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Co-precipitation synthesis of lutetium aluminum garnet (LuAG) powders: The influence of ethanol
Optical Materials xxx (2016) 1e6
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Co-precipitation synthesis of lutetium aluminum garnet (LuAG) powders: The influence of ethanol Liangjie Pan a, b, Benxue Jiang a, *, Jintai Fan a, **, Pande Zhang a, c, Xiaojian Mao a, Long Zhang a, *** a b c
Key Laboratory of Materials for High-Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Science, Shanghai 201800, China School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 February 2016 Received in revised form 29 April 2016 Accepted 12 June 2016 Available online xxx
Aluminum Garnet (LuAG) precursors were co-precipitated by using ethanolewater as the precipitant solvent. The effect of different volume ratios of ethanol to water (R) on the preparation of pure-phase LuAG powders has been mainly studied. The evolution of phase, composition and micro-structure of the as-synthesized LuAG powders were characterized by TG/DTA, FTIR, XRD, BET, and SEM. The BETequivalent diameter of LuAG nano particles increased with R. The ethanolewater solvent does not change the main composition of the LuAG precursors, but has great influence on the morphology of the final LuAG nano particles. Uniformly dispersed LuAG powders calcined at 1200 C for 3 h with a particle size of approximately 120 nm were obtained by using ethanolewater solvent with proper R ¼ 1. The mechanism of ethanol in the preparation process was discussed. © 2016 Elsevier B.V. All rights reserved.
Keywords: Ethanolewater Morphology Co-precipitation LuAG Powder technology
1. Introduction Presently, transparent polycrystalline ceramic materials have attracted much attention for optical applications [1]. On one hand, LuAG is, besides YAG, a promising host lattice for solid state lasers [2e5]. On the other hand, LuAG-based scintillator crystals, which are similar to YAG, has received great interest for scintillating application due to its excellent physical and chemical properties [6]. However, it is difficult to obtain large single crystals, and it requires long time, high temperature and expensive experimental apparatus for crystal growth [7]. As an alternative, polycrystalline LuAG can be used as a substitute for single crystal LuAG because of its excellent scintillation performance, low-cost, short manufacturing period, and other characteristics [8,9]. Sintering from LuAG nano powders is one promising way to fabricate LuAG polycrystalline LuAG. Therefore, nano-scaled LuAG powders with no or little agglomeration are good candidates in order to achieve
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (B. Jiang), [email protected] (J. Fan), [email protected] (L. Zhang).
high transmittance, fine grain structure, and full density in polycrystalline LuAG ceramics. There is a lot of interest in finding ways to synthesize pure and uniformly dispersed ultrafine powders. Wet-chemical approaches have been developed and successfully used in the preparation of fine LuAG powder, including solegel method [10,11], solvothermal method [7,12], co-precipitation method [13,14], and homogeneous precipitation [15]. Compared with the other methods, the coprecipitation method is one of the most promising techniques because of its advantages, such as atomic level mixing of highpurity precursors, low processing temperature, low cost, ease of mass production, and so on [16]. In the co-precipitation process, parameters during the synthesis process of YAG powder are of important reference to LuAG powder. In previous [16,17], the proper volume ratio of alcohol to water R (0.2e1.2) is crucial for the formation of pure-phase YAG, and the results indicate that alcohol can play the same role as the surfactants and contribute to the well-dispersion of the powder. However, the size of the elliptical YAG particles is only about 30 nm [16], resulting in agglomeration problem, which takes place at high calcination temperature. The powders of small crystallite size has high sintering activity and leads to abnormal grain growth and intragranular porosity in the sintering process [18].
http://dx.doi.org/10.1016/j.optmat.2016.06.020 0925-3467/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: L. Pan, et al., Co-precipitation synthesis of lutetium aluminum garnet (LuAG) powders: The influence of ethanol, Optical Materials (2016), http://dx.doi.org/10.1016/j.optmat.2016.06.020
2
L. Pan et al. / Optical Materials xxx (2016) 1e6
At present, the synthesis of LuAG powder using AHC as the precipitant with good dispersion at high calcination temperature is rarely reported. In this paper, the LuAG powders of moderate particle size and with good dispersion were synthesized by using ethanolewater as the precipitant solvent at high calcination temperature (1200 C). The effects of different volume ratios of ethanol to water (R) on the preparation of LuAG powder have been mainly studied.
