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Structure, sintering behavior and dielectric properties of silica-coated BaTiO3
June 2002
Materials Letters 54 (2002) 314 – 317 www.elsevier.com/locate/matlet
Structure, sintering behavior and dielectric properties of silica-coated BaTiO3 RenZheng Chen a,*, AiLi Cui b, XiaoHui Wang a, ZhiLun Gui a, LongTu Li a a
Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China b Department of Chemistry, Tsinghua University, Beijing 100084, PR China Received 28 August 2000; received in revised form 28 August 2001; accepted 2 September 2001
Abstract Silica was homogeneously coated on the surface of barium titanate (BaTiO3, BT) fine particles via sol – gel method. The thickness of silica film was about 5 nm. High-resolution transmission electron microscope (HRTEM) investigation and energy dispersion spectrometry (EDS) analysis proved that there was silica coated on BaTiO3. The coating mechanism is that after BaTiO3 hydrolyzed slightly, Na2SiO3 gelatinized on the surface of TiO2x. Compared with pure BaTiO3, the coating process can improve sintering behavior and flatten temperature coefficients (TC) curves of ceramics based on BaTiO3. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Coating; SiO2; BaTiO3; Sintering; Dielectric
1. Introduction In the ceramics industry, the additives of ceramics are normally doped by the milling method to control the structure and properties of ceramics. The homogeneity scale depends on many aspects, such as density, particle size and shape of the starting particles and additives. Compared with milling method, chemical mixing techniques such as sol –gel coating, precipitation coating and solution coating show an advantage in enhancing the microhomogeneity of additives, modifying the surface states and then have an effect on the properties of coated ceramics. In the paint industry,
*
Corresponding author.
coating processes have been widely used [1]. However, in the ceramics field, the coating technique has not been paid enough attention yet. There are some literatures on coating of structure ceramics, such as AlN [2], Si3N4 [3], ZrO2 [4], SnO2 [5], Al2O3 [6] and so on. Vo¨ltzke and Abicht [7] and Shih et al. [8] reported on the additive surface modification of BaTiO3 powders. At present, the coating mechanisms are not discussed deeply. BaTiO3 is a well-known material for capacitors. One way to reduce cost is to reduce the sintering temperature so that cheap internal electrodes such as Ag can be used. The coating process will produce a core-shell-like structure of BaTiO3 powders with the additive. Since silica is known to be a good sintering aid, silica coating may improve the sintering behavior of BaTiO3 particles. In addition, due to the coating
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 1 ) 0 0 5 8 4 - 5
R. Chen et al. / Materials Letters 54 (2002) 314–317
315
layer, sintering is performed with minimal grain growth, then the desired core-shell microstructure is achieved in fine-grain BaTiO3 ceramics. We can control the dielectric properties by adjusting the core-shell structure. In this paper, we studied the sol – gel mechanism of silica coating of BaTiO3. Silica nanofilm with the thickness of 5 nm was coated on BaTiO3 powders. The influences on sintering behavior and dielectric properties of coating were also discussed.
testing dielectric properties. The disks were sintered at 1260 jC for 2 h. Then, disk samples were electroded using silver paste applied on opposing surfaces and fired at 750 jC. Dielectric properties of the sintered disks were studied as a function of temperature using an automatic measurement system with an LCR meter (HP 4291A) at a frequency of 1 kHz. The measurements were performed during heating cycles from 60 to 130 jC at a rate of 3 jC/min. An alternating voltage of 1 V was applied.
2. Experimental
3. Results and discussions
The BaTiO3 starting powders used in this study were prepared by oxalate coprecipitation method [Ti(OC4H9)4, A.R., Ba(CH3COO)2, A.R., Ba/Ti=1:1]. The size of the powders is about 110– 130 nm. The mole ratio of water glass is Na2O/SiO2=1:3.
3.1. Characterization of the coated BaTiO3 powders Fig. 1 shows the morphology of the coated BaTiO3 powders. From Fig. 1, the homogeneous silica film can be seen clearly on the powders’ surfaces. The thickness of the film is about 5 nm.
