1. Introduction

Yellow phosphors, such as broadband YAG phosphors, have been extensively studied on the integration with the complementary blue LEDs to create white light. To homogeneously disperse phosphors onto the LED chips and keep the formation, matrix materials are essential to support phosphor particles. High optical transmittance and low fabricating temperature make curable silicone the most popular carrier material for phosphors in the wavelength converters. However, because of the low glass transition temperature of silicone, high heat radiation from the LED chips usually detaches the methyl groups in the silicone [1]. The bond breakage creates sub-band defects, making the silicone yellow and decreasing the transmittance of the wavelength converters [2]. The yellowish silicone consequently degrades the optical performance of WLEDs such as in the form of lumen loss and chromaticity shift [3]. Development of new thermal-stable carriers for phosphors in the high power WLEDs is consequently necessary, since the poor thermal stability of silicone makes reliability no longer one of the advantages of WLEDs, especially in the applications such as interior lighting and automotive headlights where high power LEDs are essentially required. Recently, glass-based phosphors have been developed for the high-temperature resistant WLEDs since glass-based materials exhibit better thermal stability compared to silicone because of the high glass transition temperature, high thermal conductivity, and low thermal expansion coefficient [4–6]. Fujita et. al. have developed a Ce³⁺:YAG glass-ceramic phosphor for WLEDs [7–9]. The glass-ceramic phosphor showed higher heat-resistant property than resin, expected to greatly improve the thermal stability of the WLEDs utilizing the glass-ceramic phosphor [10]. However, since the fabrication temperature of the glass-ceramic is between 1200°C and 1500°C, high cost of the fabrication facility (such as high temperature furnaces and platinum crucibles) weakens the practicability of the glass-ceramic phosphor. Dispersing phosphors directly into glass matrices is therefore an alternative way to realize glass-based phosphors. The temperature during dispersing can be considerably low, which is determined by the melting temperature of the glass. Borate and tellurite glasses have been studied as the matrices of phosphors [11,12], the sintering temperature is 1000°C and 500°C, respectively. Although the sintering temperature can be reduced to 500°C for tellurite glass, the pale yellow appearance of tellurite glass could limit the optical performance of the tellurite glass phosphors. Furthermore, strong crystallization tendency of tellurite glass usually degrades the transparency of the glass phosphors due to light-scattering poly-crystals formed during sintering [13,14]. Silicate glass is a good candidate considering optical transparency, but the melting temperature of silicate glass (~1000°C) is much higher than tellurite glass.

In this study, we report the fabrication and characterization of thermally stable Ce³⁺:YAG doped sodium glass (CeYDG), which show low-sintering temperature of 700°C and high- quantum yield of 68%. The high quantum yield of the sodium glass phosphors can be attributed to the reduction of the inter-diffusion between silicon atoms and Ce³⁺:YAG. The Si- Ce³⁺:YAG inter-diffusion has been found to significantly depend on temperature. Reduction of Si-Ce³⁺:YAG inter-diffusion is thus critical to achieve high quantum yield of the sodium glass phosphors. This new low-temperature sodium glass phosphor with high quantum yield is essentially beneficial to the applications for next-generation solid-state lighting in the area where high power and absolute reliability are required.

2. Experiments

Figure 1 illustrates the fabrication sequence of the sodium phosphors. The fabrication procedures consist of two key steps: i) preparation of sodium mother glass by melting mixtures of raw materials at 1300°C; ii) dispersing Ce³⁺:YAG phosphor crystals (particle size ~13μm) into the glass matrix by gas-pressure sintering under different temperature. The composition of the sodium mother glass was 60 mol% SiO₂, 25 mol% Na₂CO₃, 9 mol% Al₂O₃, and 6 mol% CaO. The mixed raw materials were heated at 1300°C for 1h in a platinum crucible to melt. After cooling, the cullet glass (SiO₂-Na₂O-Al₂O₃-CaO) was dried and milled into glass powders. Ce³⁺:YAG crystals were uniformly mixed into the mother glass then sintered for 30 min at 700°C, 750°C, 800°C, 850°C, and 900°C in air atmosphere to form CeYDG-700, CeYDG-750, CeYDG-800, CeYDG-850, and CeYDG-900, respectively. The concentration of Ce³⁺:YAG in the glass phosphors not specified was 4wt%. The glass phosphors were then polished to 0.5mm of thickness after quenching to room temperature.

 

Fig. 1 Outline of the fabrication scheme of glass phosphors: CeYDG-700, CeYDG-750, CeYDG-800, CeYDG-850, and CeYDG-900.

