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New evidence of a dissolution–precipitation mechanism in hydrothermal synthesis of barium titanate powders
December 2002
Materials Letters 57 (2002) 490 – 494 www.elsevier.com/locate/matlet
New evidence of a dissolution–precipitation mechanism in hydrothermal synthesis of barium titanate powders Huarui Xu, Lian Gao* State Key Laboratory on High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Science, 1295 Ding Xi Road, Shanghai 200050, People’s Republic of China Received 2 March 2002; accepted 8 April 2002
Abstract Accomplished by varying the precursors of BaTiO3 powders as hydrothermal preparation in the strong basic solution, this paper has observed the following new pieces of evidence of a mechanism by dissolution – precipitation: (1) SEM observations of the TiO2 powders of fore-and-aft hydrothermal reaction, using TiCl4 and NaOH as precursors, show a dissolution process; (2) TEM observations of incompletely reacted powders, using TiCl4, BaCl2 and NaOH as precursors, show that the homogeneous nucleation occurs instead of heterogeneous nucleation; (3) uniform BaTiO3 powders with the average particle size of 0.5 Am could be quickly prepared by the hydrothermal method, using BaTiO3 powders prepared by oxalate coprecipitation method as precursor. These three experimental observations provide new strong evidence of dissolution – precipitation as the hydrothermal preparation mechanism of BaTiO3 powders in the strong basic solution. D 2002 Elsevier Science B.V. All rights reserved. Keywords: BaTiO3 (barium titanate); Powders; Hydrothermal synthesis; Evidence; Mechanism
1. Introduction In the past years, extensive discussion has focused on BaTiO 3 powders prepared by hydrothermal method [1 –4]. A question has been raised in the literature, that is, with which mechanism the BaTiO3 powders are prepared by the hydrothermal method? Two possible mechanisms have been generally proposed: (1) in situ transformation, which was assumed by Hertl [5] and Hu et al. [6], proposes that TiO2
*
Corresponding author. Tel.: +86-21-62512990; fax: +86-2162513903. E-mail address: [email protected] (L. Gao).
particle reacts initially with dissolved barium to produce a continuous layer of BaTiO3 and the additional barium must diffuse this layer and react with TiO2 until the TiO2 supply is exhausted; (2) dissolution – precipitation, which was suggested by Ovramenko et al. [7] and Pinceloup et al. [8], proposes that TiO2 particle must dissolve to form hydroxytitanium complexes (Ti(OH)n ) and then react with barium ions in the solution to precipitate BaTiO3, which may either originate on the TiO2 substrate or form directly in the bulk solution. Hu et al. [6] first synthesized monodispersed titania powders with a diameter of 0.1 – 1 Am and then used it as the precursor and successfully prepared the BaTiO3 powders by hydrothermal conversion at the temperature
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 8 1 7 - 0
H. Xu, L. Gao / Materials Letters 57 (2002) 490–494
V 100 jC. They found that the size and morphology of the BaTiO3 particles remained the same as the precursor titania particles and thought that this result agreed with the in situ mechanism. Pinceloup et al. [8] studied the system of barium hydroxide –titanium tetraisopropoxide – water – isopropanol between 85 and 150 jC, and found the following results: (1) the grain size of the BaTiO3 powder decreased when the solubility of the precursor decreased; (2) TEM observations of incompletely reacted powder showed that the grains were either amorphous or entirely crystalline BaTiO3; (3) high-resolution TEM observations of fully reacted powders revealed the presence of necks between particles. And then they thought that these three observations provided the evidence of dissolution – precipitation mechanism. Eckert et al. [9] observed that the mechanism evolved from a dissolution – precipitation process at the beginning of the reaction to an in situ mechanism for longer reaction times. The reason for several contradictive experimental observations in the literatures is probably the difference from the hydrothermal conditions. Now, in order to quickly synthesize the tetragonal BaTiO3 powders by hydrothermal method, the strong basic solution had better be used [10 – 14]. Confirmation the hydrothermal reaction mechanism in the strong basic solution is important for the choice of the best precursor for the various particle size BaTiO3 powder [1]. In this paper, accomplished by varying the precursors of BaTiO3 powders as hydrothermal preparation in the strong basic solution, we have observed some new evidence of a mechanism by dissolution –precipitation. It is the purpose of this paper to present the evidence.
