Improving the Thermal Properties of BaSi2O2N2∶Eu2+ Phosphor by SiO2 Coating

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(1.Beijing University of Chemical Technology, Beijing 100029, China；2.Jiangsu Bree Optronics Co., Ltd., Nanjing 241000, China)

Abstract:In this paper, the effect of SiO2 coating on the luminescence and thermal properties of BaSi2O2N2∶Eu2+ blue-green phosphors was investigated. The BaSi2O2N2∶Eu2+ blue-green phosphor was coated with SiO2 by sol-gel method. The experimental results show that the optimal coating amount is 6wt%, the phosphor brightness is rapidly decrease when the amount larger than this value. After coating SiO2, the thermal quenching performance of BaSi2O2N2∶Eu2+ phosphor at 150 ℃ was increased by 2.4%, and the luminescence performance of the phosphor after thermal degradation at 500 ℃ was increased by 15%. SiO2 coating significantly improves the thermal stability of BaSi2O2N2∶Eu2+ phosphors. The mechanism of SiO2 coating is that a blocking layer is formed between the surface of the phosphor and the oxidizing atmosphere protects the luminescent center of Eu2+ from being oxidizing during thermal heating.

Key words:LED lighting； full-spectrum； BaSi2O2N2∶Eu2+； SiO2 coating

1 Introduction

Nowdays, general lighting is the main application area of white LED (WLED)[1-2]. In the field of general lighting, white LEDs are moving from the original medium color rendering index (CRI: 70-80) to high color rendering index (CRI: above 90). And due to the rapid development of phosphor technology[3-4], the full-spectrum LED lighting concept has came into being in recent years. In a full-spectrum LED, the spectral structure of the LEDs will be more inclined to the spectral structure of the sunlight, which is beneficial to human’s visual health.

At present, the lighting industry has the following requirements for blue chip based full-spectrum WLEDs： (1) spectral coverage is as wide as possible; (2) color rendering indexRa> 95; (3) all special color rendering indexes R1-R15 are greater than 90. Although the WLED with a color rendering index of 95 can be packaged using an aluminate YGG yellow green phosphor and a nitride CASN red phosphor combined with a blue chip, the blue-green light spectral range (480～510 nm) in the WLED spectrum is insufficient, resulting in the value of the special color rendering index R12 is low, which does not meet the requirements of the full spectrum WLED.

Nitrogen oxide phosphors have been widely used in white LEDs due to their good chemical stability[5-10], wide excitation spectrum and emission spectrum coverage. Such as Eu-activated α-SIALON yellow phosphors[11]and Eu-activated β-SIALON green phosphors[12], etc. Among them, BaSi2O2N2∶Eu2+(BASON) blue-green phosphor[13-15]can effectively absorb blue light in the range of 440-460 nm, and emit blue-green light with peak wavelength at about 490 nm, which has great application prospects in full-spectrum WLED lighting. However, due to the instability of the crystal structure, BaSi2O2N2∶Eu2+phosphor has poor thermal stability and aging performance. Long-term use on LEDs can cause large light decay and color shift, which limits the application of the BaSi2O2N2∶Eu2+blue-green phosphor used in the full spectrum WLED.

Improving the luminescent stability of BaSi2O2N2∶Eu2+blue-green phosphor in high temperature and high humidity environment has become the key issues. Wang et al[16-17]found that The thermal properties of BaSi2O2N2∶Eu2+blue-green fluorescence can be enhanced by doping with trace elements such as Mg, La and Ge. Xie et al[18]has studied the degradation mechanism of SrSi2O2N2∶Eu2+green phosphor in high temperature and high humidity environment. However, so far there has been no report on how to improve the luminescence stability of BaSi2O2N2∶Eu2+blue-green phosphor in high temperature and high humidity environment. Our research found that a layer of dense inorganic film coated on the surface of BaSi2O2N2∶Eu2+blue-green phosphor can effectively improve the performance of the phosphor in high temperature and high humidity environment.

The present study focuses on improving the thermal reliability of BaSi2O2N2∶Eu2+phosphor, By coating with SiO2blocking layer prepared with sol-gel method, the luminescent properties, temperature dependent properties as well as thermal degradation performance of BaSi2O2N2∶Eu2+phosphor were measured. In addition, the mechanism of SiO2coating to enhance thermal reliability was investigated.

