a drive current of 150 mA, and subsequently the blue light excites the Al2O3-Ce:YAG ceramic phosphor to produce emission light within 500~700nm. The absorption at ~450 nm comes from the transition of Ce3+, and the photoluminescence spectrum of the Al2O3-Ce:YAG ceramic exhibits a typical broadband emission centered at 540nm under the 450 nm excitation in the YAG host. The emission light and the blue light which are not absorbed by Ce3+ form white light together, as shown in Fig. 5.
The measured luminous efficiency of WLED with the Al2O3-Ce:YAG ceramic phosphors is listed in Table 2 and all the samples were measured at room temperature (25°C). The Al2O3-Ce:YAG ceramics with different mole ratios of Al2O3/YAG were cut into appropriate thicknesses to emit white light when packed on the blue LED chip. When the content of YAG in weight is more than 90%, on increasing the Ce:YAG content, the thicknesses of the samples increase. Meanwhile, the transparency of the samples increases gradually, shown as Fig. 1. However, when the content of Ce:YAG in weight is less than 90%, on increasing the Ce:YAG content, the thicknesses of the samples decrease, as shown in Table 2. The Ce:YAG matrix in different ceramic phosphors has the same Ce3+ concentration at a mole ratio of Ce/Y = 0.1%. Thus the Ce3+ doping amount increases with the increasing of Ce:YAG content. Therefore, the samples turn thinner gradually to achieve the white light when packed on the blue chip . A lot of Al2O3 particles in the Ce:YAG matrix lead to the ceramic opaque, but the Al2O3 particles and the grain boundary can change the direction of light propagation in ceramic, which may increase the absorption probability of the blue light (Fig. 6a). So the proper amount of Al2O3 particles can reduce the thickness of samples to get white light. More importantly, Luminous efficiency was evaluated under a drive current of 150mA, and the highest luminous efficiency, ~95lm/W, was achieved at a mole ratio of Al2O3/YAG = 0.65, as shown in Table 2. Notably, the Al2O3-Ce:YAG ceramic phosphor has a better luminous efficacy than the transparent Ce:YAG ceramic phosphor under the same test condition. The Al2O3 particles play an important role in promoting the luminous efficacy. The Al2O3 particle not only changes the propagation of the light in ceramics, but also improves the extraction efficiency of light that may be equivalent to the photonic crystal and surface roughening . The light extraction of the transparent Ce:YAG ceramic phosphor is relatively low in accordance with trapping of light by total internal reflection and the waveguide effect. As seen from Fig. 6(b), the structure of the Al2O3-Ce:YAG ceramic makes the propagation direction of the light more variable, thus the extraction efficiency is increased. Obviously, the size and distribution density of Al2O3 particles in the YAG matrix have great influence on the luminous efficacy of WLED.
In summary, we have successfully fabricated Al2O3-Ce:YAG ceramics by solid-state reactions. The Al2O3 particles sized 2-3μm are homogeneously dispersed in the Ce:YAG matrix whose grain size is 5-15μm. The ceramic sample with the mole ratio of Al2O3/YAG = 0.65 has a high luminous efficacy, ~95lm/W. In the Ce:YAG matrix, the existence of the Al2O3 particle can change the propagation of the light in ceramics to increase the probability of blue light to be absorbed and converted by Ce3+. Furthermore, appropriate amount of Al2O3 particles interspersed in Ce:YAG matrix can also improve luminous efficacy of the WLED. The Al2O3 particles improve the extraction efficiency of light as the photonic crystal and surface roughening, and meanwhile the fabrication of the Al2O3-Ce:YAG ceramic phosphor is easier and with lower cost than the photonic crystal and surface roughening. The result indicates that the investigated Al2O3-Ce:YAG ceramic phosphor should be an excellent alternative to the transparent Ce:YAG ceramic phosphor for high-power WLED application. In the next work, we will adjust the distribution density of the Al2O3 particle further uniformity and its size more appropriate to improve the luminous efficacy.
This work was supported by Nature Science Foundation of China (No. 51172254, 51202269 and 61475172).
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