2. Experimental Reagents (99.999% pure) of BaCl22H2O and titanium(IV) tetrachloride and NaOH were obtained from Shanghai Chemical. The precursor BaTiO3 powders, which were prepared by oxalate coprecipitation method, were provided by Xingtai Titanium Chemical Industry. This paper used (1) 0.3 mol/l TiCl 4 and 2.2 mol/l NaOH; (2) 0.48 mol/l BaCl22H2O, 0.3 mol/l TiCl4 and 2.2 mol/l NaOH; (3) 0.3 mol/l BaTiO3, 0.16 mol/l BaCl22H2O and 1.0 mol/l NaOH as precursors, respectively.
491
The precursor solution was purged with N2 gas for f 5 min, then transferred to a 50-ml Teflon-lined stainless steel vessel. The sealed vessel was placed in an oven at 240 jC for 12 h. The samples were removed and extensively washed with hot nanopure water to remove any adsorbed ions, and then dried at 90 jC for 12 h. The powders were characterized by X-ray diffraction (XRD, Rigaku Geigerflex D/Max 2200) using CuKa radiation for phase and X-ray fluorescence (XRF, Philips PW2400) for chemical composition. Observations of the barium titanate powder were made by transmission electronic microscopy (TEM, JEOL JEM-2010) and scanning electronic microscopy (SEM, Philips XL30 and DX-4I).
3. Results and discussion 3.1. Using TiCl4 and NaOH as precursors Fig. 1 shows the morphologies of the prepared powders fore-and-aft hydrothermal reaction using TiCl4 and NaOH as precursors. The morphologies are much different from each other. Before the hydrothermal reaction, the newly formed TiO2 powder prepared by mixing TiCl4 and NaOH is seriously agglomerated, and the original particles can be easily found. However, after the hydrothermal reaction, the TiO2 powder came out with fibre-like morphology, and the original particles cannot be found. It is obvious that the prepared fibre-like TiO2 could not be prepared by easily growing the raw materials, and the dissolution process must occur in the hydrothermal process. The reasons for the agglomeration in Fig. 1a could be attributed to the large surface area of newly formed titanium dioxide particles. The agglomeration of the TiO2 powder does not change even after another 48 h of reaction at 90 jC. However, after 12 h of hydrothermal treatment, the agglomeration phenomenon decreased and the fibre-like powders formed. Because the original particles could not be found in Fig. 1b, it is obviously impossible that TiO2 particles in Fig. 1a disperse from the agglomeration and then self-assemble to form the fibre-like powders. The formation of fibre-like TiO2 powder must be formed through the dissolution of the agglomeration particles and the
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H. Xu, L. Gao / Materials Letters 57 (2002) 490–494
morphology of the incomplete hydrothermal reacted powders (for 1 h) is shown in Fig. 2a. In Fig. 2a, the bigger particles are the unreacted titania. Barium is not resolved in the EDS spectra from these particles. The morphology of the smaller particles in Fig. 2a (pointed by the arrow) is magnified and is shown in Fig. 2b. Characterized by EDS, the particles in Fig. 2b prove to be BaTiO3 particles. These TEM results clearly suggest that a homogenous nucleation occurs. From Fig. 2a, the boundary of the bigger particles is not clearer than that of the particles in Fig. 1a, which are the newly formed TiO2 powder by hydrolysis with [OH ] = 1.0 mol/l. This also proves the dissolution process in the hydrothermal treatment. Although the free energy of the heterogeneous nucleation is smaller than that of the homogeneous nucleation because of the dissolution of amorphous TiO2 particles, the homogeneous nucleation still takes place in the hydrothermal synthesis of BaTiO3 powders. According to the in situ mechanism, the BaTiO3 nuclei must occur at the surface of the TiO2 particles through the heterogeneous nucleation and the homogeneous nucleation could not happen [5,6]. However, this conclusion does not agree with the above result. 3.3. Using BaTiO3, BaCl2 and NaOH as precursors
Fig. 1. SEM images of TiO2 powder of (a) before and (b) after the hydrothermal reaction.
