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Ethanol-dependent solvothermal synthesis of monodispersed YAG powders with precursor obtained through bubbling ammonia
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CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 16317–16321 www.elsevier.com/locate/ceramint
Ethanol-dependent solvothermal synthesis of monodispersed YAG powders with precursor obtained through bubbling ammonia Xianxue Lia, Tareque Odoom-Wubahb, Zhangxu Chena, Bingyun Zhenga,n, Jiale Huangb,n b
a Department of Chemical Engineering, College of Environmental & Biological Engineering, Putian University, Putian 351100, PR China Department of Chemical and Biochemical Engineering, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, PR China
Received 28 May 2014; received in revised form 7 July 2014; accepted 14 July 2014 Available online 22 July 2014
Abstract Yttrium aluminum garnet (YAG) precursor precipitates, which were produced through bubbling ammonia gas approach, were solvothermally treated in alcoholic solution to synthesize monodispersed YAG powders in this paper. A variety of techniques, such as thermogravimetry/ differential scanning calorimetry (TG/DSC), X-ray diffraction (XRD), Fourier transform infrared spectra (FT-IR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to characterize the as-synthesized YAG precursor and powders. The results showed that the crystallization temperature of the YAG phase was largely dependent on the ethanol. Increasing the ethanol content in the solvent could decrease the YAG crystallization temperature. When the ratio of ethanol to water was 3:1, single-phase YAG was accurately available at a relatively lower temperature of 320 1C. Moreover, the obtained YAG powders were well-defined monodispersed spheres with mean particle size of about 274 nm, and showed excellent sinterability. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: YAG; Solvothermal; Alcohol; Powder
1. Introduction Yttrium aluminum garnet (Y3Al5O12, YAG), in view of its high-temperature chemical stability (melting point at approximately 1970 1C) and exceptional high creep resistance, is regarded as an attractive ceramic material [1]. In the last decades, YAG-based transparent ceramics have inspired intensive enthusiasm because of their several advantages, such as low cost, short preparation time, high doping concentration and large size, etc. [2,3]. By far, it has already been established that, to obtain transparent YAG ceramics, synthesizing high-quality polycrystalline YAG powders is of primary significance [4,5]. As a result, a variety of techniques including solid-state reaction and wet chemical methods have been employed towards the preparation of well-defined YAG powders. Among these, solid-state reaction method, starting with respective oxide powders (Al2O3 and n
Corresponding authors. Tel.: þ86 594 2696445; fax: þ 86 594 2631931. E-mail addresses: [email protected] (B. Zheng), [email protected] (J. Huang). http://dx.doi.org/10.1016/j.ceramint.2014.07.070 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Y2O3), usually suffers from high calcining temperature of over 1600 1C and long aging time [2]. By contrast, wet chemical methods not only have the advantages of low synthesis temperature and short calcining time, but also are more flexible and versatile. Thus, a great number of wet chemical approaches have been extensively investigated up to now [6–13]. Nevertheless, to promote the phase transformation from the preliminary precursor to stoichiometric YAG, high calcination temperature (4800 1C) is usually necessary even for wet chemical methods. Consequently, severe agglomerations, which are disadvantageous to the acquisition of well-sinterable YAG powders, are easily formed among the YAG particles. Herein, to achieve highquality YAG powders, synthesis schedules are highly preferable to be innovated. In the last decade, both hydrothemal and solvothermal methods have proven to be effective approaches to produce well-defined powders with few agglomerations [14,15]. Due to the fact that hydrothermal (solvothermal) reactions are performed at high pressure (40–100 MPa), the phase transformation from the preliminary precursor to stoichiometric YAG can be realized at
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relatively lower temperatures (360–600 1C), thus avoiding the formation of hard agglomerations at higher calcination temperatures (4800 1C) and facilitating the obtainment of monodispersed YAG powders with high sintering ability [16]. Generally, hydrothermal method uses pure water as the medium. On the contrary, the media that solvothermal method utilizes are usually organic solvents, such as ethylene glycol, alcohol, butanediol, etc. Because organic solvents possess lower supercritical temperature and pressure than pure water [15], it is reasonably anticipated that YAG phase transformation could be solvothermally achieved under even more moderate conditions. Herein, in this present work, solvothermal method was adopted for the synthesis of monodispersed YAG powders. In addition, due to the easy controllability of ammonia flow rate with the aid of a gas flow meter, bubbling ammonia method rather than the commonly used titration technique was introduced to obtain the precursor precipitates. The detailed synthesis process and characterization of the produced YAG precursor and powders have been elaborated in this paper. 