Elsevier

Journal of Rare Earths

Volume 36, Issue 3, March 2018, Pages 248-256
Journal of Rare Earths

Combustion synthesis of YAG:Ce phosphors via the thermite reaction of aluminum

https://doi.org/10.1016/j.jre.2017.06.014Get rights and content

Abstract

Cerium-doped yttrium aluminum garnet (YAG:Ce) as a yellow phosphor for white light-emitting diodes (LEDs) was synthesized via a facile combustion method using Y2O3, CeO2, Al2O3, Al, and NaClO4 as raw materials. The combustion synthesis approach utilizes the strong exothermic oxidation of aluminum to realize a self-sustaining reaction. In this study, we investigated the effects of the ratios of Al2O3 to Al, fluxes, and coprecipitated materials as raw materials on the luminescence properties of the synthesized YAG:Ce phosphors. When the amount of Al2O3 x is varied, the combustion reaction proceeds at x ≤ 1.8, with x = 1.725 being the optimum condition for producing a high-performance product. When 5 wt% BaF2 is added, the luminescence intensity is significantly improved owing to a decrease of YAP (YAlO3) formation with improved uniformity. However, the addition of CaF2 and NaF does not improve the luminescence properties. To suppress the segregation of CeO2, we used the coprecipitated material Y2O3–CeO2 as a raw material. Unlike with separate addition of Y2O3 and CeO2, Ce ions are uniformly distributed in the coprecipitated material, resulting in improved luminescence properties. The combination of BaF2 and coprecipitated material significantly improves the internal quantum efficiency to 83.0%, which is close to that of commercial phosphors.

Graphical abstract

Schematic diagram of the experimental apparatus for combustion synthesis and Emission spectra for the combustion-synthesized products under different conditions: no flux, 5 wt% BaF2, and Y2O3–CeO2 + 5 wt% BaF2. The inset shows a photograph of the synthesized powders.

  1. Download : Download high-res image (223KB)
  2. Download : Download full-size image

Introduction

Ce-doped Y3Al5O12 (YAG:Ce) phosphors have been widely used as yellow phosphors for white light-emitting diodes (LEDs) owing to their high emission efficiency under blue light excitation. In conventional solid-state reactions, the synthesis of pure YAG:Ce phosphors requires high-temperature (>1500 °C) treatment, which is both energy- and time-consuming. Therefore, lower temperature syntheses, such as spray pyrolysis,1 sol–gel processing,2, 3, 4 coprecipitation method,5, 6 citrate sol–gel combustion preparation,7, 8, 9, 10 and hydrothermal synthesis,11 have been proposed. However, these processes require the use of nitrates, such as Al(NO3)3 and Y(NO3)3, which still necessitate the use of high temperature treatments. To resolve this problem, this paper proposes a combustion synthesis (self-propagation high-temperature synthesis) of YAG:Ce phosphor using aluminum oxidation heat. Combustion synthesis using the thermite reaction of Al employs the propagation of a strong exothermic reaction, namely, Al oxidation, which is a self-sustaining reaction. This method has advantages of low energy consumption and short reaction times, and has been applied to produce a variety of advanced materials, such as oxides,12, 13, 14 nitride/oxynitride ceramics,15, 16, 17 and intermetallics.18, 19

In this study, we examined the facile and effective combustion synthesis of YAG:Ce phosphors via the thermite reaction of Al, where the oxidation heat of Al is used for self-propagation of the high-temperature synthesis. The key for successful combustion synthesis of YAG:Ce phosphors is regulating the reaction temperature, which can be optimized by controlling the ratio of Al and Al2O3 in the raw materials. We also investigated the effects of fluxes and coprecipitated materials on the luminescence properties of the synthesized YAG:Ce phosphors. It is known that flux addition greatly influences ion diffusion and crystallization processes, resulting in improved phase uniformity and luminescence properties.1, 20 For single crystalline YAG growth, fluxes such as PbOsingle bondPbF2 and PbOsingle bondPbF2single bondB2O3 have been used.21 Among the various available fluxes, fluoride-type fluxes such as BaF2, CaF2, MgF2, and AlF3 have been found to be effective for aluminate-type phosphors.20, 22, 23 However, the effect of these fluxes on combustion-synthesized phosphors is unknown. In addition, we studied the effect of utilizing coprecipitated Y2O3–CeO2 as a raw material. Because Y and Ce ions are uniformly distributed in the coprecipitated material, suppression of CeO2 segregation and increased uniformity are expected.

