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Improved production of ZnS blue phosphor powder
Materials Chemistry and Physics 93 (2005) 481–486
Improved production of ZnS blue phosphor powder Lyuji Ozawa a , Mika Makimura b , Minoru Itoh c,∗ b
a L. L. Technology, 8 Old Grange Rd, Hopewell JCT, NY 12533, USA Industrial Research Institute of Nagano Prefecture, 1-18-1 Wakasato, Nagano 380-0928, Japan c Department of Electrical and Electronic Engineering, Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan
Received 12 November 2004; received in revised form 8 March 2005; accepted 28 March 2005
Abstract The production procedure of ZnS:Ag:Cl blue phosphor powder is still complicated, although it is widely used for color displays. The complication mainly comes from handling of the by-product Na2 S4 , which covers the surface of grown ZnS particles in a crucible. The formation of Na2 S4 introduces serious problems in many factors such as (i) CL intensity and color, (ii) particle size distribution, (iii) sintering of heated products, (iv) photodarkening of powders, and (v) stability of PVA phosphor slurry. These problems are clarified in this paper. It is shown that the blue phosphor powder can be produced by a more simplified procedure than before, with high reproducibility. The ZnS:Ag:Cl phosphor powder produced by the present procedure is well suitable for blue primary in color screens of FED and CRT. © 2005 Elsevier B.V. All rights reserved. Keywords: Optical materials; Crystal growth; Electron microscopy; Microstructure
1. Introduction ZnS:Ag:Cl phosphor powder is used as blue primary in color screens of cathode ray tube (CRT) and field emission display (FED) [1–3]. The intensity and color of cathodoluminescence (CL), the separation of sintered products to primary ZnS particles, the particle size distribution, and the stability of poly(vinyl alcohol) (PVA) phosphor slurry are essentially important for ZnS phosphor production. Blue ZnS phosphor powder of average size 4 m is commonly produced by the procedure established by Leverentz more than 50 years ago [4]. According to his procedure, the blend mixture consisting of powdered amorphous (a-)ZnS, a few weight percentages of NaCl, and sulfur is softly charged (∼0.5 g cm−3 ) in a quartz crucible. The crucible is directly put in a furnace heated at around 970 ◦ C, and after a while, taken out from it rapidly. The heated products in the crucible are highly sintered. After soaking the sintered products in water, they are rinsed by deionized (DI) water several times. Small and large particles
∗
DOI of original article:10.1016/j.matchemphys.2005.03.041. Corresponding author. Tel.: +81 26 269 5261; fax: +81 26 269 5220. E-mail address: [email protected] (M. Itoh).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.03.033
are separated from the rinsed powder with the help of the difference of settling time in water, according to the Stokes’ law; that is a water sieve. The sieved powder is dried in a heated oven at 110 ◦ C for practical use. However, there remain some serious problems in the blue phosphor powder produced by the Leverentz’s procedure. They include (1) poor reproducibility of CL color and intensity, (2) wide distribution of phosphor particle size, (3) darkening of powder under UV-light illumination, and (4) instability of PVA phosphor slurry. All these problems originate from the loosely sealed quartz crucible and the existence of NaCl of a few weight percentages of a-ZnS in the blend mixture. In the 1940s, a quartz crucible (500 mL) was only available as a high-grade crucible for ZnS phosphor production, which has gaps between the crucible top and the cover plate. Leverentz established his production technique with use of such a loosely sealed quartz crucible. Since the melting point Tm (=801 ◦ C) of NaCl is lower than 970 ◦ C, he considered that melted NaCl has the flux action to growth of ZnS particles [4]. This hypothesis requires a large amount of melted NaCl in the heated crucible. Our recent study [5], however, clearly indicated that vaporized ZnCl2 gas is really acting as the
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fluxing agent for ZnS particle growth, in accord with a very earlier study by Houben [6]. ZnCl2 is one of the by-products of chemical reactions proceeding in the heating mixture, and NaCl is not essential for generation of ZnCl2 . The crucial roles of the by-products have been overlooked by ZnS phosphor producers for 50 years. In this paper, we have studied the improvement of the production procedure of blue ZnS:Ag:Cl phosphor powder by using two different kinds of crucibles; quartz and alumina. It should be noted that a quartz crucible loosely seals in the gases produced by chemical reactions taking place in itself, because of the existence of narrow gaps between the crucible top and the cover plate. On the other hand, an alumina crucible tightly seals in the gases, because it has the well-polished top and cover plate. 2. Experiment 2.1. Preparation of samples In the present experiments, we investigated three kinds of blend mixtures, the raw materials of which are listed in Tables 1–3. The raw materials were blended in a V-shaped mill for 1 h. The blended mixture was charged into either quartz or alumina crucible (electronics grade) of 2 L. The heat program used was different between the recipes of Table 1 and of Tables 2 and 3. The difference will be described in Section 3. The heated product in the crucible was transferred to a plastic container of 5 L, which contained DI water. The suspenTable 1 Blend mixture with NaCl for ZnS phosphor production Materials
Weight (g)
Percentage (wt.%)
Mole ratio
a-ZnS (containing 10−4 Ag) NaCl S
3400 53 70
100 1.6 2.1
1.0 2.6 × 10−2 0.07
Table 2 Blend mixture with AlCl3 for ZnS phosphor production Materials
Weight (g)
Percentage (wt.%)
Mole ratio
a-ZnS (containing 10−4 Ag) AlCl3 ·6H2 O S
3400 223 34
100 6.5 1.0
1.0 2.6 × 10−2 0.03
Table 3 Blend mixture for improved ZnS phosphor production Materials
Weight (g)
Percentage (wt.%)
Mole ratio
a-ZnS (containing 10−4 Ag) MgCl3 ·6H2 O S Water
2000 2.5 64 20
100 0.1 3 1.0
1.0 500 ppm 0.1 0.05
sion of the phosphor powder was poured on a 140 mesh sieve (open aperture 126 m). The ZnS particles, passed through the sieve, were soaked in DI water for one overnight. The powder was rinsed by DI water, and then experienced a wet ball-mill using small glass balls (diameter 3 mm) for 2 h, in which the weight ratio of phosphor powder, glass ball, and DI water was 1:1:0.8. After rinsing the ball-milled powder several times, it was dried at 110 ◦ C for one overnight. The resulting ZnS:Ag:Cl phosphor powders were examined systematically. 2.2. Measurements For measurements of CL properties, the phosphor powder put on a small plate was mounted on a handmade demountable CRT. The electron beam of 15 kV, 1 A cm−2 was used for the irradiation. The CL color and intensity were measured by a colorimeter (Minolta CM-3700d). The shape of particles was observed with an optical microscope (×100–500) or a scanning electron microscope (SEM), and their distribution was determined by using a particle sizer (Coulter Multisizer-II). The materials were identified by X-ray diffraction (Rigaku Denki, RAD-IIA).
