Preparation of Y3Al5O12:Eu powders by microwave-induced combustion process and their luminescent properties

Journal of Alloys and Compounds 402 (2005) 233–236

Preparation of Y3Al5O12:Eu powders by microwave-induced combustion process and their luminescent properties Yen-Pei Fu ∗ Department of Materials Science and Engineering, National Dong-Hwa University, Shou-Feng, Hualien 974, Taiwan Received 3 February 2005; accepted 18 February 2005 Available online 24 May 2005

Abstract A novel ceramic synthesis technique, microwave-induced combustion process was investigated for the production of Y3 Al5 O12 :Eu powders with improved physical and luminescence properties. This technique involves the reaction of aluminum nitrate (Al(NO3 )3 ·9H2 O), yttrium nitrate hexahydrate (Y(NO3 )3 ·6H2 O), europium nitrate (Eu(NO3 )3 ·6H2 O), and carbohydrazide (CO(N2 H3 )2 ) at microwave oven and the whole process took only 30 min. The powders of Y3 Al5 O12 :Eu were further investigated by X-ray diffractometer (XRD), scanning electron microscope (SEM), infrared spectroscopy (IR), and photoluminescence emission spectra (PL). The results showed that the formation temperature of YAG phase is significantly low, compared to solid-state reaction route of constituent oxides. For luminescence property, the emission intensity of YAG doped with 3 mol% Eu synthesized by microwave-induced combustion reaches the maximum emission intensity. © 2005 Elsevier B.V. All rights reserved. PACS: 74.25.Gz; 74.62.Bf; 78.55.Hx Keywords: YAG; Microwave-induced combustion; Luminescence

1. Introduction Inorganic luminescence materials are crystalline compounds that absorb energy and subsequently emit this absorbed energy as light [1]. Phosphors are composed of an inert host lattice and an optically excited activator, typically a 3d or 4f electron metal such as Ce3+ , Cr3+ , Eu3+ , and Tb3+ . Oxide phosphors were found to be suitable for field emission display (FED), vacuum fluorescent display (VFD), electroluminescent (EL) devices, and plasma panel display (PDP) devices. Y3 Al5 O12 (YAG) is an advanced ceramic with interesting optical and mechanical properties [2,3]. YAG is a refractory, hard oxide ceramic that does not damage easily under condition of high irradiance with an electron beam. Moreover, YAG doped with a lanthanide element can be widely used as solid-state laser material in the luminescence field and as ∗

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window material for a variety of lamps [4–6]. Because YAGbased phosphors have high temperature chemical stability, they are expected to replace sulfide-based materials, which are currently used as main components in cathode ray tube (CRT) [7]. YAG and YAG-based phosphors have been prepared by several techniques, such as solid-state reaction [8,9], sol–gel [10], coprecipitation method [11,12], and hydrothermal method [13]. In this study, we used a new method called microwave-induced combustion synthesis to YAG:Eu powder. Microwave processing of materials is fundamentally different from conventional processing in terms of the heat generation mechanism. In a microwave oven, heat is generated within the sample itself by interaction of microwaves with material. In conventional heating, heat is generated by heating elements and then transferred to the sample surfaces [14]. Microwave-induced combustion synthesis involves the dissolution of metal nitrate and carbohydrazide in water [15], and then heating the resulting solution in a microwave oven. After the solution reaches the point of spontaneous combus-

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tion, it begins to burn in solid form, burning at high temperature. The combustion is not complete until all the flammable substances are all burnt yielding a powder that is highly friable and shows voids and pores formed by escaping gases during the combustion reaction. The whole process takes only a few minutes to produce YAG:Eu powder.