2. Experimental 2.1. Preparation of the precursor Y(NO3)3 6H2O (499.9% purity), Al(NO3)3 9H2O (499.9% purity) and NH3 gas (analytical grade) were used as the raw materials for the synthesis of the YAG precursor precipitates. First, concentrated solutions were obtained by dissolving Y (NO3)3 6H2O and Al(NO3)3 9H2O in distilled water, and Y3 þ and Al3 þ concentrations of the nitrate solutions were assayed by inductively coupled plasma (ICP) spectrophotometric technique. Subsequently, the above concentrated salt solutions were mixed homogeneously with appropriate volume according to the stoichiometry of YAG (Y3Al5O12). Next, NH3 gas was bubbled at a flow rate of 20 mL/min into the mixed solution under mild stirring at room temperature. The whole process was monitored with a pHmeter until the pH value reached 8.8. Afterwards, the resultant precursor precipitate was filtered, washed with distilled water and used for the following solvothermal reaction. During the total precipitation process, no ammonia escaped into air due to its ease of solubility in water. 2.2. Solvothermal synthesis In a typical synthesis, the above-obtained precursor precipitates and 25 ml solvent mixture of ethanol and water were added to a 50 ml stainless steel autoclave all at once. The system was sealed and then solvothermally treated at the designated temperature (290–330 1C) for different hours. After the reaction was cooled to room temperature, the products were collected at the bottom of the vessel, filtered, washed with distilled water, and then dried in vacuum at 60 1C for further characterizations. 2.3. Sintering The YAG powders thus obtained were then ball milled for 12 h with .5 wt% sintering aid tetraethoxysilane. The milled
slurry was dried, sieved, dry-pressed manually under 10 MPa into small cylinders and then cold-isostatically pressed under 200 MPa. The pressed specimens were finally sintered at 1700 1C for 3 h in a molybdenum wire-heated vacuum furnace. During the sintering period, the vacuum degree was 10 3 Pa. After sintering, the specimens were annealed at 1450 1C for 6 h in air. 2.4. Characterization methods and instruments Thermogravimetry/differential scanning calorimetry (TG/ DSC) of the original precursor was performed at constant pressure up to 1000 1C at a heating rate of 10 1C/min using a NETZSCH STA 449C thermal analyzer. The crystalline development of the precursor heat-treated at different temperatures was identified by X-ray diffraction (XRD) in a MAC Science MXP21VAHF diffractometer. Fourier transform infrared spectroscopy (FT-IR) was recorded employing a Bruker TENSOR-27 FT-IR spectrometer by a KBr disk method. Morphologies of the resultant YAG powders and microstructures of the sintered ceramic specimens were examined using scanning electron microscopy (SEM) (Model S-4300, Hitachi, Tokyo, Japan). For sintered bodies, the surface of the sample was polished to 1-mm finish with diamond paste and thermally etched at 1500 1C for 2 h to reveal the grain boundaries. Sintered density was measured by the Archimedes method, using deionized water as the immersion medium. The samples for transmission electron microscopy (TEM) were prepared by placing a drop of YAG suspension solution on carbon coated copper grids. The images were taken, and selected area electron diffraction (SAED) analysis was performed on a microscope (Tecnai F30, FEI, Netherlands). 3. Results and discussion 3.1. TG/DSC analysis TG/DSC traces of the precursor are given in Fig. 1. As shown, two major peaks were identified on the DSC curve. The broad endothermic peak centered at 202 1C was assigned to the discharge of molecular water, accompanied by a steeper weight loss of about 40%. The sharp exotherm at 920 1C was caused by the crystallization of YAG phase. Thus, to produce stoichiometric YAG phase at constant pressure, the calcination temperature should be at least elevated to as high as 920 1C. Meanwhile, for the TG curve, no significant weight loss was observed after 920 1C, indicating no change in phase composition. 3.2. Powder properties The XRD spectra of the powders calcined at various temperatures for different ethanol–water ratios (volume ratio) are shown in Fig. 2. As seen, the ethanol–water ratio had significant influence on the transformation temperature needed to produce pure YAG powders. First, in the case of ethanol– water ratio of 2:1, the XRD spectra corresponding to 320 1C were still composed of several intermediate impurity phases,
X. Li et al. / Ceramics International 40 (2014) 16317–16321
Fig. 1. TG/DSC traces of the precursor produced with ammonia gas.
