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Synthesis of (Bi0.5Na0.5)TiO3 nanocrystalline powders by stearic acid gel method
Materials Chemistry and Physics 90 (2005) 282–285
Synthesis of (Bi0.5Na0.5)TiO3 nanocrystalline powders by stearic acid gel method Junjie Hao∗ , Xiaohui Wang, Renzheng Chen, Longtu Li State Key Laboratory of New Ceramics and Fine Processing, Department of Material Science and Engineering, Tsinghua University, Beijing 100084, PR China Received 23 September 2003; received in revised form 6 January 2004; accepted 20 May 2004
Abstract Bismuth sodium titanate (Na0.5 Bi0.5 )TiO3 (NBT), is considered to be an excellent candidate for a key material of lead-free piezoelectric ceramics. In this study, we propose a stearic acid gel method for the preparation of nanocrystalline single phase NBT powder at relatively low treatment-temperature. Infrared (IR) spectroscopy, differential thermal analysis (DTA), thermogravimetric analysis (TG) and X-ray diffraction (XRD) were used to characterize the process of crystallization. The particle size and morphology of the calcined powders were examined by TEM. It shows that pure single phase NBT powders could be obtained at 700 ◦ C for 1 h, and the particle size is about 20 nm. With an increase in the calcination temperature, crystallite size increased. The powders were further pressed into disk and sintered at 1120 ◦ C for 2 h in air, and its density and microstructure were compared with traditionally prepared samples. © 2004 Elsevier B.V. All rights reserved. Keywords: Stearic acid gel method; Nanocrystal; Lead free; Piezoelectric
1. Introduction Ever since the discovery of piezoelectric effect, piezoelectric materials have been rapidly developed and widely used [1]. At present, the most widely used piezoelectric materials are Pb(Zr, Ti)O3 (PZT)-based ceramics because of their superior piezoelectric properties. However, because the evaporation of harmful lead oxides during the preparation of Pb-contained ceramics has detrimental influence on environment, lead-free piezoelectric materials have been studied in order to replace PZT-based ceramics. Bismuth sodium titanate (Na0.5 Bi0.5 )TiO3 (abbreviated to NBT), is considered to be an excellent candidate for a key material of lead-free piezoelectric and/or pyroelectric ceramics because of the ferroelectric properties [2–4]. The NBT shows strong ferroelectric properties of a large remanent polarization, Pr = 38 C cm−2 , and has a Curie temperature, Tc = 320 ◦ C and a phase transition point from ferroelectric to antiferroelectric, Tp = 200 ◦ C [5,6]. However, conventional solid-state method for synthesizing NBT ceramic powder often format large grains which are difficult to disperse and affect the sintering properties of NBT [7]. Re∗ Corresponding author. Tel.: +86 10 6278 2195; fax: +86 10 6277 3650. E-mail address: [email protected] (J. Hao).
0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.05.019
cently, alternate methods of powder synthesis such as sol–gel and hydrothermal method have been developed by many authors [8,9]. We developed the stearic acid gel method to synthesize nanocrystalline NBT powders. In this route, the carboxylic acid group and long carbon chain in stearic acid endow it with strong ability to disperse metal precursors. In addition, the mixing process is performed in a melted state, therefore, metallic ions were well dispersed and separated by stearic acid in the mixing system. It is reported that each component was uniformly even in the resulting mixture after removing organic substance by combustion [10,11]. Moreover, this synthetic process is easily controlled and convenient in comparison with other methods.
