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The transmittance of phosphor-in-glass (PIG) color converter material was studied as a factor affecting the luminescence properties of light emitting diode packaging; it is closely related to the residual pores of sintered glass. In this study, the correlation between porosity and optical properties of the glass and PIG plates was investigated. The transmittance, luminescence properties, and porosity were measured by UV-visible spectrometer, integrating sphere and scanning electron microscope, respectively. Transmittance of the sintered glass plate and the luminous efficacy of the PIG plate both increase with decreased porosity, while the light scattering coefficient decreases. Luminescence properties such as emission intensity and color coordinates are also influenced by transmittance of the PIG plate.
© 2015 Optical Society of America
1. Introduction
Recently, there have been studies on the use of inorganic color converters with high-power white light emitting diodes (LEDs); the merits of these materials are high heat-resistance and prevention of yellowing. The intent of these studies is to eliminate the use of epoxy and silicone as encapsulants. Color converters are classified into three types, depending on their production method: phosphor-in-glass (PIG), which is fabricated by sintering a mixture of glass frit and phosphor [1–3]; glass-ceramic phosphor, for which a glass matrix containing fluorescent substances is crystalized under heat-treatment [4,5]; and ceramic plate phosphor, for which a co-precipitated phosphor is sintered in vacuum [6–8].
PIGs based on inorganic materials have luminescence characteristics that can be controlled by adding heterogeneous phosphors with various compositions; the phosphor can be dispersed in a glass matrix regardless of its type. However, it is necessary to use a sintering temperature that is below the firing temperature of the color converter phosphors [9]. Furthermore, PIG can be used in separated-phosphor designs, where placing the material in a position apart from the blue LED chip can improve system efficiency and provide uniform illumination.
Studies on enhancing the luminescence characteristics of PIG used in LED packages have focused on varying the glass composition and thickness and the phosphor/glass ratio [9–11], and on increasing light extraction efficiency by reducing the refractive index gap between glass matrix and phosphor [12–14]. Transparency, however, is also an important parameter of LED encapsulants because the color converter transmits blue and yellow wavelengths emitted from the LED chip and yellow-emitting phosphor, respectively [15]. Increasing the transmittance of sintered glass turns out to be strongly related to minimizing the residual porosity [16–18] and the associated light scattering in the matrix, thereby enhancing PIG luminous efficacy.
In this study, we investigate the relation between residual porosity and transmittance of sintered glass and PIG plates, and determine how this determines the resulting luminous efficacy of LED packages. High refractive index glass was chosen to minimize the mismatch between glass and phosphor, and thus the study focused on the effect of residual pores. The variation of pore characteristics with various sintering conditions was observed, and the transmittances of sintered glass and PIG were calculated from Mie scattering theory.
2. Experimental methods
High refractive index glass was selected as described in previous work, where silicate glasses were obtained by addition of oxides such as La2O3 and WO3 [14]. A mixture of SiO2 (11.4 mol%), B2O3 (24.3 mol%), ZnO (35 mol%), LiO2 (5.3 mol%), La2O3 (12 mol%), and WO3 (12 mol%) powders was weighed, mixed for 2 h in a tubular shaker-mixer (Model T2F, Glen Mills Inc., USA), and melted in an alumina crucible at 1200 °C for 1 h. The melted glass was injected into a ribbon roller, quenched in air, and made into a glass cullet. The glass cullet was crushed to glass frit using a Planetary Mono Mill (Pulverisette-7, Fritsch, Germany) and dry ball milled for 10 min. The glass transition temperature (Tg) was measured by differential scanning calorimetry (DSC) in air, (STA 449, Netzsch, Germany), with a temperature ramp from room temperature to 800 °C at the rate of 10 °C/min. A hot-stage microscope (Misura HSM, Expert System Solutions, Inc., Italy) was used to analyze the glass softening point (Ts). The glass showed a glass transition temperature of 520°C and a softening point of 620°C.