2.1. Coating process The BaTiO3 powders were added into the water glass solution first. Then, the pH value of the solution was adjusted to about 10 using 1 mol/l HCl. The solution became silica sol. The concentration of BaTiO3 was 5 wt.%. The gelation time must be longer than 2 h. During the sol –gel process, electromagnetic stirring was used. The sol temperature was kept at 80 jC. After coating, the slurry was filtered then washed using deionized water and dried in a 150 jC oven for 12 h. 2.2. Characterization The coated powders were investigated using a high-resolution transmission electron microscope (JEOL-2010F). The particle size was measured by the HRTEM. The composition of the particle was analyzed by energy dispersion spectrometry (LINKISIS-300EDS). The powders were cold-pressed uniaxially into disks 5 mm in diameter and 3.5 mm in thickness for the study of the sintering behavior of coated BaTiO3. The dilatometric investigation was performed with a dilatometer (TMA92) in air, with a heating rate of 10 jC/min up to 1300 jC. The powders were cold-pressed uniaxially into disks 10 mm in diameter and 1 mm in thickness for
Fig. 1. HRTEM investigation of silica-coated powders.
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R. Chen et al. / Materials Letters 54 (2002) 314–317
structure that is easy to be coated with silica, silica is stable on BaTiO3. Given enough time, the silica film derived by gelation will stack to the required thickness. 3.3. Effect on the sintering behavior of coating
Fig. 2. EDS analyses of silica-coated BaTiO3 powders.
EDS was used to identify the elements of the powder. Fig. 2 is the EDS curve. The Si element peak at 1.76 keV can be seen from the figure. Thus it proved the existence of silica on BaTiO3.
Fig. 3 shows the shrinkage curves of coated BaTiO3 and pure BaTiO3, respectively. Fig. 3a shows that DL/L at 1300 jC of coated BaTiO3 disk reaches 19.3% greater than that of pure BaTiO3, which is 15.0% shown in Fig. 3b. The temperatures of shrinkage maxima for the coated BaTiO3 and pure BaTiO3 are 1190 and 1260 jC, respectively. At 800 jC, there is a reaction between SiO2 and BaTiO3 matrix that resulted in the little shrinkage there. The resultant is a secondary phase fresnoite, Ba2TiSi2O3 [7], which benefits by forming a eutectic above 1200 jC.
3.2. Coating mechanism Based on the instability of BaTiO3 in water and silica coating on TiO2, the coating on BaTiO3 can be divided into two steps: 3.2.1. Slight hydrolysis of BaTiO3 in aqueous solution At room temperature, BaTiO3 is unstable in water having a pH value lower than 12 [9]. Under the influence of water during stirring, Ba2+ ions are dissolved superficially from the BaTiO3 particles according to BaTiO3 ðsÞ þ H2 O ¼ Ba2þ ðaqÞ þ TiO2 ðsÞ þ 2OH ðaqÞ
ð1Þ
Therefore, the dissolved BaTiO3 surface is rich in TiO2. Silica is very easy to coat on the TiO2 [1]. 3.2.2. Na2SiO3 sol – gel process on BaTiO3 Na2SiO3 solution will gelate at pH=10 and BaTiO3 will hydrolyze at this condition. The bigger the silica concentration is, the shorter the gelation time. Thus, a high concentration is not good for the coating process. By controlling the pH value and concentration, we can control the speed of gelation. There are great areas on the BaTiO3 powders so that Silica will gelate preferentially on the surface of BaTiO3. Because the surface of BaTiO3 is rich in TiO2, forming a TiO2-like
Fig. 3. Sintering behavior of coated BaTiO3 and pure BaTiO3. (a) Sintering behavior of coated BaTiO3. (b) Sintering behavior of pure BaTiO3.
R. Chen et al. / Materials Letters 54 (2002) 314–317
317
grains grow up to micrometer scale. So the fine-grain ceramic shows less permittivity. The temperature coefficients (TC) of permittivity of coated BT ceramics and pure BT ceramics are shown in Fig. 4. The TC of coated BT ceramics is flatter than that of the pure BT. Thus, the silica coated BaTiO3 is promising in thin-layer BaTiO3-based capacitors.
4. Conclusions
Fig. 4. TC comparison between coated BT and pure BT ceramics.
Because of the small quantity of SiO2, the shrinkage at 800 jC is very little. Therefore, it can be concluded that coated BaTiO3 has a better sintering property than pure BaTiO3.
By sol – gel method, BaTiO3 particles were coated with silica film of 5 nm thickness. The coating mechanism is discussed which can be divided into two steps: (1) hydrolysis of BaTiO3 and (2) sol – gel process on BaTiO3. The coating process increases the shrinkage rate so it can improve the sintering behavior of BaTiO3, and coating film inhibits the grain growth. The e– T curve of coated BaTiO3 ceramics is flatter than that of pure BaTiO3, thus, it can be applied in fine-grain BaTiO3-based capacitors.