2. Experimental 2.1. Preparation Fine NBT powders were prepared by the stearic acid gel method using stearic acid, tetrabutyl titanate, sodium acetate, and bismuth nitrate as raw materials. Firstly, an appropriate amount of stearic acid was melted at 80 ◦ C by heating. Secondly, CH3 COONa, Bi(NO3 )3 and Ti(OC4 H9 ) were added stoichiometrically into the molten stearic acid, according to
J. Hao et al. / Materials Chemistry and Physics 90 (2005) 282–285
283
the formula of Na0.5 Bi0.5 TiO3 . After that, the solution was heated at 80–100 ◦ C for about 2 h to obtain a transparent sol. The sol was then cooled to room temperature and the gel was formed. Finally, the gel precursor was decomposed at a temperature higher than 600 ◦ C in air; white nanocrystals were obtained. 2.2. Characterization Infrared (IR) spectroscopy was used for monitoring the structural changes occurring during the synthetic process. Differential thermal analysis (DTA) and thermogravimetric analysis (TG) were used to characterize the process of crystallization. The phase identification and structure analysis for the thermally treated samples were performed by powder X-ray diffraction (XRD). The particle size and morphology of the calcined powders were examined by transmission electron microscope (TEM). The powders were further pressed into disk (10 mm o.d., 0.5–1.5 mm in thickness) and sintered at 1120 ◦ C for 2 h in air. The density and microstructure were compared with samples prepared via conventional process.
3. Results and discussion The structure changes during the synthetic process were monitored by Fourier transform infrared spectroscopy (FT-IR) analysis. The IR spectra of the stearic acid, the precursor and powders calcined in different temperature are shown in Fig. 1. It can be seen that the stearic and the precursor gel have four strong band at 940, 1730, 2849 and 2921 cm−1 , indicating the presence of carbonate groups [12]. After firing more than 600 ◦ C, the band intensity of the carbonate group was reduced significantly, but a large band appeared at 600–650 cm−1 . Music et al. [13] have assigned
Fig. 1. FT-IR spectra of the stearic acid, gel, and powders calcined at different temperature.
Fig. 2. DTA and TG curve of the gel.
650–550 cm−1 to Ti–O vibration. With the peak appeared at 650 cm−1 , revealed that the formation of a large amount of NBT [14]. All carbonate groups disappeared after the sample was heated at 600 ◦ C, at which the precursor attained a stable weight. Fig. 2 shows the result of DTA and TG analysis of the gel. For DTA curve, there are three peaks, the first one appears at about 130 ◦ C, and it is due to the evaporation of water and the melting of the gel. The second peak at about 430 ◦ C was caused by burning of organic substances. The last one (612 ◦ C) is regarded as the temperature of the solid-state reaction. The TG curve of stearic acid (Fig. 3) shows that all stearic acid burnt out by 450 ◦ C and there is no loss in weight thereafter, this correspond to the second peak of DTA curve. In Fig. 2, the weight loss of NBT gel at the temperature range of 450–600 ◦ C is caused by the decomposition of nitrate material such as Bi(NO3 )3 or sodium acetate. From the analysis of Figs. 2 and 3, all combustion and decomposition of the material and the crystallization of NBT has finished by 600 ◦ C.
Fig. 3. TG curve of stearic acid.
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Fig. 4. XRD of NBT powder calcined at different temperatures: (a) 600 ◦ C; (b) 700 ◦ C; (c) 800 ◦ C; (d) solid-state reaction NBT.
Fig. 4 shows XRD patterns of NBT powders calcined at different temperature. It can be seen that nanocrystalline NBT was formed at 600 ◦ C and developed with increasing temperatures. Comparing with solid-state reaction NBT powders, the structure of NBT nanocrystalline belongs to pure pervoskite type, and no other intermediate phase is found. The particle sizes and morphologies of NBT powders fired at different temperatures were examined by TEM. As shown in Fig. 5(a)–(c), all NBT particles were very small, with an average size of 10–40 m. The formation of nanosized particles might be due to the combustion of organic present in the gel, along with the evolution of gases. Particle morphology of calcined powders at 600 ◦ C for 1 h is not very clear, with an average primary particle size around 10 nm. The average particle size increases with calcining temperature. At 700 ◦ C for 1 h, the particle size is about 10–20 nm with clear morphology. The primary paricle size of powders calcined at 800 ◦ C for 1 h is about 20–40 nm and the grains became
Fig. 6. (a) SEM of NBT ceramic prepared by fine particles. (b) SEM of NBT prepared by traditional process.
