Phosphor-in-glass (PIG) has recently been receiving attention for use in light emitting diode encapsulants because of the remarkable chemical and mechanical properties of the resulting encapsulants. The PIG plate, however, contains pores that interact with the light and lead to changes in transmittance and chromaticity of the plate. In this study, therefore, PIG was sintered via spark plasma sintering (SPS) to reduce the number of pores via the densification process, and the factors for improving the luminous efficacy of PIG were confirmed by analyzing the pore size and scattering coefficient. The luminous efficacy and color rendering index were dramatically improved in all of the PIG samples subjected to SPS. The purpose of this study is to reduce the pores and improve the optical properties of PIG via the densification process. Further, it also aims to understand the effect of the pores on the increase in the efficacy of the PIGs based on the scattering events.
© 2017 Optical Society of America
Inorganic materials have recently been attracting attention for preparing LED encapsulants to replace conventional resin and silicone materials because of their superior thermal, mechanical, and chemical properties that protect the LED chips from the external environment and improve the strength of the package [1-3]. In particular, phosphor-in-glass (PIG) sintered using glass materials can be used to achieve the desired color by simply mixing different types of phosphors at different concentrations . Several studies, therefore, are being conducted to improve the PIG efficacy in various ways [2,5,6] for applying the LED package to outdoor lamps or in the automotive field with the aim of realizing high weather resistance and durability . Unlike conventional polymer encapsulants, however, PIG contains pores formed from voids that exist between the particles during sintering . The pores present inside the PIG plate interfere with the passage of light or the absorption of light by the phosphor, thereby causing changes in the transmittance and the chromaticity [9-11]. Therefore, studies have been conducted to improve the luminous efficacy and control the chromaticity by analyzing the influence of the pores existing in the plate on the optical properties of the PIGs [12,13].
In order to efficiently increase the light extraction of the PIG, it is necessary to reduce the number of pores that obstruct the path of light, by realizing the densification of the matrix . In addition, several phosphors used to obtain the desired color of the LED package exhibit a poor thermal stability [14,15]. Therefore, to improve the luminous efficacy of PIG by maintaining the phosphor properties after sintering, it is essential to minimize the exposure time to heat by lowering the sintering temperature and the total sintering time. As the spark plasma sintering (SPS) process has been widely used for the densification of ceramic materials [16,17], it can be effective for the microstructural modification of the PIG. In addition, lowering the total sintering time and the sintering temperature by controlling the applied pressure during the process, can prevent the deterioration of the phosphors used in the PIG [5,18].
In this study, PIG was sintered using the SPS process (SPS PIG) and its microstructure and optical properties were compared with those of the PIG sintered in an electric furnace. The porosity and the pore size of the SPS PIG were effectively reduced, and the light extraction efficiency was significantly improved in all the yellow and red PIGs. In addition, the relationship between the pore properties and the optical properties of the PIGs was investigated based on the sintering temperature and the heating rate, and the factors for improving the luminous efficacy of the SPS PIG were determined by studying the scattering coefficient. The purpose of this study was to dramatically reduce the porosity and improve the luminous efficacy of the PIGs using the SPS process and to understand the role of pores in improving the efficacy by considering the scattering events.
2. Experimental procedure
The 20SiO2-30B2O3-45ZnO-5Li2O (mol%) glass frit was prepared using SiO2 (99.5%; Sigma-Aldrich), H3BO3 (99.97%; Sigma-Aldrich), ZnO (99.9%, Sigma-Aldrich), and Li2CO3 (99.0%; Sigma-Aldrich). The raw materials mixed using a tubular shaker mixer (T2F; Willy A. Bachofen, Switzerland) for 1 h, were melted in an alumina crucible using an electric furnace (S-27; Ajeon, Korea) at 1200 °C for 30 min in an air. A cullet was made from melting glass poured on rotating rollers, and pulverized using a zirconia ball and a planetary mono mill (Pulverisette-7; Fritsch, Germany) under dry condition. The milled glass frit was screened using a 100-µm-size mesh (No. 149; Nonaka Rikaki Co. Ltd., Japan) to remove the relatively large particles that interrupt the mixing and packing of the phosphors. Two types of phosphors, namely, yellow phosphor (Y phosphor, Y3Al5O12:Ce3+) and red phosphor (R phosphor, Ca0.2AlSiSr0.8N3:Eu2+) (Intematix, USA) were selected to make the PIG. The particle size of the glass frit and the two types of phosphors were analyzed using a particle size analyzer (Mastersizer 2000; Malvern, UK) (Fig. 1 and Table 1). The specific surface area and the density were determined by BET analysis (ASAP 2020; Micromeritics, USA) and by using a pycnometer (AccuPyc II 1340; Micromeritics, USA), respectively. The glass frit was mixed with 5 vol% of the phosphor considering each glass and phosphor density by using a tubular shaker mixer (T2F; Willy A. Bachofen, Switzerland) for 12 h at 300 rpm. The glass transition temperature (Tg) and the mass change of the glass frit, phosphor, and phosphor-containing glass frit were analyzed by differential scanning calorimetry (DSC, STA 449; Netzsch, Germany) from 30 °C to 800 °C at a heating rate of 10 °C/min. The viscosity-temperature curves including characteristic viscosity points (first-shrinkage temperature (Tfs), glass-softening temperature (Ts), half-ball point (Th), and flow point (Tf)) of the glass frit and phosphor-containing glass frit were measured using a hot-stage microscope (HSM, Misura HSM; Expert System Solutions Inc., Italy) .
