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Dielectric properties of fine-grained BaTiO3 prepared by spark-plasma-sintering
Materials Chemistry and Physics 83 (2004) 23–28
Dielectric properties of fine-grained BaTiO3 prepared by spark-plasma-sintering Baorang Li∗ , Xiaohui Wang, Longtu Li, Hui Zhou, Xingtao Liu, Xiuquan Han, Yingchun Zhang, Xiwei Qi, Xiangyun Deng State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China Received 2 June 2003; received in revised form 1 August 2003; accepted 1 August 2003
Abstract In our previous studies, an investigation aiming at finding what affects the microstructure and density of BaTiO3 by spark-plasma-sintering (SPS) has been reported. However, in this paper, BaTiO3 with different grain size ranging from 80 nm to several micrometers are prepared by controlling SPS conditions and the relations between the grain size and dielectric properties have been studied. XRD and Raman spectroscopy analysis suggest tetragonality decreases with smaller grain size because the orthorhombic structure starts to present in the pellets. For quantifying the tetragonal content, the XRD profiles around 2θ = 10–90◦ of BaTiO3 were analyzed according to Gaussian–Lorentzian distribution after the elimination of K␣2 component and in a further step the tetragonal content in the pellets with different grain size were calculated. The temperature dependence of relative permittivity indicated that decreased tetragonal content can deteriorate the dielectric properties and Tc shifts to lower temperature with reduced grain size. The former can be attributed to the decreased area of the domain wall per volume with decreased tetragonal content while the latter can be interpreted as the internal stress produced from phase transformation. © 2003 Elsevier B.V. All rights reserved. Keywords: BaTiO3 ; Gaussian–Lorentzian distribution; Tetragonal content; Dielectric property
1. Introduction Since the discovery of the high permittivity of ferroelectric barium titanate in 1943, ceramic materials based on this compound are utilized in the manufacture of ceramic capacitors. The permittivity of ceramic BaTiO3 strongly depends on the grain size and grain size effects in BaTiO3 ceramics have been investigated extensively. It is reported that the room temperature permittivity of BaTiO3 obviously has a maximum value at a grain size of about 1 um [1]. High density BaTiO3 with fine grain size, especially for grain size less than 100 nm, is usually difficult to obtain because of the uncontrolled grain growth during the final stage of sintering process. So studies of the influence of grain size on the electric properties referred above are actually limited in BaTiO3 with grain size in micro-scale. There are still other reports on the particle size effect upon the dielectric properties [2,3]. However, these studies are also indispens∗ Corresponding author. E-mail address: [email protected] (B. Li).
0254-0584/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2003.08.009
able to understand the relations of grain size with the properties because the grains, usually surrounded by its neighboring in polycrystalline BaTiO3 ceramics, are considered to be in a significantly different situation from the individual powders with the same size. All these indicate preparation of fine-grain sized BaTiO3 with high density is the key to resolve the above problems. The-spark-plasma-sintering (SPS) system was first designed by Inoue in 1960s and developed in 1990 [4,5]. It is a process which makes use of microscopic electrical discharge between particles under pressure, So it is a combination of the hot-press and the plasma generator. Compared with the conventional sintering method the SPS process enables a compact powders to be sintered under uniform heating to high density at relatively low temperatures and in a much shorter sintering periods typically a few minutes. Shorter sintering periods and lower sintering temperature carried out in the SPS process are advantageous in suppressing exaggerated grain growth. In our previous study [6], in which SPS have been applied on BaTiO3, high density BaTiO3 with different grain size ranging from 80 nm to several micrometers prepared
24
B. Li et al. / Materials Chemistry and Physics 83 (2004) 23–28
successfully. In present study, the dielectric properties of ceramic BaTiO3 with grain size of 80 nm to several micro-meters are also investigated. The relationship between grain size and dielectric properties is also analyzed.