Fig. 5. TEM of NBT powders calcined at different temperatures. (a) 600 ◦ C; (b) 700 ◦ C; (c) 800 ◦ C.
J. Hao et al. / Materials Chemistry and Physics 90 (2005) 282–285 Table 1 Piezoelectric and dielectric properties of samples prepared by stearic acid gel method and solid-state method
ε (at 1 MHz) d33 (PC N−1 )
Stearic acid gel method sample
Conventional solid-state method sample
466 60
550 85
285
more than traditional process. The particle size is controlled by the calcining temperature. The sintered samples by the synthesized powders are less porous in microstructure and have higher piezoelectric constant than that by traditional process.
Acknowledgements square. All particle size is much smaller than that by the solid-state method. The 700 ◦ C synthesized powder was dry-pressed at a pressure of about 100 MPa to form pellets. Then, the specimens were sintered at 1120 ◦ C for 2 h in air. The bulk density of the sintered samples was measured by the Archimedes method. The measured density ratio is 97%, higher than that 90% of the solid phase reaction samples. Fig. 6(a) and (b) shows the SEM images of sintered sample by the synthesized powder and the surface of traditional NBT. The sintering temperature was 1120 and 1150 ◦ C, respectively. Compared with the sample prepared by traditional process, this sample derived from nanoNBT is less porous and has the homogeneous microstructure with the grain size of about 200 nm. This result shows that the improved microstructure homogeneity and high density NBT ceramics can be obtained by using nanocrystalline powders. The dielectric constant ε and piezoelectric constant d33 at room temperature was measured respectively. The results are shown in Table 1. It can be seen that the ε and d33 values of the specimen of stearic acid gel method are approximately comparable with, that of conventional solid-state method samples.
4. Conclusion A stearic acid gel method is an effective route to synthesize reactive nanosize NBT powders, which can be sintered
This work was supported by the Ministry of Sciences and Technology of China through 973-project under Grant 2002CB613301 and High Technology Research and Development project of PR China under Grant 2001AA325010. References [1] B.J. Chu, D.R. Chen, G.R. Li, Q.R. Yin, J. Eur. Ceram. Soc. 22 (2002) 2115. [2] A. Herabut, A. Dafan, J. Am. Ceram. Soc. 80 (1997) 2954. [3] M.S. Hagiyev, I.H. Ismaizade, A.K. Abiyev, Ferroelectrics 56 (1984) 215. [4] J.V. Zvirgzds, P.P. Kapostis, V.A. Isupov, Ferroelectrics 40 (1980) 75. [5] K. Sakata, Y. Masuda, Ferroelectrics 95 (1974) 347. [6] T. Takenaka, K. Maruyama, K. Sakata, Jpn. J. Appl. Phys. 30 (1991) 2263. [7] P. Pookmaneea, S. Phanichphanta, R.B. Heimann, Ceram. Forum Int. 78 (2001) E27. [8] X.Z. Jing, Y.X. Li, Q.R. Yin, Mater. Sci. Eng. B 99 (2003) 506. [9] M.L. Zhao, C.L. Wang, W.L. Zhong, et al., Acta Phys. Sin. 52 (2003) 229. [10] J. Yang, D. Li, X. Wang, et al., J. Solid State Chem. 165 (2002) 193. [11] X.H. Wang, D. Li, L.D. Lu, et al., J. Alloys Compd. 45 (1996) 237. [12] R. Urlaub, U. Posset, R. Thull, J. Non-Cryst. Solids 265 (2000) 276. [13] S. Music, M. Gotic, M. Ivanda, S. Popovic, A. Rukovic, et al., Mater. Sci. Eng. B 47 (1997) 33. [14] C.Y. Kim, T. Sekino, K. Niihara, J. Am. Ceram. Soc. 86 (2003) 1464.