3.4. Dielectric property of coated BaTiO3 References Because of the disappearance of unit cell tetragonality and multidomain structure, the dielectric permittivity drops significantly with decrease in grain size less than 0.6 Am, which is the size effect of BaTiO3. Wang and Dayton [10] reported that a dense ceramic with fine grained size (<0.2 Am) and permittivity of 2800, meeting the EIA X7R temperature characteristic had been obtained. The disk samples were sintered at 1260 jC for 2 h. The permittivity of coated BaTiO3 ceramics at room temperature is 2900, less than that of pure BT which is 4030. Because the thin films isolate adjacent BT grains from each other, the coating silica films can inhibit the growth of BT. Thereafter, the ceramic grain kept the size of the starting powders at about 130 nm. Under the same sintering condition, the pure BaTiO3
[1] A. Cui, T. Wang, Y. Jin, Chem. J. Chin. Univ. 19 (11) (1998) 1727. [2] W.J. Kim, Y.T. Moon, C.H. Kim, D.K. Kim, H. Lee, J. Mater. Sci. Lett. 13 (1994) 1349. [3] C.M. Wang, J. Mater. Sci. 31 (1996) 4709. [4] B. Djuricic, D. Mcgarry, S. Pickering, J. Mater. Sci. Lett. 12 (1993) 1320. [5] F.A. Selmi, V.R.W. Amarakoon, J. Am. Ceram. Soc. 71 (1988) 934. [6] D.H. Pearce, A.J. Jickells, C.B. Ponton, in: P. Duran, J.F. Fernandez (Eds.), Third Euro-Ceramics vol. 1, Faenza Editrice, Iberica, Madrid, 1993, p. 231. [7] D. Vo¨ltzke, H.-P. Abicht, Solid State Sci. 2 (2000) 149. [8] W.-H. Shih, D. Kisailus, Y. Wei, Mater. Lett. 24 (1995) 13. [9] D. Vo¨lzke, S. Gablenz, H.-P. Abicht, Mater. Chem. Phys. 61 (1999) 110. [10] S.-F. Wang, G.O. Dayton, J. Am. Ceram. Soc. 82 (10) (1999) 2677.
Materials Letters 54 (2002) 314 – 317 www.elsevier.com/locate/matlet
Structure, sintering behavior and dielectric properties of silica-coated BaTiO3 RenZheng Chen a,*, AiLi Cui b, XiaoHui Wang a, ZhiLun Gui a, LongTu Li a a
Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China b Department of Chemistry, Tsinghua University, Beijing 100084, PR China Received 28 August 2000; received in revised form 28 August 2001; accepted 2 September 2001
Abstract Silica was homogeneously coated on the surface of barium titanate (BaTiO3, BT) fine particles via sol – gel method. The thickness of silica film was about 5 nm. High-resolution transmission electron microscope (HRTEM) investigation and energy dispersion spectrometry (EDS) analysis proved that there was silica coated on BaTiO3. The coating mechanism is that after BaTiO3 hydrolyzed slightly, Na2SiO3 gelatinized on the surface of TiO2x. Compared with pure BaTiO3, the coating process can improve sintering behavior and flatten temperature coefficients (TC) curves of ceramics based on BaTiO3. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Coating; SiO2; BaTiO3; Sintering; Dielectric
1. Introduction In the ceramics industry, the additives of ceramics are normally doped by the milling method to control the structure and properties of ceramics. The homogeneity scale depends on many aspects, such as density, particle size and shape of the starting particles and additives. Compared with milling method, chemical mixing techniques such as sol –gel coating, precipitation coating and solution coating show an advantage in enhancing the microhomogeneity of additives, modifying the surface states and then have an effect on the properties of coated ceramics. In the paint industry,
*
Corresponding author.