The PIGs were prepared using the SPS process (LABOX315; Sinterland, Japan) at various sintering temperatures and heating rates using a pulse cycle with an on-period of 40 ms and an off-period of 7 ms under a vacuum of 20 Pa (Table 2). The glass frit with each phosphor was loaded in a graphite mold with a diameter of 15 mm. The first target temperature T1 was set below the glass transition temperature. The heating rate up to T1 was maintained at 20 or 50 °C/min in order to reduce the total sintering time. The sintering temperature of the PIGs is denoted as T2, and the heating rate was varied from T1 to T2. The uniaxial pressure was gradually increased from 20 °C and maintained at 40 MPa during the whole sintering process, and the temperature was decreased to 20 °C within 5 min while gradually reducing the pressure. To compare the SPS PIGs with the reference glass plate and the PIGs, the glass frit and the mixed powder were pressed using a metal mold (Hantech Inc., Korea) with a diameter of 25 mm and sintered in an electric furnace (S-27; Ajeon, Korea) at 630 °C for 30 min at a heating rate of 10 °C/min in air, and then naturally cooled. Both the reference and the SPS PIGs were polished to a thickness of 500 µm for analyzing the optical properties.
The transmittance of the PIGs was analyzed using a UV-visible spectrophotometer (UV 2450; Shimadzu Corp., Japan). The luminous efficacy (lm/W), color rendering index (CRI), color chromaticity, and electroluminescence (EL) spectra were measured using a spectroradiometer with an integrating sphere of diameter 50 cm (GS-1290-3 Spectroradiometer; Gamma Scientific, USA). The PIG samples were placed at a certain distance from the blue LED chip using a reflector holder with a diameter of 10 mm. The center of the PIG samples was cut to observe the cross-section, and the cross-sectional images (analysis area of about 6400 × 500 μm) were obtained by scanning electron microscopy (SEM, S-4300; Hitachi, Japan). The SEM images were analyzed by employing the image analysis software Image-Pro Plus (Ver. 6.0, Media Cybernetics Inc., USA) to detect the pore size, porosity, and number of PIG pores (pore-size range of approximately 0.97‒11.54 μm).
3. Results and discussion
The stable state between the phosphor and the glass can be confirmed through the DSC curves of the glass frit with phosphor (Fig. 2(a)). The Y phosphor and the Y phosphor-containing glass frit were more thermally stable than the R phosphor and the R phosphor-containing glass frit (Fig. 2(b)). The mass change of the R phosphor was also higher than that of the Y phosphor and the glass frit because of the oxidation of the doping ions and the structural collapse [14,15]. The viscosity of the Y phosphor-containing glass frit was similar to that of the glass frit, whereas the viscosity of the R phosphor-containing glass frit increased from approximately Ts owing to the higher surface area of the R phosphor, which hinders the viscous flow of the glass (Table 1 and Fig. 3) .
The PIGs were highly densified after the SPS process; however, the pore size and the porosity of the PIG were slightly different depending on the phosphor types (Fig. 4). The microstructure with two clear phases of the glass and phosphor can be seen in the SEM images.
The SPS_Y PIGs showed a smaller mean pore size and lower porosity than the glass plate and the Ref_Y PIG sintered in an electric furnace (Fig. 5(a)). In the case of Y1–Y3 PIGs, a slightly higher porosity and larger pore size than those in the case of Y4–Y7 were observed because of the faster T1 rate (50 °C/min to 450 °C), even though Y2 and Y3 were sintered at higher temperatures. The Y4 and the Y5 PIGs sintered at lower heating rates (20 °C/min to 420 °C, and 2 and 5 °C/min, respectively, from 420 °C to the sintering temperature) showed lower porosities, indicating that the pore properties of the SPS PIGs were highly dependent on the sintering rate during the process. However, the Y8 PIG, sintered at the highest rate (50 °C/min) during the SPS process, showed a lower porosity and pore size than Y1–Y3 PIGs. Since the Y phosphor-containing glass frit exhibited a low viscosity, similar to that of the glass frit, the densification of the Y8 PIG is believed to be affected by overheating due to the rapid heating rate. The SPS_R PIGs showed a lower porosity and smaller pore size than the glass plate and the Ref_R PIG (Fig. 5(b)). Samples R1–R3 and R8 PIGs with a fast initial heating rate (T1: 50 °C/min) and the R7 PIG with a heating rate of 20 °C/min during SPS, showed a higher porosity of about 0.2% than the other SPS_R PIGs regardless of the sintering temperature because of insufficient densification. The pore properties of the SPS_R PIGs are influenced by the heating rate rather than the temperature, as indicated by the lower porosity of ~0.1% and the smaller pore size of about 3 µm corresponding to the slow heating rates of T1 and T2. The porosity of the SPS_R PIGs was slightly higher than that of the SPS_Y PIGs owing to the increased viscosity that prevents the densification based on the mass change and the shape of the phosphor .