At the same glass frit weight ratio, 5 wt% of commercial Y3Al5O12:Ce3+ phosphor (FORCE4 Co. Ltd., Korea) was added and mixed using a vortex mixer (G-560, Scientific Industries Inc., USA) for 30 min, then uniaxially pressed into a 32-mm-diameter mold. The glass frit and the glass-phosphor composite were sintered in a furnace with a heating rate of 10 °C/min (Table 1).
Table 1. Sintering conditions of the sintered glass and PIG specimens
The sintered glass and PIG specimens were ground to a thickness of 500 μm. The transmittance and reflectance were measured over the range 300–800 nm using a UV-visible spectrophotometer (UV2450, Shimadzu Corp., Japan). The luminescence characteristics, including the luminous efficacy, CIE color coordinates, color rendering index (CRI), and correlated color temperature (CCT), were determined with an integrating sphere (LMS-400, Lab sphere Inc., USA). The sintered glass was sectioned and its microstructure observed by scanning electron microscopy (SEM), (S-4200, Hitachi, Japan). To quantitatively analyze the pore size and porosity, seven images (the dimensions of each sample area were 500 μm x 850 μm) from each specimen were selected and the parameters measured using image analysis software (Image-Pro Plus, Version 6.0, Media Cybernetics Inc., USA).
3. Results and discussion
To establish the relation between residual porosity and sintering conditions (Table 1), the cross-sections of sintered glass and PIG were observed by SEM (Fig. 1). The voids between glass frit particles that did not merge became pores; the shape and size of the pores, randomly formed during sintering, were dependent on the sintering temperature and time. The initial shape of pores is non-spherical but it changes to spherical with increasing holding time (Fig. 1(c) and (d)).
Fig. 1 SEM image of a cross-section of a glass plate sintered at (a) 600 °C, (b) 620 °C for 30 min; sintered at 620 °C for (c) 0 min, (d) 20 min, and PIG (e) 600 °C, (f) 620 °C for 30 min; sintered at 620 °C for (g) 0 min, (h) 20 min
The average pore diameters of sintered glass samples range from approximately 4 μm to 9 μm, and porosities from 0.92 ± 0.24% to 1.65 ± 0.42% (Fig. 2(a)). The PIG samples have an average pore diameter ranging from approximately 4 μm to 9 μm, and porosity from 0.90 ± 0.18% to 3.13 ± 1.07% (Fig. 2(b)). The pore size and porosity of PIG were larger than those of the sintered glass, which is due to the difference between the size distributions of phosphor and glass frit.
Fig. 2 Number of pores (inset), porosity, and average pore diameter as a function of sintering temperature (from 600 °C to 640 °C, closed symbols) and holding time (from 0 min to 40 min, open symbols) for (a) sintered glass and (b) PIG
When the temperature was being controlled, owing to the nearly constant number of pores (in the inset of Fig. 2(a) and 2(b)), the porosity increased with an increase in the average pore diameter of samples. However, a slight increase in porosity occurred with increasing sintering time; this result can be attributed to a decrease in the average pore diameter and an increase in the number of pores. The average pore size was more strongly affected by sintering temperature than by holding time. In the final stage of viscous sintering, with an increased gas pressure in each pore due to a higher temperature, or a reduction of the driving forces for sintering due to pore coalescence, the pores expand, leading to an increase in porosity [19,20].