2. Experimental BaTiO3 powders were prepared by oxalic acid precipitation method and its synthesis route has been described in our previous study [7]. The obtained powders were then disposed according the following route: Powders synthesized by oxalic acid precipitation
holding times in order to obtain BaTiO3 with different grain size. The heating rate was chosen as 400 ◦ C min−1 and the applied mechanical pressure was 40 MPa. To remove the carbon contamination, the as-sintered pellets were annealed in air at 850 ◦ C for 24 h. Density of the sintered samples was determined by Archimedes method. The X-ray diffraction data were collected on D/max-RB using Cu K␣ radiation. Raman scattering spectra were obtained using microscopic confocal Raman Spectrometer (RM2000). The fracture surface of the sintered samples was observed by scanning electron microscopy and the grain size was calculated using the Scherrer equation. Silver paste was painted to the surfaces of the as-sintered samples followed by heat treatment at 600 ◦ C for 10 min to form an electode for electrical measurement. Dielectric properties including dielectric constant and dielectric loss were measured by using HP4194A impedance analyzer.
Manual milling
3. Results and discussion 3.1. Microstructure Sieving
Ultrasonic disperse (2)
Washing (1) Alcohol
Centrifugal Filter (2)
Ultrasonic disperse (1)
Drying (2)
By controlling sintering conditions, BaTiO3 ceramics with different grain sizes ranging from 80 nm to several micrometers were prepared and their densities were shown in Table 1. It is easily found from Table 1 that all the densities were higher than 95% of theoretical density. Corresponding SEM images of fracture surface of the annealed BaTiO3 pellets are shown in Fig. 1. Just as pointed out in our previous studies, the enhanced densities and remained fine grain size might result from special sintering way of SPS because its fast heating and cooling rate can change some mechanism of sintering. 3.2. X-ray patterns
Centrifugal Filter (1)
Drying (1)
Washing (2) Isopropanol
After disposal, the particle size becomes smaller and more uniform. The final particle size is about 20 nm. Six gram of the disposed BaTiO3 powders was poured into the graphite mold and then sintered at different temperatures for different
Fig. 2 shows the X-ray diffraction patterns for BaTiO3 pellets with different grain size. Fig. 2 also shows the XRD profiles around 2θ = 45◦ . For the pellets with larger grains, the XRD profiles show obvious splitting of the tetragonal (0 0 2) and (2 0 0) which indicates the pure tetragonal structure is present in the pellets while for grain size less than 100 nm symmetrical (2 0 0) reflection suggests pure cubic structure is stable. The complicated XRD profiles
Table 1 Grain size
Density (%)
>1 um 800 nm 500 nm 200 nm <100 nm
97 97 97.8 95.6 94.6
B. Li et al. / Materials Chemistry and Physics 83 (2004) 23–28
25
Fig. 1. SEM images of fracture surface of the annealed BaTiO3 pellets with different grain size.
around 2θ = 45◦ may be regarded as being composed of a mixed of tetragonal and cubic phase. This indicates that the grain size greatly influence the phase transformation from the cubic to tetragonal structure. Raman scattering spectra, as shown in Fig. 3, further confirmed the results of XRD. For orthorhombic phase, a positive intensity Raman peak at 180 cm−1 usually can be observed while a negative peak is indicative of a tetragonal phase. Therefore, Raman spectra in our studies, as shown in Fig. 3, indicated that a population of the tetragonal phase in the pellets is increased with grain size increasing, just as found in Fig. 2. In contrast,
for grain size less than 100 nm, Raman spectra suggested it is mainly composed of pure orthorhombic structure. However, this is contradictory to the X-ray results in which the structure for grain size less than 100 nm is shown to be cubic, as shown in Fig. 2. This discrepancy between the results of XRD and Raman in nano-grain sized BaTiO3 can be attributed to the difference of the sensitivity of the both measurements. For identification of both cubic and tetragonal phases and determination of the relative amounts of tetragonal phase in the present pellets, the XRD profiles around 2θ = 45◦ were analyzed according to Gaussian–Lorentzian distribution
26
B. Li et al. / Materials Chemistry and Physics 83 (2004) 23–28
obvious ansotropic in the single crystals and the dielectric properties were influenced greatly by the internal stress produced by the phase transformation from the cubic to the tetragonal type. Therefore, the increased tetragonal content with the increased grain size must affect the dielectric properties of BaTiO3 . 3.3. Dielectric properties Fixed-frequency (1 kHz) permittivity data for BaTiO3 with different grain size over the temperature range 25–180 ◦ C are illustrated in Fig. 6. When grain size was reduced, the Curie temperature (Tc ) shifts to lower temperature while the corresponding k decrease. The permittivity of BaTiO3 ceramics can be regarded as the sum of a volume contribution, which is based on the single-crystal dielectric constant and of the contribution of the ferro-electric domain walls in the electric field [10]. Fig. 2. X-ray diffraction patterns for BaTiO3 pellets with different grain size.