coating processes have been widely used [1]. However, in the ceramics field, the coating technique has not been paid enough attention yet. There are some literatures on coating of structure ceramics, such as AlN [2], Si3N4 [3], ZrO2 [4], SnO2 [5], Al2O3 [6] and so on. Vo¨ltzke and Abicht [7] and Shih et al. [8] reported on the additive surface modification of BaTiO3 powders. At present, the coating mechanisms are not discussed deeply. BaTiO3 is a well-known material for capacitors. One way to reduce cost is to reduce the sintering temperature so that cheap internal electrodes such as Ag can be used. The coating process will produce a core-shell-like structure of BaTiO3 powders with the additive. Since silica is known to be a good sintering aid, silica coating may improve the sintering behavior of BaTiO3 particles. In addition, due to the coating
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 1 ) 0 0 5 8 4 - 5
R. Chen et al. / Materials Letters 54 (2002) 314–317
315
layer, sintering is performed with minimal grain growth, then the desired core-shell microstructure is achieved in fine-grain BaTiO3 ceramics. We can control the dielectric properties by adjusting the core-shell structure. In this paper, we studied the sol – gel mechanism of silica coating of BaTiO3. Silica nanofilm with the thickness of 5 nm was coated on BaTiO3 powders. The influences on sintering behavior and dielectric properties of coating were also discussed.
testing dielectric properties. The disks were sintered at 1260 jC for 2 h. Then, disk samples were electroded using silver paste applied on opposing surfaces and fired at 750 jC. Dielectric properties of the sintered disks were studied as a function of temperature using an automatic measurement system with an LCR meter (HP 4291A) at a frequency of 1 kHz. The measurements were performed during heating cycles from 60 to 130 jC at a rate of 3 jC/min. An alternating voltage of 1 V was applied.
2. Experimental
3. Results and discussions
The BaTiO3 starting powders used in this study were prepared by oxalate coprecipitation method [Ti(OC4H9)4, A.R., Ba(CH3COO)2, A.R., Ba/Ti=1:1]. The size of the powders is about 110– 130 nm. The mole ratio of water glass is Na2O/SiO2=1:3.
3.1. Characterization of the coated BaTiO3 powders Fig. 1 shows the morphology of the coated BaTiO3 powders. From Fig. 1, the homogeneous silica film can be seen clearly on the powders’ surfaces. The thickness of the film is about 5 nm.
2.1. Coating process The BaTiO3 powders were added into the water glass solution first. Then, the pH value of the solution was adjusted to about 10 using 1 mol/l HCl. The solution became silica sol. The concentration of BaTiO3 was 5 wt.%. The gelation time must be longer than 2 h. During the sol –gel process, electromagnetic stirring was used. The sol temperature was kept at 80 jC. After coating, the slurry was filtered then washed using deionized water and dried in a 150 jC oven for 12 h. 2.2. Characterization The coated powders were investigated using a high-resolution transmission electron microscope (JEOL-2010F). The particle size was measured by the HRTEM. The composition of the particle was analyzed by energy dispersion spectrometry (LINKISIS-300EDS). The powders were cold-pressed uniaxially into disks 5 mm in diameter and 3.5 mm in thickness for the study of the sintering behavior of coated BaTiO3. The dilatometric investigation was performed with a dilatometer (TMA92) in air, with a heating rate of 10 jC/min up to 1300 jC. The powders were cold-pressed uniaxially into disks 10 mm in diameter and 1 mm in thickness for
Fig. 1. HRTEM investigation of silica-coated powders.
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R. Chen et al. / Materials Letters 54 (2002) 314–317
structure that is easy to be coated with silica, silica is stable on BaTiO3. Given enough time, the silica film derived by gelation will stack to the required thickness. 3.3. Effect on the sintering behavior of coating
Fig. 2. EDS analyses of silica-coated BaTiO3 powders.
EDS was used to identify the elements of the powder. Fig. 2 is the EDS curve. The Si element peak at 1.76 keV can be seen from the figure. Thus it proved the existence of silica on BaTiO3.
Fig. 3 shows the shrinkage curves of coated BaTiO3 and pure BaTiO3, respectively. Fig. 3a shows that DL/L at 1300 jC of coated BaTiO3 disk reaches 19.3% greater than that of pure BaTiO3, which is 15.0% shown in Fig. 3b. The temperatures of shrinkage maxima for the coated BaTiO3 and pure BaTiO3 are 1190 and 1260 jC, respectively. At 800 jC, there is a reaction between SiO2 and BaTiO3 matrix that resulted in the little shrinkage there. The resultant is a secondary phase fresnoite, Ba2TiSi2O3 [7], which benefits by forming a eutectic above 1200 jC.