2. Experimental Lu2O3 (99.99%, diyang, Shanghai, China), Al(NO3)3$9H2O (99.99%, aladdin, Shanghai, China) and ammonium hydrogen carbonate (99.995, aladdin, Shanghai, China) were used as starting materials. All these chemicals were used as received without further purification. The stock solution of Lu(NO3)3 was prepared by dissolving Lu2O3 into hot nitric acid and diluting with deionized water. Al(NO3)3 aqueous solution stock was made by dissolving Al(NO3)3$9H2O into deionized water. Cation contents in the both stock solutions were assayed with titration method. These solutions were weighed according to the stoichiometric ratio of Lu3Al5O12, diluted to achieve a Lu3þ content of 0.1 mol/L and mixed homogeneously. (NH4)2SO4 (analytical purity, aladdin, Shanghai, China) was added into the mixed solutions as dispersant. Precipitant solution with concentration of 2.0 mol/L and pH ¼ 8.50 was obtained by dissolving NH4HCO3 in ethanolewater solvents with different R (R ¼ 0e1.5, higher ratio R results in ethanol waste and greatly reduces the solubility of NH4HCO3, which makes the experiment difficult to be done.). The mixed nitrate solutions were dripped into the precipitant at a speed of 6 ml/min. After titration, the suspensions were aged for 1 h with agitation subsequently centrifuged and washed six times, of which former three times were with deionized water, latter three times with ethanol. The precipitates were dried at 70 C for 24 h. The dried cakes were
crushed with a corundum pestle and mortar, and sieved through a 200-mesh screen. The sieved precursor powders were calcined at different temperatures for 3 h to form garnet phase of LuAG powders, respectively. The pH value of solutions before precipitation and after aging was monitored using a digital pH meter with an accuracy of 0.01 (SevenEasy S20, Mettler Toledo, Shanghai, China). The phase formation process and microstructure of LuAG powders were investigated by X-ray diffraction (XRD, Rigaku Co., Tokyo, Japan) and the scanning electron microscopy (SEM, JEOL, Tokyo, Japan). Thermal behaviors of the LuAG precursors were studied using thermogravimetry analysis and differential thermal analysis (TG/DTA7300, EXSTAR Series, JEOL, Tokyo, Japan) from room temperature to 1200 C at a heating rate of 10 C min1 in air, and the alpha alumina was used as a reference. The compositions of the precipitates were investigated by the Fourier transform infrared (FTIR) spectrometer (Thermo Nicolet Nexus, Youngstown, OH, USA) with KBr pellet. Specific surface area was measured on the LuAG powders with surface area equipment (Quadrasorb SI, USA), with N2 as the absorbate gas.
3. Results and discussion The SEM images of the LuAG precursors with different R are shown in Fig. 1. With the increase of R, the particle of LuAG precursors tended to become finer, more homogeneous and rounder. When R ¼ 1, the precursors were finest. Then precursors grew up a little with R increased to 1.5. As can be seen, the particle size decreases with increasing the ethanol composition roughly. The variation tendency of precursors are consistent with the results of the preparation of nano particles using alcoholewater method [19,20]. This change in particle size of LuAG precursors can be interpreted by the changes in electrostatic interactions and nucleation rate with varying the dielectric constant of ethanolewater
Fig. 1. SEM images of the LuAG precursors with different R. (a) R ¼ 0; (b) R ¼ 0.5; (c) R ¼ 1; (d) R ¼ 1.5.
Please cite this article in press as: L. Pan, et al., Co-precipitation synthesis of lutetium aluminum garnet (LuAG) powders: The influence of ethanol, Optical Materials (2016), http://dx.doi.org/10.1016/j.optmat.2016.06.020
L. Pan et al. / Optical Materials xxx (2016) 1e6
3
1 B ¼Aþ r ε
Fig. 2. FTIR spectra of LuAG precursors obtained from different solvents. (a): distilled water (R ¼ 0); (b): ethanolewater solvent (R ¼ 1).