3. Results and discussion 3.1. Troublesome by-products in ZnS phosphor powder 3.1.1. Formation of Na2 S4 layer: coloration At first, we studied the Leverentz’s recipe of Table 1 as reference. The blend mixture was softly charged (∼0.5 g cm−3 ) in a quartz crucible. The crucible was directly put in and taken out from a furnace heated above 800 ◦ C. These treatments are the same as employed by Leverentz [4]. The heated product was found to have white body color. If the same crucible was slowly heated from room temperature (RT) to 970 ◦ C, the volume of the heated product markedly shrank (about 60% of the original). The surface of the shrunken product was white, but the inside was dark yellow. The colored volume was decreased with the holding time at 970 ◦ C. Similar result was also obtained for the densely charged blend mixture (1.1 g cm−3 ), if a quartz crucible was used. As the next step, the same blend mixture was softly charged in an alumina crucible instead of a quartz crucible. The alumina crucible was heated from RT to 970 ◦ C and then slowly cooled down to RT in the furnace. The obtained product totally exhibited dark yellow. The colored phosphor powder results in low CL intensity. For obtaining brighter ZnS:Ag:Cl phosphor, the powder must be white. It should be noted that white powder is only obtainable by putting directly a quartz crucible in the furnace heated above 800 ◦ C, as far as the recipe of Table 1 is used as raw material. We must identify the colored materials for finding appropriate heat condition of the blend mixture. To our knowl-
L. Ozawa et al. / Materials Chemistry and Physics 93 (2005) 481–486
edge, there is no report on the coloration of ZnS:Ag:Cl phosphor powders. When the grown particles were well soaked in heated water, their color was white. These particles were confirmed to be cubic ZnS crystals by X-ray diffraction analysis. No sodium was detected in ZnS particles by a wet chemical analysis (atomic absorption). It is inferred that the ZnS particle itself is colorless, but its surface is covered with colored layer of by-product. In the crucible, possible chemical reaction of the heated blend mixture of Table 1 is ZnS + 2NaCl + 3S → ZnCl2 + Na2 S4
(1)
The by-products of this reaction are ZnCl2 and Na2 S4 . ZnCl2 is colorless and has Tm = 283 ◦ C and the boiling temperature Tb = 732 ◦ C [7]. Na2 S4 (cubic) has body color of dark yellow, and does not evaporate even at 1300 ◦ C [7]. In the crucible at 970 ◦ C, the melted material is only Na2 S4 with Tm = 275 ◦ C. It is thus supposed that the surface of grown ZnS particles is covered with the layer of melted Na2 S4 . In the cooling process of an alumina crucible from 970 ◦ C, ZnCl2 vapor condenses at 732 ◦ C, forming the mixture of melted Na2 S4 and ZnCl2 . The melts are solidified below Tm (Na2 S4 ) = 275 ◦ C. The solidified thick layer of mixed Na2 S4 and ZnCl2 binds grown ZnS particles together. The solidified layer has the color of Na2 S4 ; i.e., dark yellow. The above idea is supported by the following experiment. As mentioned before, NaCl is not necessary to generate the fluxing agent ZnCl2 [5]. We, therefore, prepared the blend mixture of a-ZnS, AlCl3 ·6H2 O, and S, as shown in Table 2, where NaCl is replaced by AlCl3 . The mole ratio of AlCl3 ·6H2 O is the same as that of NaCl in Table 1. In this case, the by-products are ZnCl2 and Al2 S3 (Tm = 1100 ◦ C, light yellow, hexagonal) [7], and no melted material exists at 970 ◦ C. Only ZnCl2 is in the melted phase below 732 ◦ C. The blend mixture was charged in an alumina crucible, and then the crucible was heated from RT to 970 ◦ C at a slow heating rate (5 ◦ C min−1 ). The heated product exhibited white body color. The microscope observation revealed that the heated products are well-crystallized ZnS particles, with a small amount of micro-sized particles (20–300 m) of light yellow. These microparticles, collected from the powder by a pair of tweezers, were confirmed to be Al2 S3 by the Xray diffraction analysis. The by-product Al2 S3 does not contribute to the coloration, because it cannot make its melted layer on ZnS particle since Tm (Al2 S3 ) > 970 ◦ C. This result provides indirect evidence that the coloration of the heated product of Table 1 originates from the layer of solidified mixture of Na2 S4 and ZnCl2 . 3.1.2. Decomposition of Na2 S4 to Na2 S: bleaching The bleaching of the heated product is achieved by decomposition of Na2 S4 to Na2 S that is a colorless material [7]. Although there is no available data in the literature, it is supposed that Na2 S4 at high temperatures is stable under a certain pressure of sulfur vapor, but it becomes unstable when the sulfur vapor is lacking. If the crucible cannot hold an ap-
483
