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Synthesis, Crystal Growth, and Photoluminescence Properties of YAG:Eu3+ Phosphors by High-Energy Ball Milling and Solid-State Reaction Hyun Kyoung Yang and Jung Hyun Jeong* Department of Physics, Pukyong National UniVersity, Busan 608-737, Republic of Korea ReceiVed: September 15, 2009; ReVised Manuscript ReceiVed: October 19, 2009
Y3Al5O12:Eu3+ powders were synthesized by using a high-energy ball mill (HEB) and conventional solidstate reaction method (SSR). The effects of the synthesis procedure on the crystallinity, morphology, structure, and luminescence spectra were examined by X-ray diffraction, field emission-scanning electron microscopy, and photoluminescence spectroscopy. Compared to the solid-state reaction method, synthesis by high-energy ball milling is simple and easy to carry out, and the raw materials are commercially available. Introduction Yttrium aluminum garnet doped with europium ions (Y3Al5O12: Eu3+, YAG) is an important phosphor with a variety of applications in many luminescent and optical devices. YAG doped with lanthanide elements is used widely as a solid-state laser material in the luminescence field and as a window material in a variety of lamps.1-5 YAG-based phosphors are expected to replace sulfide-based materials, which are currently the main components in cathode ray tubes (CRT), on account of their high-temperature chemical stability.6 YAG powders are produced traditionally by a solid-state reaction7,8 between the component oxides, which requires repeated mechanical mixing and extensive heat-treatment to eliminate any intermediate phases, such as Y4Al2O9 (YAM) and YAlO3 (YAP). Wet chemical roots for producing YAG powders, such as co-precipitation,9-11 sol-gel,12,13 and spray pyrolysis14,15 methods, have been developed in recent years. The synthesis and luminescence properties of YAG:Eu3+ phosphor prepared via the conventional solid-state route (SSR) have been reported.16-18 The powders with a pure YAG phase are normally synthesized by a conventional solid-state reaction method with Y2O3 and Al2O3 as the raw materials. However, this method has some unavoidable disadvantages, such as a high temperature, agglomeration, and impurities due to the flux introduced, etc. The grinding process is essential for obtaining small particles. The processing of materials by high-energy ball milling (HEB) is an attractive method for preparing novel materials,19,20 which decreases the crystallite and particle size and induces the continuous formation of structural defects through the cycling cut and deformation of large crystallites.21 The most important advantage of the high-energy ball milling process is that it can be used to synthesize the designed compounds at room temperature with a grain size on the nanometer scale. HEB-YAG:Eu3+ and SSR-YAG:Eu3+ phosphors were synthesized and characterized, and their luminescence properties and crystallinity were examined. The effects of the process on the crystalline phase, surface morphology, and photoluminescence spectra of YAG:Eu3+ powders are discussed. * To whom correspondence should be addressed. E-mail: jhjeong@ pknu.ac.kr.
2. Experimental Sections 2.1. Preparation of HEB-YAG:Eu3+ and SSR-YAG:Eu3+. Materials. Y2O3 (99.99%, Aldrich), Al2O3 (nanopowder (40-47 nm) and 99.9%, Aldrich) and Eu2O3 (99.99%, Aldrich) were purchased from Aldrich. All chemicals were used without further purification. Synthesis. HEB-YAG:Eu3+ powders were prepared from stoichiometric amounts of Y2O3, Al2O3, and Eu2O3. Planetary ball milling was performed using hard material ZrO balls (diameter of 10 mm) with a rotation speed of 350 rpm for 50 min and an interruption time of 10 min. The process was repeated 100 times. The powder samples were sintered in air at 1400 °C for 2 h to form polycrystalline YAG:Eu3+. For comparison, SSR-YAG:Eu3+ powders were obtained by using a conventional solid-state reaction method (SSR). The starting materials were Y2O3, Al2O3, and Eu2O3. The appropriate amount of starting materials according to the nominal composition of YAG:Eu3+ were mixed thoroughly, ground in an agate mortar, and heated to 1400 °C for 2hrs. 2.2. Characterization. The crystalline structures of the HEBand SSR-YAG:Eu3+ powders were measured from the X-ray diffraction (XRD) patterns using a Philips XPert/MPD diffraction system with Cu KR1 radiation (λ ) 1.54056 Å) and Fourier transform-near-infrared spectroscopy (FT-IR, Perkin-Elmer Spectrum GX). The surface morphology of the powders was observed by field emission-scanning electron microscopy (FESEM: HITACHI, S-4200) operated at 20 kV. The roomtemperature photoluminescence (PL) spectra of the HEB- and SSR-YAG:Eu3+ phosphors were recorded on a PTI (Photon Technology International) fluorimeter using a Xe-arc lamp with a power of 60 W. The lifetimes were measured using a phosphorimeter attachment to the main system with a Xe-flash lamp (25 W power) with a dominant excitation wavelength of 245 nm. 3. Results and Discussion 3.1. Synthesis and Morphology of -YAG:Eu3+ and SSRYAG:Eu3+. Figure 1 shows the XRD patterns of (a) HEB- and (b) SSR-YAG:Eu3+ powders at different sintering temperatures. The diffraction peaks in Figure 1a, HEB-YAG:Eu3+, and Figure 1b, SSR-YAG:Eu3+, powders are in good agreement with the standard JCPDS cards of Y3Al5O12 (08-0178), YAlO3 (YAP, 33-0041), and Y4Al2O9 (YAM, 22-0987), respectively. The XRD patterns of both samples showed a polycrystalline phase of
10.1021/jp908903t 2010 American Chemical Society Published on Web 11/11/2009
Syntheses of YAG:Eu3+ Phosphors
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Figure 2. FT-IR spectra of HEB and SSR-YAG:Eu3+, Y2O3, and Al2O3.
Figure 1. XRD pattern of (a) HEB and (b) SSR-YAG:Eu3+ powders as a function of various sintering temperatures.
(400), (420), (521), (611), or (640) peaks. The (420) plane was the preferred orientation of YAG:Eu3+ powders. However, the XRD pattern of the SSR-YAG:Eu3+ powder showed impurity peaks at all sintering temperatures. Several reactions occurred simultaneously, including the grain growth of Al2O3 and Y2O3, diffusion of Al3+ into the Y2O3 lattice, and a phase formation reaction of YAMfYAPfYAG (the reaction of YAG).8 The crystallinity of the HEB-YAG:Eu3+ powders improved with increasing sintering temperature. However, the HEB-YAG:Eu3+ powders sintered at