after the elimination of K␣2 component. The results are shown in Fig. 4. The observed diffraction profile was expressed by solid line and the calculated one was represented by dash line. Other peak profiles from 2θ = 10–90◦ were also analyzed in the same way. The tetragonal content is defined as the ratio of the integrated intensities of the tetragonal peaks to those of the total profile for each sample. The grain size dependence of tetragonal content is shown in Fig. 5. It is reported [8–12] that the dielectric constant show
>1um
800nm
500nm
200nm
<100nm
0
200
400
600
800
1000
Fig. 3. Raman scattering spectra as a function of grain size.
εr (T) = εr,vol (T) + εr,dom (T) It is reported that 90◦ domain can be formed to minimize the transformation stress from cubic to tetragonal phase transformation [13]. It is also reported that 90◦ domain was expected to be vanished at grain size less than 400 nm [1]. Therefore, with increased grain size, gradually increased tetragonal content suggest the increased area of the domain wall per volume, which can enhance the dielectric properties. As for fine grain size, especially less than 400 nm, however, this is not applicable. Disappearance of domains makes transformation from cubic to tetragonal type more difficult because a localized shear strain presented at the grain boundaries, which hindered the transformation cannot be relieved by domains [14]. So metastable orthorhombic structure starts to present in BaTiO3 mixed with tetragonal structure with reduced grain size since the unit cell of the orthorhombic is characterized by a shear deformation of the cubic perovskite cell, which is relatively easy to happen compared with tetragonal structure [15]. For nano-grained BaTiO3 , pure orthorhombic structure can be expected. The gradually decreased tetragonal content with reduced grain size can deteriorate the dielectric properties resulting in the lower dielectric constant. Another feature that is observed in Fig. 6 is with decreased grain size Tc tends to shift to low temperature. The Curie temperature is reported to have close relations to the internal stress [16]. On the one hand, internal stresses develop in the constrained grains at the phase transition temperature. With decreased grain size, they can shift Tc to lower temperature. The internal stresses are expected in the pellets because of the fast heating and cooling rate of SPS. However, the induced internal stresses can be relieved for large grains because in larger grained ferroelectric ceramics, the internal stresses concentration is higher and enough to
B. Li et al. / Materials Chemistry and Physics 83 (2004) 23–28
27
Fig. 4. X-ray diffraction line profiles of (0 0 2) and (2 0 0) peaks for (a–e).
form micro-cracks at the grain boundaries which can relieve the stresses [17]. In contrast, less or no micro-cracks can be formed in smaller grain sized BaTiO3 because increased grain boundaries with reduced grain size can alleviate the
internal stresses concentration in a degree. So they are remained in the pellets with smaller grains, which are usually compressive and also contribute to the decreasing of Tc [18].
Fraction of tetragonal content(%)
28
B. Li et al. / Materials Chemistry and Physics 83 (2004) 23–28
sintering conditions. Investigations upon the grain size dependence of dielectric properties shows that the dielectric constant of Tc decreases and Tc shifts to lower temperature with decreased grain size. This can be explained by the decreased tetragonal content and internal stress remained in the pellets with reduced grain size. The decreased tetragonal contents result in the decreased area of the domain wall per volume, which deteriorates the dielectric properties while the unrelieved internal stress shift Tc to low temperature.
1 .0 0 .8 0 .6 0 .4 0 .2 0 .0
-2
0
2
4
6
8
10 12 14 16 18 20 22
References
G ra in s ize (X 1 0 0 n m )
Fig. 5. Tetragonal content via grain size.
Dielectric constant
10000
>1000nm 800nm 500nm 200nm <100nm
8000 6000 4000 2000 0 -50
0
50 100 o Temperature( C)
150
Fig. 6. Fixed-frequency (1 kHz) relative permitivity data for BaTiO3 with different grain size over the temperature range 25–180 ◦ C.