Fig. 3. DTA-TG curves of the precursor (R ¼ 1).
solvents. The dependence of particle radius (r) on the dielectric constant (ε) of the solution can be simplified as
where values of A and B can be regarded as constants [19]. As the R increases, the dielectric constant (ε) decrease [21], leading to the decrease of the particle size of LuAG precursors with R. Fig. 2 shows the FTIR spectra of LuAG precursors prepared by using different solvents. Though the intensity of peaks of a and b seem different, the positions of the main peaks in the two curves are basically consistent with each other. And the positions of the main peaks (584 cm1, 849 cm1, 1416 cm1, 1539 cm1, 3419 cm1) is basically in line with the work of Yi-kun Liao et al. [8]. It indicates the main compositions of the precipitate is not sensitive to the variations of R. Fig. 3 shows the TG/DTA curves of the precursor produced under proper conditions (R ¼ 1). Two major peaks are identified on the DTA curve. The endothermic valley that appeared at 121 C is assigned to the removal of molecular water. The exothermic peak around 1001 C in the DTA curve is caused by the crystallization of LuAG phase, which is consistent with the XRD results in Fig. 4. At this temperature, a weight loss of 37.78% is clearly seen, which is attributed to the final decomposition of precursor. There is no weight loss in the TG curve above 1063 C, which implies complete decomposition of the precursor. XRD patterns of calcined precursor produced under proper conditions (R ¼ 1) at different temperatures for 3 h to form the garnet phase of LuAG (JCPDS card No. 73-1368) powders are shown in Fig. 4a. The precipitate remains amorphous when the calcining temperature is 800 C and crystallization begins at 900 C. The crystallization temperature (900 C) measured by XRD is lower than that (1001 C) given by TG/DTA analyzer, which is caused by the different holding time in high temperature and the hysteresis of the TG/DTA analyzer. Above 900 C, continued refinement of peak shapes and intensities are observed, indicating crystallite growth of the LuAG powder as temperature increases. No formation of other phases is found at any calcining temperature in Fig. 4a. The XRD patterns of the other three samples (R ¼ 0, 0.5, 1.5) calcined at different temperature are similar to the sample prepared with R ¼ 1. The XRD patterns of them calcined at 900 C are shown in Fig. 4b. It can be seen that the powders are pure, no impure phase can be observed. It indicates cations distribution uniformity at micro scale for different R. The morphology of LuAG powders calcined at 800e1100 C for
Fig. 4. XRD patterns of LuAG powders. (a) R ¼ 1, calcined at 800e1200 C; (b) R ¼ 0, 0.5, 1.5, calcined at 900 C.
Please cite this article in press as: L. Pan, et al., Co-precipitation synthesis of lutetium aluminum garnet (LuAG) powders: The influence of ethanol, Optical Materials (2016), http://dx.doi.org/10.1016/j.optmat.2016.06.020
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L. Pan et al. / Optical Materials xxx (2016) 1e6
Fig. 5. SEM images of the LuAG powders calcined at different temperatures for 3 h with different R. (a) R ¼ 0, 800 C; (b) R ¼ 1, 800 C; (c) R ¼ 0, 900 C; (d) R ¼ 1, 900 C; (e) R ¼ 0, 1100 C; (f) R ¼ 1, 1100 C.
3 h with different R is revealed in Fig. 5. By comparing the precursors with the samples calcined at 800 C, evident decomposition can be observed. Yet, the samples are amorphous according to the XRD results. Increasing the calcining temperature to 900 C lead to evident change of morphology. The change is due to the crystallization at this temperature. The growth of particles is observed when the calcining temperature is further increase of to 1100 C. It also can be seen from Fig. 5 that the powders prepared with R ¼ 1 exhibit better dispersity than that prepared with R ¼ 0. The results shows that the addition of ethanol can effectively influence the dispersion of the particles. In order to investigate the influence of ethanol to the dispersion of the LuAG nano powders, the morphology of all the samples calcined at 1200 C is examined. The dispersion state of the LuAG powders calcined at 1200 C for 3 h changes significantly with different R are shown in Fig. 6. It can be seen that adding ethanol can improve the dispersion of the powders. The sample prepared with R ¼ 1 is the least agglomerated. However, further increase the content of ethanol results in re-agglomeration. The mean primary particle size estimated from SEM images is denoted as DSEM. The
DSEM is 100e130 nm in Fig. 6. With the increase of R (R ¼ 0, 0.5, 1), the particle shape of LuAG powders tended to be ellipsoid and the average size of particles was increasing. When R ¼ 1.5, the average size of particles was maximum. The results indicate that ethanol can play the same role as the surfactants and contribute to the welldispersion and the increasing of particle size of powder at high calcination temperature (1200 C). The SBET values and DBET values of the resultant LuAG powders with different R are listed in Table 1. The particle size DBET was calculated from the following formula
DBET ¼
6
r$SBET
where r (6.72 g/cm3) is the theoretical density of LuAG, and SBET is the specific surface area measurement. For the powders synthesized with R ¼ 0, 0.5, 1 and 1.5, the corresponding SBET value is 8.268 m2/g, 8.316 m2/g,7.357 m2/g and 7.163 m2/g, respectively; DBET value is 108 nm, 107 nm, 121 nm and 125 nm, respectively. With the increase of R, the particle size of powders determined by
Please cite this article in press as: L. Pan, et al., Co-precipitation synthesis of lutetium aluminum garnet (LuAG) powders: The influence of ethanol, Optical Materials (2016), http://dx.doi.org/10.1016/j.optmat.2016.06.020
L. Pan et al. / Optical Materials xxx (2016) 1e6
5
Fig. 6. SEM images of the LuAG powders calcined at 1200 C for 3 h with different R. (a) R ¼ 0; (b) R ¼ 0.5; (c) R ¼ 1; and (d) R ¼ 1.5.