2. Experimental procedures 2.1. Sample synthesis The synthesis process of the YAG:Eu powders involved the combustion of redox mixtures, in which metal nitrate acted as an oxidizing agent and carbohydrazide as a reducing agent. The initial composition of the solution containing aluminum nitrate, yttrium nitrate, europium nitrate, and carbohydrazide was based on the total oxidizing and reducing valences of the oxidizer and fuel using concepts in propellant chemistry [16]. The stoichiometry of the redox mixtures used for combustion was determined using the total oxidizing and reducing valences of the components, which serve as numerical coefficients for the stoichmetric balance so that the equivalence ratio (Φe ) is unity and the energy released by combustion is maximum. According to this concept, the valences of some elements and ions are as follows: C = +4, H = +1, O = −2, divalent metal ions = −2, and trivalent metal ions = −3 [17]. The valence of nitrogen is considered to be zero. Accordingly, the oxidizing and reducing valences of the compounds used in the combustion mixtures can be calculated. In order to have the same oxidizing and reducing valences, the total ratio of the oxidizing and reducing valences was 1 [18]. A flowchart for the preparation of as-prepared YAG:Eu powders is described in Fig. 1. Stoichiometric amounts of yttrium nitrate hexahydate, aluminum nitrate, europium nitrate, and carbohydrazide were dissolved in 15 ml of deionized–distilled water in a crucible. The atomic ratios of Y:Al were set to be 3:5. As the literature report that dopant ratio was varied between 0.1 and 6 mol% in the YAG powder to produce phosphor materials. In this study, the mole ratio of YAG:Eu was set from 100:1 to 100:5. The crucible containing the solution was placed in a microwave oven (CEM, MDS 81D, 650 W). Initially, the solution boiled and underwent dehydration followed by decomposition with the evolution of large amount of gases. After the solution reached the point of spontaneous combustion, it began to burn with the release of much heat, vaporized all the solution instantly and burnt in solid at high temperature. The entire combustion process for producing YAG:Eu powders in the microwave oven took only 30 min. A stoichiometric combustion reaction of metal nitrate with carbohydrazide to from YAG is as follows: 3Y(NO3 )3 + 5Al(NO3 )3 + 15CO(N2 H3 )2 → Y3 Al5 O12 + 15CO2 + 45H2 O + 42N2

Fig. 1. Flowchart for the preparation of as-prepared YAG:Eu powders by microwave-induced combustion.

2.2. Characterization A computer-interface X-ray powder diffractometer (XRD) with Cu K␣ radiation (Rigaku D/Max-II) was used to identify the crystalline phase. Particle morphological features were observed using a scanning electron microscope (SEM; JEOL JSM-6500F). The emission and excitation spectra were obtained on a spectrofluorimeter (PL, Hitach F-4500), equipped with a 450 W xenon lamp as the excitation source. Fourier transform infrared spectroscopy (FTIR, Nicolet Magna 550 FTIR) were used to measured YAG:Eu powder with the KBr pellet technique.

3. Results and discussion Fig. 2 shows the X-ray diffraction patterns of as-prepared YAG:Eu powder with various Eu amount annealed at 1100 ◦ C for 2 h. It reveals that the single crystalline phase of YAG (Y3 Al5 O12 ) can be obtained for Eu-doped amount less than 3 mol%, further increase the Eu-doped amount (≥4 mol%) for YAG:Eu powder, which both YAG (Y3 Al5 O12 ) and YAP (YAlO3 ) phases can be detected. YAP with a perovskite structure is an intermediate phase of Y2 O3 –Al2 O3 system. These results show that the formation of YAG phase using the microwave-induced combustion (∼1100 ◦ C) is rather low as compared to the conventional solid-state reaction. Since much higher temperatures (>1600 ◦ C) are required to prepared YAG in a solid-state reaction of constituent oxide to form YAG phase. The morphology of the YAG:Eu-doped with 3 mol% Eu powders annealing at 1100 ◦ C for 2 h is shown in Fig. 3. Evidently, it can be seen pores and voids in the specimen, which result from the escaping gases during combustion. The particles of these specimens show highly

Y.-P. Fu / Journal of Alloys and Compounds 402 (2005) 233–236

Fig. 2. XRD pattern of as-prepared YAG:Eu phosphors with various europium concentrations annealed at 1100 ◦ C for 2 h.