precipitation when the supersaturated concentration at the hydrothermal condition is reached. Since the dissolution process of the TiO2 powders occurs in the absence of Ba2 + ions, after adding the Ba2 + ions and because of the occurrence of BaTiO3 precipitation, the dissolution rate should be accelerated. According to the in situ mechanism, the titanium and oxygen ionic species with TiO2 particles are believed to be immobile [6]. However, this conclusion does not agree with the above result. 3.2. Using BaCl2, TiCl4 and NaOH as precursors Using BaCl2, TiCl4 and NaOH as precursors and repeating the experiment several times, the typical
The precursor BaTiO3 powder is sheet and its morphology is not uniform with the average particle size of 0.4 Am and broad particle size distribution (see Fig. 3a). Using the above BaTiO3 powder and BaCl2, NaOH as precursors, the BaTiO3 powder had a uniform morphology and an average particle size of 0.5 Am and relative narrow particle size distribution is prepared by the hydrothermal method (Fig. 3b). Clearly, it is seen that the smaller BaTiO3 particles disappeared and the bigger BaTiO3 particles grew simultaneously. This result is the same as in the works of Asiaie et al. [4]. At first, BaTiO3 powders of 0.3-Am average particle size were obtained after 1 week of hydrothermal treatment, starting with 0.5 mol/l of NaOH, and then BaTiO3 powders of 0.5-Am average particle size were prepared by adding another 0.3 mol/l of NaOH to the product mixture after 1 week of synthesis and continuous hydrothermal treatment for another week. The solubility of BaTiO3 powder in the strong basic hydrothermal condition should be so
H. Xu, L. Gao / Materials Letters 57 (2002) 490–494
493
Fig. 3. SEM images of the BaTiO3 powder (a) before and (b) after the hydrothermal reaction.
high that the BaTiO3 precursor with a minimal particle size of 0.1 Am also dissolves and the Ba2 + and Ti(OH)n ions grow on the surface of the bigger BaTiO3 particles. Because the BaTiO3 precursor is in the tetragonal phase, the prepared BaTiO3 powders should grow with a special direction so as to form the uniform morphology. The relevant chemical reaction could be written as: BaTiO3 ðsmallÞ ! Ba2þ þ TiðOHÞ2 6 ! BaTiO3 ðlargeÞ
Fig. 2. TEM images of the sample reacted for 1 h [(b) is the magnification of the arrow in (a)].
According to the in situ mechanism, the preparation process of BaTiO3 powder by hydrothermal method is similar to the solid-state reaction process
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H. Xu, L. Gao / Materials Letters 57 (2002) 490–494
between BaCO3(s) and TiO2(s) [6]. However, this conclusion does not agree with the above result. According to the above evidence, the dissolution – precipitation mechanism could be confirmed, and then the best precursor also could be chosen. Because the rate-controlling process is the dissolution of the titania precursor, the precursor solution, including the muchactive titania, and much strong basicity is recommended for the quick preparation of BaTiO3 powders by the hydrothermal method. Compared with anatase and rutile titania, a newly formed amorphous TiO2 powder by the hydrolysis of TiCl4 is an excellent Ti precursor to use in the hydrothermal synthesis of the BaTiO3. These conclusions were also proved by the previous workers [1,4,14].
4. Conclusions From the hydrothermal synthesis of BaTiO3 powders varying the precursors, three new pieces of evidence have been collected which must be explained by the dissolution – precipitation mechanism. These new pieces of evidence are: (1) TiO2 particles have a dissolution process, using TiCl4 and NaOH as precursors, and characterizes the TiO2 morphology of fore-and-aft hydrothermal reaction; (2) TEM observations of incompletely reacted powders, using the TiCl4, BaCl2 and NaOH as precursors, shows that the homogeneous nucleation occurs instead of heterogeneous nucleation; (3) uniform
BaTiO3 powders with the average particle size of 0.5 Am has been quickly prepared by the hydrothermal method, using the BaTiO3 powder prepared by oxalate coprecipitation method as the precursor.
