- Email: [email protected]
Low-temperature synthesis of BaMgAl10O17:Eu2+ blue phosphors
Journal of Physics and Chemistry of Solids 75 (2014) 163–167
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Low-temperature synthesis of BaMgAl10O17:Eu2 þ blue phosphors Wen Zhang a, Diping He a, Gang Ma b, Shujie Cui a, Gang Li a, Huan Jiao a,n a
Key Laboratory of Macromolecular Science of Shaanxi Province, School of Chemistry & Chemical Engineering, Shaanxi Normal University, Xi'an 710062, Shaanxi Province, PR China Teaching and Research Department of Military Training, Border Defence Academy of PLA, Xi'an 710106, Shaanxi Province, PR China
b
art ic l e i nf o
a b s t r a c t
Article history: Received 3 July 2012 Received in revised form 27 August 2013 Accepted 5 September 2013 Available online 19 September 2013
Using H3BO3 as the flux, pure BaMgAl10O17:Eu2 þ (BAM) blue phosphors were successfully prepared via a solid-state reaction. By this approach, well-crystallized submicron BAM particles were obtained at temperatures as low as 1100 1C with a sintering duration of 2 h. The sintering temperature required by this approach was at least 400 1C lower than that required by the conventional solid-state approach for preparing BAM; moreover, the sintering time required by the former approach was also considerably shorter than that required by the latter approach. These factors are expected to lower the cost for largescale production of BAM phosphors. Crystal structures and luminescence properties of the synthesized samples were characterized by XRD and TG–DTA, and photoluminescence spectroscopy, respectively. The reactivity of an intermediate, BaAl2O4, is thought to be the key factor influencing the synthesis temperature for BAM. & 2013 Elsevier Ltd. All rights reserved.
Keywords: D. Luminescence
1. Introduction It is important to improve the performance and lower the cost of large-scale production of blue-emitting BaMgAl10O17:Eu2 þ (BAM:Eu2 þ ) phosphors for continuing the recent progress in plasma display panels, tricolor lamps, and LEDs and backlights for liquid crystal displays [1,2]. Conventionally, BAM:Eu2 þ phosphors are prepared by a solidstate reaction process, i.e., firing a mixture of BaCO3, Eu2O3, Mg (OH)2 4MgCO3 4H2O, and Al2O3 together with a small amount of additional flux such as AlF3 or MgF2 at about 1600 1C in a reducing atmosphere. This approach has two intrinsic disadvantages: (1) it imposes high cost and energy requirements on the production process; and (2) the high-temperature process often results in irregular-shaped particles with hard aggregation that are detrimental to phosphor efficiency [3]. Thus, many alternative preparation technique such as combustion synthesis, hydrothermal process, spray pyrolysis [5] and sol–gel route [6] have been developed to decrease the synthesis temperature. However, the use of organic species is often involved in these approaches and their removal is often harmful for the environment. In addition, a subsequent additional heat treatment (in range of 1200–1600 1C) is also required to achieve good crystallinity for the phosphor particles [4–6]. The fabrication of high-purity BAM at low temperatures by a simple process has still not been realized. The fabrication of BAM with well-crystallized particles by a low-
n
Corresponding author. Tel.: þ 86 29 81530766; fax: þ 86 29 81530727. E-mail address: [email protected] (H. Jiao).
0022-3697/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jpcs.2013.09.002
temperature solid-state reaction is still one of the most desirable industrial processes. In the present study, H3BO3 was selected as flux for the preparation of BAM because it decreases the synthesis temperature for well-crystallized BAM and has potential applications in the synthesis of similar materials.
2. Experimental section A BAM:Eu2 þ phosphor with a composition similar to those of commercial phosphors (containing 10 mol% Eu2 þ ions) was synthesized and examined in this study. The starting materials were Eu2O3 (99.99%), Al2O3 (99%), BaCO3 (99%), and Mg(OH)2 4MgCO3 6H2O (99%). H3BO3 (99%) was added as flux to improve the reaction process. The amount of added H3BO3 was 5, 6, 7, 8, and 9 wt% with respect to the total weight of the materials. In order to obtain a homogeneous mixture, distilled ethanol was added as a dispersing liquid. All raw materials were thoroughly mixed by grinding. All samples were fired in reductive atmospheres, and the calcination temperature and time were in the range of 600–1100 1C and 0.5–5 h, respectively. The crystal structures of all samples were investigated by X-ray powder diffraction using Cu Kα (λ ¼1.5406 Å) radiation on a Rigaku D/MAX 2000 X-ray diffractometer. The photoluminescence of the synthesized samples was measured using a Hitachi F4600 fluorescentometer. The thermal reactions of the raw materials were analyzed under an ambient atmosphere by simultaneous thermogravimetry (TG) and differential thermal analysis (DTA) performed using a Q1000DSC þ LNCS þ FACS Q600SDT instrument. For this
164
W. Zhang et al. / Journal of Physics and Chemistry of Solids 75 (2014) 163–167
analysis, 10 mg of a sample at a heating rate of 10 1C/min was reacted in the temperature range of 25–1200 1C.
