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Novel synthesis of YAG by solvothermal method
ARTICLE IN PRESS
Journal of Crystal Growth 275 (2005) e1913–e1917 www.elsevier.com/locate/jcrysgro
Novel synthesis of YAG by solvothermal method Xudong Zhanga,b, Hong Liua, Wen Heb, Jiyang Wanga,, Xia Lia,b, Robert I. Boughtonc a State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China Department of Materials Science and Engineering, Shandong Institute of Light Industry, Jinan, 250100, China c Center for Material Science, Bowling Green State University, Bowling Green, Ohio 43403, USA
b
Available online 19 December 2004
Abstract Yttrium aluminum garnet (YAG) powder was synthesized by a mixed solvothermal method under moderate conditions (2602290 C; 2–4 h) with inexpensive aluminum and yttrium hydroxides as the precursors, and a low-cost ethanol–water solution (the volume ratio of ethanol to water X2: 1) as the solvent. The presence of ethanol in mixed solvent is beneficial to the formation of YAG. The synthesized powder consists of well-dispersed and nearly spherical fine grains (80 nm on average) with a relatively narrow grain size distribution. The powder form is the most desirable for the compacting and sintering of YAG ceramics. r 2004 Elsevier B.V. All rights reserved. PACS: 42.70.Hj Keywords: A1. X-ray diffraction; A2. Solvothermal crystal growth; B1. Yttrium aluminum garnet; B2. Phosphors; B3. Solid state lasers
1. Introduction Yttrium aluminum garnet (YAG) is now used widely in optical materials because it has a variety of good optical properties [1–3]. Translucent YAG doped with rare-earth ions (Nd:YAG) ceramics is a promising material for large-size solid-state lasers, as a substitute for single-crystal YAG because of its excellent laser performance, low cost, short preparation period, and other charCorresponding author. Fax: +86 531 8968495.
E-mail address: [email protected] (X. Zhang).
acteristics [4,5]. YAG powders doped with transition and rare-earth metal elements can be used as ultra short afterglow phosphors for cathode ray tubes and high-resolution displays [2,6]. Moreover, YAG ceramics are a variety of potentially advanced structural materials in view of their high creep and oxidation resistance at high temperatures and their low heat conductivity. YAG powders can be synthesized by many methods, such as solid-state reaction [3,5], coprecipitation [4,7], sol–gel [2,4,8], and spray thermal decomposition [9]. The solid-state reaction method is a simple process suitable for large-scale
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.11.274
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X. Zhang et al. / Journal of Crystal Growth 275 (2005) e1913–e1917
production, but it requires a high temperature ð41650 CÞ and a long reaction time. Most important, the size of the YAG powder particles synthesized by the solid-state method is very large (normally larger than 1 mm), and the composition of the particles is not homogeneous, which is difficult for sintering. Although YAG powders synthesized by chemical methods have the advantage of high purity, homogeneous composition, and fine grains, they require a complicated preparation process, a long preparation period, and additional thermal treatment at high temperatures ð4800 CÞ: Therefore, hard aggregation of the powder is easily caused. The hydrothermal synthesis [6] of YAG powders can avoid the problems described above. However, it requires complicated and expensive facilities due to higher temperatures and pressures ð4400 C; 430 MPaÞ needed [6]. YAG powders can be obtained at lower temperatures and pressures by using organic solvothermal synthesis [10]. However, the powder synthesized by this method consists of aggregates of irregular grains, which is not beneficial to the ceramic sintering process, and some of the organic solvents used, e.g. ethylene glycol, butanediol, etc., increase the cost of the synthesized powder, and even may pollute the environment. In this work, inexpensive metal nitrates and a mixed aqueous solution of harmless ethyl alcohol were used as the starting materials and the reaction solvent, respectively, to synthesize monodispersed spherical YAG powder with good properties under moderate preparation conditions.