References [1] I.J. Clark, T. Takeuchi, N. Ohtori, D.C. Sinclair, J. Mater. Chem. 9 (1999) 83 – 91. [2] P. Pinceloup, C. Courtois, A. Leriche, B. Thierry, J. Am. Ceram. Soc. 82 (11) (1999) 3049 – 3056. [3] S.W. Lu, B.I. Lee, Z.L. Wang, W.D. Samuels, J. Cryst. Growth 219 (2000) 269 – 276. [4] R. Asiaie, W. Zhu, S.A. Akbar, P.K. Dutta, Chem. Mater. 8 (1996) 226 – 234. [5] W. Hertl, J. Am. Ceram. Soc. 71 (1988) 879 – 883. [6] M.Z. Hu, V. Kurian, E.A. Payzant, C.J. Rawn, R.D. Hunt, Powder Technol. 110 (2000) 2 – 14. [7] N.A. Ovramenko, L.I. Shvets, F.D. Ovcharenko, B.Y. Kornilovich, Izv. Akad. Nauk. SSSR, Neorg. Mater. 15 (1979) 1982 – 1984. [8] P. Pinceloup, C. Courtois, J. Vicens, A. Leriche, B. Thierry, J. Eur. Ceram. Soc. 19 (1999) 973 – 977. [9] J.O. Eckert, C.C. Hung-Houston, B.L. Gersten, M.M. Lencka, R.R. Riman, J. Am. Ceram. Soc. 79 (11) (1996) 2929 – 2939. [10] P.K. Dutta, J.R. Gregg, Chem. Mater. 4 (1992) 843 – 846. [11] P.K. Dutta, R. Asiaie, S.A. Akbar, W. Zhu, Chem. Mater. 6 (1994) 1542 – 1548. [12] Y. Ma, E. Vileno, S.L. Suib, P.K. Dutta, Chem. Mater. 9 (1997) 3023 – 3031. [13] M. Wu, J. Long, G. Wang, A. Huang, Y. Luo, J. Am. Ceram. Soc. 82 (11) (1999) 3254 – 3256. [14] H. Xu, L. Gao, J. Guo, J. Eur. Ceram. Soc. 22 (7) (2002) 1163 – 1170.
Materials Letters 57 (2002) 490 – 494 www.elsevier.com/locate/matlet
New evidence of a dissolution–precipitation mechanism in hydrothermal synthesis of barium titanate powders Huarui Xu, Lian Gao* State Key Laboratory on High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Science, 1295 Ding Xi Road, Shanghai 200050, People’s Republic of China Received 2 March 2002; accepted 8 April 2002
Abstract Accomplished by varying the precursors of BaTiO3 powders as hydrothermal preparation in the strong basic solution, this paper has observed the following new pieces of evidence of a mechanism by dissolution – precipitation: (1) SEM observations of the TiO2 powders of fore-and-aft hydrothermal reaction, using TiCl4 and NaOH as precursors, show a dissolution process; (2) TEM observations of incompletely reacted powders, using TiCl4, BaCl2 and NaOH as precursors, show that the homogeneous nucleation occurs instead of heterogeneous nucleation; (3) uniform BaTiO3 powders with the average particle size of 0.5 Am could be quickly prepared by the hydrothermal method, using BaTiO3 powders prepared by oxalate coprecipitation method as precursor. These three experimental observations provide new strong evidence of dissolution – precipitation as the hydrothermal preparation mechanism of BaTiO3 powders in the strong basic solution. D 2002 Elsevier Science B.V. All rights reserved. Keywords: BaTiO3 (barium titanate); Powders; Hydrothermal synthesis; Evidence; Mechanism
1. Introduction In the past years, extensive discussion has focused on BaTiO 3 powders prepared by hydrothermal method [1 –4]. A question has been raised in the literature, that is, with which mechanism the BaTiO3 powders are prepared by the hydrothermal method? Two possible mechanisms have been generally proposed: (1) in situ transformation, which was assumed by Hertl [5] and Hu et al. [6], proposes that TiO2
*
Corresponding author. Tel.: +86-21-62512990; fax: +86-2162513903. E-mail address: [email protected] (L. Gao).