3. Results and discussion 3.1. Powder X-ray diffraction analysis In order to understand the crystallization behavior of BAM by using the mixture of raw materials, temperature-resolved X-ray diffraction patterns of the BAM:Eu powders were measured and are represented in Fig. 1. The compositions of all raw material samples were set as BAM:Eu2þ , to which H3BO3 was added at a fixed amount of 7 wt%. Further, the sintering duration was set as 2 h. The main reaction process was deduced with increasing calcination temperatures. The XRD pattern of the powder obtained after firing the mixture of raw materials at 600 1C showed characteristics typical of an amorphous phase and crystallization, the main crystal being BaCO3, whereas aluminum and magnesium compounds were not detected by XRD. The samples maintained their amorphous character until a sintering temperature of 800 1C, even though a phase transformation from BaCO3 to an intermediate phase (BaAl2O4) occurred. When the temperature increased to 900 1C, only patterns corresponding to BaAl2O4 could be detected. When the sintering temperature was further increased up to 1000 1C, patterns corresponding to BaMgAl10O17 began to appear, whereas BaAl2O4 could be still detected. With the temperature increasing to 1100 1C, a pure BAM phase was obtained. It should be noted that all the XRD patterns perfectly matched the reported one in JCPDS (26-0163). This result indicates that the use of H3BO3 as flux reduces the synthesis temperature for the BAM phase to be at least 400 1C lower than that required in the conventional solid-state reaction method [7,8]. Normally, the temperature required for synthesizing BAM by the solid-state method is in the range of 1400–1600 1C. The most significant obstacle to the low-temperature synthesis of BAM by the conventional solid-state method is that stable BaAl2O4 intermediates with a spinel structure are formed at firing temperatures lower than 1200 1C [7]. The transformation of BaAl2O4 and residual Al2O3 to BAM normally occurs from 1300 to 1600 1C. The reactivity of BaAl2O4 plays an important role in the sintering process. BaAl2O4 formed at a relatively high temperature would result in a high synthesis temperature for BAM. Thus, it can be assumed that the temperature required for the transformation of BaAl2O4 to BAM can be decreased by the formation of highly active BaAl2O4. In this study, BaAl2O4 was formed at 800–900 1C, which is much lower than the temperatures reported in other studies [7,8]. Therefore, it can be deduced that BaAl2O4 formed in this study was highly active and induced a significant decrease in the BAM synthesis temperature. In order to determine the optimum firing time, BAM:Eu2 þ powders were prepared at 1100 1C and different heating times,
Fig. 1. XRD patterns of the raw materials with a composition of BAM:Eu powders calcined at different temperatures for 2 h.
Fig. 2. XRD patterns of the BAM:Eu2 þ different times.
particles sintered at 1100 1C for
and were evaluated by XRD. XRD patterns of the BAM:Eu2 þ particles sintered at 1100 1C and different times are shown in Fig. 2. It can be seen that the main phase of the product is BAM, even for samples sintered for only 0.5–1 h. When the sintering time was shorter than 2 h, weak diffraction peaks corresponding to BaAl2O4 and Al2O3 in the synthesized samples were detected, which indicates that the transformation of BaAl2O4 to BAM was not completed and a longer reaction time was needed. When the sintering time was 2 h or longer, a pure BAM phase was obtained. These results show that the time needed to obtain a pure BAM phase in this study was significantly shorter than that required by the conventional solid-state reaction method. Flux plays an important role in the synthesis of phosphors. In this study, H3BO3 was used as the flux, which influenced not only the formation of BaAl2O4 but also the formation of BAM. Experimental results indicated that BaAl2O4 could not be formed after the raw materials were sintered at 900 1C for 2 h without using H3BO3. The content of added H3BO3 also influences the formation of BaAl2O4; high-purity BaAl2O4 could be obtained only when the H3BO3 content was higher than 5 wt%. On the other hand, a high H3BO3 content could induce impurities. Therefore, the H3BO3 content was varied in a narrow range of 5–8 wt%. BAM: Eu2 þ phosphors doped with different amounts of H3BO3 were prepared in this study, and their photoluminescence properties were investigated. The results exhibited that the phosphor prepared with 7 wt% H3BO3 showed the highest PL intensity. Thus, the content of H3BO3 was set as 7 wt%. Normally, flux plays an important role in phase formation, amelioration properties, and control of the particle shape of materials. For inorganic luminescent materials, the effect of flux on synthetic techniques and luminescent properties has become a hot research topic. The mechanism of improvement in the photoluminescence characteristics of BAM by the use of flux in this study can be explained as follows. At the synthesis temperatures of (1073 and 1373 K) employed in this study, H3BO3 decomposed and it was one of the products, B2O3, that played an important part in the reaction. The melting point of B2O3 is 718.13 K. At the reaction temperatures, it melted and a certain amount of liquid was introduced in the reaction system. The liquid phase caused a strong fluxing action, enabling an increase in process flexibility, accelerated the reaction with the addition of flux, and decreased the sintering temperature. Initially, it was thought that H3BO3 promotes only crystallization by acting as a high-temperature solvent. The liquid phase surrounded the solid particles in the reaction system. With an increase in the temperature, the solid particles dissolved in the liquid. This means that the reactions between solid particles were replaced by reactions between ions in the liquid, which inevitably accelerated the chemical reaction and decreased the reaction temperature. At the same time, the liquid phase also facilitated the uniform distribution of the activator ions, Eu2þ , which improved the PL intensity. Furthermore, it was easy to obtain regularly shaped BAM particles from the liquid phase [9–13].
W. Zhang et al. / Journal of Physics and Chemistry of Solids 75 (2014) 163–167
Fig. 3. TG–DTA curve (heating rate 10 1C/min) of the raw materials with a composition of BAM:Eu2 þ .
