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Luminescent Properties of B-Sialon:Eu2＋ Green Phosphors Synthesized by Gas Pressured Sintering

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Luminescent Properties of B-Sialon:Eu2＋ Green Phosphors Synthesized by Gas Pressured Sintering Journal of the Ceramic Society of Japan 116 [3] 389-394 2008 Paper Luminescent properties of b-SiAlON:Eu2＋ green phosphors synthesized by gas pressured sintering JeongHoRYU,† Youn­Gon PARK, Hyong Sik WON, Hideo SUZUKI, Sang Hyun KIM and Chulsoo YOON Corporate R&D Institute, Samsung Electro­Mechanics Co., LTD., 314, Maetan 3­Dong, Yeongtong­Gu, Suwon, Guunggi­Do 443–743, South Korea 2＋ b-SiAlON: Eu oxynitride green phosphors with compositions of EuxSi6－zAlzOzN8－z (x＝0.0065 －0.0390, z＝0.231) were successfully prepared using GPS (gas pressured sintering) method. The phase purity, microstructure, luminescent and thermal quenching properties for the prepared b-SiAlON:Eu2＋ phosphors were investigated in detail. The prepared b-SiAlON:Eu2＋ phosphor samples with Eu2＋ doping concentration of x＜0.0238 showed pure single b-SiAlON phase. The prepared b-SiAlON: Eu2＋ phosphors absorbed UV-visible spectral region, and showed a single intense broadband emission in a range from 525 to 540nm.TheeffectsoftheEu2＋ doping concentration on the optical properties for the b-SiAlON: Eu2＋ phosphors were dis- cussed under consideration of concentration quenching. The temperature dependence of photoluminescence (PL) properties was investigated from 25 to 300°C, and the activation energies (DE) for thermal quenching of the prepared b-SiAlON:Eu2＋ phosphors were determined by Arrhenius fitting. The experimental results clearly indicates that the prepared b-SiAlON:Eu2＋ has great potentials as a down-conversion green phosphor for white light emitting diodes (LEDs) utilizing near UV or blue LEDs as the primary light source. 2008 The Ceramic Society of Japan. All rights reserved. Key-words : b-SiAlON:Eu2＋,GPS(gas pressured sintering), Photoluminescence, Concentration quenching, Temperature depen- dence, Activation energy for thermal quenching [Received November 13, 2007; Accepted January 17, 2008] 1. Introduction red colors, and were not accepted for general illuminations. The white LEDs (light-emitting diodes) are promising To improve the color-rendering properties of LEDs for new-generation light source which can replace conventional general illumination, appropriate green and red phosphors incandescent and fluorescent lamps due to their reliability should be incorporated simultaneously. Therefore, it is thus and low energy consumption.1),2) At the present time, the necessary to develop highly efficient green or red lumines- commercial white LEDs comprising a blue LED chip and cent nitride or oxynitride materials that are suitable for white YAG:Ce3＋ yellow phosphor are widely used as outdoor LEDs. 3) lighting sources. However, these white LEDs are not b-SiAlON has a hexagonal crystal structure (P63 or P63/m applicable for indoor lighting purpose due to their less red space group), which is derived structure from b-Si3N4 by luminescence and relatively lower color rendering index equivalent substitution of Al–O for Si–N, and its chemical (CRI75) value. Improvement of CRI is possible by doping compositioncanbewrittenasSi6－zAlzOzN8－z (z represents ionsthatcanemitinorange/red region or by mixing of the number of Al–O pairs substituting for Si–N pairs and another orange/red phosphor.4) 0＜Z4.2).10) Previously, Hirosaki research group already In recent times, rare-earth-doped oxynitride or nitride reported a green emitting property of Eu2＋-doped b-SiAlON compounds have been reported to be photoluminescent and with a composition of Eu0.00296Si0.41395Al0.01334O0.0044N0.56528 may then serve as new phosphors because of their good ther- under the near UV or blue light excitation.11) Recently, mal and chemical stabilities.5)–7) Their luminescent property extensive and detailed experimental results for b-SiAlON: is attributable to the strong nephelauxetic effect and large Eu2＋ were reported with dopant concentration varying in a crystal field splitting as activator ions are coordinated to range of 0.02–1.5 mol and z value from 0.1 to 2.0 by same nitrogen. Besides this, the oxynitride or nitride phosphors research group.12) In that report, it was found that Eu2＋ are expected to have high thermal and chemical stabilities solubility go down with increasing z value (z＞0.5). Besides, because the crystal structure of the host lattice is built on the powders coarsened and platelike SiAlON polytypoids stiff frameworks consisting of Si–N or Al–N tetrahedra. were detected in high z value samples. Hirosaki et al.8),9) have developed a yellow oxynitride phos- In a viewpoint of application to LEDs, phosphors with phor based on Eu2＋-doped Ca-a-SiAlON which absorbs high phase purity, fine particle size, uniform particle size strongly over a broad range from UV to blue spectral region, distribution and high dopant solubility are recommended. and reported white LED devices using