4. Conclusions BaTiO3 with different grain size ranging from 80 nm to several micrometers were prepared by controlling proper
[1] G. Arlt, D. Hennings, G. deWith, J. Appl. Phys. 58 (1985) 1619– 1625. [2] K. Uchino, iji Sandanaga, T. Hirose, J. Am. Ceram. Soc. 72 (1989) 1555–1558. [3] K. Ishikawa, K. Yoshikawa, N. Okaca, Phys. Rev. B 37 (1988) 5852–5855. [4] K. Inoue, US Patent 3,241,956 (March 1962). [5] K. Inoue, US Patent 3,250,892 (May 1966). [6] B. Li, X. Wang, L. Li, Mater. Chem. Phys, in press. [7] B. Li, X. Wang, L. Li, Mater. Chem. Phys. 78 (2002) 292– 298. [8] W.J. Merz, in: F. Jona, G. Shirane, (Eds.) Ferroelectric Crystals, (Pergamon Press, London, 1962) p. 115. [9] H. Diamond, J. Appl. Phys. 12 (1961) 909. [10] G. Arlt, H. Peusens, Ferroelectrics 48 (1983) 213. [11] A.V. Turik, Bull. Acad. Sci. USSR, Phys. Ser. 29 (1965) 91. [12] A.V. Turik, J.J. Bondarenko, Ferroelectrics 17 (1974) 303. [13] W.R. Buessem, L.E. Cross, A.K. Goswami, J. Am. Ceram. Soc. 49 (1966) 33–39. [14] G. Arlt, Ferroelectrics 104 (1990) 217–219. [15] M.H. Frey, D.A. Payne, Phys. Rev. B 54 (1996) 3158–3167. [16] T. Hiroshima, K. Tanaka, T. Kimurra, J. Am. Ceram. Soc. 79 (1996) 3235–3242. [17] H.-T. Chung, B.-C. Shin, Ho-Gi Kim, J. Am. Ceram. Soc. 72 (1989) 327. [18] H.J. Hwang, T. Nagai, Tatsuki, M. Toriyama, J. Am. Ceram. Soc. 81 (1998) 709.
Dielectric properties of fine-grained BaTiO3 prepared by spark-plasma-sintering Baorang Li∗ , Xiaohui Wang, Longtu Li, Hui Zhou, Xingtao Liu, Xiuquan Han, Yingchun Zhang, Xiwei Qi, Xiangyun Deng State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China Received 2 June 2003; received in revised form 1 August 2003; accepted 1 August 2003
Abstract In our previous studies, an investigation aiming at finding what affects the microstructure and density of BaTiO3 by spark-plasma-sintering (SPS) has been reported. However, in this paper, BaTiO3 with different grain size ranging from 80 nm to several micrometers are prepared by controlling SPS conditions and the relations between the grain size and dielectric properties have been studied. XRD and Raman spectroscopy analysis suggest tetragonality decreases with smaller grain size because the orthorhombic structure starts to present in the pellets. For quantifying the tetragonal content, the XRD profiles around 2θ = 10–90◦ of BaTiO3 were analyzed according to Gaussian–Lorentzian distribution after the elimination of K␣2 component and in a further step the tetragonal content in the pellets with different grain size were calculated. The temperature dependence of relative permittivity indicated that decreased tetragonal content can deteriorate the dielectric properties and Tc shifts to lower temperature with reduced grain size. The former can be attributed to the decreased area of the domain wall per volume with decreased tetragonal content while the latter can be interpreted as the internal stress produced from phase transformation. © 2003 Elsevier B.V. All rights reserved. Keywords: BaTiO3 ; Gaussian–Lorentzian distribution; Tetragonal content; Dielectric property
1. Introduction Since the discovery of the high permittivity of ferroelectric barium titanate in 1943, ceramic materials based on this compound are utilized in the manufacture of ceramic capacitors. The permittivity of ceramic BaTiO3 strongly depends on the grain size and grain size effects in BaTiO3 ceramics have been investigated extensively. It is reported that the room temperature permittivity of BaTiO3 obviously has a maximum value at a grain size of about 1 um [1]. High density BaTiO3 with fine grain size, especially for grain size less than 100 nm, is usually difficult to obtain because of the uncontrolled grain growth during the final stage of sintering process. So studies of the influence of grain size on the electric properties referred above are actually limited in BaTiO3 with grain size in micro-scale. There are still other reports on the particle size effect upon the dielectric properties [2,3]. However, these studies are also indispens∗ Corresponding author. E-mail address: [email protected] (B. Li).