For BAM:Eu phosphors, the introduction of H3BO3 can change the sublattice structure and influence the crystal field around Eu2 þ ions. It was thus expected that the introduction of H3BO3 would optimize the photoluminescence properties of these phosphors. Hence, the BAM:Eu2 þ phosphors doped with different amounts of H3BO3 were prepared, and their photoluminescence properties were investigated. However, the usage of H3BO3 was limited to a narrow range of 5–9 wt% in order to ensure the purity of BAM. The related results indicated that the sample with 7 wt% H3BO3 showed the highest PL intensity. However, further research is needed to explain the specific mechanism for this increase. TG–DTA measurements for a mixture of the starting materials were carried out to study the mechanism of the low-temperature solid-state reaction, and the results are shown in Fig. 3. The mole ratio of BaCO3, Mg(OH)2 4MgCO3 6H2O, and Al2O3 as the starting materials was 1:0.2:5, which was equal to that of BAM; 7 wt% H3BO3 was added as flux. A continuous weight loss was detected in the reaction process, which can be divided into four temperature regions: room temperature to 500 1C, 500–740 1C, 740– 1030 1C, and higher than 1030 1C. A gradual weight loss of about 5.21% in the TG curve and endothermic peaks at 75 1C and 125 1C in the DTA curve were attributed to the dehydration of free and crystalline water in Mg(OH)2 4MgCO3 6H2O (theoretical weight loss is 2.54%) [14] and decomposition of H3BO3 (theoretical weight loss is 2.52%). In the 500–740 1C range, a rapid weight loss of about 4.44% and an endothermic peak at 740 1C in the DTA curve were observed and were attributed to the decomposition of Mg (OH)2 4MgCO3 (theoretical weight loss is 4.56%). In the 740–1030 1C range, a weight loss of about 4.45% and an endothermic peak at 820 1C in the DTA curve were detected and were ascribed to the decomposition of BaCO3 (theoretical weight loss is 4.65%). Moreover, a persistent exothermic process was observed when the temperature was higher than 800 1C, which corresponded to the formation of BaAl2O4. The exothermic process later resulted in a prominent exothermic peak at 1050 1C. Since there was no distinct weight change in this temperature region, this exothermic reaction was assigned to the phase transformation of BaAl2O4 to BAM, which was also confirmed by the XRD measurements for the sample heated at different temperatures. Similar results have been reported in a previous study [1]. Normally, the heating process of the BAM raw materials is divided into three to four temperature regions based on the weight loss, which is ascribed to the removal of free and crystalline water, the evaporation and decomposition of fluxes, and the decomposition of remaining carbonate. However, in Ref. [1], the temperature
165
range employed for measurements was from room temperature to 1100 1C, and the exothermic process ascribed to the phase transformation of BaAl2O4 to BAM was not recorded. However, in our study, this exothermic process is demonstrated, which is a direct proof of decrease in the synthesis temperature. To understand the mechanism of decrease in the synthesis temperature for BAM, samples with different baryta/alumina ratio (1:1, 1:2, 1:3, 1:4, and 1:5) were prepared at 900 1C for 2 h. Here too 7 wt% H3BO3 was used as flux for all samples. The XRD patterns of these samples are shown in Fig. 4. After sintering at 900 1C for 2 h, the XRD patterns of these samples were quite different from one another, depending on the baryta/alumina ratio. For samples with baryta/alumina ratios of 1:1 and 1:2, the XRD patterns were identical or similar to that for BaAl2O4; BaAl2O4 was obtained as the dominant phase after the reaction. However, impurities, namely, BaCO3, Ba17Al3O7, and Ba1.17Al10.67O17.2, were also detected. When the baryta/alumina ratios in the samples were increased from 1:3 to 1:5, only the BaAl2O4 phase was detected. This comparison indicates that redundant alumina favors the formation of BaAl2O4. Normally, the reaction of Al3 þ and Ba2 þ ions occurs at the alumina particles surface. Therefore, the reaction area is an important factor influencing the reaction rate between solids. The reaction process is likely to be accelerated if the reaction area between the particles is increased. When the ratio of a raw material with a certain particle size is increased, the reaction area between particles is increased accordingly. Moreover, the transfer path for matter away from the reaction surface, which is thought to be one of the ratecontrolling steps in a reaction, is also omitted or shortened, thereby the reaction rate is increased. The sintering temperature to obtain BaAl2O4 in this study was in range of 800–900 1C, which was about 400 1C lower than that required in a conventional solid-state reaction [7,8]. BaAl2O4 prepared under such a condition should be highly reactive. As an intermediate, this may decrease the synthesis temperature for BAM. On the other hand, the composition of intermediate BaAl2O4 is complex. Though only the BaAl2O4 phase was detected, superfluous Al2O3 also co-existed, which resulted in a large amount of lattice defects. This also enhances the reaction activity of the intermediate. This result indicates that the key to decrease the synthesis temperature for BAM is to decrease the synthesis temperature of the BaAl2O4 intermediate. The structure of BaMgAl10O17 mainly consists of Al spinel blocks and BaO layers. Mg atoms, however, may partially substitute for two kinds of Al sites surrounded by oxygen atoms, namely, the tetrahedral
Fig. 4. XRD patterns of the samples with different baryta/alumina ratio sintered at 900 1C for 2 h.
166
W. Zhang et al. / Journal of Physics and Chemistry of Solids 75 (2014) 163–167
Fig. 6b shows the dependence of the PL intensity of the BAM:Eu2 þ phosphors on the concentration of H3BO3 flux excited by a wavelength of 304 nm. It can be observed that the PL emission intensities of BAM:Eu2 þ initially increased as the concentration of H3BO3 increased. However, the PL intensity decreased when the H3BO3 concentration was further increased beyond 7 wt%. These results indicate that the optimum concentration of H3BO3 for preparing good BAM:Eu2 þ phosphors is about 7 wt%. It should also be noted that the shape and position of the emission peaks were not influenced by the H3BO3 flux concentration. Fig. 7 shows the SEM images of BAM:Eu particles prepared at temperatures as low as 1100 1C with a sintering duration of 2 h, using 7 wt% H3BO3 as flux. Particles of the as-prepared BAM:Eu phosphor, with a smooth surface, were irregular with a size of around 5 μm.
