Synthesizing Phosphors Through Microwave Process

Transcription

1 Mater. Res. Soc. Symp. Proc. Vol Materials Research Society 0988-QQ06-20 Synthesizing Phosphors Through Microwave Process Chris Y. Fang 1, Dinesh K. Agrawal 1, Ming Fu 1, Joan M. Coveleskie 2, Chung-nin Chau 2, James Walck 2, Robert T. McSweeney 2, and Rustum Roy 1 1 Materials Research Institute, The Pennsylvania State University, University Park, PA, OSRAM SYLVANIA, Inc., Hawes Street, Towanda, PA, ABSTRACT Various Lamp phosphors, including [Ca 10 (PO 4 ) 6 (Cl,F):Sb:Mn], (Y,Eu) 2 O 3 (YOE), BaMgAl 10 O 17 :Eu (BAM), and (La,Ce)PO 4 :Ce:Tb (LAP), with or without flux, have been synthesized by a microwave processing technique in a multimode microwave furnace operating at 2.45 GHz. The microwave-synthesized phosphors were comprehensively characterized for particle size, specific surface area, brightness, and luminescence. Although most properties of the microwave-synthesized phosphors were comparable to that of the conventional products, the kinetics of the phosphor synthesis was substantially enhanced in the microwave processing. As a result, the soaking time at the final temperature was reduced by up to 90% compared to a conventional process. In addition, the required synthesis temperature was also lowered by C in microwave process, compared to the conventional process for these lamp phosphors. Certain improved property was also observed in some microwave synthesized samples. The mechanism and advantages of microwave process for the lamp phosphor synthesis through solidstate reaction are addressed. INTRODUCTION In order for the activators to be incorporated into the crystal lattice structure of the host material, a high temperature thermal treatment is necessary in the synthesis of lamp phosphors. Conventional processing of fluorescent lamp phosphors includes blending of the starting materials, loading the mixtures into crucibles, and firing at a high temperature for several hours. Additional finishing steps may include milling, washing to remove inert materials, filtering, drying, and blending. The total firing process may take 8-12 hours. In order to lower the firing temperature and accelerate the synthesis, flux is normally used. The conventional process of phosphor synthesis is not only complex, but also requires significant time and energy. Further, contamination due to the volatiles from the process has been a concern. Microwave processing of ceramic materials is referred to Sutton [1]. Research on microwave processing of materials has been active in the Materials Research Institute at the Pennsylvania State University since early 1980 s [2-8]. Microwave processing of various materials including ceramics, composites, cermets, hard metals, electronic ceramics, metallic materials, etc. have been investigated in this laboratory with success.