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Thermal mechanical analysis (TMA) was performed at a heating rate of 10°C min⁻¹. Photoluminescence (PL) spectra were measured in room temperature. Absolute photoluminescence internal quantum yield (PL Q.Y._int) experiments, expressed the proportion of excited Ce³⁺:YAG that deactivated by emitting a fluorescence photon, were carried out using a GaN-based blue LED as pumping source, a integrate sphere equipped with an optical fiber and a CCD detector as the spectrum recorder. The crystallographic phase of the sample was determined by X-ray diffraction (XRD) with a Bruker D8 diffractometer. High resolution transmission electron microscopy was performed on a FEI Tecnai-F20 equipped with a LaB6 electron gun operating at 200kV and an energy dispersive analysis X-ray system. The glass phosphors for HRTEM sample were approximately 60nm-thick, which were coated with ~90nm-thick platinum to prevent charge accumulation.

WLED module was realized by combining a LED chip, a reflector cup, and a glass phosphor. The 5W GaN-based LED chip with peak emission at 455nm, had a dimension of square base of 1.1mm and a height of 0.23mm, which was placed at the center of a reflector cup. The reflector cup had a depth, inner diameter, and outer diameter of 0.95 mm, 10 mm, and 16 mm, respectively. The glass phosphor plate with a circular base of a diameter of 16mm and a thickness of 0.5mm completely covered on the LED and the reflective cup to form a WLED module. An integrating sphere equipped with an optical fiber and a CCD detector was employed to measure the optical spectra of the WLED module.

3. Results and discussion

The thermal and photophysical properties of the sodium glass phosphors are collected in Table 1The excitation and PL spectra of the glass phosphors are also shown in Figs. 2. The melting points (T_m) and glass transition temperature (T_g) of these glass phosphors ranging from 693 to 710°C and 578 to 597°C, respectively. There is no apparent variation in T_m and T_g for these glass phosphors prepared under different sintering temperature, since the composition of all the glass phosphors is the same.

Table 1. Physical data for CeYDG-700, CeYDG-750, CeYDG-800, CeYDG-850, and CeYDG-900.

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Fig. 2 Excitation (left) and Emission (right) spectra of CeYDG-700, CeYDG-750, CeYDG-800, CeYDG-850, and CeYDG-900.

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In Figs. 2, two major excitation bands locate at 340 nm and 470 nm, which are attributed to Ce³⁺:YAG. The broad excitation band covering between 400 and 500 nm provides a basis to use Ce³⁺:YAG phosphor along with general blue LEDs to generate white light. All of the glass phosphors are highly emissive in the yellow region. The emission band at 530 nm is attributed to the typical 5d to 4f^7/2 and 4f^5/2 transition of Ce³⁺ ions. The internal quantum yield of the 4wt% glass phosphors increases as the sintered temperature decreases: CeYDG-700>CeYDG-750>CeYDG-800>CeYDG-850>CeYDG-900. The internal quantum yield of CeYDG-700 is enhanced 24% compared to it of CeYDG-900. The internal quantum yield of 4wt% CeYDG-700 is up to 68%, which is much higher than that of other known glass phosphors, including YAG glass-ceramic (37%) [9] and tellurite glass phosphors (32%) [12]. In YAG glass-ceramic, forming of Ce³⁺:YAG microcrystal is essential, which requires high-temperature annealing process [9]. The high temperature (up to 1500°C) required during annealing of glass-ceramic may raise the fabricating cost and reduce the quantum yield. The mechanism of how the processing temperature affects the quantum yield of the glass-based phosphors will be discussed in the next section. The low quantum yield of the tellurite glass phosphors can be ascribed to both nonzero absorption in blue region and strong scattering nature of the tellurite glass matrix. The nonzero absorption in blue region of tellurite glass attenuates the light emitted from the blue LEDs. The strong scattering of the tellurite glass hinders both the blue light emitted from the LEDs and the yellow light emitted from the phosphors to escape from the wavelength converters. The notably high internal quantum yield of the sodium glass phosphors we proposed is benefited by not only the high optical transparency of the glass but also the low sintering temperature of the glass phosphors. Conventional glass exhibits excellent optical transparency but the melting point is up to 1000°C. The additive Na₂CO₃ effectively lowers the melting point of the mother glass to successfully retain the internal quantum yield of phosphor crystals after the sintering process. To further evaluate the temperature effect on the luminescence efficiency of Ce³⁺:YAG, 4wt% Ce³⁺:YAG doped silicone was fabricated. The baking and curing temperature of the Ce³⁺:YAG doped silicone was 150°C. The Q.Y._int of the silicone phosphors was about 68%, which means there was no obvious temperature effect on the luminescence Figure 3 shows X-ray diffraction (XRD) which confirms the presence of Ce³⁺:YAG and a glass material under different sintering temperature. However, peak intensity degrades clearly with increasing of sintering temperature, which indicates that the glass phosphor sintered at high temperature shows much weak crystalline phase.

 

Fig. 3 XRD patterns of CeYDG-700, CeYDG-800 and CeYDG-900.

