Chemical Engineering Journal 198–199 (2012) 449–456
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Solid combustion wave with two successive reactions to produce phosphor powders H.H. Nersisyan a, H.I. Won a, C.W. Won a,⇑, A.G. Kirakosyan b, D.Y. Jeon b a b
Department of Nano Materials Engineering, Chungnam National University, Yuseong, Daejeon 305-764, South Korea Department of Materials Science and Engineering, KAIST, Yuseong, Daejeon 305-701, South Korea
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
" Two-successive reaction pathway is
developed for rapid synthesizing phosphor powders. " Temperature distributions were analyzed and a combustion mechanism is proposed. " Single phase phosphor microparticles at low temperature (700–1200 °C) are prepared. " Phosphors show controlled morphology, good dispersion and high luminescence efficiency.
a r t i c l e
i n f o
Article history: Received 5 January 2012 Received in revised form 14 May 2012 Accepted 23 May 2012 Available online 2 June 2012 Keywords: Phosphor Combustion synthesis Luminescence Exothermic mixture Morphology
a b s t r a c t A solid combustion approach consisting of two successive reactions is developed for producing metal oxide (Y3Al5O12:Ce3+), silicate (Zn2SiO4:Mn2+), and borate (YBO3:Eu3+) phosphor powders. The typical precursors for the phosphor synthesis were the corresponding metal oxides. The entire combustion process was driven by a KClO3 + CO(NH2)2 exothermic mixture preliminarily admixed with precursor oxide powders. Small amounts of NH4F were also used to accelerate the phosphor formation and crystallization processes. The optimal synthesis temperatures estimated from the temperature distributions were between 700 and 1200 °C, and the combustion velocity varied from 0.04 to 0.5 cm/s. From the synthesis, well-dispersed phosphor microparticles with a controlled morphology were obtained. The roles of the KClO3 + CO(NH2)2 exothermic reaction and the NH4F additive were examined in the context of the reaction mechanism and the phosphor powder characteristics. The obtained combustion-synthesized phosphors were of high quality and single phase having high luminescence characteristics. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Metal oxide, silicate, and borate phosphors have recently gained much attention for their role in applications such as screens in plasma display panels, field emission displays, and white color light emitting diodes because of their high chemical stability and high luminescence efficiency. Over the last decade, many of these phosphors such as yellow-emitting yttrium aluminum garnet (Y3Al5O12:Eu2+), green-emitting zinc silicate (Zn2SiO4:Mn2+), redemitting strontium-barium silicate (SrBaSiO4:Eu2+), and yttriumboron oxide (YBO3:Eu3+) were synthesized using different ⇑ Corresponding author. Tel.: +82 42 821 6587; fax: +82 42 822 9401. E-mail address:
[email protected] (C.W. Won). 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.05.085
techniques to obtain phosphor powders with improved luminescent performance, including color purity, emission intensity, and quantum efficiency. These synthesis techniques include solid-state reaction [1–3], sol–gel [4–6], hydrothermal [7–9], spray pyrolysis [10–12], and combustion [13,14]. Among the above mentioned synthesis techniques we address the combustion process in powder mixture (also known self-propagating high temperature synthesis) for synthesizing micrometer sized phosphor powders from corresponding metal oxide precursors. Generally the reactions between metal oxides are not exothermic and cannot by carried out under a self-propagating combustion mode. Therefore an external or internal energy source always is required for heating oxide particles to higher temperature to start
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H.H. Nersisyan et al. / Chemical Engineering Journal 198–199 (2012) 449–456
reacting. One of the known and attractive routes is to combine metal oxides with a highly exothermic mixture consisting of strong oxidant (metal chlorate, nitrite, peroxide, etc.) and inorganic or organic fuel (metal, carbon, organic fuels, etc.). Kingsley and Patil et al. [15] reported the synthesis of a-alumina and related oxide materials using metal nitrate–urea mixture in the combustion process. In our early investigation [16] we demonstrated the synthesis of metal carbide powders (SiC, WC, etc.) using Me + C (Me is Si, W, Mo) low exothermic mixtures combined with highly exothermic redox mixtures of KNO3 + Al and KNO3 + Si composition. The application of redox mixtures for synthesizing inorganic nanoparticles by solution combustion synthesis (SCS) technique is very well known in the literature [17]. Among the materials synthesized by SCS method luminescent materials and catalysts occupy the lion share [18–21]. In SCS method metal nitrates oxidants are dissolved in the aqueous solution of an organic fuel (citric acid, urea, ethylene glycol, etc.) and as-prepared solution after the evaporation of water results a solid exothermic mixture which can be ignited by heating at elevated temperatures. Depending on the type of the precursors, as well as on conditions used for the process organization, the SCS may occur as volume or layer-by-layer propagating combustion modes, yielding nanosized phosphor powders. The phosphor nanoparticles produced by SCS method have a low emission intensity in comparison to its micrometer size counterparts produced the traditional solid phase method. Therefore