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Citrate sol–gel method to prepare nanoparticles of a piezoelectric crystal material: La3Nb0.5Ga5.5O14 at low temperature
Journal of Alloys and Compounds 459 (2008) L1–L4
Letter
Citrate sol–gel method to prepare nanoparticles of a piezoelectric crystal material: La3Nb0.5Ga5.5O14 at low temperature F.P. Yu a , D.R. Yuan a,∗ , X.L. Duan a , L.M. Kong b , X.Z. Shi a , S.Y. Guo a , L.H. Wang a , X.F. Cheng a , X.Q. Wang a a
State Key Laboratory of Crystal Materials and Institute of Crystal Materials, Shandong University, Jinan 250100, PR China b Institute 53 of China’s Ordnance Industry, P.O. Box 108, Jinan 250031, PR China Received 19 December 2006; accepted 14 April 2007 Available online 19 April 2007
Abstract The nanoparticles of a piezoelectric crystal material: La3 Nb0.5 Ga5.5 O14 (LGN) have been prepared via a citrate sol–gel method, the process of which was simple and the crystallization temperature of LGN nanoparticles was low. The sintered samples were characterized by means of thermogravimetry/differential thermal analysis (TG/DTA), X-ray powder diffraction (XRPD), Fourier transform infrared (FT-IR) spectra, transmission electron microscope (TEM) and high-resolution transmission electron micrography (HRTEM). All the results illustrate that the trigonal crystallographic phase LGN nanoparticles were well crystallized. It is significant to the LGN crystal growth, as in which, the problem of decompensate and evaporate of Ga2 O3 at high temperatures may be resolved. © 2007 Elsevier B.V. All rights reserved. Keywords: Sol–gel synthesis; X-ray diffraction; Thermal analysis
1. Introduction Recently, progress of electric and communication technology requires new piezoelectric crystals with high thermal stability of the frequency and large electromechanical coupling factors and leads to the development of new piezoelectric crystal. Compounds with Ca3 Ga2 Ge4 O14 (CGG)-type [1] structure have been reported to be new promising piezoelectric materials for fabrication of filters with large pass bandwidths and oscillators with large shift or high frequency stability. Gallate crystals such as La3 Ga5 SiO14 (LGS), La3 Ga5.5 Nb0.5 O14 (LGN) and La3 Ga5.5 Ta0.5 O14 (LGT) have been actively investigated for application in the above-mentioned devices [2–5]. However, because one of the starting materials Ga2 O3 is apt to decompensate and evaporate at high temperatures, extra Ga2 O3 of 1–2 wt.% must be added to the starting materials in order to compensate for the volatilization of Ga2 O3 during the crystal growth process [6] or control the growth atmosphere in order to avoid the evaporation of gallium suboxide from melt during growth [3,7]. An alterna-
∗
Corresponding author. Tel.: +86 531 88362822; fax: +86 531 88564337. E-mail address: [email protected] (D.R. Yuan).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.04.216
tive way to get rid of these disadvantages is to use. Therefore, pure phase polycrystalline becomes important to the growth of high-quality single crystals. To our knowledge, there is no report about the preparation of LGN polycrystalline raw materials by sol–gel method. In the present work, we focus on the preparation and characterization of the LGN nanoparticles by citric sol–gel method using citric acid as a chelating agent. 2. Experimental procedure After Nb2 O5 (99.99%) was dissolved by HF acid fully, ammonia was added to form the white Nb(OH)5 deposition, which was washed, filtrated and then dissolved into citric acid, after the Nb–citric acid transparent solution was formed, La(NO3 )3 , Ga(NO3 )3 solutions were added, the final solutions were mixed with La3+ :Ga3+ :citric acid molar ratio of 3:5:16. After further mixing, the obtained solution was hearted and concentrated by slow evaporation at 80 ◦ C under continuous stirring using a magnetic agitator for several hours until the yellowish transparent gels were obtained. Finally, the gels were dried at 110 ◦ C for 24 h till the colour turned into brown. The dried mass, which was the so-called precursor, was then moved into a furnace and heated to different temperatures, pure phase LGN nanocrystals were obtained after sintered the precursor at 800 ◦ C for 6 h. The heating rate was controlled by FP21 at a speed of 100 ◦ C/h. The optimized procedures by citric sol–gel method are illustrated in Fig. 1. Thermogravimetric analysis/differential thermal analysis (TGA/DTA) of the original precursor was carried out on a SETA-RAM Labsys TGA-DTA16