2. Experimental procedure Aluminum nitrate solution (1.5 mol/l) was prepared by dissolving A1ðNO3 Þ3 9H2 O (analytical grade) in deionized water. Yttrium nitrate solution (1.3 mol/l) was obtained by dissolving high purity Y2 O3 (99.99%) in HNO3 and some deionized water. Subsequently, the above two aqueous solutions were mixed homogeneously according to the stoichiometry of YAG (Y:A1 ¼ 3:5). The aluminum and yttrium hydroxides were precipitated by dropwise addition of excess NH4 OH (10%) into the mixed solution under vigorous
stirring. The precipitates were repeatedly washed with distilled water to remove the residual nitric and ammonia ions. The washed hydroxides were dispersed in the selected solvent (ethanol:water ¼ 0:1–1:0, 0.3 mol/l) and then placed in an autoclave. The autoclave was purged with nitrogen several times before sealing and heating to the desired temperature at a rate of 1:5 C= min : After being kept at the selected temperature for a fixed amount of time, the autoclave was then cooled to room temperature. The resulting powder was filtered and repeatedly washed with distilled water, and then dried in air at 80 C for 2 h. X-ray diffraction (XRD) experiments were carried out to determine the phase development of the products on a D max-rA model (Japan Rigaku) X-ray diffractometer with Cu Ka radiation and a scanning speed of 0:2 = min : The grain morphology and size distribution were examined using a transmission electron microscope (TEM; Hitachi model H-800).
3. Results and discussion 3.1. Effect of ratio of ethanol to water In order to investigate the effect of the ratio of ethanol to water on the synthesis of YAG, conditions other than the solvent composition, e.g. temperature and time, were fixed, and the ratio of ethanol to water was changed from 0:1 to 1:0 (equal to 0:1, 1:1, 2:1, 1:0, respectively). The phase development of products synthesized at 300 C for 2 h is shown in Fig. 1. When using water as a solvent, the product is composed of YAG, Y and A1 hydroxides, and yttrium aluminate (Fig. 1a). When the volume of ethanol in the solvent is increased, the content of the YAG phase in the product increases (Fig. 1b). When the ratio of ethanol to water is equal to or exceeds 2:1, the product is single-phase YAG (Fig. 1c,d). Therefore, the presence of ethanol in a mixed solvent is beneficial for the formation of YAG under moderate conditions. The synthesis of single-phase YAG by the hydrothermal method may, however, require more extreme conditions, e.g. higher temperatures and pressures. This is also consistent
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10.6–12.5 MPa) in this work was higher than the supercritical temperature and pressure of the mixed solvent mentioned above, but was much lower than that of water. Therefore, in accordance with supercritical fluid behavior [11,12], the solubility of the YAG precursor in the mixed solvent can increase markedly in the supercritical state due to the dramatic change in the density and the dielectric constant of the fluid. In particular, the mass transfer rate can accelerate due to the low viscosity and high diffusion coefficient of the supercritical fluid. This allows the solution to reach the critical nucleation value for the YAG crystallites and promotes the formation of YAG grains at a much lower temperature by the solvothermal method than by the hydrothermal method.
3.2. Effect of synthetic temperature
Fig. 1. (a–d) XRD patterns of the products synthesized for different ratios of ethanol to water at 300 C for 2 h.