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Fig. 4. Emission spectra of YAG:Eu phosphors with various europium concentrations.

agglomerated, nonuniform shape with rough surface, as well as pores and cracks. These porous powders are highly friable, which facilitates easy grinding to obtain finer particle [19]. Besides, the broad particle size distribution was probably due to the occurrence of local reaction during combustion. Fig. 4 shows the emission spectra of the as-prepared YAG:Eu powder annealed at 1100 ◦ C for 2 h. For emission spectra measurement, YAG:Eu powder was excited with 254 nm wavelength from a xenon lamp. These results reveal that YAG:Eu powder prepared by microwave-induced combustion process is luminescent, two emission bands are observed at 590, and 615 nm, which correspond to 5 D0 → 7 F1 (590 nm) and 5 D0 → 7 F2 (615 nm) transition, respectively. From the luminescence spectrum, we can observe that the YAG:Eu powder shows an orange–red emission. According to the literatures reported that the Eu3+ ions lie in centrosymmetrical sites, the characteristic line at 590 nm (5 D0 → 7 F1 ) is predominant, the forbidden transition of 5 D0 → 7 F2 (615 nm)

is secondary. The character of the emission spectrum clearly indicates the phase containing europium ions is in the yttrium aluminum garnet crystal structure. Accordingly, the microwave-induced combustion not only yields homogeneous and stoichiometric products but also gives itself to substituting Eu3+ for Y3+ ions. Fig. 5 displays the effect of Eu-doped amount on the emission (at 590 nm) intensity of YAG:Eu powders for 254 nm excitation. It indicates that the emission intensity increases with increasing Eu amount from 1 to 3 mol%. The emission intensity of YAG doped with 3 mol% Eu reaches the maximum value. As the amount of Eu increases more than 3 mol%, the emission intensity descends, which is due to the presence of YAlO3 impurity phase, as shown in XRD pattern. Therefore, an appropriate Eu-doped amount contributed to the luminescence properties for YAG:Eu powders. The emission intensity (5 D0 → 7 F1 and 5 D → 7 F ) of spectrum strongly depends on the europium 0 2 concentration. Fig. 6 plots the FTIR spectra of the as-prepared YAG doped with 3 mol% Eu annealed at 1100 ◦ C for 2 h. The band at 3200–3600 cm−1 may be ascribed to the (O–H) vibration of

Fig. 3. SEM micrograph of YAG:Eu powder doped with 3 mol% europium annealing at 1100 ◦ C for 2 h.

Fig. 5. Effects of Eu-doped amount on the primary emission peak (at 590 nm) and secondary emission peak (at 615 nm) intensity of YAG:Eu phosphors for 254 nm excitation.

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imum value as Eu-doped amount at 3 mol%. This method can be employed as a new route for synthesizing analogous phosphor materials.

Acknowledgement The authors would like to thank the National Science Council of the Republic of China for financially supporting this research under contract no. NSC 93-2212-E-259-001.

References

Fig. 6. Infrared spectra for YAG:Eu phosphors doped with 3 mol% europium.

H2 O absorbed by the specimen. The band between 1500 and 1700 cm−1 is assigned to the asymmetric vibration of –CH2 group. Moreover, the band between 1600 and 1800 cm−1 and 1570 and 1380 cm−1 is due to the asymmetric stretching vibration and the symmetric stretching vibration of the NO3 − group, respectively [7]. The peaks at 783 and 457 cm−1 represent the characteristics (Al–O) metal–oxygen vibrations, while the peaks at 706 and 563 cm−1 represent the characteristic (Y–O) metal–oxygen vibrations [20–21], which indicates the formation of YAG phase.

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4. Conclusions Using aluminum nitrate, yttrium nitrate hexahydrate, europium nitrate, and carbohydrazide as the starting materials, YAG:Eu powders have been synthesized successfully by microwave-induced combustion with low annealing temperature (1100 ◦ C). The formation temperature of YAG phase is prominently low, compared to solid-state reaction. Furthermore, results of this study show that Eu-doped amount affect greatly the luminescence properties of YAG:Eu powders. The emission and intensity of YAG:Eu powders reached the max-

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