Fig. 5. XRD patterns of BAM:Eu2 þ powders prepared using intermediates with different baryta/alumina ratios.
and octahedral sites, in the spinel block. Furthermore, there is a probability that both sites have partial occupation. In Ref. [15], it was assumed that Mg atoms partially substitute Al atoms, which occupy tetrahedral sites 4f (1/3, 2/3, z), surrounded by oxygen atoms in the spinel block. Therefore, Mg2 þ is inserted into the spinel block to maintain the charge balance. In order to verify the reaction ability of the intermediate, samples with different Al/Ba ratios were synthesized (Fig. 4). After re-calculation, re-weighting, re-mixing, and re-sintering at 1100 1C for 2 h, samples with different compositions of BAM:Eu2 þ were prepared. Fig. 5 shows the XRD patterns of the BAM:Eu2 þ powders prepared using the intermediates with different baryta/alumina ratios. It was observed that the XRD patterns of the samples match the standard JCPDS (26-0163) perfectly. This demonstrates that even though intermediates with different Al/Ba ratios were used, the BAM phase was formed and no peaks of impurity were detected. The XRD peaks in Fig. 5 are widened slightly, which might be interpreted by the lattice distortion caused by the low synthesis temperature. By increasing the sintering temperature, the crystal quality can be improved greatly. Additionally, comparative experiments were designed to verify our hypothesis that a highly active BaAl2O4 intermediate is the key to decrease the BAM synthesis temperature. If the raw materials are heated at 1500 1C with a heating speed of 50 1C/min, the time needed to form a pure BAM phase is longer than 6 h. When the reaction is used to obtain BaAl2O4 intermediates at a higher temperature, a long sintering time is needed. This implies that a low-temperature sintering process is essential to decrease the synthesis temperature for BAM. Fig. 6a shows the room-temperature PL spectrum of the BAM: Eu2 þ phosphors prepared at 1100 1C for 2 h. The blue-emission peak at 430 nm was attributed to the 5d–4f transition from Eu2 þ , whose ions partially replaced the Ba2 þ ions in BaMgAl10O17 [7]. The BAM:Eu2 þ samples synthesized below 1500 1C are thought to include the BaAl2O4:Eu2 þ phosphor, which exhibits a bluish-green emission peak at 500 nm, and would cause the longer wavelength side of the spectrum of BAM:Eu2 þ to expand [8]. In this study, no such phenomenon was observed, and this is also a proof that highpurity BAM:Eu2 þ was obtained, again confirming that BaAl2O4 and Al2O3 were transformed to BAM at 1100 1C thoroughly. The excitation spectrum of the sample shows a broad band, which corresponds to the 4f–5d transition of Eu2 þ ions, implying that this phosphor can be well excited by UV light. In order to optimize the H3BO3 usage to maximize the PL intensity, the H3BO3 usage was varied in the range of 5–9 wt%.
Fig. 6. (a) Excitation and emission spectra of the BAM:Eu2 þ phosphor. (b) The emission spectra of the BAM:Eu2 þ phosphors with various H3BO3 usage.
Fig. 7. SEM of the as prepared BAM:Eu particles.
W. Zhang et al. / Journal of Physics and Chemistry of Solids 75 (2014) 163–167
They were assembled into three or more thin layers, although the shapes of the thin layers were different. The thickness of the thin layers was similar and was less than 200 nm. In Ref. [1], the effect of flux on the morphology of the BAM phosphor during the particle formation was discussed; the flux system was believed to benefit the particles size and facilitate the control of the phosphor morphology. Though the synthesis conditions employed in Ref. [1] were different from those used in this study, the results of the two studies are similar. As is well known, the morphology of particles is mainly dependent on the temperature. The synthesis temperature for the BAM phosphor decreased to 1100 1C in this study. Although the morphology of the as-prepared BAM particles was still not satisfactory, it still provided the opportunity to control the particle morphology of the BAM phosphor. 4. Conclusions A new low-temperature method for preparing blue-emitting BaMgAl10O17:Eu2 þ (BAM:Eu2 þ ) phosphors was developed. By this method, a well-crystallized BaMgAl10O17:Eu2 þ phosphor without any impurity phase could be prepared at 1100 1C after 2 h using H3BO3 as flux. The synthesis temperature required in this method was at least 400 1C lower than that required in the conventional solid-state reaction method. The formation of BAM was significantly accelerated by the highly active BaAl2O4 intermediate. It is believed that these results could provide insight on the reaction process and lowering of the synthesis temperature for BAM:Eu2 þ and other phosphors.
167
Acknowledgments We are grateful to the National Natural Science Foundations of China (21141001, 51272151) and the Fundamental Research Funds for the Central Universities (GK20111004, GK201305013) for their financial support. References [1] J. Zhou, Y.H. Wang, B.T. Liu, J.D. Liu, Journal of Physics and Chemistry of Solids 72 (2011) 995–1001. [2] Y. Zhou, J. Lin, Journal of Solid State Chemistry 178 (2005) 441–447. [3] S. Oshio, T. Matsuaka, S. Tanaka, H. Kobayashi, Journal of the Electrochemical Society 145 (1998) 3898–3903. [4] S. Kunimi, S. Fujihara, Dyes and Pigments 91 (2011) 49–54. [5] Y.C. Kang, S.B. Park, I.W. Lenggoro, K. Okuyama, Journal of the Electrochemical Society 146 (1999) 2744–2747. [6] C.H. Lu, W.T. Hsu, C.H. Huang, S.V. Godbole, B.M. Cheng, Materials Chemistry and Physics 90 (2005) 62–68. [7] S. Oshio, K. Kitamura, T. Shigeta, T. Hori, T. Matsuoka, S. Tanaka, H. Kobayashi, Journal of the Electrochemical Society 146 (1999) 392–399. [8] J.Y. Zhang, Z.T. Zhang, Z.L. Tang, Y. Tao, X. Long, Chemistry of Materials 14 (2002) 3005–3008. [9] J. Zhou, Y.H. Wang, B.T. Liu, Y.H. Lu, Journal of Alloys and Compounds 484 (2009) 439–443. [10] Y.J. Park, Y.J. Kim, Materials Science and Engineering: B 146 (2008) 84–88. [11] A. Nag, T. Kutty, Journal of Alloys and Compounds 354 (2003) 221–231. [12] I.C. Chen, T.M. Chen, Journal of Materials Research 16 (2001) 644–651. [13] Y.C. Kang, H.S. Roh, S.B. Park, H.D. Park, Journal of the European Ceramic Society 22 (2002) 1661–1665. [14] C.Y. Wang, Z.M. Du, X.M. Cong, X.L. Tian, Applied Chemical Industry 37 (2008) 657–660. [15] Y. Kim, S.O. Kang, J.S. Lee, M.J. Jung, K.H. Kim, Journal of Materials Science Letters 21 (2002) 219–222.