with the results reported by Hakuta et al. [6]. Such a result can be explained in terms of the supercritical fluid theory [11,12]. As is well known, the supercritical temperature and pressure of water and ethanol are 374 C; 22 MPa and 243 C; 6.3 MPa, respectively [11]. The aqueous solution of ethanol can reach the supercritical state at a lower temperature and pressure. The supercritical temperature and pressure of the ethanol–water solution of 1:1 and 2:1 are 283 C; 9.2 MPa and 267 C; 7.5 MPa, respectively [12]. Obviously, the reaction temperature and pressure (300 C;
According to the results mentioned above, it is possible that the synthetic temperature continues to decrease with increase in the ratio of ethanol to water, which is beneficial to improving the properties of the powder. For ethanol to water ratios of 2:1 and 1:0, the effect of temperature on the synthetic reaction has been investigated under conditions similar to those described above. The phase composition of products synthesized at 2202290 C for 2 h are shown in Table 1. When the ethanol to water ratio was 2:1; the content of the YAG phase in the products increased with increase in reaction temperature, and single-phase YAG could be obtained at 290 C; When ethanol was used as the solvent (i.e. the ethanol/water ratio is equal to 1:0), although the development of the YAG phase in the products at the selected temperature was similar to those with an ethanol– water solution of 2:1, the formation temperature of single-phase YAG decreased to 264 C: Therefore, the beneficial effect of ethanol on the formation of YAG is further confirmed. Moreover, as seen in Table 1, it is worth noticing that single-phase YAG can be formed directly without other intermediate crystalline phases with ethanol as the solvent. This case is obviously different from those with the ethanol–water mixed solvent or
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X. Zhang et al. / Journal of Crystal Growth 275 (2005) e1913–e1917
Table 1 Phase composition of the products synthesized under selected conditions Ethanol:water
220 C
264 C
274 C
290 C
2:1
YAIO3 g-AIOOH YOOH
YAIO3 YAG g-AIOOH YOOH
YAG YAIO3
YAG
Amphours
YAG (weaka)
YAG (strong)
YAG (strong)
1:0 a
Means intensity of XRD peaks.
with water alone. Further studies on its mechanism are now under way.
3.3. Effect of holding time In the above experiments, only the holding time was changed from 1 to 8 h at 290 C with the ratio of 2:1 (ethanol:water) to investigate its effect on the synthetic reactions. The XRD patterns of the products synthesized under selected holding times are given in Fig. 2. Fig. 2 shows that a holding time of 1 h was short due to the presence of phases other than a small amount of the YAG phase. When the holding time is 2 h or longer, singlephase YAG can be obtained. In fact, according to the difference of intensity in the XRD peaks, the stable YAG phase had been formed after 4 h or longer. However, as the TEM images shown in Fig. 3 indicate, the YAG powder synthesized with a holding time of 2 h is mainly composed of welldispersed and nearly spherical fine grains (80 nm on average) (Fig. 3a), which is favorable for sintering ceramics with high density [13,14]. When the holding time is prolonged to 8 h, the YAG grains had grown markedly, and the grain size increased to 220 nm on average. Additionally, the formation of agglomerates occurred (Fig. 3b). This phenomenon is mainly caused by the dissolution of smaller YAG grains and the growth of some larger grains. When large YAG grains grow, the attractive force between them is increased and aggregation among the particles occurs. Therefore, this research indicates that a holding time of 2–4 h
Fig. 2. (a–d) XRD patterns of the products synthesized for different holding times at 290 C:
ARTICLE IN PRESS X. Zhang et al. / Journal of Crystal Growth 275 (2005) e1913–e1917
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powder with well-dispersed, nearly-spherical grains, and a relatively narrow grain size distribution can be synthesized at 2602290 C for 2–4 h. This kind of powder is most desirable for the compacting and sintering of YAG ceramics. Furthermore, this synthetic method may be a potential way for other oxide powders with good properties to be synthesized under moderate preparation conditions.
Acknowledgements This work was supported by the Key Project of Ministry of Education Science and Technology (0314), the Grant of State Key Program of China and the Natural Science Foundation of Shandong Province of China (Y2003F02).
References
Fig. 3. TEM photographs of the products synthesized for different holding times at 290 C: (a) 2 h, (b) 8 h.
should be suitable for the synthesis of YAG powder with good properties.