Both SPS_Y and R PIGs were observed to have smaller pores, usually less than 5 μm, whereas some pores larger than 10 μm were observed in the glass plate and Ref PIGs (Fig. 6). In particular, the SPS PIGs with the slower heating rate had more pores with ~1 μm diameter than the SPS PIGs with the faster heating rate. The SPS_R PIGs showed more pores than the SPS_Y PIGs due to the insufficient densification.
The SPS_Y PIGs showed a higher transmittance than the glass without phosphor and the Ref_Y PIG (Fig. 7(a)). In the case of the R_PIGs, the SPS_R PIGs showed a lower transmittance than the glass plate because of the influence of the R phosphors with a high refractive index [21,22], but a higher transmittance than Ref_R (Fig. 7(b)). As the porosity of the SPS PIGs decreased, the transmittance increased; therefore, the highest transmittance was observed in Y5 and R4 with the lowest porosity, and the Y1–3, R1–R3 and the R8 PIGs with relatively higher porosities exhibited the lower transmittance.
The Ref_Y PIG with the lowest transmittance showed the lowest light intensity owing to the low transmissions of both blue and yellow light (Fig. 8(a)). A higher intensity of the yellow light was observed when the SPS PIGs showed a relatively higher porosity, resulting in increased internal scattering which caused the increase in interaction between the blue light and the phosphors. On the other hand, the Y4 and Y5 PIGs with lower porosity and high transmittance showed a lower yellow intensity because the blue light from the LED chip transmitted the PIG plate without interacting with the phosphor. The Y1–Y3 SPS PIGs with a higher porosity and lower transmittance showed a higher yellow intensity than the other PIGs; however, the trend for blue intensity could not be confirmed as there were no significant differences in the densification between the SPS PIG plates showing a very low porosity of 0.1%. The Ref_R PIG exhibited the lowest blue intensity because of its low transmittance, and the red intensity also exhibited the lowest value because the interaction between the blue light and the R phosphor was hindered by the excessive scattering events (Fig. 8(b)) [23,24]. The blue intensity of the SPS_R PIGs was almost constant, while the red intensity decreased as the transmittance increased. In other words, R4 with the lowest porosity exhibited the lowest red intensity because of the decrease in the interaction between the blue light and the R phosphor. On the other hand, R1, R2, and R8 PIGs, which showed a relatively higher porosity and lower transmittance, exhibited a higher value of red intensity because of the increased possibility of absorption of the blue light by the phosphor .
The intensity variation related to the pore properties of each PIG affected the chromaticity of the Y_PIGs (Fig. 9(a)). The color coordinates of Y1 and Y2 SPS PIGs were closer than those of the other SPS PIGs to those of the Y phosphor (0.54, 0.54). This was because of the increase in the interaction between the blue light and the phosphor due to the higher porosity, which resulted in lower blue intensity and higher yellow intensity. The chromaticities of the other PIGs were approximately 0.31 and 0.37, which are significantly different from those of Ref_Y, viz., 0.34 and 0.43. In other words, the chromaticities of Y4–Y8 PIGs are closer to that of ideal white light (0.33, 0.33) than the chromaticity of Ref_Y PIG is, because of the increased transmission of the blue light based on the lower porosity. The Ref_R PIG, which showed a relatively lower red intensity compared to the SPS_R PIGs, is located close to the chromaticity of blue light of the LED chip (0.14, 0.06) because of the reduced light extraction caused by the excessive scattering (Fig. 9(b)). As the porosity of the PIG increased, the chromaticity of the SPS_R PIG with the increased interaction tended to transform closer to the chromaticity of the R phosphor (0.33, 0.58). These results show that the relationship between the pores and red light intensity is dominant factor in the change in chromaticity of the SPS_R PIGs.