the yellow phosphor In this work, therefore, Eu2＋ activated b-SiAlON phosphors with a blue LED chip. However, the white LEDs using a with compositions of EuxSi6－zAlzOzN8－z (x＝0.0065– single Eu2＋-doped Ca-a-SiAlON presented relatively low 0.0390) at low z value (z＝0.231) were prepared using GPS color-rendering properties due to lack of enough green and (gas pressured sintering) method, and effects of the Eu2＋ doping concentration on their optical characteristics were † Corresponding author: Jeong Ho RYU; E-mail: jimihen.ryu＠ investigated in detail. A relative high emission efficiency was samsung.com foundinhighEu2＋ doping concentration region (x＞0.015), 2008 The Ceramic Society of Japan 389 JCS­Japan Ryu et al.: Luminescent properties of b-SiAlON:Eu2＋ green phosphors synthesized by gas pressured sintering which were not investigated in the previous works.11),12) Fur- thermore, the b-SiAlON:Eu2＋ is thought to be promising for high-temperature application due to its thermal and chemical stability. However, its luminescent property in elevated tem- perature ranges was not systematically studied. In this report, temperature dependence on luminescent properties was analyzed in a temperature range from room temperature to 300°C and activation energy for thermal quenching was measured. 2. Experimental procedures 2＋ b-SiAlON:Eu phosphors with compositions of EuxSi6－z AlzOzN8－z were synthesized from a-Si3N4 (Ube Industries LTD., Japan),AlN(Tokuyama Corp., Japan) and Eu2O3 (Shin-Etsu Chemical Co., Ltd., Japan) powders. The Eu2＋ doping concentration (x) varied in a range from 0.0065 to 0.039 with a fixed z value of 0.231. The raw powder mixtures were prepared using a Si3N4 ball milling in n-hexane. After drying in vacuum oven, the powder mixture was granulated using a test sieve, and then loaded into a BN crucible. Calci- nation was carried out at 2000°Cfor2hin0.92MPaofN2 atmosphere using GPS (gas pressured sintering) furnace. After heating, the power was shut off and the samples were cooled down in the GPS furnace. The crystalline phase of the synthesized powders were identified by X-ray powder diffraction (XRD, Rigaku, Japan), operating at 40 kV using Cu Ka radiation (l＝0.15406 nm). The data were collected in the continuous scan mode at the speed of 3°at 2u/min with step size of 0.014°from 20 to 60°. The powder mor- phology was investigated by scanning electron microscopy Fig. 1. (a) Schematic projection of 56-atomic supercell in (SEM, JEOL, JSM 5900 LV, Japan). The photolumines- b-SiAlON structure onto the (001) plane. (b) X-ray diffraction cence (PL) properties of the prepared phosphor samples patterns for the EuxSi6－zAlzOzN8－z (x＝0.0065–0.0390, z＝0.231) were measured using a spectrofluorometer (Fluorolog phosphors prepared at 2000°Cfor2hwithvaryingEu2＋ doping con- Tau-3, Horiba, USA) in a temperature range of 25 and centration. 300°C with a 450 W xenon lamp as an excitation source. The excitation wavelength used for measuring PL emission was 460 nm, and the excitation spectra were measured at emis- sion maxima x＝0.0217, whereas a secondary low-temperature phase of a-SiAlON is observed in samples with composition of x 3. Results and discussion 0.0238. The remain of a-SiAlON phase in the highly Eu2＋ It is well known that b-SiAlON comprises a three-dimen- doped samples can be explained by stabilization effect of 2＋ sional network structure of corner sharing (Si, Al)(O, N)4 doped rare earth metal (Eu ) ions. It has previously been tetrahedra with a continuous channel along [001] direc- stated that the transformation between a-andb-Silicon tion13) as represented in Fig. 1(a). Unlike a-SiAlON, it is nitride, which occurs at temperatures exceeding 1350°C, is a not essential for b-SiAlON to accommodate metal ions for reconstructive polymorphic transformation, whereas the charge compensation. It means that metal ions such as transformation between a-andb-SiAlON is essentially rare-earth ions could not enter into the crystal structure of chemically controlled.15) Accordingly, the phase assemblage b-SiAlON or occupy any sites in the b-SiAlON structure. obtained in the final product is mainly determined by the This was verified by previous study that metal ions were overall composition of the starting materials. It is well located in the glass phase connecting the b-SiAlON known also that the metal cation (M) doped a-SiAlON, particles.14) However, the strong green emission from Eu2＋ where M is Li, Mg, Ca, Y and the rare-earth metals, is more clearly indicated that Eu2＋ was located somewhere in the stable at high temperatures due to incorporation of Mp＋ structure of b-SiAlON.11) This conflicting result imply that metal ions into vacant network sites, and that the ease with the solubility of metal ions in b-SiAlON is still debatable and which this transformation proceeds decreases as the atomic needs to be clearly researched through various analyses. number of the metal cation increases.16),17) The XRD results Here, we used XRD technique to roughly outline the doping in Fig.
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