2 The general features of microwave processing of materials include volumetric and selective heating, enhanced kinetics, the potential to improve product quality, process simplification, and the potential of cost reduction. Generally, microwave processing of ceramic materials is a dielectric heating process. The mechanism of microwave heating is inherently different from that of conventional heating. In microwave heating, heat is generated within the materials exposed to the microwave field through microwave-material interactions, while during conventional heating, heat is transferred from the heating element to the surface of the load by radiation and convection, then to the center of the load by thermal conduction. The absorption of microwave energy by the load in a microwave cavity depends on the dielectric loss factor of the materials in the microwave field. For the highly lossy materials, microwave processing can bring about substantial savings in time and energy with improved quality of the product. For example, regular, porous, and transparent hydroxapatite (HAp) ceramics have been fabricated by microwave processing within a few minutes; Ba(Zn 1/3 Ta 2/3 )O 3 has been sintered to full density by microwave processing within 30 minutes at C compared to conventional sintering of the same material that requires 1600 C and as long as 24 hours [9]. The present study provides a unique process for synthesizing lamp phosphors using microwave technology. This process is much faster than conventional synthesis techniques and can result in substantial time and energy savings. EXPERIMENTAL Starting materials of commercial quality, with or without flux, for various phosphors were supplied by OSRAM SYLVANIA, Inc. for the synthesis work. The microwave synthesis of these phosphors was carried out using a 6 kw maximum power batch microwave multimode system operated at 2.45 GHz. The input power can be adjusted to control temperature. The microwave furnace is equipped with a turntable and a vacuum pump. Thus the experiments can be conducted either in air or in a desired atmosphere. Typically, the sample was loaded into an alumina crucible, and the crucible was placed at the center of a thermal insulating package which was positioned on the turntable. The insulating material used in this study is FiberFrax TM board made of aluminosilicate fibers and is porous in structure, light in weight, and very low in microwave absorption. In order to facilitate heat pickup at low temperatures, a microwave susceptor (e.g. SiC) was used. The temperature was measured with an optical pyrometer through a quartz window on the top of the microwave chamber. The configuration of the insulation package in the microwave cavity is described elsewhere [10]. The general procedure started with placing grams of starting fine powder into an alumina crucible and then covered with a lid. The lid had a hole in the center to monitor the temperature of the sample. If a special atmosphere was required, the microwave chamber was evacuated to 10 Torrs before filling in with the required gas. The sample was then heated by microwave irradiation. The heating rate and the temperature were controlled through proper adjustment of the input power to the magnetron. Once the desired temperature was reached, the sample was soaked for a specified period of time and then cooled down by switching off the microwave power. The microwave-synthesized products were characterized for particle size, brightness, phase composition, morphology, luminescence emission, color coordinates, etc.

3 RESULTS AND DISCUSSION 1. Halophosphor The microwave processing technique was used for the synthesis of a halophosphor (calcium chlorofluoroapatite doped with Sb 3+ and Mn 2+ ) from a mixture of raw materials. In order to simplify the synthesis process, dicalcium phosphate (HCaPO 4 ) was used in the starting mixture instead of pyrophosphate (Ca 2 P 2 O 7 ). Thus the microwave synthesis included both a pyrolysis and the formation of chlorofluoroapatite in a one-step synthesis. The raw materials mixture was composed of HCaPO 4, CaCO 3, CaF 2, NH 4 Cl, MnCO 3, and Sb 2 O 3. Typically, the halophosphate was synthesized by microwave processing at 1000 C for 20 minutes. The microwave-synthesized phosphor showed the same phase, morphology, dopant incorporation, and properties as the control. It was found that using a shallow tray load of the starting mixture and a pure nitrogen flow helped improve the quality of the product. In a typical schedule, microwave synthesis took 20 minutes soaking at the peak temperature, and 120 minutes total in heating, compared to typically 8-12 h in typical conventional processing. Time and energy saving and productivity increase in the microwave process is obvious. Table 1 is a comparison of x and y color coordinates of the emission from a microwave-synthesized halophosphate sample excited by 254 nm UV, compared with a conventional halophosphate. Table 1. C.I.E. color coordinates on a microwave synthesized halo-phosphor sample Control Microwave sample X color Y color BaMgAl 10 O 17, or BAM Conventionally, BAM is produced by firing the mixture of raw materials in a reducing atmosphere at C for two hours with BaF 2 as flux. The total firing process lasts 6-8 hours. In the conventional production of BAM, alumina crucibles are used in a pusher furnace. Each crucible can be used only for a limited number of runs. Fluxed mixtures containing aluminum hydroxide, magnesium oxide, barium carbonate, and europium oxide with a moderate level of BaF 2 flux were fired in the microwave furnace. Temperatures ranged from 1250 C to 1500 C in an atmosphere of 25% N 2 and 75% H 2 for up to 20 min. The fired samples remained loose, soft, and fine. Below 1350 C, the loss on ignition (LOI) was 27.4%, and a 2 nd phase was detected in the as-fired samples as a minor phase, thus the reaction was not complete. At 1400 C or above, the LOI was 30.7%, and the reaction was complete. This was also confirmed by the x-ray diffraction XRD. All samples were easily screened to 160 µm without grinding. Compared to the BET of m 2 /g generally obtained by the conventional process in the commercial production, the microwave synthesized powders were finer ( m 2 /g). In addition, microwave processing of BAM lowered the processing temperature by C and the

