Solid-state lighting (SSL) has been widely used in indoor and outdoor lighting, including landscape lighting and automobile headlights. As mainstream SSL devices, phosphor-converted WLEDs (pc-WLEDs), exhibit many advantages such as low energy consumption and environmental friendliness. However, the applications of pc-WLEDs in high-power fields have been restricted because of their poor thermal stability, which originates from organic encapsulation. Glass ceramic phosphor (GC) is an important material required to resolve the problems resulting from high thermal stability by the means of remote encapsulation. Unfortunately, the chemical reaction between the red phosphors and glass powders during the co-sintering process significantly hinders the development of GCs. In this study, GCs are prepared by a screen-printing technique using a binder of borosilicate glass powder and a sapphire matrix with superior thermal conductivity. The phosphors used for the preparation of GCs are compounded with (Ca,Sr) AlSiN3∶Eu2+ (CSASN) and Y3Al5O12∶Ce3+ (YAG). The experimental processes of the GCs are simulated, and their optical properties are analyzed. Superior optical properties and color-tunable GCs are obtained. All results reveal that the as-prepared GCs are promising candidates for high-power white illumination.
GC preparation was divided into two parts. The first was the preparation of frit-seal glass, which was prepared by a rapid melt-quenching technique with a composition of SiO2-B2O3-ZnO-Al2O3-CaO. One part of the glass melt was poured into a graphite mold and annealed to remove the thermal stress. After natural cooling, the sample was polished into a small cube for detection. The other part was poured directly into deionized water to quench the cullet, which was then dried and milled into glass powder. The second part was the preparation of the GC, using a screen-printing technique utilizing slurries, substrates, and screens. The slurries, consisting of CSASN, YAG, frit-seal glass powder, and an organic binder solution, were manually printed on the sapphire substrate using a screen. The printed samples with mass fractions of 17% YAG+(17%-x) CSASN+83% glass powders (where x=0%,1%,2%,3%, and 17%) were marked as Y-GC, Y-R1-GC, Y-R2-GC, Y-R3-GC, and R-GC series, respectively. After drying in an oven, the printed samples were sintered in a muffle furnace and allowed to cool naturally. Further, a series of GCs were prepared, and related characterization was performed.
The pattern of Y-GC corresponds well to that of YAG phosphor. Compared with the pattern of the CSASN phosphor, the intensities of some peaks of R-GC become higher or lower and some diffractions disappear, which originates from the preferential orientation when CSASN phosphors are prepared into R-GC (Fig. 2). The experimental GC processes are simulated using a DSC device. All GC samples show flat lines, similar to the results for glass powder (Fig. 3). As shown in the SEM images of the GCs (Fig. 4), the YAG and CSASN particles exhibit an acceptable homogeneous distribution in the glass, and the thickness of the samples is approximately 210 μm. The values of nY∶nAl (atomic ratio) and nSr∶nAl∶nSi in the EDS spectra of the GCs correspond to those of the phosphors, and no additional elements appear in the glass regions. Meanwhile, distinct boundaries appear between the elements of CSASN and the glass powder in the EDS spectra. The above results, shown in Fig. 2 to Fig. 4, indicate that the two phosphors have no obvious chemical reactions with the sealing glass during the co-sintering process for GC preparations. With an increase in the red phosphor content, the PL spectra of the GCs exhibit a red shift (Fig. 5), and the intensities and quantum yields decrease gradually (Fig. 6). The results are ascribed to the changes in the microstructure of CSASN during GC preparation. The thermal stability of the GCs decreases gradually, but the rate of decline is small. The PL intensity at 150 ℃ is maintained at 89% of the initial value (Fig. 7), which is attributed to the high thermal conductivity of the frit-seal glass and sapphire substrate. With the increase in red phosphor content, the GC samples show a color change from yellow to red and a color-tunability from cool white to warm white encapsulated with a blue chip (Fig. 8). GCs exhibit a decrease in luminous efficiency (LE), a decline in correlated color temperature (CCT), and an increase in the color rendering index (CRI). The results are due to the small luminous flux of red light and the QY decrease in the GC. The LE gradually decreases as the operation current increases from 20 to 150 mA, which is ascribed to the phenomenon of "efficiency droop" (Table 1).
GCs containing YAG and CSASN phosphors are prepared using a screen-printing technique on a sapphire substrate. The results indicate that the two phosphors have no obvious chemical reactions with the sealing glass during the co-sintering process for the GC preparation. With increasing red phosphor content, the PL spectra exhibit a red shift. Meanwhile, the intensities and quantum yields gradually decrease. The GCs show superior thermal stability, and the PL intensity at 150 ℃ is maintained at 89% of the initial value. The GC samples exhibit color tunability from cool white to warm white with increasing red phosphor content. The optimal photoelectronic parameters of the GCs are a luminous efficiency of 147.70 lm/W, color temperature of 4915 K, and color rendering index of 73.3. These results indicate that the as-prepared GCs have a wide range of applications in the field of high-power white illumination.