propriate sulfur pressure in its space, Na2 S4 in the heating mixture (actually voids) releases S, forming colorless Na2 S. The Na2 S is stable in the heated crucible. The alumina crucible can hold the sulfur vapor pressure during the heating process because of tight sealing between the top and the cover plate. Therefore, Na2 S4 does not decompose to Na2 S in this case, thus resulting in dark-yellowed powder. On the other hand, S vapor in the quartz crucible gets away into open space of the furnace through the gap. The decomposition of Na2 S4 to Na2 S, which supplies S vapor in the crucible space, continues until the decomposition is completed. For this reason, the quartz crucibles are employed for the growth of white ZnS phosphor powder. Although commercial particles are covered with the layer of Na2 S, they achieve high CL intensity. 3.2. Clarification of remaining problems As mentioned in Section 3.1.2, the achievement of high CL intensity is a major reason that quartz crucibles are used to produce ZnS phosphor powder from the Leverentz’s recipe. However, the bleaching of phosphor products is sensitively influenced by (a) the shape and material of crucible, (b) the charged density of blend mixture, and (c) the heating condition. As far as we employ the recipe of Table 1, technical control of CL intensity causes a lot of problems in the optimization of CL color, particle size distribution, photodarkening of powder, and stability of PVA phosphor slurry. These problems are clarified below. 3.2.1. Variation of particle size distribution From a viewpoint of practical application, one of the serious problems concerns the size distribution of ZnS:Ag:Cl phosphor particles. Fig. 1(A) shows a typical example of SEM image (×1000) of commercial ZnS:Ag:Cl particles produced from the recipe of Table 1. At a glance, one can recognize that the particles indeed distribute in a wide range of size. Since ZnS particles grow through the flux action of ZnCl2 vapor in closed voids (pores) in the heating blend mixture, the size of grown particles corresponds to that of closed voids [5]. The voids are formed from air babbles in the charged blend mixture. When the blend mixture is softly charged, the distribution of void size is in a wide range. Another factor to be considered is the amount of ZnCl2 vapor in the voids. This amount is controlled by ZnCl2 vapor pressure in the crucible space. The alumina crucible holds a given pressure of ZnCl2 vapor, which is determined by the weight of the crucible cover. In the quartz crucible, ZnCl2 vapor gets away into the furnace through the gap, so that the ZnCl2 pressure cannot be controlled by the weight of the cover. Individual void must be sealed for holding ZnCl2 vapor, as far as quartz crucibles are employed. The ZnF2 with Tm = 872 ◦ C and Tb = 1500 ◦ C [7] may be useful as a sealant of the voids. However, the heated product is hardly sintered by fluorides, and the CL of the resultant ZnS powder exhibits a long decay time. These facts are not acceptable.
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Fig. 2. Heat program for improved production of ZnS:Ag:Cl phosphor powder.
Fig. 1. SEM micrographs of (A) ZnS blue phosphor produced from the recipe of Table 1 (×1000) and (B) that from the recipe of Table 2 (×5000).
Only melted Na2 S4 is the useful sealant in the heating crucible above 275 ◦ C; i.e., a thick layer of Na2 S4 reinforces the fragile seal by the melt. This is the reason why a large amount of NaCl (a few weight percentages) is added into the blend mixture (Table 1), which is not for generation of ZnCl2 . The raw material for Na2 S4 sealant is not limited to NaCl. NaOH may be useful as the raw material for generation of Na2 S4 . When the crucible is directly put in the heated furnace, the thickness of Na2 S4 sealant varies with the blending time of mixture and the size of S particles, because the sulfur gas evaporates from S particles (1–100 m) in the blend mixture without spreading of melted S. The blending condition of the mixture and S particles is thus very important for the growth of ZnS phosphor particles. 3.2.2. Control of particle size distribution by heat program As described in Section 3.1, white ZnS powders are also obtainable without Na2 S4 sealant. We studied the heat program for the recipe of Table 2, in which NaCl is not included, in order to realize a narrow distribution of the void size.
The blend mixture was softly charged (0.5 g cm−3 ) in an alumina crucible. The crucible was heated from RT to 400 ◦ C (
L. Ozawa et al. / Materials Chemistry and Physics 93 (2005) 481–486
485
Fig. 4. SEM micrograph of as-grown particles from the recipe of Table 3 (×5000).
Fig. 3. Particle size distribution plotted on log-normal graphic paper. The curve (A) corresponds to the result obtained for particles grown from densely charged blend mixture with melted sulfur and the curve (B) to that for particles grown by addition of water.