Synthesis of (Bi0.5Na0.5)TiO3 nanocrystalline powders by stearic acid gel method Junjie Hao∗ , Xiaohui Wang, Renzheng Chen, Longtu Li State Key Laboratory of New Ceramics and Fine Processing, Department of Material Science and Engineering, Tsinghua University, Beijing 100084, PR China Received 23 September 2003; received in revised form 6 January 2004; accepted 20 May 2004
Abstract Bismuth sodium titanate (Na0.5 Bi0.5 )TiO3 (NBT), is considered to be an excellent candidate for a key material of lead-free piezoelectric ceramics. In this study, we propose a stearic acid gel method for the preparation of nanocrystalline single phase NBT powder at relatively low treatment-temperature. Infrared (IR) spectroscopy, differential thermal analysis (DTA), thermogravimetric analysis (TG) and X-ray diffraction (XRD) were used to characterize the process of crystallization. The particle size and morphology of the calcined powders were examined by TEM. It shows that pure single phase NBT powders could be obtained at 700 ◦ C for 1 h, and the particle size is about 20 nm. With an increase in the calcination temperature, crystallite size increased. The powders were further pressed into disk and sintered at 1120 ◦ C for 2 h in air, and its density and microstructure were compared with traditionally prepared samples. © 2004 Elsevier B.V. All rights reserved. Keywords: Stearic acid gel method; Nanocrystal; Lead free; Piezoelectric
1. Introduction Ever since the discovery of piezoelectric effect, piezoelectric materials have been rapidly developed and widely used [1]. At present, the most widely used piezoelectric materials are Pb(Zr, Ti)O3 (PZT)-based ceramics because of their superior piezoelectric properties. However, because the evaporation of harmful lead oxides during the preparation of Pb-contained ceramics has detrimental influence on environment, lead-free piezoelectric materials have been studied in order to replace PZT-based ceramics. Bismuth sodium titanate (Na0.5 Bi0.5 )TiO3 (abbreviated to NBT), is considered to be an excellent candidate for a key material of lead-free piezoelectric and/or pyroelectric ceramics because of the ferroelectric properties [2–4]. The NBT shows strong ferroelectric properties of a large remanent polarization, Pr = 38 C cm−2 , and has a Curie temperature, Tc = 320 ◦ C and a phase transition point from ferroelectric to antiferroelectric, Tp = 200 ◦ C [5,6]. However, conventional solid-state method for synthesizing NBT ceramic powder often format large grains which are difficult to disperse and affect the sintering properties of NBT [7]. Re∗ Corresponding author. Tel.: +86 10 6278 2195; fax: +86 10 6277 3650. E-mail address: [email protected] (J. Hao).
0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.05.019
cently, alternate methods of powder synthesis such as sol–gel and hydrothermal method have been developed by many authors [8,9]. We developed the stearic acid gel method to synthesize nanocrystalline NBT powders. In this route, the carboxylic acid group and long carbon chain in stearic acid endow it with strong ability to disperse metal precursors. In addition, the mixing process is performed in a melted state, therefore, metallic ions were well dispersed and separated by stearic acid in the mixing system. It is reported that each component was uniformly even in the resulting mixture after removing organic substance by combustion [10,11]. Moreover, this synthetic process is easily controlled and convenient in comparison with other methods.