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Low-temperature synthesis of BaMgAl10O17:Eu2 þ blue phosphors Wen Zhang a, Diping He a, Gang Ma b, Shujie Cui a, Gang Li a, Huan Jiao a,n a
Key Laboratory of Macromolecular Science of Shaanxi Province, School of Chemistry & Chemical Engineering, Shaanxi Normal University, Xi'an 710062, Shaanxi Province, PR China Teaching and Research Department of Military Training, Border Defence Academy of PLA, Xi'an 710106, Shaanxi Province, PR China
b
art ic l e i nf o
a b s t r a c t
Article history: Received 3 July 2012 Received in revised form 27 August 2013 Accepted 5 September 2013 Available online 19 September 2013
Using H3BO3 as the flux, pure BaMgAl10O17:Eu2 þ (BAM) blue phosphors were successfully prepared via a solid-state reaction. By this approach, well-crystallized submicron BAM particles were obtained at temperatures as low as 1100 1C with a sintering duration of 2 h. The sintering temperature required by this approach was at least 400 1C lower than that required by the conventional solid-state approach for preparing BAM; moreover, the sintering time required by the former approach was also considerably shorter than that required by the latter approach. These factors are expected to lower the cost for largescale production of BAM phosphors. Crystal structures and luminescence properties of the synthesized samples were characterized by XRD and TG–DTA, and photoluminescence spectroscopy, respectively. The reactivity of an intermediate, BaAl2O4, is thought to be the key factor influencing the synthesis temperature for BAM. & 2013 Elsevier Ltd. All rights reserved.
Keywords: D. Luminescence
1. Introduction It is important to improve the performance and lower the cost of large-scale production of blue-emitting BaMgAl10O17:Eu2 þ (BAM:Eu2 þ ) phosphors for continuing the recent progress in plasma display panels, tricolor lamps, and LEDs and backlights for liquid crystal displays [1,2]. Conventionally, BAM:Eu2 þ phosphors are prepared by a solidstate reaction process, i.e., firing a mixture of BaCO3, Eu2O3, Mg (OH)2 4MgCO3 4H2O, and Al2O3 together with a small amount of additional flux such as AlF3 or MgF2 at about 1600 1C in a reducing atmosphere. This approach has two intrinsic disadvantages: (1) it imposes high cost and energy requirements on the production process; and (2) the high-temperature process often results in irregular-shaped particles with hard aggregation that are detrimental to phosphor efficiency [3]. Thus, many alternative preparation technique such as combustion synthesis, hydrothermal process, spray pyrolysis [5] and sol–gel route [6] have been developed to decrease the synthesis temperature. However, the use of organic species is often involved in these approaches and their removal is often harmful for the environment. In addition, a subsequent additional heat treatment (in range of 1200–1600 1C) is also required to achieve good crystallinity for the phosphor particles [4–6]. The fabrication of high-purity BAM at low temperatures by a simple process has still not been realized. The fabrication of BAM with well-crystallized particles by a low-
n
Corresponding author. Tel.: þ 86 29 81530766; fax: þ 86 29 81530727. E-mail address: [email protected] (H. Jiao).
0022-3697/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jpcs.2013.09.002
temperature solid-state reaction is still one of the most desirable industrial processes. In the present study, H3BO3 was selected as flux for the preparation of BAM because it decreases the synthesis temperature for well-crystallized BAM and has potential applications in the synthesis of similar materials.
2. Experimental section A BAM:Eu2 þ phosphor with a composition similar to those of commercial phosphors (containing 10 mol% Eu2 þ ions) was synthesized and examined in this study. The starting materials were Eu2O3 (99.99%), Al2O3 (99%), BaCO3 (99%), and Mg(OH)2 4MgCO3 6H2O (99%). H3BO3 (99%) was added as flux to improve the reaction process. The amount of added H3BO3 was 5, 6, 7, 8, and 9 wt% with respect to the total weight of the materials. In order to obtain a homogeneous mixture, distilled ethanol was added as a dispersing liquid. All raw materials were thoroughly mixed by grinding. All samples were fired in reductive atmospheres, and the calcination temperature and time were in the range of 600–1100 1C and 0.5–5 h, respectively. The crystal structures of all samples were investigated by X-ray powder diffraction using Cu Kα (λ ¼1.5406 Å) radiation on a Rigaku D/MAX 2000 X-ray diffractometer. The photoluminescence of the synthesized samples was measured using a Hitachi F4600 fluorescentometer. The thermal reactions of the raw materials were analyzed under an ambient atmosphere by simultaneous thermogravimetry (TG) and differential thermal analysis (DTA) performed using a Q1000DSC þ LNCS þ FACS Q600SDT instrument. For this
164
W. Zhang et al. / Journal of Physics and Chemistry of Solids 75 (2014) 163–167
analysis, 10 mg of a sample at a heating rate of 10 1C/min was reacted in the temperature range of 25–1200 1C.