2 Experimental

SiO2coated BASON phosphor were prepared by sol-gel method. A SiO2thin layer was coated on the phosphor via the hydrolysis of Tetra-ethyl ortho silicate (TEOS). The TEOS was first dissolved in iso-propyl alcohol (IPA). The concentration of the TEOS was varied from 1wt% to 10wt% in the IPA. Next, 0.01N HCl was added to the solution to ensure completion of the hydrolysis reaction. After 24 h, BaSi2O2N2∶Eu2+phosphor were added to the sol-gel solution. The phosphor sol-gel solution was stirred for 10 h and then filtered. The filtrate was dried at 80 ℃ for 24 h. After drying, the filtrate was annealed at 200 ℃ for 8 h in air.

X-ray diffraction patterns were examined on a D8 (BRUKE) X-ray powder diffractometer. The PL properties were measured using a fluorescent spectrophotometer (F-7000,HITACHI). The temperature-dependence luminescence properties were measured on the same F-7000 spectrophotometer, which was combined with a self-made heating attachment and a computer-controlled electric furnace. X-ray photoelectron spectroscopy (XPS) of the samples was performed with a system (Thermo Scientific, ESCALAB 250Xi) with an Al KαX-Ray source. Finally, the elemental content as a function of etch depth were calculated from the XPS spectra. All the measurements were performed at room temperature.

3 Results and discussion

The excitation and emission spectra of BASON phosphors are shown in Fig.1. The excitation peaks at 321 nm, 381 nm, and 443 nm are derived from the transitions of the 4f ground state of Eu2+to the excited 5d bands. It can be seen that the main excitation band is located at 430-455 nm, very suitable for absorbing blue light emitted by blue chip (peak wavelength 445-455 nm). From the emission spectrum, the BASON phosphor emits a broad band blue-green light with a peak wavelength at 490 nm, which can be attributed to 5d-->4f transitions of Eu2+. The light in this band can fill the blue-green light gap in the high CRI WLED, which can significantly improve the color reproduction ability of WLED and enhance the special color rendering index of R12.

Fig.2 gives the XRD patterns of uncoated BASON phosphor, various amount of SiO2coated samples and standard XRD Card of BASON (PDF card No.41-9450), respectively. It can be seen clearly from Fig.2 that the positions of the main diffraction peaks coincide well with standard cards, no other impurity peaks were found in the patterns of these selected samples. The results indicated that the synthesized phosphor is BASON pure phase and the phase structure of the coated phosphors remained the same as the original samples. However, we found that the intensity of the diffraction peaks of the sample coated with 8wt% of SiO2is only 60% of that of the original sample, which means that this sample produced a little amorphization. Therefore, we propose that SiO2layer coated on the surface of phosphor particle should be amorphous.

Fig.1 The excitation (a) and emission (b) spectra of BASON phosphor

Fig.2 XRD patterns of phosphor: (a) uncoated, (b)coated with 2wt% SiO2， (c) coated with 8wt% SiO2, and standard Card (PDF card No.41-9450) of BASON

Fig.3 shows the effect of different SiO2coatings on the luminescence brightness of BASON phosphors. It can be seen that the luminescence intensity of BASON phosphors decreases with the increase of SiO2coating amount. But the luminescence intensity of BASON phosphors decreases very small when the coating amount is less than 7wt%. The luminescence intensity of the 7wt% SiO2coated BASON phosphor is 99.3% of the original one, still maintains good luminescent performance. However, when the coating amount exceeds 7wt%, the luminescence of the BASON phosphor decreases rapidly, and the luminescent intensity of the 10wt% SiO2coated BASON phosphor is only 96% compared to that of the uncoated phosphor. In order to achieve both the luminescent properties and the coating effect, we chose the optimum coating amount to be 6wt%. Subsequent discussion focused on 6wt% SiO2coated BASON phosphor.