particle reacts initially with dissolved barium to produce a continuous layer of BaTiO3 and the additional barium must diffuse this layer and react with TiO2 until the TiO2 supply is exhausted; (2) dissolution – precipitation, which was suggested by Ovramenko et al. [7] and Pinceloup et al. [8], proposes that TiO2 particle must dissolve to form hydroxytitanium complexes (Ti(OH)n ) and then react with barium ions in the solution to precipitate BaTiO3, which may either originate on the TiO2 substrate or form directly in the bulk solution. Hu et al. [6] first synthesized monodispersed titania powders with a diameter of 0.1 – 1 Am and then used it as the precursor and successfully prepared the BaTiO3 powders by hydrothermal conversion at the temperature
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 8 1 7 - 0
H. Xu, L. Gao / Materials Letters 57 (2002) 490–494
V 100 jC. They found that the size and morphology of the BaTiO3 particles remained the same as the precursor titania particles and thought that this result agreed with the in situ mechanism. Pinceloup et al. [8] studied the system of barium hydroxide –titanium tetraisopropoxide – water – isopropanol between 85 and 150 jC, and found the following results: (1) the grain size of the BaTiO3 powder decreased when the solubility of the precursor decreased; (2) TEM observations of incompletely reacted powder showed that the grains were either amorphous or entirely crystalline BaTiO3; (3) high-resolution TEM observations of fully reacted powders revealed the presence of necks between particles. And then they thought that these three observations provided the evidence of dissolution – precipitation mechanism. Eckert et al. [9] observed that the mechanism evolved from a dissolution – precipitation process at the beginning of the reaction to an in situ mechanism for longer reaction times. The reason for several contradictive experimental observations in the literatures is probably the difference from the hydrothermal conditions. Now, in order to quickly synthesize the tetragonal BaTiO3 powders by hydrothermal method, the strong basic solution had better be used [10 – 14]. Confirmation the hydrothermal reaction mechanism in the strong basic solution is important for the choice of the best precursor for the various particle size BaTiO3 powder [1]. In this paper, accomplished by varying the precursors of BaTiO3 powders as hydrothermal preparation in the strong basic solution, we have observed some new evidence of a mechanism by dissolution –precipitation. It is the purpose of this paper to present the evidence.
2. Experimental Reagents (99.999% pure) of BaCl22H2O and titanium(IV) tetrachloride and NaOH were obtained from Shanghai Chemical. The precursor BaTiO3 powders, which were prepared by oxalate coprecipitation method, were provided by Xingtai Titanium Chemical Industry. This paper used (1) 0.3 mol/l TiCl 4 and 2.2 mol/l NaOH; (2) 0.48 mol/l BaCl22H2O, 0.3 mol/l TiCl4 and 2.2 mol/l NaOH; (3) 0.3 mol/l BaTiO3, 0.16 mol/l BaCl22H2O and 1.0 mol/l NaOH as precursors, respectively.
491
The precursor solution was purged with N2 gas for f 5 min, then transferred to a 50-ml Teflon-lined stainless steel vessel. The sealed vessel was placed in an oven at 240 jC for 12 h. The samples were removed and extensively washed with hot nanopure water to remove any adsorbed ions, and then dried at 90 jC for 12 h. The powders were characterized by X-ray diffraction (XRD, Rigaku Geigerflex D/Max 2200) using CuKa radiation for phase and X-ray fluorescence (XRF, Philips PW2400) for chemical composition. Observations of the barium titanate powder were made by transmission electronic microscopy (TEM, JEOL JEM-2010) and scanning electronic microscopy (SEM, Philips XL30 and DX-4I).