3400 g powder in Table 2, was put in a 500 mL beaker. After adding 20 g of DI water to this powder, the wet powder was well mixed by a glass bar. The wet powder was then transferred to the original mixture. The mixture was blended for 30 min. The blend mixture was softly charged in an alumina crucible, as in the previous experiment. The crucible was heated with the same program as Fig. 2. The particle size of obtained powders is depicted by a curve B in Fig. 3. From Fig. 3 it appears that the particle size is increased by addition of water to the blend mixture. The average size (≈4 m) is comparable with that of commercial ones. The void expansion can be controlled by the amount of added water and the heat program in the region between 100 and 400 ◦ C. 3.2.4. Amount of added chloride in blend mixture In the closed capsule, 500 ppm ZnCl2 per 1 mol ZnS is enough to produce ZnS particles [5]. The alumina crucible realizes a tight sealing owing to the polished top and cover. If we use the alumina crucible, the concentration (mole ratio) of added halide may be reduced from 2 × 10−2 to 5 × 10−4 . The recipe of Table 2 has a practical problem that the by-product Al2 S3 changes to Al2 O3 in water, and Al2 O3 dissolve in neither water nor acid solution. As an alternative to AlCl3 , MgCl2 was chosen. In this case, the by-product MgS (Tm > 2000 ◦ C, red-brown, cubic) changes to Mg(OH)2
in water, which is soluble in dilute acid solution. Since MgCl2 is a deliquescent material, we used MgCl2 ·6H2 O in the experiment. Table 3 gives the recipe of the blend. The blend mixture was softly charged in an alumina crucible. The crucible was heated according to the heat program in Fig. 2. Fig. 4 shows a SEM picture (×5000) of as-grown ZnS particles. It is recognized that ZnS particles are well formed with only 500 ppm MgCl3 ·6H2 O. A remarkable advantage of the recipe of Table 3 is that the particles in heated products are weakly clumped (not sintered) by an extremely small amount of solidified ZnCl2 (∼500 ppm). In the cooling process, ZnCl2 vapor preferentially condenses at the contact gaps between grown ZnS particles through the capillary action. The particles are thus bound together with weak cohesive force of the condensed ZnCl2 . Small particles, possibly MgS, adhered on ZnS particles in Fig. 4 are not involved in the clumping. The clumped particles were smoothly separated to primary particles by wet ball-mill. The separated particles were etched by dilute HCl (1% solution), and then rinsed by DI water several times to remove ZnCl2 completely. The powder was dried in a heated oven at 110 ◦ C. The size of obtained particles was found to distribute similarly to the curve B in Fig. 3. 3.2.5. Intensity and color of CL A significance of the heat program shown in Fig. 2 is that the reproducibility of CL intensity and color is sufficiently high. With respect to the recipe of Table 3, the results are not influenced by the blending time of raw materials (in the range between 10 and 60 min). The pressurized ZnCl2 vapor in the crucible smoothly penetrates into the voids that are not sealed by melted material. Furthermore, a large amount of S vapor in the voids converts oxygen to SO2 gas, which is harmless to ZnS:Ag:Cl phosphor, as mentioned before. This allows ZnS particles to be produced in the voids safely. The obtained ZnS:Ag:Cl phosphor powder has the CL intensity
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L. Ozawa et al. / Materials Chemistry and Physics 93 (2005) 481–486
with 113% of our internal standard and the CL color coordinates with x = 0.155 ± 0.02 and y = 0.053 ± 0.02 on the chromaticity diagram. 3.2.6. Photodarkening The photodarkening of commercial ZnS phosphor powder produced from the recipe with NaCl (Table 1) is a serious problem, especially in their production and screening. The commercial powder is immediately darkened under the exposure to sunlight or fluorescent lamp. In usual, lamps and windows in production and screening rooms must be covered with yellow films to prevent the darkening of ZnS phosphor powder. To investigate the cause of the photodarkening, we soaked commercial powder in heated (80 ◦ C) and agitated water for 3 days, and then rinsed by DI water five times. The resulting ZnS powder did not exhibit the photodarkening effect. This fact suggests that the darkening of ZnS powders under UV-light irradiation is caused by the presence of Na2 S on ZnS particles, although the detail of photochemical reactions responsible for the darkening is not clear at present. It should be stressed that the ZnS:Ag:Cl powder from the recipe of Tables 2 and 3 is not darkened under prolonged exposure to UV light, even under wet condition. This is probably due to the absence of Na2 S layer on ZnS particle produced without using NaCl. 3.2.7. Stability of PVA phosphor slurry When the ZnS powders produced from the recipes of Tables 2 and 3 were added into PVA phosphor slurry, no changes of the pH value and the conductivity were found for the slurry. Furthermore, there was no difference in screening result between the slurries made freshly and held for 3 days. From these results, it is allowed to say that the instability of PVA phosphor slurry is solved by the use of ZnS phosphor powder produced without using NaCl. We suppose that the instability of PVA slurry containing commercial ZnS:Ag:Cl powder is due to slow dissolution of Na2 S located on ZnS particles. To check this supposition,
we soaked the commercial powder in heated water at 80 ◦ C for 2 days with agitation. The powder soaked in this way was added in PVA phosphor slurry. The slurry obtained was certainly stable, as expected. In addition, the pH value and the conductivity did not change even after 3 days.
4. Conclusion The remaining problems in the production of ZnS:Ag:Cl phosphor powder, CL intensity, CL color, particle size distribution, photodarkening, and instability of PVA phosphor slurry, have been solved by application of (1) blend mixture without Na-compounds, (2) polished alumina crucible, (3) addition of water to blend mixture, and (4) appropriate heat program. The grown ZnS particles in heated crucible are bound together, not sintered, with preferentially condensed 500 ppm ZnCl2 in the small gaps between particles. The clumped particles are smoothly separated into the primary particles by wet ball-mill. The production procedure of ZnS:Ag:Cl phosphor powder developed in this paper is simple, but has high reproducibility of the results. The present work must be useful and important for further improvement of color displays such as FED and CRT.
References [1] L. Ozawa, Application of Cathodoluminescence to Display Devices, Kodansha, Tokyo, 1990. [2] S. Shionoya, W.M. Yen (Eds.), Phosphor Handbook, CRC Press, Boca Raton, FL, 1998. [3] L. Ozawa, M. Itoh, Chem. Rev. 103 (2003) 3835. [4] H.W. Leverentz, An Introduction to Luminescence of Solids, Wiley, New York, 1950. [5] L. Ozawa, M. Koike, M. Itoh, Mater. Chem. Phys., doi:10.1016/j.matchemphys.2005.03.041. [6] J. Houben, Mettalurgie 9 (1912) 592. [7] D.R. Lide (Ed.), CRC Handbook of Chemistry and Physics, B-146, 74th ed., CRC Press, Boca Raton, FL, 1993.