4. Conclusions YAG powder can be synthesized by the mixed solvothermal method under moderate conditions with an ethanol–water solution as the solvent. The presence of ethanol in mixed solvent is beneficial for the formation of YAG. When the ratio of ethanol to water is 2:1 or larger, single-phase YAG
[1] J.E. Geusic, H.M. Marcos, L.G.V. Uitert, Appl. Phys. Lett. 4 (1964) 182. [2] M. Veith, S. Mathur, A. Kareiva, M. Jilari, M. Zimmer, V. Huch, J. Mater. Chem. 9 (1999) 3069. [3] A. Ikesue, I. Furusato, K. Kamata, J. Am. Ceram. Soc. 78 (1995) 225. [4] S.M. Sim, K.A. Keller, J. Mater. Sci. 35 (2000) 713. [5] A. Ikesue, T. Kinoshita, K. Kammata, K. Yoshida, J. Am. Ceram. Soc. 78 (1995) 1033. [6] Y. Hakuta, K. Seino, H. Ura, T. Adschiri, H. Takizawa, K. Arai, J. Mater. Chem. 9 (1999) 2671. [7] J.G. Li, T. Ikegami, J.H. Lee, T. Moriand, Y. Yajima, J. Eur. Ceram. Soc. 20 (2000) 2395. [8] D. Hreniak, W. Strek, J. Alloys Compounds 341 (2002) 183. [9] G. De With, H. Van Dijk, J. Am. Mater. Res. Bull. 19 (1984) 1669. [10] M. Inoue, H. Otsu, H. Kominami, T. Inui, J. Am. Ceram. Soc. 74 (1991) 1452. [11] J.C. Zhang, Supercritical Fluid Extraction, Publishing House of Chemical Industry of China, Beijing, 2000, p. 7 (in Chinese). [12] Z.Q. Zhu, Publishing House of Chemical Industry of China, Beijing, 2000, p. 137 (in Chinese). [13] C.S. Fang, Y.W. Chen, Mater. Chem. Phys. 78 (2003) 739. [14] Y.T. Moon, H.K. Park, D.K. Kim, C.H. Kim, J. Am. Ceram. Soc. 78 (1995) 2690.
Journal of Crystal Growth 275 (2005) e1913–e1917 www.elsevier.com/locate/jcrysgro
Novel synthesis of YAG by solvothermal method Xudong Zhanga,b, Hong Liua, Wen Heb, Jiyang Wanga,, Xia Lia,b, Robert I. Boughtonc a State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China Department of Materials Science and Engineering, Shandong Institute of Light Industry, Jinan, 250100, China c Center for Material Science, Bowling Green State University, Bowling Green, Ohio 43403, USA
b
Available online 19 December 2004
Abstract Yttrium aluminum garnet (YAG) powder was synthesized by a mixed solvothermal method under moderate conditions (2602290 C; 2–4 h) with inexpensive aluminum and yttrium hydroxides as the precursors, and a low-cost ethanol–water solution (the volume ratio of ethanol to water X2: 1) as the solvent. The presence of ethanol in mixed solvent is beneficial to the formation of YAG. The synthesized powder consists of well-dispersed and nearly spherical fine grains (80 nm on average) with a relatively narrow grain size distribution. The powder form is the most desirable for the compacting and sintering of YAG ceramics. r 2004 Elsevier B.V. All rights reserved. PACS: 42.70.Hj Keywords: A1. X-ray diffraction; A2. Solvothermal crystal growth; B1. Yttrium aluminum garnet; B2. Phosphors; B3. Solid state lasers
1. Introduction Yttrium aluminum garnet (YAG) is now used widely in optical materials because it has a variety of good optical properties [1–3]. Translucent YAG doped with rare-earth ions (Nd:YAG) ceramics is a promising material for large-size solid-state lasers, as a substitute for single-crystal YAG because of its excellent laser performance, low cost, short preparation period, and other charCorresponding author. Fax: +86 531 8968495.