3. Results and discussion 3.1. Using TiCl4 and NaOH as precursors Fig. 1 shows the morphologies of the prepared powders fore-and-aft hydrothermal reaction using TiCl4 and NaOH as precursors. The morphologies are much different from each other. Before the hydrothermal reaction, the newly formed TiO2 powder prepared by mixing TiCl4 and NaOH is seriously agglomerated, and the original particles can be easily found. However, after the hydrothermal reaction, the TiO2 powder came out with fibre-like morphology, and the original particles cannot be found. It is obvious that the prepared fibre-like TiO2 could not be prepared by easily growing the raw materials, and the dissolution process must occur in the hydrothermal process. The reasons for the agglomeration in Fig. 1a could be attributed to the large surface area of newly formed titanium dioxide particles. The agglomeration of the TiO2 powder does not change even after another 48 h of reaction at 90 jC. However, after 12 h of hydrothermal treatment, the agglomeration phenomenon decreased and the fibre-like powders formed. Because the original particles could not be found in Fig. 1b, it is obviously impossible that TiO2 particles in Fig. 1a disperse from the agglomeration and then self-assemble to form the fibre-like powders. The formation of fibre-like TiO2 powder must be formed through the dissolution of the agglomeration particles and the
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H. Xu, L. Gao / Materials Letters 57 (2002) 490–494
morphology of the incomplete hydrothermal reacted powders (for 1 h) is shown in Fig. 2a. In Fig. 2a, the bigger particles are the unreacted titania. Barium is not resolved in the EDS spectra from these particles. The morphology of the smaller particles in Fig. 2a (pointed by the arrow) is magnified and is shown in Fig. 2b. Characterized by EDS, the particles in Fig. 2b prove to be BaTiO3 particles. These TEM results clearly suggest that a homogenous nucleation occurs. From Fig. 2a, the boundary of the bigger particles is not clearer than that of the particles in Fig. 1a, which are the newly formed TiO2 powder by hydrolysis with [OH ] = 1.0 mol/l. This also proves the dissolution process in the hydrothermal treatment. Although the free energy of the heterogeneous nucleation is smaller than that of the homogeneous nucleation because of the dissolution of amorphous TiO2 particles, the homogeneous nucleation still takes place in the hydrothermal synthesis of BaTiO3 powders. According to the in situ mechanism, the BaTiO3 nuclei must occur at the surface of the TiO2 particles through the heterogeneous nucleation and the homogeneous nucleation could not happen [5,6]. However, this conclusion does not agree with the above result. 3.3. Using BaTiO3, BaCl2 and NaOH as precursors
Fig. 1. SEM images of TiO2 powder of (a) before and (b) after the hydrothermal reaction.
precipitation when the supersaturated concentration at the hydrothermal condition is reached. Since the dissolution process of the TiO2 powders occurs in the absence of Ba2 + ions, after adding the Ba2 + ions and because of the occurrence of BaTiO3 precipitation, the dissolution rate should be accelerated. According to the in situ mechanism, the titanium and oxygen ionic species with TiO2 particles are believed to be immobile [6]. However, this conclusion does not agree with the above result. 3.2. Using BaCl2, TiCl4 and NaOH as precursors Using BaCl2, TiCl4 and NaOH as precursors and repeating the experiment several times, the typical
The precursor BaTiO3 powder is sheet and its morphology is not uniform with the average particle size of 0.4 Am and broad particle size distribution (see Fig. 3a). Using the above BaTiO3 powder and BaCl2, NaOH as precursors, the BaTiO3 powder had a uniform morphology and an average particle size of 0.5 Am and relative narrow particle size distribution is prepared by the hydrothermal method (Fig. 3b). Clearly, it is seen that the smaller BaTiO3 particles disappeared and the bigger BaTiO3 particles grew simultaneously. This result is the same as in the works of Asiaie et al. [4]. At first, BaTiO3 powders of 0.3-Am average particle size were obtained after 1 week of hydrothermal treatment, starting with 0.5 mol/l of NaOH, and then BaTiO3 powders of 0.5-Am average particle size were prepared by adding another 0.3 mol/l of NaOH to the product mixture after 1 week of synthesis and continuous hydrothermal treatment for another week. The solubility of BaTiO3 powder in the strong basic hydrothermal condition should be so
H. Xu, L. Gao / Materials Letters 57 (2002) 490–494
493
Fig. 3. SEM images of the BaTiO3 powder (a) before and (b) after the hydrothermal reaction.
high that the BaTiO3 precursor with a minimal particle size of 0.1 Am also dissolves and the Ba2 + and Ti(OH)n ions grow on the surface of the bigger BaTiO3 particles. Because the BaTiO3 precursor is in the tetragonal phase, the prepared BaTiO3 powders should grow with a special direction so as to form the uniform morphology. The relevant chemical reaction could be written as: BaTiO3 ðsmallÞ ! Ba2þ þ TiðOHÞ2 6 ! BaTiO3 ðlargeÞ
Fig. 2. TEM images of the sample reacted for 1 h [(b) is the magnification of the arrow in (a)].