3. Results and discussion 3.1. Powder X-ray diffraction analysis In order to understand the crystallization behavior of BAM by using the mixture of raw materials, temperature-resolved X-ray diffraction patterns of the BAM:Eu powders were measured and are represented in Fig. 1. The compositions of all raw material samples were set as BAM:Eu2þ , to which H3BO3 was added at a fixed amount of 7 wt%. Further, the sintering duration was set as 2 h. The main reaction process was deduced with increasing calcination temperatures. The XRD pattern of the powder obtained after firing the mixture of raw materials at 600 1C showed characteristics typical of an amorphous phase and crystallization, the main crystal being BaCO3, whereas aluminum and magnesium compounds were not detected by XRD. The samples maintained their amorphous character until a sintering temperature of 800 1C, even though a phase transformation from BaCO3 to an intermediate phase (BaAl2O4) occurred. When the temperature increased to 900 1C, only patterns corresponding to BaAl2O4 could be detected. When the sintering temperature was further increased up to 1000 1C, patterns corresponding to BaMgAl10O17 began to appear, whereas BaAl2O4 could be still detected. With the temperature increasing to 1100 1C, a pure BAM phase was obtained. It should be noted that all the XRD patterns perfectly matched the reported one in JCPDS (26-0163). This result indicates that the use of H3BO3 as flux reduces the synthesis temperature for the BAM phase to be at least 400 1C lower than that required in the conventional solid-state reaction method [7,8]. Normally, the temperature required for synthesizing BAM by the solid-state method is in the range of 1400–1600 1C. The most significant obstacle to the low-temperature synthesis of BAM by the conventional solid-state method is that stable BaAl2O4 intermediates with a spinel structure are formed at firing temperatures lower than 1200 1C [7]. The transformation of BaAl2O4 and residual Al2O3 to BAM normally occurs from 1300 to 1600 1C. The reactivity of BaAl2O4 plays an important role in the sintering process. BaAl2O4 formed at a relatively high temperature would result in a high synthesis temperature for BAM. Thus, it can be assumed that the temperature required for the transformation of BaAl2O4 to BAM can be decreased by the formation of highly active BaAl2O4. In this study, BaAl2O4 was formed at 800–900 1C, which is much lower than the temperatures reported in other studies [7,8]. Therefore, it can be deduced that BaAl2O4 formed in this study was highly active and induced a significant decrease in the BAM synthesis temperature. In order to determine the optimum firing time, BAM:Eu2 þ powders were prepared at 1100 1C and different heating times,
Fig. 1. XRD patterns of the raw materials with a composition of BAM:Eu powders calcined at different temperatures for 2 h.
Fig. 2. XRD patterns of the BAM:Eu2 þ different times.
particles sintered at 1100 1C for
and were evaluated by XRD. XRD patterns of the BAM:Eu2 þ particles sintered at 1100 1C and different times are shown in Fig. 2. It can be seen that the main phase of the product is BAM, even for samples sintered for only 0.5–1 h. When the sintering time was shorter than 2 h, weak diffraction peaks corresponding to BaAl2O4 and Al2O3 in the synthesized samples were detected, which indicates that the transformation of BaAl2O4 to BAM was not completed and a longer reaction time was needed. When the sintering time was 2 h or longer, a pure BAM phase was obtained. These results show that the time needed to obtain a pure BAM phase in this study was significantly shorter than that required by the conventional solid-state reaction method. Flux plays an important role in the synthesis of phosphors. In this study, H3BO3 was used as the flux, which influenced not only the formation of BaAl2O4 but also the formation of BAM. Experimental results indicated that BaAl2O4 could not be formed after the raw materials were sintered at 900 1C for 2 h without using H3BO3. The content of added H3BO3 also influences the formation of BaAl2O4; high-purity BaAl2O4 could be obtained only when the H3BO3 content was higher than 5 wt%. On the other hand, a high H3BO3 content could induce impurities. Therefore, the H3BO3 content was varied in a narrow range of 5–8 wt%. BAM: Eu2 þ phosphors doped with different amounts of H3BO3 were prepared in this study, and their photoluminescence properties were investigated. The results exhibited that the phosphor prepared with 7 wt% H3BO3 showed the highest PL intensity. Thus, the content of H3BO3 was set as 7 wt%. Normally, flux plays an important role in phase formation, amelioration properties, and control of the particle shape of materials. For inorganic luminescent materials, the effect of flux on synthetic techniques and luminescent properties has become a hot research topic. The mechanism of improvement in the photoluminescence characteristics of BAM by the use of flux in this study can be explained as follows. At the synthesis temperatures of (1073 and 1373 K) employed in this study, H3BO3 decomposed and it was one of the products, B2O3, that played an important part in the reaction. The melting point of B2O3 is 718.13 K. At the reaction temperatures, it melted and a certain amount of liquid was introduced in the reaction system. The liquid phase caused a strong fluxing action, enabling an increase in process flexibility, accelerated the reaction with the addition of flux, and decreased the sintering temperature. Initially, it was thought that H3BO3 promotes only crystallization by acting as a high-temperature solvent. The liquid phase surrounded the solid particles in the reaction system. With an increase in the temperature, the solid particles dissolved in the liquid. This means that the reactions between solid particles were replaced by reactions between ions in the liquid, which inevitably accelerated the chemical reaction and decreased the reaction temperature. At the same time, the liquid phase also facilitated the uniform distribution of the activator ions, Eu2þ , which improved the PL intensity. Furthermore, it was easy to obtain regularly shaped BAM particles from the liquid phase [9–13].
W. Zhang et al. / Journal of Physics and Chemistry of Solids 75 (2014) 163–167
Fig. 3. TG–DTA curve (heating rate 10 1C/min) of the raw materials with a composition of BAM:Eu2 þ .