4 processing time at peak temperature by 83% (20 min vs. 2 h). It was observed that the sample microwave fired in pure nitrogen was coarser than that fired in 25%N 2 :75%H 2. Unfluxed BAM samples were fired by microwave processing in 25%N 2 /75%H 2 at temperatures between C for 5-20 minutes. The LOI were to 30.71%. All samples were white and loose. As the temperature increased, the powder was whiter. The LOI increased very little with temperatures from 1400 to 1500 C. In all the microwave-fired unfluxed BAM mixtures, there was a tiny peak at 28.5 deg. 2-θ in the XRD pattern. Thus for the best reaction, the unfluxed BAM mixture should be fired at temperature above 1500 C. 3. (La,Ce)PO 4 :Ce:Tb, or LAP Conventionally, LAP is prepared using fluxes such as boric acid (HBO 2 ) and lithium carbonate (Li 2 CO 3 ) and is fired at 1200 C for about four hours in a reducing atmosphere. The microwave fired LAP samples were prepared from a mixed co-precipitate of lanthanum phosphate, cerium phosphate, and terbium phosphate. Samples with and without flux were synthesized. The microwave time and temperature conditions were varied. These LAP samples were characterized by measuring the powder brightness compared with a standard conventional LAP phosphor. The median particle size (MPS) was measured using a Malvern instrument before and after sonification. The span is a measure of the particle size distribution. The brightness of the unfluxed LAP microwave fired in 5%H 2 95%Ar was ±2.02%, whereas that fired in static air was 97.2±2.54%. The fluxed LAP microwave fired in 5%H 2 95%Ar showed 99.5±4.1% brightness. The sonic Malvern median particle size was between 4.51 to 5.4 µm. The thermal stability of several microwave fired LAP samples was measured and compared with a conventional LAP sample. It was observed that the unfluxed samples showed better thermal stability than the fluxed samples, and the sample fired at a lower temperature showed better thermal stability than one fired at a higher temperature. The microwave sample fired at 1020 C x 10 minutes showed the best thermal stability. Eight unfluxed samples (65g per run on average) were fired by microwave processing under the same conditions with tight control of the heating process. The designed condition was 10 minutes at 1020 C under flowing 5%H 2 /Ar. The firing process was very stable for all the samples. The total heating process in the microwave chamber (including warm-up and hold) was about 90 minutes. All the samples were white, clean, loose, and fine powders. The properties of these unfluxed LAP samples are listed in Table 2. All these LAP samples showed excellent properties. The thermal stability of this batch was even better: there was no degradation up to 600 o C. Table 2. Properties of the microwave synthesized unfluxed LAP samples. % Brightness X Y Median Particle Size, µm Span Flux Min. Temp ( o C) Average No Std Dev The unfluxed LAP samples can be fired by microwave processing at about 1020 C with a 10-minute hold, whereas the fluxed sample can be fired at a much lower temperature range

