Active glass ceramics containing functional crystals for photonics

Journal of the Australian Ceramic Society Volume 49[1], 2013, 26 – 29 26 Active glass ceramics containing functional crystals for photonics Jumpei Ueda1*, Setsuhisa Tanabe1 1) Graduate School of Human and Environmental Studies, Kyoto University, Kyoto 606-8501, Japan Email: [email protected] Available Online at: www.austceram.com/ACS-Journal Abstract We have been developing active glass ceramics (GCs) containing functional crystals such as Ce3+:Y3Al5O12, Eu2+doped silicate crystals for white LEDs and Cr3+:Al2O3 (rubyGC) for photonics. In this paper, microstructure and optical properties of the rubyGC are explained. Red-colored rubyGC was developed containing α-Al2O3 microcrystals as a single phase by ceramming an as-made glass of green-color in the system of SiO2-Al2O3-Y2O3. Absorption bands attributed to Cr3+:4A2→4T1 and 4A2→4T2 were observed at around 400nm and 555nm, respectively, which are blue-shifted from their original positions in the as-made glass. The rod-shape crystals α-Al2O3. In addition, the intense red luminescence of Cr3+ at 694 nm, called the R-line, was observed in the GCs, while a broad PL band around 900nm was weakly observed in the as-made glass. Quantum yield of the luminescence in the rubyGC was 35.6%, much improved after cerammization. Keywords: Glass ceramics, phosphor, ruby, Cr3+ INTRODUCTION Glass ceramics containing functional crystals have attracted a great deal of interest for phosphors and laser materials because the glass ceramics have both properties of glasses with good durability and transparency and functional crystals such as efficient phosphors. Recently white light emitting diodes (LEDs) generally consisting of blue-LEDs and Ce3+:Y3Al5O12 yellow phosphors are starting to be widely used as indoor illumination in place of fluorescent lamps because of their high luminous efficiency, long lifetime and energy saving ability. However, deterioration of the organic resin matrix, which is used for fixing powder phosphors onto blue LED chips, has become a serious problem with increasing temperature of the LED chip due to an increase in its output power. In order to solve this problem, we have proposed a glass ceramic (GC) process as a novel phosphor synthesis because of excellent durability, thermal stability, easy fabrication and molding into any desired shapes. We have reported the glass ceramics containing phosphors such as Ce3+:Y3Al5O12 [1,2] and Eu2+-doped silicate [5,7]. In addition, the glass ceramics for solid-state laser materials have been studied due to the easier fabrication and lower production costs compared with the single crystalline materials. Fifty years had passed in 2010 since the invention of the first laser, the ruby laser (Cr3+: α-Al2O3) by Maiman in 1960 impacting the laser optics[4]. The ruby laser utilizes the 2E-4A2 transitions of Cr3+, which are intense fluorescence lines with narrow line width, called the R-lines. The R-lines of Cr3+ are observed at room temperature only in oxide crystals that have strong α-Al2O3. Materials that have weak crystal field like glasses show broad luminescence with longer wavelength due to the 4T2 4 A2 transition, and do not exhibit the R-lines. If a glass ceramic containing Cr3+ α-Al2O3 crystals is prepared, this glass ceramic can be very useful as a phosphor and a laser material due to the intense red luminescence, R-lines. However, the glass ceramics containing the ruby crystals have never been reported. The biggest problem is the easier precipitation of other crystals -Al2O3 by usual heattreatment procedure heating from low temperature. Ueda and Tanabe We already reported a way of preparing the glass ceramics by quenching a binary phase of glass forming liquid and crystal phase, named as "frozen sorbet method" [6,7]. We have successfully developed the glass ceramics containing ruby crystals by the frozen sorbet method [8]. In this study, we report microstructures such as local elemental distributions and precise quantum yield, QY of luminescence in the glass ceramics containing ruby crystals. METHODS AND PROCEDURES A