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The possible reason is that pure alcohol possesses lower supercritical temperature and pressure than pure water, which allows the solution to reach the critical nucleation value for the YAG crystallites at a much lower temperature, thereby promoting the formation of YAG grains. Therefore, compared with the results in Fig. 1, lower YAG crystallization temperature was achieved under solvothermal conditions. To further reveal the crystallization process, FT-IR spectra of the above samples obtained at different ratios of ethanol to water were also carried out, as shown in Fig. 3. Take the case of ethanol–water ratio of 2:1 as an example. As seen, at 320 1C, the broad absorption band peak at 3450 cm 1 was associated with the stretching vibrations of the hydroxyl groups (O–H). The two small peaks at about 1516 and 1067 cm 1 were attributed to the C–H deformation mode and O–H in the primary alcohol, respectively. Besides, other peaks at about 1386, 742, 666 and 480 cm 1 corresponded to the bond-stretching mode of NH4þ and NO3 . As the reaction temperature was increased to 330 1C, the intensity of the above-mentioned absorption peaks became very weak and nearly disappeared. At the same time, in the range of 400– 800 cm 1, two broad peaks centered at 758 and 495 cm 1 were observed, which may have resulted from the Al–O, Y–O and Y–O–Al stretches, confirming the formation of a pure YAG phase. The conclusion was in good agreement with the above XRD results in Fig. 2. Furthermore, similar results were obtained in both the cases of ethanol–water ratio of 3:1 and 1:0. Therefore, taking into account both the XRD and FTIR data above, increasing the
Fig. 2. XRD pattern of the YAG powders obtained at different temperatures for various alcohol–water ratios.
namely, Y(OH)3, Y4Al2O9 (YAM) and YAlO3 (YAP). However, when the temperature was further increased to 330 1C, single-phase pure YAG could be accurately obtained, with all diffraction peaks matching well with that of YAG cubic crystal structure (ICSD Card no. 20090) and no detection of any intermediate impurity phases. By contrast, when the ethanol– water ratio was changed to 3:1, the pure YAG phase was accurately obtained only at a relatively lower temperature of 320 1C. In addition, continuously increasing the ethanol–water ratio to 1:0 (also pure alcohol), the stoichiometric YAG could be easily produced at a temperature as low as 300 1C. So, one can say that ethanol played a significant role during the crystallization process of the YAG phase. Increasing the ethanol content in the solvent could decrease the transformation temperature needed to produce pure YAG phase.
Fig. 3. FT-IR spectra of the YAG powders produced at different temperatures for various alcohol–water ratios.
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content of ethanol in the solvent was beneficial to the formation of the YAG phase under solvothermal conditions. Fig. 4 shows SEM (Fig. 4a) and TEM (Fig. 4b) micrographs of the as-synthesized YAG powders at 320 1C with the ratio of ethanol to water of 3:1. As seen, well-defined monodispersed spherical particles were observed, which may be ascribed to the lower synthesis temperature of 320 1C. The size distribution histogram (Fig. 4c) showed that the mean particle size was about 274 nm. In addition, the SAED pattern (Fig. 4d) confirmed that the YAG powders were polycrystalline in essence.
3.3. Ceramic properties In order to investigate the sinterability of the as-synthesized powders produced at 320 1C with the ratio of ethanol to water of 3:1, ceramic specimen was sintered under vacuum at 1700 1C for 3 h, and a relative density of about 99.9% of the theoretical density (4.55 g/cm3) was achieved. Moreover, SEM microstructures of both the fractured surfaces (Fig. 5a) and the polished surfaces (Fig. 5b) of the obtained ceramic specimen indicated that the specimen possesses a dense and nearly pore-free microstructure, with a mean grain size of about 6 μm. Therefore, the solvothermally synthesized monodispersed YAG powders showed excellent sinterability. However, possibly because of the bigger particle size, the higher sintering temperature of 1700 1C is still necessary.
Fig. 5. SEM micrographs of the ceramics sintered under vacuum at 1700 1C for 3 h: (a) the fractured surfaces and (b) the polished surfaces.
Fig. 4. (a) SEM, (b) TEM micrographs, (c) particle size distribution and (d) SAED pattern for the as-synthesized YAG powders at 320 1C with the ratio of ethanol to water of 3:1.