0254-0584/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2003.08.009
able to understand the relations of grain size with the properties because the grains, usually surrounded by its neighboring in polycrystalline BaTiO3 ceramics, are considered to be in a significantly different situation from the individual powders with the same size. All these indicate preparation of fine-grain sized BaTiO3 with high density is the key to resolve the above problems. The-spark-plasma-sintering (SPS) system was first designed by Inoue in 1960s and developed in 1990 [4,5]. It is a process which makes use of microscopic electrical discharge between particles under pressure, So it is a combination of the hot-press and the plasma generator. Compared with the conventional sintering method the SPS process enables a compact powders to be sintered under uniform heating to high density at relatively low temperatures and in a much shorter sintering periods typically a few minutes. Shorter sintering periods and lower sintering temperature carried out in the SPS process are advantageous in suppressing exaggerated grain growth. In our previous study [6], in which SPS have been applied on BaTiO3, high density BaTiO3 with different grain size ranging from 80 nm to several micrometers prepared
24
B. Li et al. / Materials Chemistry and Physics 83 (2004) 23–28
successfully. In present study, the dielectric properties of ceramic BaTiO3 with grain size of 80 nm to several micro-meters are also investigated. The relationship between grain size and dielectric properties is also analyzed.
2. Experimental BaTiO3 powders were prepared by oxalic acid precipitation method and its synthesis route has been described in our previous study [7]. The obtained powders were then disposed according the following route: Powders synthesized by oxalic acid precipitation
holding times in order to obtain BaTiO3 with different grain size. The heating rate was chosen as 400 ◦ C min−1 and the applied mechanical pressure was 40 MPa. To remove the carbon contamination, the as-sintered pellets were annealed in air at 850 ◦ C for 24 h. Density of the sintered samples was determined by Archimedes method. The X-ray diffraction data were collected on D/max-RB using Cu K␣ radiation. Raman scattering spectra were obtained using microscopic confocal Raman Spectrometer (RM2000). The fracture surface of the sintered samples was observed by scanning electron microscopy and the grain size was calculated using the Scherrer equation. Silver paste was painted to the surfaces of the as-sintered samples followed by heat treatment at 600 ◦ C for 10 min to form an electode for electrical measurement. Dielectric properties including dielectric constant and dielectric loss were measured by using HP4194A impedance analyzer.
Manual milling
3. Results and discussion 3.1. Microstructure Sieving
Ultrasonic disperse (2)
Washing (1) Alcohol
Centrifugal Filter (2)
Ultrasonic disperse (1)
Drying (2)
By controlling sintering conditions, BaTiO3 ceramics with different grain sizes ranging from 80 nm to several micrometers were prepared and their densities were shown in Table 1. It is easily found from Table 1 that all the densities were higher than 95% of theoretical density. Corresponding SEM images of fracture surface of the annealed BaTiO3 pellets are shown in Fig. 1. Just as pointed out in our previous studies, the enhanced densities and remained fine grain size might result from special sintering way of SPS because its fast heating and cooling rate can change some mechanism of sintering. 3.2. X-ray patterns
Centrifugal Filter (1)
Drying (1)
Washing (2) Isopropanol
After disposal, the particle size becomes smaller and more uniform. The final particle size is about 20 nm. Six gram of the disposed BaTiO3 powders was poured into the graphite mold and then sintered at different temperatures for different
Fig. 2 shows the X-ray diffraction patterns for BaTiO3 pellets with different grain size. Fig. 2 also shows the XRD profiles around 2θ = 45◦ . For the pellets with larger grains, the XRD profiles show obvious splitting of the tetragonal (0 0 2) and (2 0 0) which indicates the pure tetragonal structure is present in the pellets while for grain size less than 100 nm symmetrical (2 0 0) reflection suggests pure cubic structure is stable. The complicated XRD profiles
Table 1 Grain size
Density (%)
>1 um 800 nm 500 nm 200 nm <100 nm
97 97 97.8 95.6 94.6
B. Li et al. / Materials Chemistry and Physics 83 (2004) 23–28
25
Fig. 1. SEM images of fracture surface of the annealed BaTiO3 pellets with different grain size.