Improved production of ZnS blue phosphor powder Lyuji Ozawa a , Mika Makimura b , Minoru Itoh c,∗ b
a L. L. Technology, 8 Old Grange Rd, Hopewell JCT, NY 12533, USA Industrial Research Institute of Nagano Prefecture, 1-18-1 Wakasato, Nagano 380-0928, Japan c Department of Electrical and Electronic Engineering, Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan
Received 12 November 2004; received in revised form 8 March 2005; accepted 28 March 2005
Abstract The production procedure of ZnS:Ag:Cl blue phosphor powder is still complicated, although it is widely used for color displays. The complication mainly comes from handling of the by-product Na2 S4 , which covers the surface of grown ZnS particles in a crucible. The formation of Na2 S4 introduces serious problems in many factors such as (i) CL intensity and color, (ii) particle size distribution, (iii) sintering of heated products, (iv) photodarkening of powders, and (v) stability of PVA phosphor slurry. These problems are clarified in this paper. It is shown that the blue phosphor powder can be produced by a more simplified procedure than before, with high reproducibility. The ZnS:Ag:Cl phosphor powder produced by the present procedure is well suitable for blue primary in color screens of FED and CRT. © 2005 Elsevier B.V. All rights reserved. Keywords: Optical materials; Crystal growth; Electron microscopy; Microstructure
1. Introduction ZnS:Ag:Cl phosphor powder is used as blue primary in color screens of cathode ray tube (CRT) and field emission display (FED) [1–3]. The intensity and color of cathodoluminescence (CL), the separation of sintered products to primary ZnS particles, the particle size distribution, and the stability of poly(vinyl alcohol) (PVA) phosphor slurry are essentially important for ZnS phosphor production. Blue ZnS phosphor powder of average size 4 m is commonly produced by the procedure established by Leverentz more than 50 years ago [4]. According to his procedure, the blend mixture consisting of powdered amorphous (a-)ZnS, a few weight percentages of NaCl, and sulfur is softly charged (∼0.5 g cm−3 ) in a quartz crucible. The crucible is directly put in a furnace heated at around 970 ◦ C, and after a while, taken out from it rapidly. The heated products in the crucible are highly sintered. After soaking the sintered products in water, they are rinsed by deionized (DI) water several times. Small and large particles
∗
DOI of original article:10.1016/j.matchemphys.2005.03.041. Corresponding author. Tel.: +81 26 269 5261; fax: +81 26 269 5220. E-mail address: [email protected] (M. Itoh).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.03.033
are separated from the rinsed powder with the help of the difference of settling time in water, according to the Stokes’ law; that is a water sieve. The sieved powder is dried in a heated oven at 110 ◦ C for practical use. However, there remain some serious problems in the blue phosphor powder produced by the Leverentz’s procedure. They include (1) poor reproducibility of CL color and intensity, (2) wide distribution of phosphor particle size, (3) darkening of powder under UV-light illumination, and (4) instability of PVA phosphor slurry. All these problems originate from the loosely sealed quartz crucible and the existence of NaCl of a few weight percentages of a-ZnS in the blend mixture. In the 1940s, a quartz crucible (500 mL) was only available as a high-grade crucible for ZnS phosphor production, which has gaps between the crucible top and the cover plate. Leverentz established his production technique with use of such a loosely sealed quartz crucible. Since the melting point Tm (=801 ◦ C) of NaCl is lower than 970 ◦ C, he considered that melted NaCl has the flux action to growth of ZnS particles [4]. This hypothesis requires a large amount of melted NaCl in the heated crucible. Our recent study [5], however, clearly indicated that vaporized ZnCl2 gas is really acting as the
482
L. Ozawa et al. / Materials Chemistry and Physics 93 (2005) 481–486
fluxing agent for ZnS particle growth, in accord with a very earlier study by Houben [6]. ZnCl2 is one of the by-products of chemical reactions proceeding in the heating mixture, and NaCl is not essential for generation of ZnCl2 . The crucial roles of the by-products have been overlooked by ZnS phosphor producers for 50 years. In this paper, we have studied the improvement of the production procedure of blue ZnS:Ag:Cl phosphor powder by using two different kinds of crucibles; quartz and alumina. It should be noted that a quartz crucible loosely seals in the gases produced by chemical reactions taking place in itself, because of the existence of narrow gaps between the crucible top and the cover plate. On the other hand, an alumina crucible tightly seals in the gases, because it has the well-polished top and cover plate. 2. Experiment 2.1. Preparation of samples In the present experiments, we investigated three kinds of blend mixtures, the raw materials of which are listed in Tables 1–3. The raw materials were blended in a V-shaped mill for 1 h. The blended mixture was charged into either quartz or alumina crucible (electronics grade) of 2 L. The heat program used was different between the recipes of Table 1 and of Tables 2 and 3. The difference will be described in Section 3. The heated product in the crucible was transferred to a plastic container of 5 L, which contained DI water. The suspenTable 1 Blend mixture with NaCl for ZnS phosphor production Materials
Weight (g)
Percentage (wt.%)
Mole ratio
a-ZnS (containing 10−4 Ag) NaCl S
3400 53 70
100 1.6 2.1
1.0 2.6 × 10−2 0.07
Table 2 Blend mixture with AlCl3 for ZnS phosphor production Materials
Weight (g)
Percentage (wt.%)
Mole ratio
a-ZnS (containing 10−4 Ag) AlCl3 ·6H2 O S
3400 223 34
100 6.5 1.0
1.0 2.6 × 10−2 0.03
Table 3 Blend mixture for improved ZnS phosphor production Materials
Weight (g)
Percentage (wt.%)
Mole ratio
a-ZnS (containing 10−4 Ag) MgCl3 ·6H2 O S Water
2000 2.5 64 20
100 0.1 3 1.0
1.0 500 ppm 0.1 0.05
sion of the phosphor powder was poured on a 140 mesh sieve (open aperture 126 m). The ZnS particles, passed through the sieve, were soaked in DI water for one overnight. The powder was rinsed by DI water, and then experienced a wet ball-mill using small glass balls (diameter 3 mm) for 2 h, in which the weight ratio of phosphor powder, glass ball, and DI water was 1:1:0.8. After rinsing the ball-milled powder several times, it was dried at 110 ◦ C for one overnight. The resulting ZnS:Ag:Cl phosphor powders were examined systematically. 2.2. Measurements For measurements of CL properties, the phosphor powder put on a small plate was mounted on a handmade demountable CRT. The electron beam of 15 kV, 1 A cm−2 was used for the irradiation. The CL color and intensity were measured by a colorimeter (Minolta CM-3700d). The shape of particles was observed with an optical microscope (×100–500) or a scanning electron microscope (SEM), and their distribution was determined by using a particle sizer (Coulter Multisizer-II). The materials were identified by X-ray diffraction (Rigaku Denki, RAD-IIA).