For BAM:Eu phosphors, the introduction of H3BO3 can change the sublattice structure and influence the crystal field around Eu2 þ ions. It was thus expected that the introduction of H3BO3 would optimize the photoluminescence properties of these phosphors. Hence, the BAM:Eu2 þ phosphors doped with different amounts of H3BO3 were prepared, and their photoluminescence properties were investigated. However, the usage of H3BO3 was limited to a narrow range of 5–9 wt% in order to ensure the purity of BAM. The related results indicated that the sample with 7 wt% H3BO3 showed the highest PL intensity. However, further research is needed to explain the specific mechanism for this increase. TG–DTA measurements for a mixture of the starting materials were carried out to study the mechanism of the low-temperature solid-state reaction, and the results are shown in Fig. 3. The mole ratio of BaCO3, Mg(OH)2 4MgCO3 6H2O, and Al2O3 as the starting materials was 1:0.2:5, which was equal to that of BAM; 7 wt% H3BO3 was added as flux. A continuous weight loss was detected in the reaction process, which can be divided into four temperature regions: room temperature to 500 1C, 500–740 1C, 740– 1030 1C, and higher than 1030 1C. A gradual weight loss of about 5.21% in the TG curve and endothermic peaks at 75 1C and 125 1C in the DTA curve were attributed to the dehydration of free and crystalline water in Mg(OH)2 4MgCO3 6H2O (theoretical weight loss is 2.54%) [14] and decomposition of H3BO3 (theoretical weight loss is 2.52%). In the 500–740 1C range, a rapid weight loss of about 4.44% and an endothermic peak at 740 1C in the DTA curve were observed and were attributed to the decomposition of Mg (OH)2 4MgCO3 (theoretical weight loss is 4.56%). In the 740–1030 1C range, a weight loss of about 4.45% and an endothermic peak at 820 1C in the DTA curve were detected and were ascribed to the decomposition of BaCO3 (theoretical weight loss is 4.65%). Moreover, a persistent exothermic process was observed when the temperature was higher than 800 1C, which corresponded to the formation of BaAl2O4. The exothermic process later resulted in a prominent exothermic peak at 1050 1C. Since there was no distinct weight change in this temperature region, this exothermic reaction was assigned to the phase transformation of BaAl2O4 to BAM, which was also confirmed by the XRD measurements for the sample heated at different temperatures. Similar results have been reported in a previous study [1]. Normally, the heating process of the BAM raw materials is divided into three to four temperature regions based on the weight loss, which is ascribed to the removal of free and crystalline water, the evaporation and decomposition of fluxes, and the decomposition of remaining carbonate. However, in Ref. [1], the temperature
165
range employed for measurements was from room temperature to 1100 1C, and the exothermic process ascribed to the phase transformation of BaAl2O4 to BAM was not recorded. However, in our study, this exothermic process is demonstrated, which is a direct proof of decrease in the synthesis temperature. To understand the mechanism of decrease in the synthesis temperature for BAM, samples with different baryta/alumina ratio (1:1, 1:2, 1:3, 1:4, and 1:5) were prepared at 900 1C for 2 h. Here too 7 wt% H3BO3 was used as flux for all samples. The XRD patterns of these samples are shown in Fig. 4. After sintering at 900 1C for 2 h, the XRD patterns of these samples were quite different from one another, depending on the baryta/alumina ratio. For samples with baryta/alumina ratios of 1:1 and 1:2, the XRD patterns were identical or similar to that for BaAl2O4; BaAl2O4 was obtained as the dominant phase after the reaction. However, impurities, namely, BaCO3, Ba17Al3O7, and Ba1.17Al10.67O17.2, were also detected. When the baryta/alumina ratios in the samples were increased from 1:3 to 1:5, only the BaAl2O4 phase was detected. This comparison indicates that redundant alumina favors the formation of BaAl2O4. Normally, the reaction of Al3 þ and Ba2 þ ions occurs at the alumina particles surface. Therefore, the reaction area is an important factor influencing the reaction rate between solids. The reaction process is likely to be accelerated if the reaction area between the particles is increased. When the ratio of a raw material with a certain particle size is increased, the reaction area between particles is increased accordingly. Moreover, the transfer path for matter away from the reaction surface, which is thought to be one of the ratecontrolling steps in a reaction, is also omitted or shortened, thereby the reaction rate is increased. The sintering temperature to obtain BaAl2O4 in this study was in range of 800–900 1C, which was about 400 1C lower than that required in a conventional solid-state reaction [7,8]. BaAl2O4 prepared under such a condition should be highly reactive. As an intermediate, this may decrease the synthesis temperature for BAM. On the other hand, the composition of intermediate BaAl2O4 is complex. Though only the BaAl2O4 phase was detected, superfluous Al2O3 also co-existed, which resulted in a large amount of lattice defects. This also enhances the reaction activity of the intermediate. This result indicates that the key to decrease the synthesis temperature for BAM is to decrease the synthesis temperature of the BaAl2O4 intermediate. The structure of BaMgAl10O17 mainly consists of Al spinel blocks and BaO layers. Mg atoms, however, may partially substitute for two kinds of Al sites surrounded by oxygen atoms, namely, the tetrahedral
Fig. 4. XRD patterns of the samples with different baryta/alumina ratio sintered at 900 1C for 2 h.
166
W. Zhang et al. / Journal of Physics and Chemistry of Solids 75 (2014) 163–167
Fig. 6b shows the dependence of the PL intensity of the BAM:Eu2 þ phosphors on the concentration of H3BO3 flux excited by a wavelength of 304 nm. It can be observed that the PL emission intensities of BAM:Eu2 þ initially increased as the concentration of H3BO3 increased. However, the PL intensity decreased when the H3BO3 concentration was further increased beyond 7 wt%. These results indicate that the optimum concentration of H3BO3 for preparing good BAM:Eu2 þ phosphors is about 7 wt%. It should also be noted that the shape and position of the emission peaks were not influenced by the H3BO3 flux concentration. Fig. 7 shows the SEM images of BAM:Eu particles prepared at temperatures as low as 1100 1C with a sintering duration of 2 h, using 7 wt% H3BO3 as flux. Particles of the as-prepared BAM:Eu phosphor, with a smooth surface, were irregular with a size of around 5 μm.
Fig. 5. XRD patterns of BAM:Eu2 þ powders prepared using intermediates with different baryta/alumina ratios.