2. Experimental 2.1. Preparation Fine NBT powders were prepared by the stearic acid gel method using stearic acid, tetrabutyl titanate, sodium acetate, and bismuth nitrate as raw materials. Firstly, an appropriate amount of stearic acid was melted at 80 ◦ C by heating. Secondly, CH3 COONa, Bi(NO3 )3 and Ti(OC4 H9 ) were added stoichiometrically into the molten stearic acid, according to
J. Hao et al. / Materials Chemistry and Physics 90 (2005) 282–285
283
the formula of Na0.5 Bi0.5 TiO3 . After that, the solution was heated at 80–100 ◦ C for about 2 h to obtain a transparent sol. The sol was then cooled to room temperature and the gel was formed. Finally, the gel precursor was decomposed at a temperature higher than 600 ◦ C in air; white nanocrystals were obtained. 2.2. Characterization Infrared (IR) spectroscopy was used for monitoring the structural changes occurring during the synthetic process. Differential thermal analysis (DTA) and thermogravimetric analysis (TG) were used to characterize the process of crystallization. The phase identification and structure analysis for the thermally treated samples were performed by powder X-ray diffraction (XRD). The particle size and morphology of the calcined powders were examined by transmission electron microscope (TEM). The powders were further pressed into disk (10 mm o.d., 0.5–1.5 mm in thickness) and sintered at 1120 ◦ C for 2 h in air. The density and microstructure were compared with samples prepared via conventional process.
3. Results and discussion The structure changes during the synthetic process were monitored by Fourier transform infrared spectroscopy (FT-IR) analysis. The IR spectra of the stearic acid, the precursor and powders calcined in different temperature are shown in Fig. 1. It can be seen that the stearic and the precursor gel have four strong band at 940, 1730, 2849 and 2921 cm−1 , indicating the presence of carbonate groups [12]. After firing more than 600 ◦ C, the band intensity of the carbonate group was reduced significantly, but a large band appeared at 600–650 cm−1 . Music et al. [13] have assigned
Fig. 1. FT-IR spectra of the stearic acid, gel, and powders calcined at different temperature.
Fig. 2. DTA and TG curve of the gel.
650–550 cm−1 to Ti–O vibration. With the peak appeared at 650 cm−1 , revealed that the formation of a large amount of NBT [14]. All carbonate groups disappeared after the sample was heated at 600 ◦ C, at which the precursor attained a stable weight. Fig. 2 shows the result of DTA and TG analysis of the gel. For DTA curve, there are three peaks, the first one appears at about 130 ◦ C, and it is due to the evaporation of water and the melting of the gel. The second peak at about 430 ◦ C was caused by burning of organic substances. The last one (612 ◦ C) is regarded as the temperature of the solid-state reaction. The TG curve of stearic acid (Fig. 3) shows that all stearic acid burnt out by 450 ◦ C and there is no loss in weight thereafter, this correspond to the second peak of DTA curve. In Fig. 2, the weight loss of NBT gel at the temperature range of 450–600 ◦ C is caused by the decomposition of nitrate material such as Bi(NO3 )3 or sodium acetate. From the analysis of Figs. 2 and 3, all combustion and decomposition of the material and the crystallization of NBT has finished by 600 ◦ C.
Fig. 3. TG curve of stearic acid.
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Fig. 4. XRD of NBT powder calcined at different temperatures: (a) 600 ◦ C; (b) 700 ◦ C; (c) 800 ◦ C; (d) solid-state reaction NBT.
Fig. 4 shows XRD patterns of NBT powders calcined at different temperature. It can be seen that nanocrystalline NBT was formed at 600 ◦ C and developed with increasing temperatures. Comparing with solid-state reaction NBT powders, the structure of NBT nanocrystalline belongs to pure pervoskite type, and no other intermediate phase is found. The particle sizes and morphologies of NBT powders fired at different temperatures were examined by TEM. As shown in Fig. 5(a)–(c), all NBT particles were very small, with an average size of 10–40 m. The formation of nanosized particles might be due to the combustion of organic present in the gel, along with the evolution of gases. Particle morphology of calcined powders at 600 ◦ C for 1 h is not very clear, with an average primary particle size around 10 nm. The average particle size increases with calcining temperature. At 700 ◦ C for 1 h, the particle size is about 10–20 nm with clear morphology. The primary paricle size of powders calcined at 800 ◦ C for 1 h is about 20–40 nm and the grains became
Fig. 6. (a) SEM of NBT ceramic prepared by fine particles. (b) SEM of NBT prepared by traditional process.