References Table 1 SBET values and DBET values of LuAG powders calcined at 1200 C for 3 h with different R. R
SBET (m2/g)
DBET (nm)
0 0.5 1 1.5
8.268 8.316 7.357 7.163
108 107 121 125
BET increases, which is consistent with SEM results in Fig. 6.
4. Conclusions In the present study, ethanol-water solvent method was introduced to synthesize LuAG nano-powders. The dielectric constant of ethanolewater solvents decreases caused by the addition of ethanol. As a result, the particle of LuAG precursors tended to become finer, more homogeneous and rounder. The powders synthesized by this method have larger specific surface area and better dispersion than that from distilled water solvent method. Uniformly dispersed LuAG powders calcined at 1200 C for 3 h with DBET ¼ 121 nm were obtained by using ethanolewater solvent with proper R ¼ 1.
Acknowledgments The work is financially supported by the National Nature Science Founds of China (No. 61378069, 61405221) and the Youth Innovation Promotion Association of CAS.
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Please cite this article in press as: L. Pan, et al., Co-precipitation synthesis of lutetium aluminum garnet (LuAG) powders: The influence of ethanol, Optical Materials (2016), http://dx.doi.org/10.1016/j.optmat.2016.06.020
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Co-precipitation synthesis of lutetium aluminum garnet (LuAG) powders: The influence of ethanol Liangjie Pan a, b, Benxue Jiang a, *, Jintai Fan a, **, Pande Zhang a, c, Xiaojian Mao a, Long Zhang a, *** a b c
Key Laboratory of Materials for High-Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Science, Shanghai 201800, China School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 February 2016 Received in revised form 29 April 2016 Accepted 12 June 2016 Available online xxx
Aluminum Garnet (LuAG) precursors were co-precipitated by using ethanolewater as the precipitant solvent. The effect of different volume ratios of ethanol to water (R) on the preparation of pure-phase LuAG powders has been mainly studied. The evolution of phase, composition and micro-structure of the as-synthesized LuAG powders were characterized by TG/DTA, FTIR, XRD, BET, and SEM. The BETequivalent diameter of LuAG nano particles increased with R. The ethanolewater solvent does not change the main composition of the LuAG precursors, but has great influence on the morphology of the final LuAG nano particles. Uniformly dispersed LuAG powders calcined at 1200 C for 3 h with a particle size of approximately 120 nm were obtained by using ethanolewater solvent with proper R ¼ 1. The mechanism of ethanol in the preparation process was discussed. © 2016 Elsevier B.V. All rights reserved.
Keywords: Ethanolewater Morphology Co-precipitation LuAG Powder technology
1. Introduction Presently, transparent polycrystalline ceramic materials have attracted much attention for optical applications [1]. On one hand, LuAG is, besides YAG, a promising host lattice for solid state lasers [2e5]. On the other hand, LuAG-based scintillator crystals, which are similar to YAG, has received great interest for scintillating application due to its excellent physical and chemical properties [6]. However, it is difficult to obtain large single crystals, and it requires long time, high temperature and expensive experimental apparatus for crystal growth [7]. As an alternative, polycrystalline LuAG can be used as a substitute for single crystal LuAG because of its excellent scintillation performance, low-cost, short manufacturing period, and other characteristics [8,9]. Sintering from LuAG nano powders is one promising way to fabricate LuAG polycrystalline LuAG. Therefore, nano-scaled LuAG powders with no or little agglomeration are good candidates in order to achieve
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (B. Jiang), [email protected] (J. Fan), [email protected] (L. Zhang).