E-mail address: [email protected] (X. Zhang).
acteristics [4,5]. YAG powders doped with transition and rare-earth metal elements can be used as ultra short afterglow phosphors for cathode ray tubes and high-resolution displays [2,6]. Moreover, YAG ceramics are a variety of potentially advanced structural materials in view of their high creep and oxidation resistance at high temperatures and their low heat conductivity. YAG powders can be synthesized by many methods, such as solid-state reaction [3,5], coprecipitation [4,7], sol–gel [2,4,8], and spray thermal decomposition [9]. The solid-state reaction method is a simple process suitable for large-scale
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.11.274
ARTICLE IN PRESS e1914
X. Zhang et al. / Journal of Crystal Growth 275 (2005) e1913–e1917
production, but it requires a high temperature ð41650 CÞ and a long reaction time. Most important, the size of the YAG powder particles synthesized by the solid-state method is very large (normally larger than 1 mm), and the composition of the particles is not homogeneous, which is difficult for sintering. Although YAG powders synthesized by chemical methods have the advantage of high purity, homogeneous composition, and fine grains, they require a complicated preparation process, a long preparation period, and additional thermal treatment at high temperatures ð4800 CÞ: Therefore, hard aggregation of the powder is easily caused. The hydrothermal synthesis [6] of YAG powders can avoid the problems described above. However, it requires complicated and expensive facilities due to higher temperatures and pressures ð4400 C; 430 MPaÞ needed [6]. YAG powders can be obtained at lower temperatures and pressures by using organic solvothermal synthesis [10]. However, the powder synthesized by this method consists of aggregates of irregular grains, which is not beneficial to the ceramic sintering process, and some of the organic solvents used, e.g. ethylene glycol, butanediol, etc., increase the cost of the synthesized powder, and even may pollute the environment. In this work, inexpensive metal nitrates and a mixed aqueous solution of harmless ethyl alcohol were used as the starting materials and the reaction solvent, respectively, to synthesize monodispersed spherical YAG powder with good properties under moderate preparation conditions.
2. Experimental procedure Aluminum nitrate solution (1.5 mol/l) was prepared by dissolving A1ðNO3 Þ3 9H2 O (analytical grade) in deionized water. Yttrium nitrate solution (1.3 mol/l) was obtained by dissolving high purity Y2 O3 (99.99%) in HNO3 and some deionized water. Subsequently, the above two aqueous solutions were mixed homogeneously according to the stoichiometry of YAG (Y:A1 ¼ 3:5). The aluminum and yttrium hydroxides were precipitated by dropwise addition of excess NH4 OH (10%) into the mixed solution under vigorous
stirring. The precipitates were repeatedly washed with distilled water to remove the residual nitric and ammonia ions. The washed hydroxides were dispersed in the selected solvent (ethanol:water ¼ 0:1–1:0, 0.3 mol/l) and then placed in an autoclave. The autoclave was purged with nitrogen several times before sealing and heating to the desired temperature at a rate of 1:5 C= min : After being kept at the selected temperature for a fixed amount of time, the autoclave was then cooled to room temperature. The resulting powder was filtered and repeatedly washed with distilled water, and then dried in air at 80 C for 2 h. X-ray diffraction (XRD) experiments were carried out to determine the phase development of the products on a D max-rA model (Japan Rigaku) X-ray diffractometer with Cu Ka radiation and a scanning speed of 0:2 = min : The grain morphology and size distribution were examined using a transmission electron microscope (TEM; Hitachi model H-800).