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F.P. Yu et al. / Journal of Alloys and Compounds 459 (2008) L1–L4
Fig. 1. Procedure diagram of synthesis of LGN nanocrystal by sol–gel method. Thermal Analyser in flowing air atmosphere (50 ml/min) with a heating rate of 10 ◦ C/min. The crystallization process and phase identification of the powders calcined at different temperatures were identified by X-ray powder diffraction (XRPD) on a Bruker AXS (Advanced X-ray solutions) Inc. (D8 Advance) using Cu K␣ radiation (λ = 0.1540598 nm), and a Fourier transform infrared (FT-IR) (Nicolet 20 SX FTIR) spectrometer. The grain size and morphology of the heat-treated powders without being ground were characterized by an H-800 transmission electron microscope (TEM) and also, the microstructure of the nanocrystal was analyzed by Philips Tecnai 20U-TWIN high-resolution transmission electron microscopy (HRTEM) observation, which was performed with JEOL-2010 transmission electron microscope using an accelerating voltage of 200 kV.
Fig. 3. XRPD patterns of the samples sintered at (a) 600 ◦ C, (b) 750 ◦ C, (c) 800 ◦ C and (d) 900 ◦ C for 6 h.
strates that almost all of the weight loss occurred below 700 ◦ C and the ratio of which is about 84.83%, it is fairly consistent to the calculation in theory (85.02%). The obvious endothermic peaks around 168.1 ◦ C may be caused by the decomposition of nitrate and the loss of the crystalline water. One strong exothermic peak at the range of 400–700 ◦ C attributes to the combustion of citric acid, the other one around 700 ◦ C corresponds to the
3. Results and discussion 3.1. Thermal analyses The TGA/DTA curves of the original precursor are presented in Fig. 2. The DTA curve shows only one endothermic peak and two exothermic peaks up to 900 ◦ C. The TGA curve demon-
Fig. 2. TGA/DTA curves of the original precursor.
Fig. 4. Fourier transform infrared spectrometry of the precursors sintered at (a) 600 ◦ C, (b) 750 ◦ C, (c) 800 ◦ C for 6 h and LGN crystal (d).
F.P. Yu et al. / Journal of Alloys and Compounds 459 (2008) L1–L4
L3
crystallization of LGN phase. No significant weight loss was observed and the exothermic peak from 710 to 900 ◦ C can be presumed as the phase transformation started at about 710 ◦ C, which is consistent with the XRPD result. 3.2. X-ray powder diffraction The XRPD patterns of the LGN powders calcined at various temperatures are shown in Fig. 3. As the temperature increased, the diffraction peaks became intense, which indicates the crystallization of the powders. At 750 ◦ C, though the LGN phase has formed, Nb2 O5 phase exits too (see Fig. 3b), so the product is not pure. At 800 ◦ C, the precursor completely transformed to LGN and the diffraction peaks agreed well with those of pure LGN (JCPDS No. 840775). Trigonal La3 Nb0.5 Ga5.5 O14 was the only phase detected in the samples calcined above 800 ◦ C. These main peaks can be indexed as (2 0 0), (1 1 1), (0 2 1), (2 1 0), (0 0 2), (1 2 1) and (1 2 2) diffraction lines. According to Scherrer’s equation, the grain size of the samples calcined 800 ◦ C is about 30 nm. Compared with solid-stated synthesis method [6,8], these nanoscaled LGN powders are synthesized at fairly lower calcining temperature. The possible reasons for these cases may be the molecular level mixing of La3+ , Ga3+ and Nb5+ ions, the high degree of homogeneity shortened the ion diffusion distance [9]. 3.3. Fourier transform infrared spectrometry analysis FT-IR spectra of the raw materials sintered at various temperatures are shown in Fig. 4. The broad band centered at 3403 cm−1 is assigned to the O–H bond vibration of H2 O, indicating the existence of water absorbed in the samples. The bands
Fig. 5. TEM micrograph of the sample sintered at 800 ◦ C for 6 h; inset, corresponding SAEM pattern.