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Relevant experiment optimization is now being done in order to prepare YAG ceramics with desirable transparency. 4. Conclusions In summary, well-defined monodispersed YAG powders were solvothermally synthesized from a solvent mixture of alcohol and water. TG/DSC, XRD and FT-IR results indicated that the YAG phase transformation temperature was dependent on the alcohol content in the mixture. Increasing the ethanol content in the solvent could decrease the transformation temperature needed to produce the pure YAG powders. When the ratio of ethanol to water was 3:1, single-phase YAG could be obtained at a relatively lower temperature of 320 1C. Moreover, the as-obtained YAG powders with mean particle size of about 274 nm were monodispersed spheres, and showed excellent sinterability when sintered under vacuum at 1700 1C. Acknowledgments The authors acknowledge financial support from the National Natural Science Foundation of China (Grant no. 21206079) and Key Projects of Fujian Province Department of Science and Technology (2012H0033). References [1] J.-G. Li, T. Ikegami, J.-H. Lee, T. Mori, Y. Yajima, Co-precipitation synthesis and sintering of yttrium aluminum garnet (YAG) powders: the effect of precipitant, J. Eur. Cer. Soc. 20 (2000) 2395–2405. [2] A. Ikesue, I. Furusato, K. Kamata, Fabrication of polycrystal line, transparent YAG ceramics by a solid-state reaction method, J. Am. Ceram. Soc. 78 (1995) 225–228. [3] X. Li, B. Zheng, T. Odoom-Wubah, J. Huang, Co-precipitation synthesis and two-step sintering of YAG powders for transparent ceramics, Ceram. Int. 39 (2013) 7983–7988.
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[4] A. Ikesue, Y.L. Aung, Ceramic laser materials, Nat. Photon. 2 (2008) 721–727. [5] Y. Lv, W. Zhang, H. Liu, Y. Sang, H. Qin, J. Tan, L. Tong, Synthesis of nano-sized and highly sinterable Nd:YAG powders by the urea homogeneous precipitation method, Powder Technol. 217 (2012) 140–147. [6] H. Gong, D.-Y. Tang, H. Huang, M.-D. Han, T. Sun, J. Zhang, X-p Qin, J. Ma, Crystallization kinetics and characterization of nanosized Nd: YAG by a modified sol–gel combustion process, J. Cryst. Growth 362 (2013) 52–57. [7] L. Wang, H. Kou, Y. Zeng, J. Li, Y. Pan, X. Sun, J. Guo, The effect of precipitant concentration on the formation procedure of yttrium aluminum garnet (YAG) phase, Ceram. Int. 38 (2012) 3763–3771. [8] L. Wang, H. Kou, Y. Zeng, J. Li, Y. Pan, J. Guo, Preparation of YAG powders and ceramics through mixed precipitation method, Ceram. Int. 38 (2012) 4401–4405. [9] C. Marlot, E. Barraud, S. Le Gallet, M. Eichhorn, F. Bernard, Synthesis of YAG nanopowder by the co-precipitation method: influence of pH and study of the reaction mechanisms, J. Solid State Chem. 191 (2012) 114–120. [10] M.L. Saladino, G. Nasillo, D. Chillura Martino, E. Caponetti, Synthesis of Nd: YAG nanopowder using the citrate method with microwave irradiation, J. Alloys Compd. 491 (2010) 737–741. [11] X. Li, W. Wang, Preparation of uniformly dispersed YAG ultrafine powders by co-precipitation method with SDS treatment, Powder Technol. 196 (2009) 26–29. [12] J. Li, Y. Pan, F. Qiu, Y. Wu, J. Guo, Nanostructured Nd:YAG powders via gel combustion: the influence of citrate-to-nitrate ratio, Ceram. Int. 34 (2008) 141–149. [13] J. Su, Q.L. Zhang, C.J. Gu, D.L. Sun, Z.B. Wang, H.L. Qiu, A.H. Wang, S.T. Yin, Preparation and characterization of Y3Al5O12 (YAG) nanopowder by co-precipitation method, Mater. Res. Bull. 40 (2005) 1279–1285. [14] X. Li, H. Liu, J. Wang, H. Cui, S. Yang, I. Boughton, Solvothermal synthesis and luminescent properties of YAG: Tb nano-sized phosphors, J. Phys. Chem. Solid 66 (2005) 201–205. [15] X. Li, H. Liu, J. Wang, H. Cui, F. Han, R.I. Boughton, Production of nanosized YAG powders with spherical morphology and nonaggregation via a solvothermal method, J. Am. Ceram. Soc. 87 (2004) 2288–2290. [16] X. Zhang, H. Liu, W. He, J. Wang, X. Li, R.I. Boughton, Novel synthesis of YAG by solvothermal method, J. Cryst. Growth 275 (2005) 1913–1917.
CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 16317–16321 www.elsevier.com/locate/ceramint
Ethanol-dependent solvothermal synthesis of monodispersed YAG powders with precursor obtained through bubbling ammonia Xianxue Lia, Tareque Odoom-Wubahb, Zhangxu Chena, Bingyun Zhenga,n, Jiale Huangb,n b
a Department of Chemical Engineering, College of Environmental & Biological Engineering, Putian University, Putian 351100, PR China Department of Chemical and Biochemical Engineering, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, PR China
Received 28 May 2014; received in revised form 7 July 2014; accepted 14 July 2014 Available online 22 July 2014
Abstract Yttrium aluminum garnet (YAG) precursor precipitates, which were produced through bubbling ammonia gas approach, were solvothermally treated in alcoholic solution to synthesize monodispersed YAG powders in this paper. A variety of techniques, such as thermogravimetry/ differential scanning calorimetry (TG/DSC), X-ray diffraction (XRD), Fourier transform infrared spectra (FT-IR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to characterize the as-synthesized YAG precursor and powders. The results showed that the crystallization temperature of the YAG phase was largely dependent on the ethanol. Increasing the ethanol content in the solvent could decrease the YAG crystallization temperature. When the ratio of ethanol to water was 3:1, single-phase YAG was accurately available at a relatively lower temperature of 320 1C. Moreover, the obtained YAG powders were well-defined monodispersed spheres with mean particle size of about 274 nm, and showed excellent sinterability. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: YAG; Solvothermal; Alcohol; Powder
1. Introduction Yttrium aluminum garnet (Y3Al5O12, YAG), in view of its high-temperature chemical stability (melting point at approximately 1970 1C) and exceptional high creep resistance, is regarded as an attractive ceramic material [1]. In the last decades, YAG-based transparent ceramics have inspired intensive enthusiasm because of their several advantages, such as low cost, short preparation time, high doping concentration and large size, etc. [2,3]. By far, it has already been established that, to obtain transparent YAG ceramics, synthesizing high-quality polycrystalline YAG powders is of primary significance [4,5]. As a result, a variety of techniques including solid-state reaction and wet chemical methods have been employed towards the preparation of well-defined YAG powders. Among these, solid-state reaction method, starting with respective oxide powders (Al2O3 and n
Corresponding authors. Tel.: þ86 594 2696445; fax: þ 86 594 2631931. E-mail addresses: [email protected] (B. Zheng), [email protected] (J. Huang). http://dx.doi.org/10.1016/j.ceramint.2014.07.070 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Y2O3), usually suffers from high calcining temperature of over 1600 1C and long aging time [2]. By contrast, wet chemical methods not only have the advantages of low synthesis temperature and short calcining time, but also are more flexible and versatile. Thus, a great number of wet chemical approaches have been extensively investigated up to now [6–13]. Nevertheless, to promote the phase transformation from the preliminary precursor to stoichiometric YAG, high calcination temperature (4800 1C) is usually necessary even for wet chemical methods. Consequently, severe agglomerations, which are disadvantageous to the acquisition of well-sinterable YAG powders, are easily formed among the YAG particles. Herein, to achieve highquality YAG powders, synthesis schedules are highly preferable to be innovated. In the last decade, both hydrothemal and solvothermal methods have proven to be effective approaches to produce well-defined powders with few agglomerations [14,15]. Due to the fact that hydrothermal (solvothermal) reactions are performed at high pressure (40–100 MPa), the phase transformation from the preliminary precursor to stoichiometric YAG can be realized at
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relatively lower temperatures (360–600 1C), thus avoiding the formation of hard agglomerations at higher calcination temperatures (4800 1C) and facilitating the obtainment of monodispersed YAG powders with high sintering ability [16]. Generally, hydrothermal method uses pure water as the medium. On the contrary, the media that solvothermal method utilizes are usually organic solvents, such as ethylene glycol, alcohol, butanediol, etc. Because organic solvents possess lower supercritical temperature and pressure than pure water [15], it is reasonably anticipated that YAG phase transformation could be solvothermally achieved under even more moderate conditions. Herein, in this present work, solvothermal method was adopted for the synthesis of monodispersed YAG powders. In addition, due to the easy controllability of ammonia flow rate with the aid of a gas flow meter, bubbling ammonia method rather than the commonly used titration technique was introduced to obtain the precursor precipitates. The detailed synthesis process and characterization of the produced YAG precursor and powders have been elaborated in this paper. 