The scattering coefficient (Csca) was calculated from the effective scattering coefficient which was derived from Mie scattering theory [26-28], and the effect of pore properties on the optical properties, such as the luminous efficacy and the CRI of the PIGs were studied. The lowest luminous efficacy was observed in Ref_Y owing to the low transmittance and excessive scattering event inside the PIG plate (Fig. 10(a)). In the case of the SPS Y_PIGs, the luminous efficacy was higher than that of Ref_Y by 30% because of the increase in the transmittance caused by the high densification. A significantly higher luminous efficacy was observed in Y1 and Y2, in which the interaction between the blue light and phosphor increased due to the relatively higher porosity than that in the other PIGs. In contrast, Y4 and Y5, which exhibited a lower porosity and a higher transmittance, showed the lowest luminous efficacy. Hence, when the porosity is decreased, the luminous efficacy also decreases because the blue light passes through the PIG plate without interacting with the phosphors . The increase in the internal scattering coefficient, based on the proper distribution of the pores in the PIG, increases the luminous efficacy . In addition, the luminous efficacy of the PIGs shows a tendency to increase with an increase in the yellow light intensity. Therefore, the CRI value was lower in Y1 and Y2 PIGs showing the highest yellow intensity because the peak around 555 nm is sensitive to the human eye, but has a very poor CRI . From these results, it can be confirmed that all the SPS Y_PIGs are superior to Ref_Y in terms of the luminous efficacy and the CRI.
The SPS_R PIGs were also superior to the Ref_R PIG in terms of the luminous efficacy and the CRI value (Fig. 10(b)). Ref_R showed the lowest luminous efficacy due to the decrease in the extraction of light to the outside of the plate. The increase in the interaction between the blue light and the R phosphors caused an increase in the red light intensity, which is highly related to the luminous efficacy of the SPS PIGs. Similar to the SPS_Y PIGs, the highest efficacy was observed in the PIG showing a relatively higher porosity, and the lowest luminous efficacy was observed in R4 with the highest transmittance because of the sufficient densification at a slow heating rate. In addition, because the red intensity increased with the porosity, higher CRI values were observed in the R1 and R2 PIGs, which showed the greatest increase in the red intensity. A high CRI can be achieved by adjusting the respective peak intensity , based on the pore properties. The CRI of the SPS R_PIGs was increased by up to about 30% compared to that of the Ref_R PIG.
The variation in the size and distribution of pores based on the sintering conditions is confirmed in Figs. 5 and 6. If the incident light from the LED chip interacts with the pores that have ~1 μm diameters, the scattering angle of the blue light increases, so the possibility that the blue light can interact with the phosphor increases. On the other hand, when the pore size is ~10 μm or larger than 10 μm, the angle of refraction of the incident light becomes smaller, which decreases the scattering angle and causes scattering in the forward direction [30-32]. Therefore, the scattering angle and direction of light and its interaction with the phosphors were studied based on the difference in the pore properties of the PIG by using an interaction model (Fig. 11). In the case of SPS PIGs, the model was classified by the heating rate because the pore properties varied according to the heating rate of SPS. When the heating rate of SPS is slow, a high densification is achieved, but the blue light passes through the plate without interaction with the phosphors (Fig. 11(a)). On the other hand, a fast heating rate is disadvantageous for densification. However, the pores existing inside the plate scatter the blue light; therefore, the possibility of interaction with the phosphors increases and the intensity of the converted light also increases (Fig. 11(b)). When too many larger pores exist inside the Ref PIGs, the blue light and the light emitted by the phosphors are also trapped by the excessive internal scattering, thereby reducing the light extraction efficiency (Fig. 11(c)). An appropriate number of small pores acting as scattering agents in the PIG can enhance the luminous efficacy by changing the scattering angle and increasing the interaction between blue light and phosphors [23-25].
The Y_PIGs and the R_PIGs were fabricated at a much lower temperature and shorter duration by adjusting the sintering temperature and the heating rate of the SPS process. All the SPS PIGs were found to be superior to the Ref PIGs in terms of the luminous efficacy and the CRI because of the increased densification and lower sintering temperature and process time, which prevent the thermal degradation of the phosphors. The pore properties of the SPS PIGs depended on the heating rate of the SPS process rather than the sintering temperature. As the heating rate was increased, the internal porosity increased along with insufficient densification. Therefore, the interaction between the blue light and phosphors increased because of the increase in the scattering events, and the chromaticity of the PIGs transformed closer to the phosphor chromaticity. In contrast, when the PIG showed a low porosity because of a slow heating rate, the interaction between the blue light and the phosphor decreased with the increased transmittance. As a result, the peak intensity of the phosphor and the luminous efficacy of PIGs also decreased, and the blue light was extracted to a greater extent, resulting in the chromaticity being closer to that of the blue light. The results show that the presence of an appropriate number of small pores with diameters less than 5 μm in the PIG increases the phosphor intensity and luminous efficacy by increasing the scattering angle of the light that caused the increase in the interaction with the phosphor as compared to the PIGs with no pores.
This work 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).
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