between 1000 and 1600 cm−1 are related to NO3 − ions (see Fig. 4a–c) [9]. The bands at 2168 and 2923 cm−1 in Fig. 4a represent the stretching vibrations of the remnant organic compound of citrate ions. With the increasing of temperature, the above-mentioned bands became weaker or disappeared. When the samples were heated to 800 ◦ C, new absorption peaks at 668, 583 and 499 cm−1 (see Fig. 4c) appeared, which indicates that the formation of La3 Nb0.5 Ga5.5 O14 nanocrystals. Fig. 4d shows the FT-IR spectrum of LGN single crystal, from which, one can see that the main absorbing peaks of Fig. 4c. This hypothesis was also confirmed by XRPD analysis. The bands at about 1550 and 1490 cm−1 were present in the powder calcined at 800 ◦ C and they show the existence of a little residues of organic compounds.
Fig. 6. HRTEM image of the sample sintered at 900 ◦ C for 6 h; inset, corresponding SAEM pattern (region b in part a).
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F.P. Yu et al. / Journal of Alloys and Compounds 459 (2008) L1–L4
3.4. Morphology and microstructure characterization Fig. 5 presents the morphologies and microstructure of the as-synthesized nanoscaled LGN powders calcined at 800 ◦ C. From the TEM photograph, it can be observed that all the particles are dispersed uniformly, and their sizes are between 20 and 30 nm. The selected area electron diffraction (SAED) in Fig. 5 clearly indicated that the sample was well crystallized at 800 ◦ C. Also, the crystalline structure of the sample sintered at 900 ◦ C is demonstrated in the HRTEM image (Fig. 6a), from which, one can see that the grains grew up significantly. In Fig. 6b, only one-dimension crystalline image was observed and the crystalline face of (0 0 2) was confirmed by the parallel fringes with spacing of 0.2564 nm. 4. Conclusions Single-phase nanoscaled LGN polycrystalline powders have been prepared by citrate sol–gel method. The pure phase powders were obtained by calcining the gel at fairly lower temperature (800 ◦ C) and shorter holding time (6 h) compared with solid-stated synthesis, which may resolve the problem of decompensate and evaporate of Ga2 O3 at high temperatures in the
crystal growth process. Single crystal growth of LGN with such kind of raw materials is presently under way. Acknowledgment This work is supported by the National Nature Science Foundation of China (NNSFC No. 50372034). References [1] E.L. Beloknoeva, N.V. Belov, Sov. Phys. Dokl. 26 (1981) 931. [2] Yu.V. Pusarevsky, P.A. Senushencov, P.A. Popov, B.B. Mill, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 42 (1995) 653. [3] H. Takeda, K. Shimamura, T. Kohna, T. Fukuda, J. Cryst. Growth 169 (1996) 503. [4] Yu.V. Pusarevsky, P.A. Senushencov, B.V. Mill, N.A. Moiseeva, Proceedings of the IEEE International Frequency Control Symposium, 1998, p. 742. [5] H. Kavanaka, H. Takeda, K. Shimamura, T. Fukuda, J. Cryst. Growth 183 (1998) 274. [6] H.K. Kong, J.Y. Wang, H.J. Zhang, X. Yin, J. Cryst. Growth 292 (2006) 408. [7] Z.M. Wang, D.R. Yuan, A.J. Wei, H.F. Qi, X.Z. Shi, D. Xu, M.K. Lv, J. Cryst. Growth 263 (2004) 389. [8] J.Y. Wang, X. Yin, R.J. Han, S.J. Zhang, H.K. Kong, H.J. Zhang, X.B. Hu, M.H. Jiang, Opt. Mater. 23 (2003) 393. [9] F.P. Yu, D.R. Yuan, X.F. Cheng, X.L. Duan, X.Q. Wang, L.M. Kong, L.H. Wang, Z.F. Li, Mater. Lett. 61 (2007) 2322.