2. Experimental 2.1. Preparation of the precursor Y(NO3)3 6H2O (499.9% purity), Al(NO3)3 9H2O (499.9% purity) and NH3 gas (analytical grade) were used as the raw materials for the synthesis of the YAG precursor precipitates. First, concentrated solutions were obtained by dissolving Y (NO3)3 6H2O and Al(NO3)3 9H2O in distilled water, and Y3 þ and Al3 þ concentrations of the nitrate solutions were assayed by inductively coupled plasma (ICP) spectrophotometric technique. Subsequently, the above concentrated salt solutions were mixed homogeneously with appropriate volume according to the stoichiometry of YAG (Y3Al5O12). Next, NH3 gas was bubbled at a flow rate of 20 mL/min into the mixed solution under mild stirring at room temperature. The whole process was monitored with a pHmeter until the pH value reached 8.8. Afterwards, the resultant precursor precipitate was filtered, washed with distilled water and used for the following solvothermal reaction. During the total precipitation process, no ammonia escaped into air due to its ease of solubility in water. 2.2. Solvothermal synthesis In a typical synthesis, the above-obtained precursor precipitates and 25 ml solvent mixture of ethanol and water were added to a 50 ml stainless steel autoclave all at once. The system was sealed and then solvothermally treated at the designated temperature (290–330 1C) for different hours. After the reaction was cooled to room temperature, the products were collected at the bottom of the vessel, filtered, washed with distilled water, and then dried in vacuum at 60 1C for further characterizations. 2.3. Sintering The YAG powders thus obtained were then ball milled for 12 h with .5 wt% sintering aid tetraethoxysilane. The milled
slurry was dried, sieved, dry-pressed manually under 10 MPa into small cylinders and then cold-isostatically pressed under 200 MPa. The pressed specimens were finally sintered at 1700 1C for 3 h in a molybdenum wire-heated vacuum furnace. During the sintering period, the vacuum degree was 10 3 Pa. After sintering, the specimens were annealed at 1450 1C for 6 h in air. 2.4. Characterization methods and instruments Thermogravimetry/differential scanning calorimetry (TG/ DSC) of the original precursor was performed at constant pressure up to 1000 1C at a heating rate of 10 1C/min using a NETZSCH STA 449C thermal analyzer. The crystalline development of the precursor heat-treated at different temperatures was identified by X-ray diffraction (XRD) in a MAC Science MXP21VAHF diffractometer. Fourier transform infrared spectroscopy (FT-IR) was recorded employing a Bruker TENSOR-27 FT-IR spectrometer by a KBr disk method. Morphologies of the resultant YAG powders and microstructures of the sintered ceramic specimens were examined using scanning electron microscopy (SEM) (Model S-4300, Hitachi, Tokyo, Japan). For sintered bodies, the surface of the sample was polished to 1-mm finish with diamond paste and thermally etched at 1500 1C for 2 h to reveal the grain boundaries. Sintered density was measured by the Archimedes method, using deionized water as the immersion medium. The samples for transmission electron microscopy (TEM) were prepared by placing a drop of YAG suspension solution on carbon coated copper grids. The images were taken, and selected area electron diffraction (SAED) analysis was performed on a microscope (Tecnai F30, FEI, Netherlands). 3. Results and discussion 3.1. TG/DSC analysis TG/DSC traces of the precursor are given in Fig. 1. As shown, two major peaks were identified on the DSC curve. The broad endothermic peak centered at 202 1C was assigned to the discharge of molecular water, accompanied by a steeper weight loss of about 40%. The sharp exotherm at 920 1C was caused by the crystallization of YAG phase. Thus, to produce stoichiometric YAG phase at constant pressure, the calcination temperature should be at least elevated to as high as 920 1C. Meanwhile, for the TG curve, no significant weight loss was observed after 920 1C, indicating no change in phase composition. 3.2. Powder properties The XRD spectra of the powders calcined at various temperatures for different ethanol–water ratios (volume ratio) are shown in Fig. 2. As seen, the ethanol–water ratio had significant influence on the transformation temperature needed to produce pure YAG powders. First, in the case of ethanol– water ratio of 2:1, the XRD spectra corresponding to 320 1C were still composed of several intermediate impurity phases,
X. Li et al. / Ceramics International 40 (2014) 16317–16321
Fig. 1. TG/DSC traces of the precursor produced with ammonia gas.
