Integrated chemical process for exothermic wave synthesis of high luminance YAG:Ce phosphors
Highlights
► A new solid-flame strategy was developed for synthesizing high-luminance YAG:Ce phosphor. ► Adding KClO3+CO(NH2)2+NH4F mixture to oxide powders provides a low-temperature combustion process. ► YAG:Ce phosphor particles 10–25 μm in diameter were obtained at 1000–1100 °C within tens of seconds. ► As-prepared YAG:Ce emission intensity was 90.1–103.2% compared to that of the reference sample.
Introduction
The yttrium–aluminum garnet phosphor known as YAG:Ce (when activated by trivalent cerium) is a well-known phosphor used in the white light emitting diode (LED) commercial market. In comparison with phosphors based on silicates, sulfates, nitridosilicates, and oxonitridosilicates, YAG:Ce has a relatively high absorption efficiency of blue excitation radiation, high quantum efficiency (QE greater than about 90%), good stability in high-temperature and high-humidity environments, and a broad emission spectrum. Such unique properties of YAG:Ce make it a primary yellow-emitting phosphor material for use in white LEDs.
The synthesis methods for YAG:Ce powder production can be classified into three groups: a conventional solid-state method [1], [2], wet-chemistry methods (co-precipitation [3], [4], [5], [6], [7], sol–gel [8], [9], [10], [11], solvothermal [12], [13], [14], and ultrasonic spray pyrolysis [15], [16]); and combustion synthesis methods (flame pyrolysis [17]; and solution combustion synthesis [18], [19], [20], [21]).
The conventional solid-state method [1], [2], which is the basic production method for YAG:Ce phosphor powders, generally results in highly crystalline and micrometer-sized particles (5–25 μm) having high luminescence properties. However the solid-state method requires high calcination temperatures (1500–1600 °C) in a reducing atmosphere (N2/H2) and long reaction times (5–10 h); this negatively affects the production cost and process efficiency.
Unfortunately, the wet-chemistry techniques mentioned above [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16] cannot be considered as low-cost alternatives for solid-state reactions because they have a complicated technological procedure and cannot provide YAG:Ce3+ phosphor powders with high luminescence properties.
In this case, the solid-state combustion process is a comparatively simpler method that can be adopted for the cost-effective fabrication of YAG:Ce powders without the application of external heat sources. Recently, two combustion routes namely flame pyrolysis and solution combustion synthesis (SCS) that employ precursor nitrite solutions premixed with organic fuel have been reported for the preparation of YAG:Ce [17], [18], [19], [20], [21]. However these routes have all the characteristic disadvantages that are typical for the wet-chemistry methods: low yield, long reaction times, high energy consumption, badly agglomerated resultant powders, and poor emission intensity. For the reasons mentioned above, a novel and progressive method that reduces the production cost of YAG:Ce without negatively affecting its yield and emission characteristics is urgently required.
In this study, we report the synthesis of YAG:Ce phosphor particles using a solid-flame synthesis approach. Our synthesis strategy is novel, rapid, straightforward, and energy efficient, and it is based on the oxide precursor system (1.425Y2O3+2.5Al2O3+0.15CeO2) blended with a KClO3+CO(NH2)2 red-ox mixture. A controlled amount of NH4F was also added to the reaction mixture for in-situ formation of the Y3Al5O12 phase below 1200 °C. The solid-flame approach developed in this study provides an attractive, low-cost alternative for the production of YAG:Ce-based yellow phosphors.
Section snippets
Experimental set-up and procedure
The following raw materials were used in the synthesis of the yellow-emitting YAG:Ce phosphor: metal oxide powders (Y2O3, Al2O3, and CeO2), potassium chlorate (KClO3), urea (CO(NH2)2), and ammonium fluoride (NH4F). Other organic compounds, such as polyethylene powder (C2H4)n, teflon (C2F4)n, hexamethylenetetramine (C6H12N4), hexachlorobenzene (C6Cl6), black carbon (C), and charcoal (C7H4O), were also tested in the synthesis procedure. The properties of the raw materials are listed in Table 1. A
Thermodynamic analysis
The synthesis of pure-phase YAG from the oxide mixture was achieved with the aid of the heat generated by the KClO3+αCO(NH2)2 red-ox mixture. Therefore, a thermodynamic analysis of the KClO3+αCO(NH2)2 mixture and that mixture combined with a 1.5Y2O3+2.5Al2O3 mixture was studied using THERMO software based on the minimization of Gibbs free energy [22]. This analysis provided information about the adiabatic combustion temperature, concentrations at equilibrium, phase relationships, and
Conclusions
High-luminance yellow-emitting Y3Al5O13:Ce3+ phosphor microparticles were prepared from a 1.475Y2O3+2.5Al2O3+0.15CeO2+k(KClO3+CO(NH2)2)+mNH4F precursor mixture by a newly developed solid-flame strategy. In this process the heat evaluation from the KClO3+CO(NH2)2 red-ox reaction increased the entire system temperature to 885–1200 °C, enabling the synthesis of YAG:Ce phosphor particles. Formation of pure-phase YAG:Ce at low synthesis temperatures was activated when 0.5 mole (or above) of NH4F was
References (23)
- et al.
Powder Technol.
(2009) - et al.
Mater. Res. Bull.
(2005) - et al.
J. Alloys Compd.
(2006) - et al.
Mater. Chem. Phys.
(2005) - et al.
Mater. Lett.
(2007) - et al.
J. Alloys Compd.
(2008) - et al.
Mater. Res. Bull.
(2005) - et al.
J. Alloys Compd.
(2008) J. Alloys Compd.
(2006)- et al.
Powder Technol.
(2010)