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Figures 4(a), 4(b) and 4(c) show the high-resolution transmission electron microscope (HRTEM) images of CeYDG-700, CeYDG-800 and CeYDG-900, respectively. The results displays that Ce³⁺:YAG possesses crystalline arrangement and SiO₂ possesses amorphous structure in the glass phosphors. Figure 4(a) shows a clear boundary between SiO₂ and Ce³⁺:YAG crystal in CeYDG-700. However, Figs. 4(b) and 4(c) show a fuzzy region between amorphous SiO₂ and crystalline Ce³⁺:YAG in both CeYDG-800 and CeYDG-900, which indicates more serious diffusion between SiO₂ and Ce³⁺:YAG under higher fabrication temperature compared to CeYDG-700. The inter-diffusion distance in CeYDG-800 and CeYDG-900 are about 5nm and 30nm, respectively. Selected-area of electron diffraction (SAED, inset in Figs. 4) from white square in Fig. 4 further confirm different inter-diffusion nature of glass phosphors under different sintering temperature. The SAED pattern of CeYDG-700 indicates evident crystallization state near the interface between SiO₂ and Ce³⁺:YAG, illustrated by the clearly distinguishable diffraction spots. On the contrary, the SAED patterns of CeYDG-800 and CeYDG-900 show both the diffraction spots and amorphous rings, which indicate the existence of amorphous SiO₂ in the Ce³⁺:YAG crystals due to Inter-diffusion energy dispersive X-ray diffraction spectroscopy (EDS) was used to study the elemental composition in the glass phosphors.

 

Fig. 4 HRTEM images of a) CeYDG-700, b) CeYDG-800, and c) CeYDG-900. Insets: the selected area electron diffraction pattern from the white square.

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Table 2 lists the atomic percent of Y, Al, and Si in a Ce³⁺:YAG crystal at a distance of 100nm from the interface between the Ce³⁺:YAG crystal and amorphous SiO₂ for CeYDG-700, CeYDG-800, and CeYDG-900. The Si element comes from the SiO₂, and the Y and Al come from Ce³⁺:YAG grid. The Si/Al molar ratio for CeYDG-700, CeYDG-800, and CeYDG-900 are determined as 14.0/49.9 = 0.28, 24.7/47.9 = 0.52, and 32.2/38.4 = 0.84, respectively. The Si/Al molar ratio increases as the increasing of sintering temperature: CeYDG-900>CeYDG-800>CeYDG-700, which indicates a much stronger inter-diffusion between Si and Al atoms at higher sintering temperature. Such a strong penetration of Si atoms into Ce³⁺:YAG crystals may introduce defects around the boundary of the Ce³⁺:YAG crystals. The intrinsic properties of the Ce³⁺:YAG crystals are hindered by the surrounding defects. The trend of inter-diffusion with sintering temperature may provide useful information for the further design and implementation of glass phosphors. The lower sintering temperature may have lesser defects in YAG crystals which act as photon quenching centers, resulting in more efficient glass phosphors.

Table 2. Atomic percent of Y, Al, and Si in a Ce³⁺:YAG crystal at a distance of 100nm from the interface between the Ce³⁺:YAG crystal and amorphous SiO₂ for CeYDG-700, CeYDG-800, and CeYDG-900.

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Figure 5 shows the CeYDG-700 samples with different phosphor concentration of Ce³⁺:YAG ranging from 0 to 5 wt%. Transparency decreases with the increasing of doping concentration. The emission spectra and 1931-CIE coordinates of the WLEDs combining a 5W blue LED and glass phosphors are shown in Fig. 6 and 7.The 465nm band is attributed to the emission of the blue LED and the broad emission band around 560nm is attributed to the fluorescence of the glass phosphors and the color coordinates of the WLEDs according to the CIE 1931 chromaticity diagram are summarized in Fig. 7. A color coordinate meets the requirement of commercial WLEDs, (x, y) = (0.33, 0.33), can be achieved by utilizing the 4wt% CeYDG-700 as the wavelength converter for WLEDs.

 

Fig. 5 Top view images of CeYDG-700 with a) 0 wt%, b) 1 wt%, c) 2 wt%, d) 3 wt%, e) 4 wt%, and f) 5wt% Ce³⁺:YAG concentration.

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Fig. 6 Emission spectra of WLEDs utilizing CeYDG-700.

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Fig. 7 CIE-1931 chromaticity coordinates of the WLEDs utilizing CeYDG-700.

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4. Conclusions

In summary, we have successfully developed a highly applicable sodium phosphor-doped glass with both high thermal stability and low melting temperature. With a comprehensive study of the microstructure of the glass phosphors fabricating under different sintering temperature, the Inter-diffusion between SiO₂ and Ce³⁺:YAG was found to be effectively suppressed by lowering the sintering temperature. The glass phosphors, which have intriguing high internal quantum yield, are successfully used to fabricate WLEDs with promising color performance compared to commercial WLEDs fabricated by silicone material. The glass phosphors we proposed here can possibly generate a new class of wavelength converter especially for next-generation high-power WLEDs in the applications of solid-state lighting.