Letter
Citrate sol–gel method to prepare nanoparticles of a piezoelectric crystal material: La3Nb0.5Ga5.5O14 at low temperature F.P. Yu a , D.R. Yuan a,∗ , X.L. Duan a , L.M. Kong b , X.Z. Shi a , S.Y. Guo a , L.H. Wang a , X.F. Cheng a , X.Q. Wang a a
State Key Laboratory of Crystal Materials and Institute of Crystal Materials, Shandong University, Jinan 250100, PR China b Institute 53 of China’s Ordnance Industry, P.O. Box 108, Jinan 250031, PR China Received 19 December 2006; accepted 14 April 2007 Available online 19 April 2007
Abstract The nanoparticles of a piezoelectric crystal material: La3 Nb0.5 Ga5.5 O14 (LGN) have been prepared via a citrate sol–gel method, the process of which was simple and the crystallization temperature of LGN nanoparticles was low. The sintered samples were characterized by means of thermogravimetry/differential thermal analysis (TG/DTA), X-ray powder diffraction (XRPD), Fourier transform infrared (FT-IR) spectra, transmission electron microscope (TEM) and high-resolution transmission electron micrography (HRTEM). All the results illustrate that the trigonal crystallographic phase LGN nanoparticles were well crystallized. It is significant to the LGN crystal growth, as in which, the problem of decompensate and evaporate of Ga2 O3 at high temperatures may be resolved. © 2007 Elsevier B.V. All rights reserved. Keywords: Sol–gel synthesis; X-ray diffraction; Thermal analysis
1. Introduction Recently, progress of electric and communication technology requires new piezoelectric crystals with high thermal stability of the frequency and large electromechanical coupling factors and leads to the development of new piezoelectric crystal. Compounds with Ca3 Ga2 Ge4 O14 (CGG)-type [1] structure have been reported to be new promising piezoelectric materials for fabrication of filters with large pass bandwidths and oscillators with large shift or high frequency stability. Gallate crystals such as La3 Ga5 SiO14 (LGS), La3 Ga5.5 Nb0.5 O14 (LGN) and La3 Ga5.5 Ta0.5 O14 (LGT) have been actively investigated for application in the above-mentioned devices [2–5]. However, because one of the starting materials Ga2 O3 is apt to decompensate and evaporate at high temperatures, extra Ga2 O3 of 1–2 wt.% must be added to the starting materials in order to compensate for the volatilization of Ga2 O3 during the crystal growth process [6] or control the growth atmosphere in order to avoid the evaporation of gallium suboxide from melt during growth [3,7]. An alterna-
∗
Corresponding author. Tel.: +86 531 88362822; fax: +86 531 88564337. E-mail address: [email protected] (D.R. Yuan).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.04.216
tive way to get rid of these disadvantages is to use. Therefore, pure phase polycrystalline becomes important to the growth of high-quality single crystals. To our knowledge, there is no report about the preparation of LGN polycrystalline raw materials by sol–gel method. In the present work, we focus on the preparation and characterization of the LGN nanoparticles by citric sol–gel method using citric acid as a chelating agent. 2. Experimental procedure After Nb2 O5 (99.99%) was dissolved by HF acid fully, ammonia was added to form the white Nb(OH)5 deposition, which was washed, filtrated and then dissolved into citric acid, after the Nb–citric acid transparent solution was formed, La(NO3 )3 , Ga(NO3 )3 solutions were added, the final solutions were mixed with La3+ :Ga3+ :citric acid molar ratio of 3:5:16. After further mixing, the obtained solution was hearted and concentrated by slow evaporation at 80 ◦ C under continuous stirring using a magnetic agitator for several hours until the yellowish transparent gels were obtained. Finally, the gels were dried at 110 ◦ C for 24 h till the colour turned into brown. The dried mass, which was the so-called precursor, was then moved into a furnace and heated to different temperatures, pure phase LGN nanocrystals were obtained after sintered the precursor at 800 ◦ C for 6 h. The heating rate was controlled by FP21 at a speed of 100 ◦ C/h. The optimized procedures by citric sol–gel method are illustrated in Fig. 1. Thermogravimetric analysis/differential thermal analysis (TGA/DTA) of the original precursor was carried out on a SETA-RAM Labsys TGA-DTA16