high transmittance, fine grain structure, and full density in polycrystalline LuAG ceramics. There is a lot of interest in finding ways to synthesize pure and uniformly dispersed ultrafine powders. Wet-chemical approaches have been developed and successfully used in the preparation of fine LuAG powder, including solegel method [10,11], solvothermal method [7,12], co-precipitation method [13,14], and homogeneous precipitation [15]. Compared with the other methods, the coprecipitation method is one of the most promising techniques because of its advantages, such as atomic level mixing of highpurity precursors, low processing temperature, low cost, ease of mass production, and so on [16]. In the co-precipitation process, parameters during the synthesis process of YAG powder are of important reference to LuAG powder. In previous [16,17], the proper volume ratio of alcohol to water R (0.2e1.2) is crucial for the formation of pure-phase YAG, and the results indicate that alcohol can play the same role as the surfactants and contribute to the well-dispersion of the powder. However, the size of the elliptical YAG particles is only about 30 nm [16], resulting in agglomeration problem, which takes place at high calcination temperature. The powders of small crystallite size has high sintering activity and leads to abnormal grain growth and intragranular porosity in the sintering process [18].
http://dx.doi.org/10.1016/j.optmat.2016.06.020 0925-3467/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: L. Pan, et al., Co-precipitation synthesis of lutetium aluminum garnet (LuAG) powders: The influence of ethanol, Optical Materials (2016), http://dx.doi.org/10.1016/j.optmat.2016.06.020
2
L. Pan et al. / Optical Materials xxx (2016) 1e6
At present, the synthesis of LuAG powder using AHC as the precipitant with good dispersion at high calcination temperature is rarely reported. In this paper, the LuAG powders of moderate particle size and with good dispersion were synthesized by using ethanolewater as the precipitant solvent at high calcination temperature (1200 C). The effects of different volume ratios of ethanol to water (R) on the preparation of LuAG powder have been mainly studied.
2. Experimental Lu2O3 (99.99%, diyang, Shanghai, China), Al(NO3)3$9H2O (99.99%, aladdin, Shanghai, China) and ammonium hydrogen carbonate (99.995, aladdin, Shanghai, China) were used as starting materials. All these chemicals were used as received without further purification. The stock solution of Lu(NO3)3 was prepared by dissolving Lu2O3 into hot nitric acid and diluting with deionized water. Al(NO3)3 aqueous solution stock was made by dissolving Al(NO3)3$9H2O into deionized water. Cation contents in the both stock solutions were assayed with titration method. These solutions were weighed according to the stoichiometric ratio of Lu3Al5O12, diluted to achieve a Lu3þ content of 0.1 mol/L and mixed homogeneously. (NH4)2SO4 (analytical purity, aladdin, Shanghai, China) was added into the mixed solutions as dispersant. Precipitant solution with concentration of 2.0 mol/L and pH ¼ 8.50 was obtained by dissolving NH4HCO3 in ethanolewater solvents with different R (R ¼ 0e1.5, higher ratio R results in ethanol waste and greatly reduces the solubility of NH4HCO3, which makes the experiment difficult to be done.). The mixed nitrate solutions were dripped into the precipitant at a speed of 6 ml/min. After titration, the suspensions were aged for 1 h with agitation subsequently centrifuged and washed six times, of which former three times were with deionized water, latter three times with ethanol. The precipitates were dried at 70 C for 24 h. The dried cakes were
crushed with a corundum pestle and mortar, and sieved through a 200-mesh screen. The sieved precursor powders were calcined at different temperatures for 3 h to form garnet phase of LuAG powders, respectively. The pH value of solutions before precipitation and after aging was monitored using a digital pH meter with an accuracy of 0.01 (SevenEasy S20, Mettler Toledo, Shanghai, China). The phase formation process and microstructure of LuAG powders were investigated by X-ray diffraction (XRD, Rigaku Co., Tokyo, Japan) and the scanning electron microscopy (SEM, JEOL, Tokyo, Japan). Thermal behaviors of the LuAG precursors were studied using thermogravimetry analysis and differential thermal analysis (TG/DTA7300, EXSTAR Series, JEOL, Tokyo, Japan) from room temperature to 1200 C at a heating rate of 10 C min1 in air, and the alpha alumina was used as a reference. The compositions of the precipitates were investigated by the Fourier transform infrared (FTIR) spectrometer (Thermo Nicolet Nexus, Youngstown, OH, USA) with KBr pellet. Specific surface area was measured on the LuAG powders with surface area equipment (Quadrasorb SI, USA), with N2 as the absorbate gas.