We investigated how the residual pores influence the optical characteristics through scattering of blue light from the LED chip. Scattering results from the difference between the refractive indices of gas in the near-spherical pores and the glass matrix itself; hence, reducing the transmittance [21]. The transmittance, T(λ), can be expressed with the Lambert-Beer equation [22].
where R is the reflectivity, and t is the sample thickness (500 μm in this study). Csca, the effective scattering coefficient of non-absorbing pores, is calculated from the measured porosity, radius of pores, and scattering efficiency, which is derived using Mie scattering theory with wavelength at 550 nm and refractive indices ratio of pore/glass matrix [23]. The refractive index of the glass matrix was 1.81 [14], and the pore refractive index was 1.00.As porosity increases from 0.92% to 1.65%, the transmittance remains roughly constant at about 40% for a wavelength of 550 nm (Fig. 3). The measured transmittance was similar to that predicted by the effective scattering coefficient. The small difference may have occurred because spherical pores were assumed in Mie scattering theory, while the pores of the sintered glass samples were not all spherical [24,25].
To observe how the transmittance of sintered glass affects the transmittance of PIG, the latter was calculated using the same method as that employed for sintered glasses but using data for PIG samples. Because the particle size and concentration of phosphors have an influence on the optical properties for an LED package [26,27], we fixed the amount and type of phosphor added under each condition. The transmittance of PIG decreased from 56.3% to 27.4% at 550 nm with an increase in porosity (Fig. 4). The transmittance of PIG was lower than that of the sintered glass because the porosity and pore size of PIG were larger than those of the sintered glass; in addition, the scattering of yellow light caused by phosphor particles also enhance the transmittance decrease [28]. Mie scattering theory predicts the same trend, although it significantly overestimates compared to measured values. Since only scattering from pores in a glass matrix was modeled in the calculation without phosphor, we assume that the measured PIG transmittance was lower because of phosphor scattering.
In this study, we control the residual pores in order to increase the luminous efficacy of PIG. It is expected that reducing porosity will enhance the sintered glass transmittance, causing an increase in blue light absorption in the phosphor; this will enhance the relative intensity of yellow emission. As shown, the luminous efficacy of PIG was significantly improved with increased transmittance (Fig. 5(a)). With an increase of transmittance from 27.4% to 48.4%, the PIG luminous efficacy increased substantially, from 50.9% to 71.7%.
This result confirms that reduction of porosity in the PIG plate minimizes the loss of blue light emitted from the LED chip, and enhances the luminous efficacy. While the color rendering index (CRI) of PIG remained unchanged with increased transmittance, the CCT showed the same tendency as luminous efficacy (Fig. 5(b)). The CRIs were approximately 60 Ra, regardless of sintering conditions, and the CCTs were 3700–4200 K, indicating a warm white.
The relative photoluminescence (PL) intensities at 440nm and 550nm, (blue and yellow emissions, respectively) increase with the PIG transmittance (Fig. 6(a)). However, while both blue and yellow emissions increase; the blue intensity increases faster. As transmittance increases, the CIE color coordinates of the emission move from (0.433, 0.512) to (0.388, 0.437), i.e., from the yellow to the white region in Fig. 6(b). This indicates that an increase in PIG transmittance leads to an increase in blue light interacting with the phosphor. However, this might also result from geometric differences preferentially affecting the path of blue light to the PIG plate.
Fig. 6 (a) Variations in the relative PL intensity for blue and yellow wavelengths; (b) Chromaticity coordinate shift of PIG plates with transmittance.
4. Conclusion
Sintered glass and PIG plates were produced under various sintering conditions, and their optical characteristics were assessed according to their transmittance. The calculated and measured transmittances of sintered glass differed; however, the trends in both are in agreement. These results indicate that increased porosity in sintered glass increases light scattering and hence lowers transmittance of PIG. Similarly, the luminous efficacy increases linearly with PIG transmittance, and other luminescence properties such as emission intensity and CCT are also influenced by transmittance. This study confirms that residual pores strongly govern the transmittance of sintered glass and also that enhancing transmittance will increase the luminous efficacy of an LED package.
Acknowledgment
This research was supported by the Technology Innovation Program (10044203, Development of phosphor materials based on Blue/UV LED) funded by the Ministry of Trade, industry & Energy (MI, Korea) and was also supported by INHA University Research Grant (INHA-50269).
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