3. Results and discussion 3.1. Effect of ratio of ethanol to water In order to investigate the effect of the ratio of ethanol to water on the synthesis of YAG, conditions other than the solvent composition, e.g. temperature and time, were fixed, and the ratio of ethanol to water was changed from 0:1 to 1:0 (equal to 0:1, 1:1, 2:1, 1:0, respectively). The phase development of products synthesized at 300 C for 2 h is shown in Fig. 1. When using water as a solvent, the product is composed of YAG, Y and A1 hydroxides, and yttrium aluminate (Fig. 1a). When the volume of ethanol in the solvent is increased, the content of the YAG phase in the product increases (Fig. 1b). When the ratio of ethanol to water is equal to or exceeds 2:1, the product is single-phase YAG (Fig. 1c,d). Therefore, the presence of ethanol in a mixed solvent is beneficial for the formation of YAG under moderate conditions. The synthesis of single-phase YAG by the hydrothermal method may, however, require more extreme conditions, e.g. higher temperatures and pressures. This is also consistent
ARTICLE IN PRESS X. Zhang et al. / Journal of Crystal Growth 275 (2005) e1913–e1917
e1915
10.6–12.5 MPa) in this work was higher than the supercritical temperature and pressure of the mixed solvent mentioned above, but was much lower than that of water. Therefore, in accordance with supercritical fluid behavior [11,12], the solubility of the YAG precursor in the mixed solvent can increase markedly in the supercritical state due to the dramatic change in the density and the dielectric constant of the fluid. In particular, the mass transfer rate can accelerate due to the low viscosity and high diffusion coefficient of the supercritical fluid. This allows the solution to reach the critical nucleation value for the YAG crystallites and promotes the formation of YAG grains at a much lower temperature by the solvothermal method than by the hydrothermal method.
3.2. Effect of synthetic temperature
Fig. 1. (a–d) XRD patterns of the products synthesized for different ratios of ethanol to water at 300 C for 2 h.
with the results reported by Hakuta et al. [6]. Such a result can be explained in terms of the supercritical fluid theory [11,12]. As is well known, the supercritical temperature and pressure of water and ethanol are 374 C; 22 MPa and 243 C; 6.3 MPa, respectively [11]. The aqueous solution of ethanol can reach the supercritical state at a lower temperature and pressure. The supercritical temperature and pressure of the ethanol–water solution of 1:1 and 2:1 are 283 C; 9.2 MPa and 267 C; 7.5 MPa, respectively [12]. Obviously, the reaction temperature and pressure (300 C;
According to the results mentioned above, it is possible that the synthetic temperature continues to decrease with increase in the ratio of ethanol to water, which is beneficial to improving the properties of the powder. For ethanol to water ratios of 2:1 and 1:0, the effect of temperature on the synthetic reaction has been investigated under conditions similar to those described above. The phase composition of products synthesized at 2202290 C for 2 h are shown in Table 1. When the ethanol to water ratio was 2:1; the content of the YAG phase in the products increased with increase in reaction temperature, and single-phase YAG could be obtained at 290 C; When ethanol was used as the solvent (i.e. the ethanol/water ratio is equal to 1:0), although the development of the YAG phase in the products at the selected temperature was similar to those with an ethanol– water solution of 2:1, the formation temperature of single-phase YAG decreased to 264 C: Therefore, the beneficial effect of ethanol on the formation of YAG is further confirmed. Moreover, as seen in Table 1, it is worth noticing that single-phase YAG can be formed directly without other intermediate crystalline phases with ethanol as the solvent. This case is obviously different from those with the ethanol–water mixed solvent or
ARTICLE IN PRESS e1916
X. Zhang et al. / Journal of Crystal Growth 275 (2005) e1913–e1917
Table 1 Phase composition of the products synthesized under selected conditions Ethanol:water
220 C
264 C
274 C
290 C
2:1
YAIO3 g-AIOOH YOOH
YAIO3 YAG g-AIOOH YOOH
YAG YAIO3
YAG
Amphours
YAG (weaka)
YAG (strong)
YAG (strong)
1:0 a
Means intensity of XRD peaks.
with water alone. Further studies on its mechanism are now under way.