In order to test the coating effect and coating thickness of SiO2on BASON phosphor, long etch time XPS was performed and the results of elemental content of 6wt% SiO2coated BASON phosphor as a function of etching time (etch depth is about 3 nm per minutes) are depicted in Fig.4. As can be seen from Fig.4, the amount of silicon element gradually decreases with increasing etching time, and the amount of barium element increases with the etching time. When etching time exceed for 12 min(etching depth is about 36 nm), the amount of silicon element and barium elements reach balance. The above phenomenon indicates that the surface of the BASON is indeed covered with a layer of SiO2, and the silicon element measured after 12 min of etching comes from the silicon element contained in the BASON host. That is, for the 6wt% SiO2coated BASON sample, the SiO2coating thickness is about 36 nm. Fig.5 shows the SEM image of the uncoated BASON sample and the 6% SiO2coated BASON sample. As can be seen, the surface of the BASON sample coated with SiO2exhibits a certain roughness and a certain agglomeration phenomenon, which is a normal performance of inorganic coating on powder surface.

Fig.3 The luminescent intensity of the coated samples with different amount of SiO2 compared to that of the uncoated sample

Fig.4 The elemental content of 6wt% SiO2 coated BASON phosphor as a function of etching time that measured with XPS

Fig.5 SEM images of the uncoated (a) and SiO2 coated BASON samples (b)

In order to further determine the coating effect of SiO2, TEM image of the uncoated and SiO2coated BASON phosphors were tested, as shown in Fig.6. It can be seen from the figure that the surface of the SiO2-coated BASON phosphor particle has a thickness of about 30 nm amorphous layer (between the red lines on the right figure), while there was almost no amorphous layer on the uncoated BASON phosphor particle. Therefore, the amorphous layer on the surface of the SiO2coated BASON phosphor particles should be derived from the amorphous film produced by SiO2coating, and the thickness of about 30 nm is consistent with the XPS etching experimental results(about 36 nm). The above testes indicate that the SiO2is well coated on the BASON phosphor particles.

Fig.6 The TEM images of the uncoated (left) and SiO2 coated BASON samples (right)

Fig.7 shows the luminescence intensity of the uncoated BASON phosphor and the 6wt% SiO2coated BASON phosphor as a function of heating temperature (temperature quenching curve). In the test, we heated the tray containing the sample to the specified temperature and hold for 10 min. Then, the emission spectrum is tested. After that the emission spectrum is integrated, and the integrated area covered by the emission spectrum is taken as the luminescent intensity of the sample at that temperature. As can be seen, the luminescent intensity of the uncoated BASON phosphor at 150 ℃ is 87.1% of that at room temperature, while the luminescent efficiency maintenance rate of SiO2coated sample is 89.5% at 150 ℃. That is, after being coated with SiO2, the temperature quenching property of the BASON phosphor is slightly improved.

The thermal stability of phosphor is an important parameters for evaluating the performance of a certain phosphor. Generally, the thermal stability can be characterized by the temperature quenching of the phosphor, but it is difficult to measure the emission spectrum of the phosphor at higher temperatures (for instance, 500 ℃). Here, we attempt to investigate the photoluminescence of the phosphor samples baked in air for 5 h at temperatures up to 500 ℃ in an oven, which can help us to evaluate the ability of luminescent properties of the phosphor under high temperature thermal degradation. Fig.8 demonstrates the luminescent spectra of uncoated and SiO2coated BASON phosphors baked at 500 ℃. It can be seen from the figure that the luminescent intensity of the uncoated phosphors baked at 500 ℃ decreased by 20%, and the emission wavelength shifted 5 nm to longer wavelength. While the luminescent intensity of SiO2coated BASON phosphor decreased by only 5% at the same condition, and with very small emission wavelength change. The above test shows that the high temperature thermal degradation resistance of BASON phosphors is significantly improved after being coated with SiO2.

Fig.7 The temperature quenching curves of uncoated BASON phosphor and SiO2 coated BASON phosphor

Fig.8 The emission spectra of (a)original BASON phosphors, (b)SiO2 coated BASON phosphor baked at 500 ℃, (c)uncoated BASON phosphor baked at 500 ℃

In the application process, LED phosphors are affected by humidity in addition to heat. In some cases, LED phosphors need to work in both high temperature and high humidity environment, so it is necessary to evaluate the performance of phosphors against high temperature and high humidity. To this end, we put the phosphors into stainless-steel reaction still, and added a small amount of distilled water to the reaction still. In the end, the reaction still were heated to 250 ℃ temperature and kept for a period of time in an oven. After heating, the phosphors were naturally cooled to room temperature. Lastly, the phosphor samples were dried in the oven at 100 ℃. The luminescence spectrum of the phosphors were tested and the changes in the luminescence intensity and color coordinates of the phosphors before and after heating were compared.