around 2θ = 45◦ may be regarded as being composed of a mixed of tetragonal and cubic phase. This indicates that the grain size greatly influence the phase transformation from the cubic to tetragonal structure. Raman scattering spectra, as shown in Fig. 3, further confirmed the results of XRD. For orthorhombic phase, a positive intensity Raman peak at 180 cm−1 usually can be observed while a negative peak is indicative of a tetragonal phase. Therefore, Raman spectra in our studies, as shown in Fig. 3, indicated that a population of the tetragonal phase in the pellets is increased with grain size increasing, just as found in Fig. 2. In contrast,
for grain size less than 100 nm, Raman spectra suggested it is mainly composed of pure orthorhombic structure. However, this is contradictory to the X-ray results in which the structure for grain size less than 100 nm is shown to be cubic, as shown in Fig. 2. This discrepancy between the results of XRD and Raman in nano-grain sized BaTiO3 can be attributed to the difference of the sensitivity of the both measurements. For identification of both cubic and tetragonal phases and determination of the relative amounts of tetragonal phase in the present pellets, the XRD profiles around 2θ = 45◦ were analyzed according to Gaussian–Lorentzian distribution
26
B. Li et al. / Materials Chemistry and Physics 83 (2004) 23–28
obvious ansotropic in the single crystals and the dielectric properties were influenced greatly by the internal stress produced by the phase transformation from the cubic to the tetragonal type. Therefore, the increased tetragonal content with the increased grain size must affect the dielectric properties of BaTiO3 . 3.3. Dielectric properties Fixed-frequency (1 kHz) permittivity data for BaTiO3 with different grain size over the temperature range 25–180 ◦ C are illustrated in Fig. 6. When grain size was reduced, the Curie temperature (Tc ) shifts to lower temperature while the corresponding k decrease. The permittivity of BaTiO3 ceramics can be regarded as the sum of a volume contribution, which is based on the single-crystal dielectric constant and of the contribution of the ferro-electric domain walls in the electric field [10]. Fig. 2. X-ray diffraction patterns for BaTiO3 pellets with different grain size.
after the elimination of K␣2 component. The results are shown in Fig. 4. The observed diffraction profile was expressed by solid line and the calculated one was represented by dash line. Other peak profiles from 2θ = 10–90◦ were also analyzed in the same way. The tetragonal content is defined as the ratio of the integrated intensities of the tetragonal peaks to those of the total profile for each sample. The grain size dependence of tetragonal content is shown in Fig. 5. It is reported [8–12] that the dielectric constant show
>1um
800nm
500nm
200nm
<100nm
0
200
400
600
800
1000
Fig. 3. Raman scattering spectra as a function of grain size.
εr (T) = εr,vol (T) + εr,dom (T) It is reported that 90◦ domain can be formed to minimize the transformation stress from cubic to tetragonal phase transformation [13]. It is also reported that 90◦ domain was expected to be vanished at grain size less than 400 nm [1]. Therefore, with increased grain size, gradually increased tetragonal content suggest the increased area of the domain wall per volume, which can enhance the dielectric properties. As for fine grain size, especially less than 400 nm, however, this is not applicable. Disappearance of domains makes transformation from cubic to tetragonal type more difficult because a localized shear strain presented at the grain boundaries, which hindered the transformation cannot be relieved by domains [14]. So metastable orthorhombic structure starts to present in BaTiO3 mixed with tetragonal structure with reduced grain size since the unit cell of the orthorhombic is characterized by a shear deformation of the cubic perovskite cell, which is relatively easy to happen compared with tetragonal structure [15]. For nano-grained BaTiO3 , pure orthorhombic structure can be expected. The gradually decreased tetragonal content with reduced grain size can deteriorate the dielectric properties resulting in the lower dielectric constant. Another feature that is observed in Fig. 6 is with decreased grain size Tc tends to shift to low temperature. The Curie temperature is reported to have close relations to the internal stress [16]. On the one hand, internal stresses develop in the constrained grains at the phase transition temperature. With decreased grain size, they can shift Tc to lower temperature. The internal stresses are expected in the pellets because of the fast heating and cooling rate of SPS. However, the induced internal stresses can be relieved for large grains because in larger grained ferroelectric ceramics, the internal stresses concentration is higher and enough to
B. Li et al. / Materials Chemistry and Physics 83 (2004) 23–28
27
Fig. 4. X-ray diffraction line profiles of (0 0 2) and (2 0 0) peaks for (a–e).