silicate glass with a composition of 45SiO240Al2O3- 15Y2O3- 0.1Cr2O3 in mol% was prepared by a melt-quenching method; the well-mixed reagent powder was melted in an alumina crucible at 1700 °C for 30 minutes and the crucible containing the melt was quenched on a stainless steel plate. The as-made glass was heat treated at 1550 °C for 1hour and quenched on the stainless steel plate, and then the glass ceramics (rubyGC) were obtained. The obtained glass ceramics were cut and polished into a shape of about 10x10x3 mm3 size. The crystal phases were identified by a XRD measurement (Shimadzu, XRD6000). The detail surfaces of the glass ceramics were observed by an SEM unit (JEOL, JSM-890) and the local elemental mapping was observed by an electron probe microanalyzer (EDAX, Genesis XM2). Transmittance spectra were measured by spectrometer (Shimadzu, UV3600). Photon distribution spectra of total flux luminescence by 438 nm excitation were measured by using integrating sphere and fiber optic spectrometer (Ocean Optics, USB2000). For the QY of photoluminescence, PL spectra were measured under a 438nm laser diode (LD) excitation (Nichia, NDHB510APA) by using an integrating sphere (Labsphere, LMS-100) which was connected to two CCD detectors (Ocean Optics, USB2000+) with different wavelength sensitivity with an optical bifurcated fiber of 400 m-core. The obtained PL spectra were calibrated by using a standard halogen lamp (Labsphere, SCL-600) and an auxiliary lamp (Labsphere, AUX-30), and then the photon distribution spectra were obtained for QY calculation. RESULTS AND DISCUSSION A. Frozen sorbet method Fig.1 shows ternary phase diagram of SiO2-Al2O3Y2O3 [3] and glass forming region, which contains the glass composition in this study represented by a red point. In addition, this composition point is in a binary phase region of Al2O3 and a liquid phase existed between about 1400 and 1600°C. Therefore, 27 heat-treatment of the as-made glass in this temperature range can lead to formation of glass ceramics containing Cr3+-doped -Al2O3 (Ruby). Fig. 1. Isothermal section at 1600°C in the SiO2Al2O3-Y2O3 system. Region enclosed by a blue line shows glass forming region in the SiO2-Al2O3-La2O3. The glass composition is represented by a red dot In addition, in order to avoid precipitation of other crystals, quenching is necessary. Actually, red colored glass ceramics (rubyGC) was obtained by the quenching of the binary phase of crystals and liquid at 1550 °C while the as-made glass is green as shown in Fig. 2. Fig. 2. Schematic view of Frozen-Sorbet method and sample photos (As-made and RubyGC). B. XRD patterns Fig. 3 shows XRD patterns of the as-made glass and the glass ceramics (rubyGC). The as-made glass is completely amorphous phase because only a halo peak was observed. The XRD pattern of rubyGC contains a halo peak and several sharp peaks attributed to -Al2O3 (JCPDS PDF#43-1484). Therefore, we have successfully prepared the glass ceramics containing -Al2O3 as a single phase; the rubyGC. Journal of the Australian Ceramic Society Volume 49[1], 2013, 26 – 29 28 these atoms uniformly. Figure. 5 shows X-ray fluorescence spectra of point 1 and 2 in SEM image by EPMA. The spectrum of spot 1 has X-ray fluorescence bands attributed O, Al, Si and Y while that of spot 2 has those bands attributed O and Al. X-ray fluorescence band attributed to Cr was not observed due to the low concentration. Fig. 3. XRD patterns of as-made glass and RubyGC. C. Transmittance spectra Figure 4 shows transmittance spectra of the as-made glass and the rubyGC. Absorption bands were observed at around 640 nm and 440nm in the as-made glass while in the rubyGC those bands were observed at around 555nm and 400nm. The longer wavelength band can be attributed to the Cr3+: 4A2→4T2 transition and the shorter band to the 4A2→4T1 transition in both samples. Both 4A2→4T2 and 4A2→4T1 bands were blue-shifted after heattreatment. This is because the crystal field of the Cr3+ ion was changed by the strong crystal field of the α-Al2O3. Since the absorption band originated from the weak crystal field was not observed in the rubyGC, most of the