Fig. 5. TEM of NBT powders calcined at different temperatures. (a) 600 ◦ C; (b) 700 ◦ C; (c) 800 ◦ C.
J. Hao et al. / Materials Chemistry and Physics 90 (2005) 282–285 Table 1 Piezoelectric and dielectric properties of samples prepared by stearic acid gel method and solid-state method
ε (at 1 MHz) d33 (PC N−1 )
Stearic acid gel method sample
Conventional solid-state method sample
466 60
550 85
285
more than traditional process. The particle size is controlled by the calcining temperature. The sintered samples by the synthesized powders are less porous in microstructure and have higher piezoelectric constant than that by traditional process.
Acknowledgements square. All particle size is much smaller than that by the solid-state method. The 700 ◦ C synthesized powder was dry-pressed at a pressure of about 100 MPa to form pellets. Then, the specimens were sintered at 1120 ◦ C for 2 h in air. The bulk density of the sintered samples was measured by the Archimedes method. The measured density ratio is 97%, higher than that 90% of the solid phase reaction samples. Fig. 6(a) and (b) shows the SEM images of sintered sample by the synthesized powder and the surface of traditional NBT. The sintering temperature was 1120 and 1150 ◦ C, respectively. Compared with the sample prepared by traditional process, this sample derived from nanoNBT is less porous and has the homogeneous microstructure with the grain size of about 200 nm. This result shows that the improved microstructure homogeneity and high density NBT ceramics can be obtained by using nanocrystalline powders. The dielectric constant ε and piezoelectric constant d33 at room temperature was measured respectively. The results are shown in Table 1. It can be seen that the ε and d33 values of the specimen of stearic acid gel method are approximately comparable with, that of conventional solid-state method samples.
4. Conclusion A stearic acid gel method is an effective route to synthesize reactive nanosize NBT powders, which can be sintered
This work was supported by the Ministry of Sciences and Technology of China through 973-project under Grant 2002CB613301 and High Technology Research and Development project of PR China under Grant 2001AA325010. References [1] B.J. Chu, D.R. Chen, G.R. Li, Q.R. Yin, J. Eur. Ceram. Soc. 22 (2002) 2115. [2] A. Herabut, A. Dafan, J. Am. Ceram. Soc. 80 (1997) 2954. [3] M.S. Hagiyev, I.H. Ismaizade, A.K. Abiyev, Ferroelectrics 56 (1984) 215. [4] J.V. Zvirgzds, P.P. Kapostis, V.A. Isupov, Ferroelectrics 40 (1980) 75. [5] K. Sakata, Y. Masuda, Ferroelectrics 95 (1974) 347. [6] T. Takenaka, K. Maruyama, K. Sakata, Jpn. J. Appl. Phys. 30 (1991) 2263. [7] P. Pookmaneea, S. Phanichphanta, R.B. Heimann, Ceram. Forum Int. 78 (2001) E27. [8] X.Z. Jing, Y.X. Li, Q.R. Yin, Mater. Sci. Eng. B 99 (2003) 506. [9] M.L. Zhao, C.L. Wang, W.L. Zhong, et al., Acta Phys. Sin. 52 (2003) 229. [10] J. Yang, D. Li, X. Wang, et al., J. Solid State Chem. 165 (2002) 193. [11] X.H. Wang, D. Li, L.D. Lu, et al., J. Alloys Compd. 45 (1996) 237. [12] R. Urlaub, U. Posset, R. Thull, J. Non-Cryst. Solids 265 (2000) 276. [13] S. Music, M. Gotic, M. Ivanda, S. Popovic, A. Rukovic, et al., Mater. Sci. Eng. B 47 (1997) 33. [14] C.Y. Kim, T. Sekino, K. Niihara, J. Am. Ceram. Soc. 86 (2003) 1464.