3. Results and discussion 3.1. Troublesome by-products in ZnS phosphor powder 3.1.1. Formation of Na2 S4 layer: coloration At first, we studied the Leverentz’s recipe of Table 1 as reference. The blend mixture was softly charged (∼0.5 g cm−3 ) in a quartz crucible. The crucible was directly put in and taken out from a furnace heated above 800 ◦ C. These treatments are the same as employed by Leverentz [4]. The heated product was found to have white body color. If the same crucible was slowly heated from room temperature (RT) to 970 ◦ C, the volume of the heated product markedly shrank (about 60% of the original). The surface of the shrunken product was white, but the inside was dark yellow. The colored volume was decreased with the holding time at 970 ◦ C. Similar result was also obtained for the densely charged blend mixture (1.1 g cm−3 ), if a quartz crucible was used. As the next step, the same blend mixture was softly charged in an alumina crucible instead of a quartz crucible. The alumina crucible was heated from RT to 970 ◦ C and then slowly cooled down to RT in the furnace. The obtained product totally exhibited dark yellow. The colored phosphor powder results in low CL intensity. For obtaining brighter ZnS:Ag:Cl phosphor, the powder must be white. It should be noted that white powder is only obtainable by putting directly a quartz crucible in the furnace heated above 800 ◦ C, as far as the recipe of Table 1 is used as raw material. We must identify the colored materials for finding appropriate heat condition of the blend mixture. To our knowl-
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edge, there is no report on the coloration of ZnS:Ag:Cl phosphor powders. When the grown particles were well soaked in heated water, their color was white. These particles were confirmed to be cubic ZnS crystals by X-ray diffraction analysis. No sodium was detected in ZnS particles by a wet chemical analysis (atomic absorption). It is inferred that the ZnS particle itself is colorless, but its surface is covered with colored layer of by-product. In the crucible, possible chemical reaction of the heated blend mixture of Table 1 is ZnS + 2NaCl + 3S → ZnCl2 + Na2 S4
(1)
The by-products of this reaction are ZnCl2 and Na2 S4 . ZnCl2 is colorless and has Tm = 283 ◦ C and the boiling temperature Tb = 732 ◦ C [7]. Na2 S4 (cubic) has body color of dark yellow, and does not evaporate even at 1300 ◦ C [7]. In the crucible at 970 ◦ C, the melted material is only Na2 S4 with Tm = 275 ◦ C. It is thus supposed that the surface of grown ZnS particles is covered with the layer of melted Na2 S4 . In the cooling process of an alumina crucible from 970 ◦ C, ZnCl2 vapor condenses at 732 ◦ C, forming the mixture of melted Na2 S4 and ZnCl2 . The melts are solidified below Tm (Na2 S4 ) = 275 ◦ C. The solidified thick layer of mixed Na2 S4 and ZnCl2 binds grown ZnS particles together. The solidified layer has the color of Na2 S4 ; i.e., dark yellow. The above idea is supported by the following experiment. As mentioned before, NaCl is not necessary to generate the fluxing agent ZnCl2 [5]. We, therefore, prepared the blend mixture of a-ZnS, AlCl3 ·6H2 O, and S, as shown in Table 2, where NaCl is replaced by AlCl3 . The mole ratio of AlCl3 ·6H2 O is the same as that of NaCl in Table 1. In this case, the by-products are ZnCl2 and Al2 S3 (Tm = 1100 ◦ C, light yellow, hexagonal) [7], and no melted material exists at 970 ◦ C. Only ZnCl2 is in the melted phase below 732 ◦ C. The blend mixture was charged in an alumina crucible, and then the crucible was heated from RT to 970 ◦ C at a slow heating rate (5 ◦ C min−1 ). The heated product exhibited white body color. The microscope observation revealed that the heated products are well-crystallized ZnS particles, with a small amount of micro-sized particles (20–300 m) of light yellow. These microparticles, collected from the powder by a pair of tweezers, were confirmed to be Al2 S3 by the Xray diffraction analysis. The by-product Al2 S3 does not contribute to the coloration, because it cannot make its melted layer on ZnS particle since Tm (Al2 S3 ) > 970 ◦ C. This result provides indirect evidence that the coloration of the heated product of Table 1 originates from the layer of solidified mixture of Na2 S4 and ZnCl2 . 3.1.2. Decomposition of Na2 S4 to Na2 S: bleaching The bleaching of the heated product is achieved by decomposition of Na2 S4 to Na2 S that is a colorless material [7]. Although there is no available data in the literature, it is supposed that Na2 S4 at high temperatures is stable under a certain pressure of sulfur vapor, but it becomes unstable when the sulfur vapor is lacking. If the crucible cannot hold an ap-