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The possible reason is that pure alcohol possesses lower supercritical temperature and pressure than pure water, which allows the solution to reach the critical nucleation value for the YAG crystallites at a much lower temperature, thereby promoting the formation of YAG grains. Therefore, compared with the results in Fig. 1, lower YAG crystallization temperature was achieved under solvothermal conditions. To further reveal the crystallization process, FT-IR spectra of the above samples obtained at different ratios of ethanol to water were also carried out, as shown in Fig. 3. Take the case of ethanol–water ratio of 2:1 as an example. As seen, at 320 1C, the broad absorption band peak at 3450 cm 1 was associated with the stretching vibrations of the hydroxyl groups (O–H). The two small peaks at about 1516 and 1067 cm 1 were attributed to the C–H deformation mode and O–H in the primary alcohol, respectively. Besides, other peaks at about 1386, 742, 666 and 480 cm 1 corresponded to the bond-stretching mode of NH4þ and NO3 . As the reaction temperature was increased to 330 1C, the intensity of the above-mentioned absorption peaks became very weak and nearly disappeared. At the same time, in the range of 400– 800 cm 1, two broad peaks centered at 758 and 495 cm 1 were observed, which may have resulted from the Al–O, Y–O and Y–O–Al stretches, confirming the formation of a pure YAG phase. The conclusion was in good agreement with the above XRD results in Fig. 2. Furthermore, similar results were obtained in both the cases of ethanol–water ratio of 3:1 and 1:0. Therefore, taking into account both the XRD and FTIR data above, increasing the
Fig. 2. XRD pattern of the YAG powders obtained at different temperatures for various alcohol–water ratios.
namely, Y(OH)3, Y4Al2O9 (YAM) and YAlO3 (YAP). However, when the temperature was further increased to 330 1C, single-phase pure YAG could be accurately obtained, with all diffraction peaks matching well with that of YAG cubic crystal structure (ICSD Card no. 20090) and no detection of any intermediate impurity phases. By contrast, when the ethanol– water ratio was changed to 3:1, the pure YAG phase was accurately obtained only at a relatively lower temperature of 320 1C. In addition, continuously increasing the ethanol–water ratio to 1:0 (also pure alcohol), the stoichiometric YAG could be easily produced at a temperature as low as 300 1C. So, one can say that ethanol played a significant role during the crystallization process of the YAG phase. Increasing the ethanol content in the solvent could decrease the transformation temperature needed to produce pure YAG phase.
Fig. 3. FT-IR spectra of the YAG powders produced at different temperatures for various alcohol–water ratios.
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content of ethanol in the solvent was beneficial to the formation of the YAG phase under solvothermal conditions. Fig. 4 shows SEM (Fig. 4a) and TEM (Fig. 4b) micrographs of the as-synthesized YAG powders at 320 1C with the ratio of ethanol to water of 3:1. As seen, well-defined monodispersed spherical particles were observed, which may be ascribed to the lower synthesis temperature of 320 1C. The size distribution histogram (Fig. 4c) showed that the mean particle size was about 274 nm. In addition, the SAED pattern (Fig. 4d) confirmed that the YAG powders were polycrystalline in essence.
3.3. Ceramic properties In order to investigate the sinterability of the as-synthesized powders produced at 320 1C with the ratio of ethanol to water of 3:1, ceramic specimen was sintered under vacuum at 1700 1C for 3 h, and a relative density of about 99.9% of the theoretical density (4.55 g/cm3) was achieved. Moreover, SEM microstructures of both the fractured surfaces (Fig. 5a) and the polished surfaces (Fig. 5b) of the obtained ceramic specimen indicated that the specimen possesses a dense and nearly pore-free microstructure, with a mean grain size of about 6 μm. Therefore, the solvothermally synthesized monodispersed YAG powders showed excellent sinterability. However, possibly because of the bigger particle size, the higher sintering temperature of 1700 1C is still necessary.
Fig. 5. SEM micrographs of the ceramics sintered under vacuum at 1700 1C for 3 h: (a) the fractured surfaces and (b) the polished surfaces.
Fig. 4. (a) SEM, (b) TEM micrographs, (c) particle size distribution and (d) SAED pattern for the as-synthesized YAG powders at 320 1C with the ratio of ethanol to water of 3:1.