Cr3+ ions in the glass were selectively incorporated into the α-Al2O3 crystals by heat-treatment. From the quantitative analysis by the ZAF correction in EPMA, the spot 1 has cation ratio of Al(39%)Si(36%)- Y(25%) and the spot 2 has cation ratio of Al(99%)- Si(0%)-Y(1%). The composition of the spot 1 is different from the starting composition of the glass. Figure 4. (a) SEM image of the RubyGC and (b) local elemental mapping of Al,Si,Y and O. Fig. 4. Transmittance spectra of the as-made glass and RubyGC. D. SEM and EPMA Figure 4 (a) shows the SEM image and Figure 4 (b) shows the elemental mapping of Al, Si, Y and O of the rubyGC using electron probe microanalysis (EPMA). From the SEM analysis, the crystals of rod shape were distributed uniformly in the glass matrix. The width and length of the rods are about 10 m and 200 m, respectively. In addition, the rod is Al rich, not including Si and Y, while the matrix includes all Figure. 5. X-ray fluorescence spectra of point 1 and 2 in SEM image by EPMA. Ueda and Tanabe However, the composition of spot 2 is essentially composed of only Al and O. These results show that the rod crystals are -Al2O3. We showed that the rubyGC is slightly translucent (Fig. 2). This is because the crystallite size of -Al2O3 is large and the refractive index between the ruby and glass matrix is different. The nucleation and subsequent crystallization can occur for the rod crystals of Al2O3 in the glass matrix. Subsequent crystal growth of the ruby can be promoted by heat-treatment time. Since the crystallite size is controlled by the heattreatment time, it would be possible to prepare glass ceramics with better transparency by optimization of preparation conditions. E. Photon distribution spectra of total flux PL Figure 6 shows calibrated photon distribution spectra of the as-made glass and rubyGC under 438nm excitation. The intense fluorescence peak attributed to the Cr3+: 2E→4A2 transition, the R-line, was observed at 694 nm in the rubyGC. In the as-made glass, the broad fluorescence band was observed at around 900 nm. This fluorescence band in the asmade glass is attributed to the Cr3+: 4T2 →4A2. Actually, the PL spectrum of the as-made glass is magnified by a hundred times because of its very low intensity. Since the number of absorption photons in the as-made glass and the rubyGC are not so different, the integrated intensities of PL spectra are almost representing the QY of both samples. The QY of the as-made glass was 0.9% and much lower than QY of 35.6% in the rubyGC. These results show that the crystal field of the Cr3+ ions in the as-made glass was changed into the stronger field and the quantum yield was enhanced by incorporation into the Al2O3 crystalline phase after cerammization. Figure 6. Photon distribution spectra of absorption and luminescence under 438 nm LD excitation in the asmade glass and RubyGC. The luminescence spectrum of as-made glass are magnified by a hundred times. CONCLUSION The red-colored Cr3+-doped glass ceramics containing -Al2O3 crystals were prepared from a green-colored 29 as-melted aluminosilicate glass. Analyzed chemical composition of precipitated rod-shape crystals have cation ratio of Al (99%)- Si(0%)-Y(1%), almost corresponding to that of -Al2O3, which was also detected by XRD. The absorption bands due to Cr3+: 4 A2→ 4T1 and 4A2→4T2 in the as-made glass were blue-shifted after heat- treatment, indicating that Cr3+ ions were selectively incorporated into the -Al2O3 crystals precipitated. The PL spectrum of the rubyGC was identical to that of the ruby crystal based on the R-line at 694nm, while that of the as-made glass was broad in the longer wavelength region due to weaker crystal fields of Cr3+ ions in glass. The quantum yield of the as-made glass and the rubyGC were 0.9% and 35.6%, respectively. ACKNOWLEDGEMENTS This work was supported by JST-PRESTO and Grantin-Aid for Scientific Research from JSPS Fellows (No. 22-2797). REFERENCES 1. S. Fujita, A. Sakamoto, andS. Tanabe, "Luminescence characteristics of YAG glassceramic phosphor for white LED," IEEE J. Sel. 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