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propriate sulfur pressure in its space, Na2 S4 in the heating mixture (actually voids) releases S, forming colorless Na2 S. The Na2 S is stable in the heated crucible. The alumina crucible can hold the sulfur vapor pressure during the heating process because of tight sealing between the top and the cover plate. Therefore, Na2 S4 does not decompose to Na2 S in this case, thus resulting in dark-yellowed powder. On the other hand, S vapor in the quartz crucible gets away into open space of the furnace through the gap. The decomposition of Na2 S4 to Na2 S, which supplies S vapor in the crucible space, continues until the decomposition is completed. For this reason, the quartz crucibles are employed for the growth of white ZnS phosphor powder. Although commercial particles are covered with the layer of Na2 S, they achieve high CL intensity. 3.2. Clarification of remaining problems As mentioned in Section 3.1.2, the achievement of high CL intensity is a major reason that quartz crucibles are used to produce ZnS phosphor powder from the Leverentz’s recipe. However, the bleaching of phosphor products is sensitively influenced by (a) the shape and material of crucible, (b) the charged density of blend mixture, and (c) the heating condition. As far as we employ the recipe of Table 1, technical control of CL intensity causes a lot of problems in the optimization of CL color, particle size distribution, photodarkening of powder, and stability of PVA phosphor slurry. These problems are clarified below. 3.2.1. Variation of particle size distribution From a viewpoint of practical application, one of the serious problems concerns the size distribution of ZnS:Ag:Cl phosphor particles. Fig. 1(A) shows a typical example of SEM image (×1000) of commercial ZnS:Ag:Cl particles produced from the recipe of Table 1. At a glance, one can recognize that the particles indeed distribute in a wide range of size. Since ZnS particles grow through the flux action of ZnCl2 vapor in closed voids (pores) in the heating blend mixture, the size of grown particles corresponds to that of closed voids [5]. The voids are formed from air babbles in the charged blend mixture. When the blend mixture is softly charged, the distribution of void size is in a wide range. Another factor to be considered is the amount of ZnCl2 vapor in the voids. This amount is controlled by ZnCl2 vapor pressure in the crucible space. The alumina crucible holds a given pressure of ZnCl2 vapor, which is determined by the weight of the crucible cover. In the quartz crucible, ZnCl2 vapor gets away into the furnace through the gap, so that the ZnCl2 pressure cannot be controlled by the weight of the cover. Individual void must be sealed for holding ZnCl2 vapor, as far as quartz crucibles are employed. The ZnF2 with Tm = 872 ◦ C and Tb = 1500 ◦ C [7] may be useful as a sealant of the voids. However, the heated product is hardly sintered by fluorides, and the CL of the resultant ZnS powder exhibits a long decay time. These facts are not acceptable.
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Fig. 2. Heat program for improved production of ZnS:Ag:Cl phosphor powder.
Fig. 1. SEM micrographs of (A) ZnS blue phosphor produced from the recipe of Table 1 (×1000) and (B) that from the recipe of Table 2 (×5000).
Only melted Na2 S4 is the useful sealant in the heating crucible above 275 ◦ C; i.e., a thick layer of Na2 S4 reinforces the fragile seal by the melt. This is the reason why a large amount of NaCl (a few weight percentages) is added into the blend mixture (Table 1), which is not for generation of ZnCl2 . The raw material for Na2 S4 sealant is not limited to NaCl. NaOH may be useful as the raw material for generation of Na2 S4 . When the crucible is directly put in the heated furnace, the thickness of Na2 S4 sealant varies with the blending time of mixture and the size of S particles, because the sulfur gas evaporates from S particles (1–100 m) in the blend mixture without spreading of melted S. The blending condition of the mixture and S particles is thus very important for the growth of ZnS phosphor particles. 3.2.2. Control of particle size distribution by heat program As described in Section 3.1, white ZnS powders are also obtainable without Na2 S4 sealant. We studied the heat program for the recipe of Table 2, in which NaCl is not included, in order to realize a narrow distribution of the void size.
The blend mixture was softly charged (0.5 g cm−3 ) in an alumina crucible. The crucible was heated from RT to 400 ◦ C (
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Fig. 4. SEM micrograph of as-grown particles from the recipe of Table 3 (×5000).
Fig. 3. Particle size distribution plotted on log-normal graphic paper. The curve (A) corresponds to the result obtained for particles grown from densely charged blend mixture with melted sulfur and the curve (B) to that for particles grown by addition of water.