X. Li et al. / Ceramics International 40 (2014) 16317–16321
Relevant experiment optimization is now being done in order to prepare YAG ceramics with desirable transparency. 4. Conclusions In summary, well-defined monodispersed YAG powders were solvothermally synthesized from a solvent mixture of alcohol and water. TG/DSC, XRD and FT-IR results indicated that the YAG phase transformation temperature was dependent on the alcohol content in the mixture. Increasing the ethanol content in the solvent could decrease the transformation temperature needed to produce the pure YAG powders. When the ratio of ethanol to water was 3:1, single-phase YAG could be obtained at a relatively lower temperature of 320 1C. Moreover, the as-obtained YAG powders with mean particle size of about 274 nm were monodispersed spheres, and showed excellent sinterability when sintered under vacuum at 1700 1C. Acknowledgments The authors acknowledge financial support from the National Natural Science Foundation of China (Grant no. 21206079) and Key Projects of Fujian Province Department of Science and Technology (2012H0033). References [1] J.-G. Li, T. Ikegami, J.-H. Lee, T. Mori, Y. Yajima, Co-precipitation synthesis and sintering of yttrium aluminum garnet (YAG) powders: the effect of precipitant, J. Eur. Cer. Soc. 20 (2000) 2395–2405. [2] A. Ikesue, I. Furusato, K. Kamata, Fabrication of polycrystal line, transparent YAG ceramics by a solid-state reaction method, J. Am. Ceram. Soc. 78 (1995) 225–228. [3] X. Li, B. Zheng, T. Odoom-Wubah, J. Huang, Co-precipitation synthesis and two-step sintering of YAG powders for transparent ceramics, Ceram. Int. 39 (2013) 7983–7988.
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[4] A. Ikesue, Y.L. Aung, Ceramic laser materials, Nat. Photon. 2 (2008) 721–727. [5] Y. Lv, W. Zhang, H. Liu, Y. Sang, H. Qin, J. Tan, L. Tong, Synthesis of nano-sized and highly sinterable Nd:YAG powders by the urea homogeneous precipitation method, Powder Technol. 217 (2012) 140–147. [6] H. Gong, D.-Y. Tang, H. Huang, M.-D. Han, T. Sun, J. Zhang, X-p Qin, J. Ma, Crystallization kinetics and characterization of nanosized Nd: YAG by a modified sol–gel combustion process, J. Cryst. Growth 362 (2013) 52–57. [7] L. Wang, H. Kou, Y. Zeng, J. Li, Y. Pan, X. Sun, J. Guo, The effect of precipitant concentration on the formation procedure of yttrium aluminum garnet (YAG) phase, Ceram. Int. 38 (2012) 3763–3771. [8] L. Wang, H. Kou, Y. Zeng, J. Li, Y. Pan, J. Guo, Preparation of YAG powders and ceramics through mixed precipitation method, Ceram. Int. 38 (2012) 4401–4405. [9] C. Marlot, E. Barraud, S. Le Gallet, M. Eichhorn, F. Bernard, Synthesis of YAG nanopowder by the co-precipitation method: influence of pH and study of the reaction mechanisms, J. Solid State Chem. 191 (2012) 114–120. [10] M.L. Saladino, G. Nasillo, D. Chillura Martino, E. Caponetti, Synthesis of Nd: YAG nanopowder using the citrate method with microwave irradiation, J. Alloys Compd. 491 (2010) 737–741. [11] X. Li, W. Wang, Preparation of uniformly dispersed YAG ultrafine powders by co-precipitation method with SDS treatment, Powder Technol. 196 (2009) 26–29. [12] J. Li, Y. Pan, F. Qiu, Y. Wu, J. Guo, Nanostructured Nd:YAG powders via gel combustion: the influence of citrate-to-nitrate ratio, Ceram. Int. 34 (2008) 141–149. [13] J. Su, Q.L. Zhang, C.J. Gu, D.L. Sun, Z.B. Wang, H.L. Qiu, A.H. Wang, S.T. Yin, Preparation and characterization of Y3Al5O12 (YAG) nanopowder by co-precipitation method, Mater. Res. Bull. 40 (2005) 1279–1285. [14] X. Li, H. Liu, J. Wang, H. Cui, S. Yang, I. Boughton, Solvothermal synthesis and luminescent properties of YAG: Tb nano-sized phosphors, J. Phys. Chem. Solid 66 (2005) 201–205. [15] X. Li, H. Liu, J. Wang, H. Cui, F. Han, R.I. Boughton, Production of nanosized YAG powders with spherical morphology and nonaggregation via a solvothermal method, J. Am. Ceram. Soc. 87 (2004) 2288–2290. [16] X. Zhang, H. Liu, W. He, J. Wang, X. Li, R.I. Boughton, Novel synthesis of YAG by solvothermal method, J. Cryst. Growth 275 (2005) 1913–1917.