3.3. Effect of holding time In the above experiments, only the holding time was changed from 1 to 8 h at 290 C with the ratio of 2:1 (ethanol:water) to investigate its effect on the synthetic reactions. The XRD patterns of the products synthesized under selected holding times are given in Fig. 2. Fig. 2 shows that a holding time of 1 h was short due to the presence of phases other than a small amount of the YAG phase. When the holding time is 2 h or longer, singlephase YAG can be obtained. In fact, according to the difference of intensity in the XRD peaks, the stable YAG phase had been formed after 4 h or longer. However, as the TEM images shown in Fig. 3 indicate, the YAG powder synthesized with a holding time of 2 h is mainly composed of welldispersed and nearly spherical fine grains (80 nm on average) (Fig. 3a), which is favorable for sintering ceramics with high density [13,14]. When the holding time is prolonged to 8 h, the YAG grains had grown markedly, and the grain size increased to 220 nm on average. Additionally, the formation of agglomerates occurred (Fig. 3b). This phenomenon is mainly caused by the dissolution of smaller YAG grains and the growth of some larger grains. When large YAG grains grow, the attractive force between them is increased and aggregation among the particles occurs. Therefore, this research indicates that a holding time of 2–4 h
Fig. 2. (a–d) XRD patterns of the products synthesized for different holding times at 290 C:
ARTICLE IN PRESS X. Zhang et al. / Journal of Crystal Growth 275 (2005) e1913–e1917
e1917
powder with well-dispersed, nearly-spherical grains, and a relatively narrow grain size distribution can be synthesized at 2602290 C for 2–4 h. This kind of powder is most desirable for the compacting and sintering of YAG ceramics. Furthermore, this synthetic method may be a potential way for other oxide powders with good properties to be synthesized under moderate preparation conditions.
Acknowledgements This work was supported by the Key Project of Ministry of Education Science and Technology (0314), the Grant of State Key Program of China and the Natural Science Foundation of Shandong Province of China (Y2003F02).
References
Fig. 3. TEM photographs of the products synthesized for different holding times at 290 C: (a) 2 h, (b) 8 h.
should be suitable for the synthesis of YAG powder with good properties.
4. Conclusions YAG powder can be synthesized by the mixed solvothermal method under moderate conditions with an ethanol–water solution as the solvent. The presence of ethanol in mixed solvent is beneficial for the formation of YAG. When the ratio of ethanol to water is 2:1 or larger, single-phase YAG
[1] J.E. Geusic, H.M. Marcos, L.G.V. Uitert, Appl. Phys. Lett. 4 (1964) 182. [2] M. Veith, S. Mathur, A. Kareiva, M. Jilari, M. Zimmer, V. Huch, J. Mater. Chem. 9 (1999) 3069. [3] A. Ikesue, I. Furusato, K. Kamata, J. Am. Ceram. Soc. 78 (1995) 225. [4] S.M. Sim, K.A. Keller, J. Mater. Sci. 35 (2000) 713. [5] A. Ikesue, T. Kinoshita, K. Kammata, K. Yoshida, J. Am. Ceram. Soc. 78 (1995) 1033. [6] Y. Hakuta, K. Seino, H. Ura, T. Adschiri, H. Takizawa, K. Arai, J. Mater. Chem. 9 (1999) 2671. [7] J.G. Li, T. Ikegami, J.H. Lee, T. Moriand, Y. Yajima, J. Eur. Ceram. Soc. 20 (2000) 2395. [8] D. Hreniak, W. Strek, J. Alloys Compounds 341 (2002) 183. [9] G. De With, H. Van Dijk, J. Am. Mater. Res. Bull. 19 (1984) 1669. [10] M. Inoue, H. Otsu, H. Kominami, T. Inui, J. Am. Ceram. Soc. 74 (1991) 1452. [11] J.C. Zhang, Supercritical Fluid Extraction, Publishing House of Chemical Industry of China, Beijing, 2000, p. 7 (in Chinese). [12] Z.Q. Zhu, Publishing House of Chemical Industry of China, Beijing, 2000, p. 137 (in Chinese). [13] C.S. Fang, Y.W. Chen, Mater. Chem. Phys. 78 (2003) 739. [14] Y.T. Moon, H.K. Park, D.K. Kim, C.H. Kim, J. Am. Ceram. Soc. 78 (1995) 2690.