5 (about 800 C). Compared to the conventional firing, microwave processing can save processing hold time by more than 95% and yet at a lower temperature. Microwave processing makes it possible to synthesize a LAP phosphor of high quality without using any flux. 4. (Y,Eu) 2 O 3 (YOE) The conventional process of producing YOE requires the firing of the raw materials at a maximum of 1300 o C in air for up to 7 hours. The starting material was a mixed co-precipitate of yttrium oxide and europium oxide. Fluxes were normally used in this firing and can consist of lithium carbonate, potassium carbonate, and boric acid. Several YOE samples, both fluxed and unfluxed, were prepared by microwave process at 1100 to 1350 o C from 10 to 40 minutes. The XRD patterns of the microwave-synthesized samples are identical to the control. The emission property, measured after excitation at 254 nm, is also the same, although the intensity varies with the firing conditions. The morphology of the co-precipitate and the microwave synthesized unfluxed YOE samples were studied by SEM. Compared with the unfired co-precipitate (Fig. 1), that is loose and jagged, the synthesized samples showed bonding and coalition between primary particles (Figs. 2-4). The morphology of the microwave fired unfluxed YOE is different from the YOE synthesized with flux. The particles of the fluxed samples are rounded whereas the unfluxed are angular. The microwave fluxed YOE shows similar morphology as the conventional one. Figure 1. Morphology of unfluxed, unfired YOE co-precipitate. Figure 2. Conventionally fired YOE (fluxed). Figure 3. Microwave fired YOE (unfluxed). Figure 4. Microwave fired YOE (fluxed).

6 CONCLUSIONS 1. Microwave processing at 2.45 GHz can substantially enhance kinetics of the synthesis of various lamp phosphors. 2. One-step synthesis of chloro/fluoroapatite fluorescent lamp phosphor by microwave processing was successfully achieved. The properties of the obtained phosphor are comparable to the standard, while the processing time was reduced by about 90% (soak at the peak temperature). 3. Microwave synthesized BAM has been prepared with processing temperatures lowered by o C from conventional furnace temperatures, and the processing time at peak temperature reduced by 83% (20 minutes vs. 2 hours). Both fluxed and unfluxed BAM were synthesized. 4. Microwave synthesized LAP samples showed excellent powder properties compared to conventional LAP. Both fluxed and unfluxed LAP samples were prepared. The unfluxed LAP showed a substantial enhancement in thermal stability. The processing time at peak temperature was <10% of the time required in the conventional process. 5. Microwave synthesized YOE samples were prepared at times much shorter than the conventional synthesis. Both fluxed and unfluxed samples were made. 6. Microwave processing has demonstrated the potential for time and energy savings, improvement in the quality of the products, and potential to produce various phosphors without using fluxes to reduce contamination and lower cost in the synthesis of fluorescent lamp phosphors. 7. Conditions such as microwave temperature, soaking time, atmosphere, type of flux (or no flux), and finishing steps can be optimized to produce quality phosphor powders. REFERENCES 1. W.H. Sutton, Am. Ceram. Soc. Bull., 68 (2), 376 (1989). 2. R. Roy, L.J. Yang, and S. Komarneni, Am. Ceram. Soc. Bull., (1984). 3. J. Cheng, Y. Fang, D.K. Agrawal, Z. Wan, L. Chen, Y. Zhang, J. Zhou, X. Dong, and J. Qiu, in Microwave Processing of Materials IV, MRS Symp. Proc. Vol. 347, (MRS Publ. Pittsburgh, PA), pp (1994). 4. Y. Fang, D.K. Agrawal, D.M. Roy, and R. Roy, Materials Letters, (1995). 5. Y. Fang, J. Cheng, R. Roy, D.M. Roy, and D.K. Agrawal, J. Materials Science, 32, (1997). 6. R. Roy, D. Agrawal, J.P. Cheng, and M. Mathis, in Microwave: Theory and Application in Materials Processing IV, Eds, D.E. Clark, W.H. Sutton and D.A. Lewis, Ceramic Trans. Vol. 80, ACS Publ., pp3-26 (1997). 7. R. Roy, D. Agrawal, J. Cheng, and S. Gedevanishvili, Nature, 399, 668 (1999). 8. Y. Fang, J. Cheng, D. Agrawal, Materials Letters, 58, (2004). 9. Y. Fang, D.K. Agrawal, W. Hackenberger, T. Shrout, R. Roy, unpublished data. 10. Y. Fang, D.K. Agrawal, G.Y. Yang, M.T. Lanagan, C.A. Randall, T.R. Shrout, A. Henderson, M. Randall, A. Tajuddin, J. Electroceramics, 15, (2005).