form micro-cracks at the grain boundaries which can relieve the stresses [17]. In contrast, less or no micro-cracks can be formed in smaller grain sized BaTiO3 because increased grain boundaries with reduced grain size can alleviate the
internal stresses concentration in a degree. So they are remained in the pellets with smaller grains, which are usually compressive and also contribute to the decreasing of Tc [18].
Fraction of tetragonal content(%)
28
B. Li et al. / Materials Chemistry and Physics 83 (2004) 23–28
sintering conditions. Investigations upon the grain size dependence of dielectric properties shows that the dielectric constant of Tc decreases and Tc shifts to lower temperature with decreased grain size. This can be explained by the decreased tetragonal content and internal stress remained in the pellets with reduced grain size. The decreased tetragonal contents result in the decreased area of the domain wall per volume, which deteriorates the dielectric properties while the unrelieved internal stress shift Tc to low temperature.
1 .0 0 .8 0 .6 0 .4 0 .2 0 .0
-2
0
2
4
6
8
10 12 14 16 18 20 22
References
G ra in s ize (X 1 0 0 n m )
Fig. 5. Tetragonal content via grain size.
Dielectric constant
10000
>1000nm 800nm 500nm 200nm <100nm
8000 6000 4000 2000 0 -50
0
50 100 o Temperature( C)
150
Fig. 6. Fixed-frequency (1 kHz) relative permitivity data for BaTiO3 with different grain size over the temperature range 25–180 ◦ C.
4. Conclusions BaTiO3 with different grain size ranging from 80 nm to several micrometers were prepared by controlling proper
[1] G. Arlt, D. Hennings, G. deWith, J. Appl. Phys. 58 (1985) 1619– 1625. [2] K. Uchino, iji Sandanaga, T. Hirose, J. Am. Ceram. Soc. 72 (1989) 1555–1558. [3] K. Ishikawa, K. Yoshikawa, N. Okaca, Phys. Rev. B 37 (1988) 5852–5855. [4] K. Inoue, US Patent 3,241,956 (March 1962). [5] K. Inoue, US Patent 3,250,892 (May 1966). [6] B. Li, X. Wang, L. Li, Mater. Chem. Phys, in press. [7] B. Li, X. Wang, L. Li, Mater. Chem. Phys. 78 (2002) 292– 298. [8] W.J. Merz, in: F. Jona, G. Shirane, (Eds.) Ferroelectric Crystals, (Pergamon Press, London, 1962) p. 115. [9] H. Diamond, J. Appl. Phys. 12 (1961) 909. [10] G. Arlt, H. Peusens, Ferroelectrics 48 (1983) 213. [11] A.V. Turik, Bull. Acad. Sci. USSR, Phys. Ser. 29 (1965) 91. [12] A.V. Turik, J.J. Bondarenko, Ferroelectrics 17 (1974) 303. [13] W.R. Buessem, L.E. Cross, A.K. Goswami, J. Am. Ceram. Soc. 49 (1966) 33–39. [14] G. Arlt, Ferroelectrics 104 (1990) 217–219. [15] M.H. Frey, D.A. Payne, Phys. Rev. B 54 (1996) 3158–3167. [16] T. Hiroshima, K. Tanaka, T. Kimurra, J. Am. Ceram. Soc. 79 (1996) 3235–3242. [17] H.-T. Chung, B.-C. Shin, Ho-Gi Kim, J. Am. Ceram. Soc. 72 (1989) 327. [18] H.J. Hwang, T. Nagai, Tatsuki, M. Toriyama, J. Am. Ceram. Soc. 81 (1998) 709.