3400 g powder in Table 2, was put in a 500 mL beaker. After adding 20 g of DI water to this powder, the wet powder was well mixed by a glass bar. The wet powder was then transferred to the original mixture. The mixture was blended for 30 min. The blend mixture was softly charged in an alumina crucible, as in the previous experiment. The crucible was heated with the same program as Fig. 2. The particle size of obtained powders is depicted by a curve B in Fig. 3. From Fig. 3 it appears that the particle size is increased by addition of water to the blend mixture. The average size (≈4 m) is comparable with that of commercial ones. The void expansion can be controlled by the amount of added water and the heat program in the region between 100 and 400 ◦ C. 3.2.4. Amount of added chloride in blend mixture In the closed capsule, 500 ppm ZnCl2 per 1 mol ZnS is enough to produce ZnS particles [5]. The alumina crucible realizes a tight sealing owing to the polished top and cover. If we use the alumina crucible, the concentration (mole ratio) of added halide may be reduced from 2 × 10−2 to 5 × 10−4 . The recipe of Table 2 has a practical problem that the by-product Al2 S3 changes to Al2 O3 in water, and Al2 O3 dissolve in neither water nor acid solution. As an alternative to AlCl3 , MgCl2 was chosen. In this case, the by-product MgS (Tm > 2000 ◦ C, red-brown, cubic) changes to Mg(OH)2
in water, which is soluble in dilute acid solution. Since MgCl2 is a deliquescent material, we used MgCl2 ·6H2 O in the experiment. Table 3 gives the recipe of the blend. The blend mixture was softly charged in an alumina crucible. The crucible was heated according to the heat program in Fig. 2. Fig. 4 shows a SEM picture (×5000) of as-grown ZnS particles. It is recognized that ZnS particles are well formed with only 500 ppm MgCl3 ·6H2 O. A remarkable advantage of the recipe of Table 3 is that the particles in heated products are weakly clumped (not sintered) by an extremely small amount of solidified ZnCl2 (∼500 ppm). In the cooling process, ZnCl2 vapor preferentially condenses at the contact gaps between grown ZnS particles through the capillary action. The particles are thus bound together with weak cohesive force of the condensed ZnCl2 . Small particles, possibly MgS, adhered on ZnS particles in Fig. 4 are not involved in the clumping. The clumped particles were smoothly separated to primary particles by wet ball-mill. The separated particles were etched by dilute HCl (1% solution), and then rinsed by DI water several times to remove ZnCl2 completely. The powder was dried in a heated oven at 110 ◦ C. The size of obtained particles was found to distribute similarly to the curve B in Fig. 3. 3.2.5. Intensity and color of CL A significance of the heat program shown in Fig. 2 is that the reproducibility of CL intensity and color is sufficiently high. With respect to the recipe of Table 3, the results are not influenced by the blending time of raw materials (in the range between 10 and 60 min). The pressurized ZnCl2 vapor in the crucible smoothly penetrates into the voids that are not sealed by melted material. Furthermore, a large amount of S vapor in the voids converts oxygen to SO2 gas, which is harmless to ZnS:Ag:Cl phosphor, as mentioned before. This allows ZnS particles to be produced in the voids safely. The obtained ZnS:Ag:Cl phosphor powder has the CL intensity
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with 113% of our internal standard and the CL color coordinates with x = 0.155 ± 0.02 and y = 0.053 ± 0.02 on the chromaticity diagram. 3.2.6. Photodarkening The photodarkening of commercial ZnS phosphor powder produced from the recipe with NaCl (Table 1) is a serious problem, especially in their production and screening. The commercial powder is immediately darkened under the exposure to sunlight or fluorescent lamp. In usual, lamps and windows in production and screening rooms must be covered with yellow films to prevent the darkening of ZnS phosphor powder. To investigate the cause of the photodarkening, we soaked commercial powder in heated (80 ◦ C) and agitated water for 3 days, and then rinsed by DI water five times. The resulting ZnS powder did not exhibit the photodarkening effect. This fact suggests that the darkening of ZnS powders under UV-light irradiation is caused by the presence of Na2 S on ZnS particles, although the detail of photochemical reactions responsible for the darkening is not clear at present. It should be stressed that the ZnS:Ag:Cl powder from the recipe of Tables 2 and 3 is not darkened under prolonged exposure to UV light, even under wet condition. This is probably due to the absence of Na2 S layer on ZnS particle produced without using NaCl. 3.2.7. Stability of PVA phosphor slurry When the ZnS powders produced from the recipes of Tables 2 and 3 were added into PVA phosphor slurry, no changes of the pH value and the conductivity were found for the slurry. Furthermore, there was no difference in screening result between the slurries made freshly and held for 3 days. From these results, it is allowed to say that the instability of PVA phosphor slurry is solved by the use of ZnS phosphor powder produced without using NaCl. We suppose that the instability of PVA slurry containing commercial ZnS:Ag:Cl powder is due to slow dissolution of Na2 S located on ZnS particles. To check this supposition,
we soaked the commercial powder in heated water at 80 ◦ C for 2 days with agitation. The powder soaked in this way was added in PVA phosphor slurry. The slurry obtained was certainly stable, as expected. In addition, the pH value and the conductivity did not change even after 3 days.
4. Conclusion The remaining problems in the production of ZnS:Ag:Cl phosphor powder, CL intensity, CL color, particle size distribution, photodarkening, and instability of PVA phosphor slurry, have been solved by application of (1) blend mixture without Na-compounds, (2) polished alumina crucible, (3) addition of water to blend mixture, and (4) appropriate heat program. The grown ZnS particles in heated crucible are bound together, not sintered, with preferentially condensed 500 ppm ZnCl2 in the small gaps between particles. The clumped particles are smoothly separated into the primary particles by wet ball-mill. The production procedure of ZnS:Ag:Cl phosphor powder developed in this paper is simple, but has high reproducibility of the results. The present work must be useful and important for further improvement of color displays such as FED and CRT.
References [1] L. Ozawa, Application of Cathodoluminescence to Display Devices, Kodansha, Tokyo, 1990. [2] S. Shionoya, W.M. Yen (Eds.), Phosphor Handbook, CRC Press, Boca Raton, FL, 1998. [3] L. Ozawa, M. Itoh, Chem. Rev. 103 (2003) 3835. [4] H.W. Leverentz, An Introduction to Luminescence of Solids, Wiley, New York, 1950. [5] L. Ozawa, M. Koike, M. Itoh, Mater. Chem. Phys., doi:10.1016/j.matchemphys.2005.03.041. [6] J. Houben, Mettalurgie 9 (1912) 592. [7] D.R. Lide (Ed.), CRC Handbook of Chemistry and Physics, B-146, 74th ed., CRC Press, Boca Raton, FL, 1993.