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F.P. Yu et al. / Journal of Alloys and Compounds 459 (2008) L1–L4
Fig. 1. Procedure diagram of synthesis of LGN nanocrystal by sol–gel method. Thermal Analyser in flowing air atmosphere (50 ml/min) with a heating rate of 10 ◦ C/min. The crystallization process and phase identification of the powders calcined at different temperatures were identified by X-ray powder diffraction (XRPD) on a Bruker AXS (Advanced X-ray solutions) Inc. (D8 Advance) using Cu K␣ radiation (λ = 0.1540598 nm), and a Fourier transform infrared (FT-IR) (Nicolet 20 SX FTIR) spectrometer. The grain size and morphology of the heat-treated powders without being ground were characterized by an H-800 transmission electron microscope (TEM) and also, the microstructure of the nanocrystal was analyzed by Philips Tecnai 20U-TWIN high-resolution transmission electron microscopy (HRTEM) observation, which was performed with JEOL-2010 transmission electron microscope using an accelerating voltage of 200 kV.
Fig. 3. XRPD patterns of the samples sintered at (a) 600 ◦ C, (b) 750 ◦ C, (c) 800 ◦ C and (d) 900 ◦ C for 6 h.
strates that almost all of the weight loss occurred below 700 ◦ C and the ratio of which is about 84.83%, it is fairly consistent to the calculation in theory (85.02%). The obvious endothermic peaks around 168.1 ◦ C may be caused by the decomposition of nitrate and the loss of the crystalline water. One strong exothermic peak at the range of 400–700 ◦ C attributes to the combustion of citric acid, the other one around 700 ◦ C corresponds to the
3. Results and discussion 3.1. Thermal analyses The TGA/DTA curves of the original precursor are presented in Fig. 2. The DTA curve shows only one endothermic peak and two exothermic peaks up to 900 ◦ C. The TGA curve demon-
Fig. 2. TGA/DTA curves of the original precursor.
Fig. 4. Fourier transform infrared spectrometry of the precursors sintered at (a) 600 ◦ C, (b) 750 ◦ C, (c) 800 ◦ C for 6 h and LGN crystal (d).
F.P. Yu et al. / Journal of Alloys and Compounds 459 (2008) L1–L4
L3
crystallization of LGN phase. No significant weight loss was observed and the exothermic peak from 710 to 900 ◦ C can be presumed as the phase transformation started at about 710 ◦ C, which is consistent with the XRPD result. 3.2. X-ray powder diffraction The XRPD patterns of the LGN powders calcined at various temperatures are shown in Fig. 3. As the temperature increased, the diffraction peaks became intense, which indicates the crystallization of the powders. At 750 ◦ C, though the LGN phase has formed, Nb2 O5 phase exits too (see Fig. 3b), so the product is not pure. At 800 ◦ C, the precursor completely transformed to LGN and the diffraction peaks agreed well with those of pure LGN (JCPDS No. 840775). Trigonal La3 Nb0.5 Ga5.5 O14 was the only phase detected in the samples calcined above 800 ◦ C. These main peaks can be indexed as (2 0 0), (1 1 1), (0 2 1), (2 1 0), (0 0 2), (1 2 1) and (1 2 2) diffraction lines. According to Scherrer’s equation, the grain size of the samples calcined 800 ◦ C is about 30 nm. Compared with solid-stated synthesis method [6,8], these nanoscaled LGN powders are synthesized at fairly lower calcining temperature. The possible reasons for these cases may be the molecular level mixing of La3+ , Ga3+ and Nb5+ ions, the high degree of homogeneity shortened the ion diffusion distance [9]. 3.3. Fourier transform infrared spectrometry analysis FT-IR spectra of the raw materials sintered at various temperatures are shown in Fig. 4. The broad band centered at 3403 cm−1 is assigned to the O–H bond vibration of H2 O, indicating the existence of water absorbed in the samples. The bands
Fig. 5. TEM micrograph of the sample sintered at 800 ◦ C for 6 h; inset, corresponding SAEM pattern.