and octahedral sites, in the spinel block. Furthermore, there is a probability that both sites have partial occupation. In Ref. [15], it was assumed that Mg atoms partially substitute Al atoms, which occupy tetrahedral sites 4f (1/3, 2/3, z), surrounded by oxygen atoms in the spinel block. Therefore, Mg2 þ is inserted into the spinel block to maintain the charge balance. In order to verify the reaction ability of the intermediate, samples with different Al/Ba ratios were synthesized (Fig. 4). After re-calculation, re-weighting, re-mixing, and re-sintering at 1100 1C for 2 h, samples with different compositions of BAM:Eu2 þ were prepared. Fig. 5 shows the XRD patterns of the BAM:Eu2 þ powders prepared using the intermediates with different baryta/alumina ratios. It was observed that the XRD patterns of the samples match the standard JCPDS (26-0163) perfectly. This demonstrates that even though intermediates with different Al/Ba ratios were used, the BAM phase was formed and no peaks of impurity were detected. The XRD peaks in Fig. 5 are widened slightly, which might be interpreted by the lattice distortion caused by the low synthesis temperature. By increasing the sintering temperature, the crystal quality can be improved greatly. Additionally, comparative experiments were designed to verify our hypothesis that a highly active BaAl2O4 intermediate is the key to decrease the BAM synthesis temperature. If the raw materials are heated at 1500 1C with a heating speed of 50 1C/min, the time needed to form a pure BAM phase is longer than 6 h. When the reaction is used to obtain BaAl2O4 intermediates at a higher temperature, a long sintering time is needed. This implies that a low-temperature sintering process is essential to decrease the synthesis temperature for BAM. Fig. 6a shows the room-temperature PL spectrum of the BAM: Eu2 þ phosphors prepared at 1100 1C for 2 h. The blue-emission peak at 430 nm was attributed to the 5d–4f transition from Eu2 þ , whose ions partially replaced the Ba2 þ ions in BaMgAl10O17 [7]. The BAM:Eu2 þ samples synthesized below 1500 1C are thought to include the BaAl2O4:Eu2 þ phosphor, which exhibits a bluish-green emission peak at 500 nm, and would cause the longer wavelength side of the spectrum of BAM:Eu2 þ to expand [8]. In this study, no such phenomenon was observed, and this is also a proof that highpurity BAM:Eu2 þ was obtained, again confirming that BaAl2O4 and Al2O3 were transformed to BAM at 1100 1C thoroughly. The excitation spectrum of the sample shows a broad band, which corresponds to the 4f–5d transition of Eu2 þ ions, implying that this phosphor can be well excited by UV light. In order to optimize the H3BO3 usage to maximize the PL intensity, the H3BO3 usage was varied in the range of 5–9 wt%.
Fig. 6. (a) Excitation and emission spectra of the BAM:Eu2 þ phosphor. (b) The emission spectra of the BAM:Eu2 þ phosphors with various H3BO3 usage.
Fig. 7. SEM of the as prepared BAM:Eu particles.
W. Zhang et al. / Journal of Physics and Chemistry of Solids 75 (2014) 163–167
They were assembled into three or more thin layers, although the shapes of the thin layers were different. The thickness of the thin layers was similar and was less than 200 nm. In Ref. [1], the effect of flux on the morphology of the BAM phosphor during the particle formation was discussed; the flux system was believed to benefit the particles size and facilitate the control of the phosphor morphology. Though the synthesis conditions employed in Ref. [1] were different from those used in this study, the results of the two studies are similar. As is well known, the morphology of particles is mainly dependent on the temperature. The synthesis temperature for the BAM phosphor decreased to 1100 1C in this study. Although the morphology of the as-prepared BAM particles was still not satisfactory, it still provided the opportunity to control the particle morphology of the BAM phosphor. 4. Conclusions A new low-temperature method for preparing blue-emitting BaMgAl10O17:Eu2 þ (BAM:Eu2 þ ) phosphors was developed. By this method, a well-crystallized BaMgAl10O17:Eu2 þ phosphor without any impurity phase could be prepared at 1100 1C after 2 h using H3BO3 as flux. The synthesis temperature required in this method was at least 400 1C lower than that required in the conventional solid-state reaction method. The formation of BAM was significantly accelerated by the highly active BaAl2O4 intermediate. It is believed that these results could provide insight on the reaction process and lowering of the synthesis temperature for BAM:Eu2 þ and other phosphors.
167
Acknowledgments We are grateful to the National Natural Science Foundations of China (21141001, 51272151) and the Fundamental Research Funds for the Central Universities (GK20111004, GK201305013) for their financial support. References [1] J. Zhou, Y.H. Wang, B.T. Liu, J.D. Liu, Journal of Physics and Chemistry of Solids 72 (2011) 995–1001. [2] Y. Zhou, J. Lin, Journal of Solid State Chemistry 178 (2005) 441–447. [3] S. Oshio, T. Matsuaka, S. Tanaka, H. Kobayashi, Journal of the Electrochemical Society 145 (1998) 3898–3903. [4] S. Kunimi, S. Fujihara, Dyes and Pigments 91 (2011) 49–54. [5] Y.C. Kang, S.B. Park, I.W. Lenggoro, K. Okuyama, Journal of the Electrochemical Society 146 (1999) 2744–2747. [6] C.H. Lu, W.T. Hsu, C.H. Huang, S.V. Godbole, B.M. Cheng, Materials Chemistry and Physics 90 (2005) 62–68. [7] S. Oshio, K. Kitamura, T. Shigeta, T. Hori, T. Matsuoka, S. Tanaka, H. Kobayashi, Journal of the Electrochemical Society 146 (1999) 392–399. [8] J.Y. Zhang, Z.T. Zhang, Z.L. Tang, Y. Tao, X. Long, Chemistry of Materials 14 (2002) 3005–3008. [9] J. Zhou, Y.H. Wang, B.T. Liu, Y.H. Lu, Journal of Alloys and Compounds 484 (2009) 439–443. [10] Y.J. Park, Y.J. Kim, Materials Science and Engineering: B 146 (2008) 84–88. [11] A. Nag, T. Kutty, Journal of Alloys and Compounds 354 (2003) 221–231. [12] I.C. Chen, T.M. Chen, Journal of Materials Research 16 (2001) 644–651. [13] Y.C. Kang, H.S. Roh, S.B. Park, H.D. Park, Journal of the European Ceramic Society 22 (2002) 1661–1665. [14] C.Y. Wang, Z.M. Du, X.M. Cong, X.L. Tian, Applied Chemical Industry 37 (2008) 657–660. [15] Y. Kim, S.O. Kang, J.S. Lee, M.J. Jung, K.H. Kim, Journal of Materials Science Letters 21 (2002) 219–222.