According to the in situ mechanism, the preparation process of BaTiO3 powder by hydrothermal method is similar to the solid-state reaction process
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H. Xu, L. Gao / Materials Letters 57 (2002) 490–494
between BaCO3(s) and TiO2(s) [6]. However, this conclusion does not agree with the above result. According to the above evidence, the dissolution – precipitation mechanism could be confirmed, and then the best precursor also could be chosen. Because the rate-controlling process is the dissolution of the titania precursor, the precursor solution, including the muchactive titania, and much strong basicity is recommended for the quick preparation of BaTiO3 powders by the hydrothermal method. Compared with anatase and rutile titania, a newly formed amorphous TiO2 powder by the hydrolysis of TiCl4 is an excellent Ti precursor to use in the hydrothermal synthesis of the BaTiO3. These conclusions were also proved by the previous workers [1,4,14].
4. Conclusions From the hydrothermal synthesis of BaTiO3 powders varying the precursors, three new pieces of evidence have been collected which must be explained by the dissolution – precipitation mechanism. These new pieces of evidence are: (1) TiO2 particles have a dissolution process, using TiCl4 and NaOH as precursors, and characterizes the TiO2 morphology of fore-and-aft hydrothermal reaction; (2) TEM observations of incompletely reacted powders, using the TiCl4, BaCl2 and NaOH as precursors, shows that the homogeneous nucleation occurs instead of heterogeneous nucleation; (3) uniform
BaTiO3 powders with the average particle size of 0.5 Am has been quickly prepared by the hydrothermal method, using the BaTiO3 powder prepared by oxalate coprecipitation method as the precursor.
References [1] I.J. Clark, T. Takeuchi, N. Ohtori, D.C. Sinclair, J. Mater. Chem. 9 (1999) 83 – 91. [2] P. Pinceloup, C. Courtois, A. Leriche, B. Thierry, J. Am. Ceram. Soc. 82 (11) (1999) 3049 – 3056. [3] S.W. Lu, B.I. Lee, Z.L. Wang, W.D. Samuels, J. Cryst. Growth 219 (2000) 269 – 276. [4] R. Asiaie, W. Zhu, S.A. Akbar, P.K. Dutta, Chem. Mater. 8 (1996) 226 – 234. [5] W. Hertl, J. Am. Ceram. Soc. 71 (1988) 879 – 883. [6] M.Z. Hu, V. Kurian, E.A. Payzant, C.J. Rawn, R.D. Hunt, Powder Technol. 110 (2000) 2 – 14. [7] N.A. Ovramenko, L.I. Shvets, F.D. Ovcharenko, B.Y. Kornilovich, Izv. Akad. Nauk. SSSR, Neorg. Mater. 15 (1979) 1982 – 1984. [8] P. Pinceloup, C. Courtois, J. Vicens, A. Leriche, B. Thierry, J. Eur. Ceram. Soc. 19 (1999) 973 – 977. [9] J.O. Eckert, C.C. Hung-Houston, B.L. Gersten, M.M. Lencka, R.R. Riman, J. Am. Ceram. Soc. 79 (11) (1996) 2929 – 2939. [10] P.K. Dutta, J.R. Gregg, Chem. Mater. 4 (1992) 843 – 846. [11] P.K. Dutta, R. Asiaie, S.A. Akbar, W. Zhu, Chem. Mater. 6 (1994) 1542 – 1548. [12] Y. Ma, E. Vileno, S.L. Suib, P.K. Dutta, Chem. Mater. 9 (1997) 3023 – 3031. [13] M. Wu, J. Long, G. Wang, A. Huang, Y. Luo, J. Am. Ceram. Soc. 82 (11) (1999) 3254 – 3256. [14] H. Xu, L. Gao, J. Guo, J. Eur. Ceram. Soc. 22 (7) (2002) 1163 – 1170.