Section snippets

Material and methods

YAG:Ce phosphors with a Ce ratio of 1.0 at% were prepared from commercially available Y2O3 (99.99% purity), CeO2 (99.99% purity), Al2O3 (99.99% purity), Al (99.99% purity), and NaClO4 (98% purity) powders. Al2O3 acts as a diluent to control the combustion flame temperature. The reaction formula for the combustion synthesis can be written as shown in Eq. 1.1.485Y2O3 + 0.03CeO2 + xAl2O3 + (5–2x)Al + yNaClO4 → Y3Al5O12 + yNaCl; (y = (7.485–3x)/4)

Here, x refers to the amount of Al2O3. The adiabatic

Effect of Al2O3 ratio in raw materials

In this work, the combustion synthesis of YAG:Ce was promoted by the oxidation of Al by O2, which was released from NaClO4. The reaction temperature for the oxidation of Al can be in excess of 5300 °C, which can cause melting and extreme sintering of the products. Therefore, we added Al2O3 as a diluent, and studied the optimum amount x of Al2O3. Fig. 2 shows photographs of samples obtained by combustion synthesis at different x values, as indicated in Eq. 1. The raw materials were in the form

Conclusions

YAG:Ce phosphors were successfully prepared by combustion synthesis via the thermite reaction of Al. The reaction temperature was controlled by varying x, the ratio of Al2O3 to Al. The combustion reaction proceeded at x ≤ 1.8, and x = 1.725 was the optimum condition for producing a high-performance product. When 5 wt% BaF2 was added, the luminescence intensity was significantly improved owing to decreased YAP formation with improved elemental uniformity through the effect of the flux on the

Acknowledgements

Part of this work was conducted at the Laboratory of XPS analysis, Hokkaido University, supported by the “Nanotechnology Platform” Program of the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT). We gratefully acknowledge Mr. K. Suzuki, Mr. K. Ohkubo, Mr. R. Oota, Mr. T. Tanioka, Ms. Y. Yamanouchi, and Ms. E. Obari for their technical support in SEM experiments. We also thank Mr. K. Harada (Combustion Synthesis Co., Ltd.) for assistance. The measurements of

References (31)

Cited by (15)

  • Fabrication of high-efficiency YAG:Ce<sup>3+</sup> phosphors via concurrent optimization of firing atmosphere and fluxing agent

    2022, Optical Materials
    Citation Excerpt :

    This review provides an insight into the relationship between crystal chemistry and PL in an important class of Ce3+-doped garnet phosphors [5]. When oxides are used as starting materials, a high calcination temperature of at least 1600 °C is required to obtain the YAG phase via the solid-state reaction method [6,7]. The fluxing agent is generally expected to increase the PL intensity by transforming into a melt during the reaction and consequently enhancing the elemental diffusion between oxide powders, promoting the formation of the YAG phase during synthesis [8–10].

  • Effect of process parameters on YAG:Ce phosphor properties obtained by co-precipitation method

    2020, Ceramics International
    Citation Excerpt :

    The adaptation of some already known processes is a way of improving the emission parameters or the production price. For example in the solid-state method, reduced fabrication costs can be obtained by partial or complete replacement of the classical raw materials (i.e: to replace Al2O3 with Al), by using addition (i.e: fuel as BaF2, NH4F etc) or different types of sintering atmospheres (i.e: reducing atmosphere), leading to the decrease of the sintering temperature [9,37–39]. Most of the YAG:Ce synthesis processes suffer from the rather high price raw materials, the methods are difficult to control, they require high temperatures, being more or less toxic due to nitrogen oxide emissions or the use of toxic solvents.

  • Transparent YAG material prepared from nano-powder with core-shell morphology

    2019, Ceramics International
    Citation Excerpt :

    The synthesis of the phase-pure YAG powders by a solid-state reaction between respective oxides needs a high temperature, even exceeding 1600 °C, which creates difficulties in controlling their morphology and causes a formation of hard agglomerates [3]. To avoid such inconveniences, several soft chemistry methods can be successfully used: sol-gel [4] co-precipitation with ammonium carbonate, ammonia or urea [4,5], the modified Pecchini method [6], microwave irradiation [7], solvothermal synthesis [8], supercritical water synthesis [9], spray pyrolysis [10] and combustion synthesis [11]. However, even with such fine powders preparation methods, to obtain a fully dense material by pressureless sintering, a high temperature, long sintering times, and/or the use of HP, HIP, microwave sintering or SPS were necessary [12,13].

View all citing articles on Scopus
View full text