3.2. Coating mechanism Based on the instability of BaTiO3 in water and silica coating on TiO2, the coating on BaTiO3 can be divided into two steps: 3.2.1. Slight hydrolysis of BaTiO3 in aqueous solution At room temperature, BaTiO3 is unstable in water having a pH value lower than 12 [9]. Under the influence of water during stirring, Ba2+ ions are dissolved superficially from the BaTiO3 particles according to BaTiO3 ðsÞ þ H2 O ¼ Ba2þ ðaqÞ þ TiO2 ðsÞ þ 2OH ðaqÞ
ð1Þ
Therefore, the dissolved BaTiO3 surface is rich in TiO2. Silica is very easy to coat on the TiO2 [1]. 3.2.2. Na2SiO3 sol – gel process on BaTiO3 Na2SiO3 solution will gelate at pH=10 and BaTiO3 will hydrolyze at this condition. The bigger the silica concentration is, the shorter the gelation time. Thus, a high concentration is not good for the coating process. By controlling the pH value and concentration, we can control the speed of gelation. There are great areas on the BaTiO3 powders so that Silica will gelate preferentially on the surface of BaTiO3. Because the surface of BaTiO3 is rich in TiO2, forming a TiO2-like
Fig. 3. Sintering behavior of coated BaTiO3 and pure BaTiO3. (a) Sintering behavior of coated BaTiO3. (b) Sintering behavior of pure BaTiO3.
R. Chen et al. / Materials Letters 54 (2002) 314–317
317
grains grow up to micrometer scale. So the fine-grain ceramic shows less permittivity. The temperature coefficients (TC) of permittivity of coated BT ceramics and pure BT ceramics are shown in Fig. 4. The TC of coated BT ceramics is flatter than that of the pure BT. Thus, the silica coated BaTiO3 is promising in thin-layer BaTiO3-based capacitors.
4. Conclusions
Fig. 4. TC comparison between coated BT and pure BT ceramics.
Because of the small quantity of SiO2, the shrinkage at 800 jC is very little. Therefore, it can be concluded that coated BaTiO3 has a better sintering property than pure BaTiO3.
By sol – gel method, BaTiO3 particles were coated with silica film of 5 nm thickness. The coating mechanism is discussed which can be divided into two steps: (1) hydrolysis of BaTiO3 and (2) sol – gel process on BaTiO3. The coating process increases the shrinkage rate so it can improve the sintering behavior of BaTiO3, and coating film inhibits the grain growth. The e– T curve of coated BaTiO3 ceramics is flatter than that of pure BaTiO3, thus, it can be applied in fine-grain BaTiO3-based capacitors.
3.4. Dielectric property of coated BaTiO3 References Because of the disappearance of unit cell tetragonality and multidomain structure, the dielectric permittivity drops significantly with decrease in grain size less than 0.6 Am, which is the size effect of BaTiO3. Wang and Dayton [10] reported that a dense ceramic with fine grained size (<0.2 Am) and permittivity of 2800, meeting the EIA X7R temperature characteristic had been obtained. The disk samples were sintered at 1260 jC for 2 h. The permittivity of coated BaTiO3 ceramics at room temperature is 2900, less than that of pure BT which is 4030. Because the thin films isolate adjacent BT grains from each other, the coating silica films can inhibit the growth of BT. Thereafter, the ceramic grain kept the size of the starting powders at about 130 nm. Under the same sintering condition, the pure BaTiO3
[1] A. Cui, T. Wang, Y. Jin, Chem. J. Chin. Univ. 19 (11) (1998) 1727. [2] W.J. Kim, Y.T. Moon, C.H. Kim, D.K. Kim, H. Lee, J. Mater. Sci. Lett. 13 (1994) 1349. [3] C.M. Wang, J. Mater. Sci. 31 (1996) 4709. [4] B. Djuricic, D. Mcgarry, S. Pickering, J. Mater. Sci. Lett. 12 (1993) 1320. [5] F.A. Selmi, V.R.W. Amarakoon, J. Am. Ceram. Soc. 71 (1988) 934. [6] D.H. Pearce, A.J. Jickells, C.B. Ponton, in: P. Duran, J.F. Fernandez (Eds.), Third Euro-Ceramics vol. 1, Faenza Editrice, Iberica, Madrid, 1993, p. 231. [7] D. Vo¨ltzke, H.-P. Abicht, Solid State Sci. 2 (2000) 149. [8] W.-H. Shih, D. Kisailus, Y. Wei, Mater. Lett. 24 (1995) 13. [9] D. Vo¨lzke, S. Gablenz, H.-P. Abicht, Mater. Chem. Phys. 61 (1999) 110. [10] S.-F. Wang, G.O. Dayton, J. Am. Ceram. Soc. 82 (10) (1999) 2677.