Table 1 Changes in luminescence intensity and color drift of uncoated and SiO2 coated samples after high temperature and high pressure heating

Table 1 shows the change in luminescence intensity and color coordinates drift of uncoated and SiO2-coated BASON phosphors after high temperature and high pressure heating. It can be seen from the table that the luminescent intensity of the uncoated phosphor after heating is reduced by 4%, and the color coordinates drift is 0.0042. While the luminescent intensity of the SiO2-coated BASON phosphor was only reduced by 1.8% under the same conditions, and there was almost no color drift. The above tests show that the high temperature and high humidity resistance of BASON phosphors is also significantly improved after SiO2coating. In other words, SiO2coating can prevent high temperature water vapor erosion to BASON phosphor.

Table 2 Eu valence composition of uncoated BASON phosphors and SiO2 coated BASON phosphors after thermal degradation at 500 ℃

The uncoated BASON blue-green phosphor and the SiO2-coated BASON phosphor were packaged into LEDs using blue chips, and the light attenuation and color shift of the LEDs after 1000 h aging were tested. The results are shown in Fig.9 after 1000 h aging, the light decay and the color shift of the LED packaged with SiO2coated BASON is far better than that of the LED packaged with the uncoated BASON phosphor. This phenomenon indicates that the SiO2-coated BASON phosphor has better long time aging properties.

Fig.9 The light decay and color shift of the LEDs packaged with uncoated and SiO2 coated BASON after 1000 h aging

Wang et al[19]believe that the thermal degradation of SrSi2O2N2∶Eu2+phosphors is mainly caused by the oxidation of activator Eu2+on the phosphor surface to Eu3+in high temperature oxidation environment. In order to confirm the mechanism of improving of thermal degradation performance of BASON phosphors by SiO2coating, we used X-ray electron spectroscopy to investigate the activator’s valence composition of uncoated BASON phosphors and SiO2coated BASON phosphors after thermal degradation at 500 ℃. The results are shown in Table 2, in which the SiO2coated BASON phosphor was etched for 12 min before testing. It can be seen from the table that the ratio of Eu2+/Eu3+on the surface of the uncoated BASON phosphor is 82∶18, while the ratio of Eu2+/Eu3+on the surface of the BASON phosphor coated with SiO2is 91∶9. The above results show that the decrease of luminescence performance after surface thermal degradation of BASON phosphor is mainly due to the reduction of Eu2+on the surface of the phosphor to Eu3+. At such condition, the content of luminescent center(Eu2+)is reduced, and in which aspect, the increase of Eu3+content will cause defects generated in the phosphor matrix to further reduce the luminescent properties of the phosphor. After coating with SiO2, a blocking layer is formed between the surface of the phosphor and the oxidizing atmosphere to protect the luminescent center of Eu2+from being oxidizing during thermal heating.

4 Conclusion

BaSi2O2N2∶Eu2+blue-green phosphor was synthesized by high temperature solid phase reaction, and investigated the effect of SiO2coating on the luminescence and thermal properties. The BASON blue-green phosphor was coated with SiO2by sol-gel method. In order to achieve both the luminescent properties and the coating effect, 6wt% amount SiO2is the optimal coating amount. From long etch time XPS test, we find that for the 6wt% SiO2coated BASON sample, the SiO2coating thickness is about 36 nm.

After coating, the thermal quenching performance of BASON phosphor at 150 ℃ was increased by 2.4%, and the luminescence performance of the phosphor after thermal degradation at 500 ℃ was increased by 15%. After high temperature and high pressure heating, the SiO2coated sample show better stability than the uncoated sample, which also was confirmed by 1000 h aging process. The mechanism of SiO2coating is that a blocking layer is formed between the surface of the phosphor and the oxidizing atmosphere protects the luminescent center of Eu2+from being oxidizing during thermal heating.