between 1000 and 1600 cm−1 are related to NO3 − ions (see Fig. 4a–c) [9]. The bands at 2168 and 2923 cm−1 in Fig. 4a represent the stretching vibrations of the remnant organic compound of citrate ions. With the increasing of temperature, the above-mentioned bands became weaker or disappeared. When the samples were heated to 800 ◦ C, new absorption peaks at 668, 583 and 499 cm−1 (see Fig. 4c) appeared, which indicates that the formation of La3 Nb0.5 Ga5.5 O14 nanocrystals. Fig. 4d shows the FT-IR spectrum of LGN single crystal, from which, one can see that the main absorbing peaks of Fig. 4c. This hypothesis was also confirmed by XRPD analysis. The bands at about 1550 and 1490 cm−1 were present in the powder calcined at 800 ◦ C and they show the existence of a little residues of organic compounds.
Fig. 6. HRTEM image of the sample sintered at 900 ◦ C for 6 h; inset, corresponding SAEM pattern (region b in part a).
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F.P. Yu et al. / Journal of Alloys and Compounds 459 (2008) L1–L4
3.4. Morphology and microstructure characterization Fig. 5 presents the morphologies and microstructure of the as-synthesized nanoscaled LGN powders calcined at 800 ◦ C. From the TEM photograph, it can be observed that all the particles are dispersed uniformly, and their sizes are between 20 and 30 nm. The selected area electron diffraction (SAED) in Fig. 5 clearly indicated that the sample was well crystallized at 800 ◦ C. Also, the crystalline structure of the sample sintered at 900 ◦ C is demonstrated in the HRTEM image (Fig. 6a), from which, one can see that the grains grew up significantly. In Fig. 6b, only one-dimension crystalline image was observed and the crystalline face of (0 0 2) was confirmed by the parallel fringes with spacing of 0.2564 nm. 4. Conclusions Single-phase nanoscaled LGN polycrystalline powders have been prepared by citrate sol–gel method. The pure phase powders were obtained by calcining the gel at fairly lower temperature (800 ◦ C) and shorter holding time (6 h) compared with solid-stated synthesis, which may resolve the problem of decompensate and evaporate of Ga2 O3 at high temperatures in the
crystal growth process. Single crystal growth of LGN with such kind of raw materials is presently under way. Acknowledgment This work is supported by the National Nature Science Foundation of China (NNSFC No. 50372034). References [1] E.L. Beloknoeva, N.V. Belov, Sov. Phys. Dokl. 26 (1981) 931. [2] Yu.V. Pusarevsky, P.A. Senushencov, P.A. Popov, B.B. Mill, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 42 (1995) 653. [3] H. Takeda, K. Shimamura, T. Kohna, T. Fukuda, J. Cryst. Growth 169 (1996) 503. [4] Yu.V. Pusarevsky, P.A. Senushencov, B.V. Mill, N.A. Moiseeva, Proceedings of the IEEE International Frequency Control Symposium, 1998, p. 742. [5] H. Kavanaka, H. Takeda, K. Shimamura, T. Fukuda, J. Cryst. Growth 183 (1998) 274. [6] H.K. Kong, J.Y. Wang, H.J. Zhang, X. Yin, J. Cryst. Growth 292 (2006) 408. [7] Z.M. Wang, D.R. Yuan, A.J. Wei, H.F. Qi, X.Z. Shi, D. Xu, M.K. Lv, J. Cryst. Growth 263 (2004) 389. [8] J.Y. Wang, X. Yin, R.J. Han, S.J. Zhang, H.K. Kong, H.J. Zhang, X.B. Hu, M.H. Jiang, Opt. Mater. 23 (2003) 393. [9] F.P. Yu, D.R. Yuan, X.F. Cheng, X.L. Duan, X.Q. Wang, L.M. Kong, L.H. Wang, Z.F. Li, Mater. Lett. 61 (2007) 2322.