3. Results and discussion The SEM images of the LuAG precursors with different R are shown in Fig. 1. With the increase of R, the particle of LuAG precursors tended to become finer, more homogeneous and rounder. When R ¼ 1, the precursors were finest. Then precursors grew up a little with R increased to 1.5. As can be seen, the particle size decreases with increasing the ethanol composition roughly. The variation tendency of precursors are consistent with the results of the preparation of nano particles using alcoholewater method [19,20]. This change in particle size of LuAG precursors can be interpreted by the changes in electrostatic interactions and nucleation rate with varying the dielectric constant of ethanolewater
Fig. 1. SEM images of the LuAG precursors with different R. (a) R ¼ 0; (b) R ¼ 0.5; (c) R ¼ 1; (d) R ¼ 1.5.
Please cite this article in press as: L. Pan, et al., Co-precipitation synthesis of lutetium aluminum garnet (LuAG) powders: The influence of ethanol, Optical Materials (2016), http://dx.doi.org/10.1016/j.optmat.2016.06.020
L. Pan et al. / Optical Materials xxx (2016) 1e6
3
1 B ¼Aþ r ε
Fig. 2. FTIR spectra of LuAG precursors obtained from different solvents. (a): distilled water (R ¼ 0); (b): ethanolewater solvent (R ¼ 1).
Fig. 3. DTA-TG curves of the precursor (R ¼ 1).
solvents. The dependence of particle radius (r) on the dielectric constant (ε) of the solution can be simplified as
where values of A and B can be regarded as constants [19]. As the R increases, the dielectric constant (ε) decrease [21], leading to the decrease of the particle size of LuAG precursors with R. Fig. 2 shows the FTIR spectra of LuAG precursors prepared by using different solvents. Though the intensity of peaks of a and b seem different, the positions of the main peaks in the two curves are basically consistent with each other. And the positions of the main peaks (584 cm1, 849 cm1, 1416 cm1, 1539 cm1, 3419 cm1) is basically in line with the work of Yi-kun Liao et al. [8]. It indicates the main compositions of the precipitate is not sensitive to the variations of R. Fig. 3 shows the TG/DTA curves of the precursor produced under proper conditions (R ¼ 1). Two major peaks are identified on the DTA curve. The endothermic valley that appeared at 121 C is assigned to the removal of molecular water. The exothermic peak around 1001 C in the DTA curve is caused by the crystallization of LuAG phase, which is consistent with the XRD results in Fig. 4. At this temperature, a weight loss of 37.78% is clearly seen, which is attributed to the final decomposition of precursor. There is no weight loss in the TG curve above 1063 C, which implies complete decomposition of the precursor. XRD patterns of calcined precursor produced under proper conditions (R ¼ 1) at different temperatures for 3 h to form the garnet phase of LuAG (JCPDS card No. 73-1368) powders are shown in Fig. 4a. The precipitate remains amorphous when the calcining temperature is 800 C and crystallization begins at 900 C. The crystallization temperature (900 C) measured by XRD is lower than that (1001 C) given by TG/DTA analyzer, which is caused by the different holding time in high temperature and the hysteresis of the TG/DTA analyzer. Above 900 C, continued refinement of peak shapes and intensities are observed, indicating crystallite growth of the LuAG powder as temperature increases. No formation of other phases is found at any calcining temperature in Fig. 4a. The XRD patterns of the other three samples (R ¼ 0, 0.5, 1.5) calcined at different temperature are similar to the sample prepared with R ¼ 1. The XRD patterns of them calcined at 900 C are shown in Fig. 4b. It can be seen that the powders are pure, no impure phase can be observed. It indicates cations distribution uniformity at micro scale for different R. The morphology of LuAG powders calcined at 800e1100 C for
Fig. 4. XRD patterns of LuAG powders. (a) R ¼ 1, calcined at 800e1200 C; (b) R ¼ 0, 0.5, 1.5, calcined at 900 C.