an additional calcination procedure at elevated temperatures (800– 1400 °C) is frequently required in order to increase luminescence characteristics of phosphor powders. In our recent publications [22,23] we reported the combustion synthesis of blue-emitting BaMgAl10O17:Eu2+ (BAM) and yellowemitting Y3Al5O12:Ce3+ (YAG) phosphor powders by combined chemical reactions. In these reports metal oxides precursors were blended with defined amount of KClO3 + organic fuel (carbon, hexamethylenetetramine, urea, polyethylene, etc.) redox mixture and as prepared mixture was compacted into a cylindrical cup. After a local ignition of the mixture a combustion wave was formed and self-propagated along the sample converting the oxide precursors into phosphor powders. Here, the high energy of redox reaction provides the driving force for the combustion process. This approach is scientifically attractive because it can be initiated at a point source by a heated filament, is easy to control, and allows for varying of the reaction temperature, combustion rate, and reaction equilibrium by shifting the proportion of oxides and redox mixtures. In addition, the KCl by-product formed after the decomposition of KClO3, can be easily separated from the phosphor material with a simple water washing procedure. In this paper we have attempted to describe the general aspects and specific features of phosphor synthesis by combined chemical reactions using a solid-flame synthesis approach. Three phosphor systems were selected for investigation: yellow-emitting yttrium aluminum garnet (Y3Al5O12:Ce3+), green–emitting zinc silicate (Zn2SiO4:Mn2+), and red emitting yttrium-boron oxide (YBO3:Eu3+). Beside KClO3 + CO(NH2)2 redox mixture (shortly K/C), a small amount of ammonium fluoride (0.5–5 wt.%) was also used to activate the diffusion processes in the combustion wave and to obtain high crystalline and uniform phosphor particles. From several of our fundamental studies on phosphor production in such a combustion process, we have identified the following key features of a developed combustion process: (i) a controlled morphology and highly crystalline particles are produced, (ii) the morphology of the produced particles can be controlled to some extent with temperature, redox mixture, and fluoride concentration, and (iii) homogeneous reducing or oxidizing atmospheres can be provided by varying the amounts of KClO3 and organic fuel. To understand the mechanism of the combustion process with specified synthesis conditions, our group has conducted temperature–time measurements
and analyzed the sequence of chemical reactions in the combustion wave, the product characteristics including reaction phases, particles shape and morphology, and the luminescence characteristics.
2. Experimental Metal oxide, silicate, and borate phosphors were prepared using the solid combustion process. The starting materials used were ZnO, SiO2, MnCO3, Y2O3, Al2O3, CeO2, B2O3, and Eu2O3. Urea was selected as the main fuel and KClO3 was used as the oxidizer. Ammonium fluoride (NH4F) was used as an active additive for accelerating the phosphor phase formation in the combustion wave. Detailed information on the starting materials is shown in Table 1. All metal oxide constituents in stoichiometric proportions, along with fuel, oxidizer, and NH4F, were dry mixed and approximately 60–100 g of the mixed powder was hand pressed into a cylindrical cup with a diameter of 4 cm and height of 6–8 cm. Approximately 2–3 g of Ti + C + 0.1(C2F4)n powder was then placed on top of the sample as an easy ignition agent. The cup with the reaction mixture was subsequently placed into a combustion chamber under a nickel–chromium filament. The chamber was then evacuated and filled with argon to a pressure of 2.0 MPa to suppress the possible elongation of the sample by reaction gases. Ignition was achieved using power added into the nickel–chromium filament that had been placed on top of the green mixture. Depending on the composition and amount of the mixture used, the combustion process brings an increase in pressure in the combustion chamber up to 3.0 MPa. After the combustion process, the reaction product was cooled, washed with distilled water (to remove KCl), and dried at 100 °C. The combustion process was monitored using a thermocouple technique in which two tungsten–rhenium thermocouples (W/ Re-5 vs. W/Re-20, 100 lm in diameter) were inserted into the reaction pellet. The first thermocouple was placed in the middle of the sample to measure the temperature changes in the combustion wave accurately. The second thermocouple was installed in the bottom of the sample, and its signal was mainly used to estimate the distance between the thermocouples to calculate the combustion velocity. The signals from the thermocouples were continuously recorded by the data acquisition system and used to develop temperature/time profiles (GL100A, Graphtec Co., Japan).
Table 1 Powder characteristics. Reactant Yttrium oxide (Y2O3) Aluminum oxide (Al2O3) Cerium oxide (CeO2) Zinc oxide (ZnO) Silicon dioxide (SiO2) Manganese carbonate (MnCO3) Boron oxide (B2O3) Europium oxide (Eu2O3) Potassium chlorate (KClO3) Urea (CO(NH2)2) Ammonium fluoride (NH4F)
Supplier Grand Chemical, Korea Grand Chemical, Korea Grand Chemical, Korea Grand Chemical, Korea Grand Chemical, Korea Kanto Chemicals, Japan Kanto Chemicals, Japan Grand Chemical, Korea Kanto Chemicals, Japan Aldrich, USA Samchun Chemicals, Korea
Particle size,
lm
Purity, wt.%
99.8
99