Please cite this article in press as: L. Pan, et al., Co-precipitation synthesis of lutetium aluminum garnet (LuAG) powders: The influence of ethanol, Optical Materials (2016), http://dx.doi.org/10.1016/j.optmat.2016.06.020
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Fig. 5. SEM images of the LuAG powders calcined at different temperatures for 3 h with different R. (a) R ¼ 0, 800 C; (b) R ¼ 1, 800 C; (c) R ¼ 0, 900 C; (d) R ¼ 1, 900 C; (e) R ¼ 0, 1100 C; (f) R ¼ 1, 1100 C.
3 h with different R is revealed in Fig. 5. By comparing the precursors with the samples calcined at 800 C, evident decomposition can be observed. Yet, the samples are amorphous according to the XRD results. Increasing the calcining temperature to 900 C lead to evident change of morphology. The change is due to the crystallization at this temperature. The growth of particles is observed when the calcining temperature is further increase of to 1100 C. It also can be seen from Fig. 5 that the powders prepared with R ¼ 1 exhibit better dispersity than that prepared with R ¼ 0. The results shows that the addition of ethanol can effectively influence the dispersion of the particles. In order to investigate the influence of ethanol to the dispersion of the LuAG nano powders, the morphology of all the samples calcined at 1200 C is examined. The dispersion state of the LuAG powders calcined at 1200 C for 3 h changes significantly with different R are shown in Fig. 6. It can be seen that adding ethanol can improve the dispersion of the powders. The sample prepared with R ¼ 1 is the least agglomerated. However, further increase the content of ethanol results in re-agglomeration. The mean primary particle size estimated from SEM images is denoted as DSEM. The
DSEM is 100e130 nm in Fig. 6. With the increase of R (R ¼ 0, 0.5, 1), the particle shape of LuAG powders tended to be ellipsoid and the average size of particles was increasing. When R ¼ 1.5, the average size of particles was maximum. The results indicate that ethanol can play the same role as the surfactants and contribute to the welldispersion and the increasing of particle size of powder at high calcination temperature (1200 C). The SBET values and DBET values of the resultant LuAG powders with different R are listed in Table 1. The particle size DBET was calculated from the following formula
DBET ¼
6
r$SBET
where r (6.72 g/cm3) is the theoretical density of LuAG, and SBET is the specific surface area measurement. For the powders synthesized with R ¼ 0, 0.5, 1 and 1.5, the corresponding SBET value is 8.268 m2/g, 8.316 m2/g,7.357 m2/g and 7.163 m2/g, respectively; DBET value is 108 nm, 107 nm, 121 nm and 125 nm, respectively. With the increase of R, the particle size of powders determined by
Please cite this article in press as: L. Pan, et al., Co-precipitation synthesis of lutetium aluminum garnet (LuAG) powders: The influence of ethanol, Optical Materials (2016), http://dx.doi.org/10.1016/j.optmat.2016.06.020
L. Pan et al. / Optical Materials xxx (2016) 1e6
5
Fig. 6. SEM images of the LuAG powders calcined at 1200 C for 3 h with different R. (a) R ¼ 0; (b) R ¼ 0.5; (c) R ¼ 1; and (d) R ¼ 1.5.
References Table 1 SBET values and DBET values of LuAG powders calcined at 1200 C for 3 h with different R. R
SBET (m2/g)
DBET (nm)
0 0.5 1 1.5
8.268 8.316 7.357 7.163
108 107 121 125
BET increases, which is consistent with SEM results in Fig. 6.
4. Conclusions In the present study, ethanol-water solvent method was introduced to synthesize LuAG nano-powders. The dielectric constant of ethanolewater solvents decreases caused by the addition of ethanol. As a result, the particle of LuAG precursors tended to become finer, more homogeneous and rounder. The powders synthesized by this method have larger specific surface area and better dispersion than that from distilled water solvent method. Uniformly dispersed LuAG powders calcined at 1200 C for 3 h with DBET ¼ 121 nm were obtained by using ethanolewater solvent with proper R ¼ 1.
Acknowledgments The work is financially supported by the National Nature Science Founds of China (No. 61378069, 61405221) and the Youth Innovation Promotion Association of CAS.
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Please cite this article in press as: L. Pan, et al., Co-precipitation synthesis of lutetium aluminum garnet (LuAG) powders: The influence of ethanol, Optical Materials (2016), http://dx.doi.org/10.1016/j.optmat.2016.06.020