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Microstructure and electrical properties of niobium doped Bi4Ti3O12 layer-structured piezoelectric ceramics
Materials Science and Engineering B 116 (2005) 99–103
Short communication
Microstructure and electrical properties of niobium doped Bi4Ti3O12 layer-structured piezoelectric ceramics Lina Zhang, Ruiqing Chu, Suchuan Zhao, Guorong Li∗ , Qingrui Yin The State Key Lab of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai 200050, PR China Received 14 July 2004; accepted 12 September 2004
Abstract Layer-structured Bi4 Ti3−x Nbx O12+x/2 (BiTN) ceramics, where x = 0.02–0.20, have been prepared by the conventional sintering technique. XRD results reveal the existence of an orthorhombic structure. The grain size decreases gradually and grain growth anisotropy is limited when x increases. The domain structures of BiTN ceramics differ on x. Nb5+ donor doping decreases markedly electrical conductivity of the materials and Tc shifts gradually to lower temperatures. In addition, low dielectric losses and good temperature stability of dielectric constant are obtained in a wide temperature range. Ferroelectric hysteresis loops of the materials are also determined on x. The best properties can be found in x = 0.08 and 0.11, indicating lower electrical conductivity, good temperature stability of dielectric properties and adequate piezoelectric properties. © 2004 Elsevier B.V. All rights reserved. Keywords: Domain structure; Electrical conductivity; Temperature coefficient of dielectric constant; Piezoelectric properties; Nb5+ -doped; Bi4 Ti3 O12 ceramics
1. Introduction Lead zirconate titanate (PZT) based piezoelectric devices are employed widely in the range of 0–200 ◦ C. Recently, there was a need for sensors that can operate at higher temperatures (>400 ◦ C) without significant changes in sensing properties. The compounds of Aurivillius bismuth layer-structure ferroelectric (BLSF) are suitable candidate materials due to their high Curie temperatures, strong anisotropic characters, low dielectric losses and low aging rates [1]. Bismuth titanate (Bi4 Ti3 O12 ) belongs to the family of BLSF with a general formula (Bi2 O2 )2+ (Am−1 Bm O3m+1 )2− . It undergoes a ferroelectric to paraelectric phase transition at the Curie temperature (Tc = 675 ◦ C). At room temperature the symmetry of Bi4 Ti3 O12 is monoclinic structure (C1h = m) [2], while it can be considered as orthorhombic structure with a = 0.5448, b = 0.5411 and c = 3.283 nm ∗
Corresponding author. Tel.: +86 21 5241 2034; fax: +86 21 5241 3122. E-mail address: [email protected] (G. Li).
0921-5107/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2004.09.007
from X-ray and neutron diffraction patterns [3]. The crystal structure is characterized by three perovskite-like unit cells sandwiched between (Bi2 O2 )2+ layers along the c-axis. Bi4 Ti3 O12 single crystal has two independent spontaneous polarizations with a major component of 50 C/cm2 in the a–b plane [2]. The piezoelectric coefficient is also very high in this plane. These characteristics suggest that Bi4 Ti3 O12 is a good material for high-temperature ferroelectric and piezoelectric applications. Moreover, it is considered to be a promising, environmentally friendly material alternative to PZT. Effort has been devoted to Bi4 Ti3 O12 ceramics and thin films fabricated by various methods [4–7]. The applications have been limited because of its high electrical conductivity, which is maximum in the a–b plane. It is difficult to polarize Bi4 Ti3 O12 to obtain high piezoelectric activity. It is well known that donor dopants decrease the conductivity of Bi4 Ti3 O12 material with a p-type mechanism. There have been correlative investigations about modifying electrical conductivity of Bi4 Ti3 O12 by Nb5+ , W6+ substituting Ti4+ [8–12].
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L. Zhang et al. / Materials Science and Engineering B 116 (2005) 99–103
According to the theory of Fousek and Janovec [13], there were 18 possibilities of domain-wall structures in Bi4 Ti3 O12 . These complex domain-wall structures may lead to a considerable contribution to the piezoelectric properties. Cummins [2,14] studied the domain walls in Bi4 Ti3 O12 single crystal by optical method and electron microscopy, respectively, and many kinds of domain-wall structures had been verified. But there is no report on the domain structures of Bi4 Ti3 O12 ceramics. Excellent temperature stability of piezoelectric materials is one of important factors for high temperature applications. Ogawa et al. [15] has investigated the temperature dependence of resonance frequency of the grain-oriented CaBi4 Ti4 O15 ceramics. In generally, if dielectric properties change remarkably with elevated temperature, the poled ceramics would be depolarized and the piezoelectric properties will be worse. Therefore, analyzing the dependence of dielectric constant versus temperature is an important subject. In the present study, microstructure, electrical conductivity, dielectric and piezoelectric properties of Nb5+ -doped Bi4 Ti3 O12 prepared by the conventional sintering behavior have been studied, in particular ferroelectric domain structures and temperature stability of dielectric properties were discussed.
2. Experimental Bi4 Ti3−x Nbx O12+x/2 (x = 0.02, 0.05, 0.08, 0.11 and 0.20) (BiTN) ceramics were prepared by the conventional sintering technique. Bi2 O3 , TiO2 and Nb2 O5 reagents were thoroughly ball-milled and calcined at 850 ◦ C for 2 h in alumina crucibles. Then the powders were ground again to gain fine granularity. The dried powders were sieved and added with about 8 wt.% of a 5% PVA binder. After that, compacted disks of 12 mm diameter and 1.5 mm thickness were obtained by uniaxially pressed at 150 MPa. These disks were sintered with a heating rate of 2.5 ◦ C min−1 at 1000–1150 ◦ C for 1 h in air. The sintered disks were ground to 0.5 mm in thickness. Silver electrodes were pasted on both sides of the samples and fired at 740 ◦ C for 20 min. The samples were polarized in a silicon oil bath at 140 ◦ C in a field of 60–100 kV/cm for 20 min. The bulk density was measured using the Archimedes method. X-ray diffraction (RAX-10, Rigaku, Japan) using Cu K␣ radiation was conducted to determine the phase of sintered samples. The microstructures and ferroelectric domain structures were examined on polished and thermally etched surfaces by a commercial scanning force microscopy (SFM) instrument (Seiko SPA 300/SPI 3800). A high resistance meter (KEITHLEY 6517A) was used to measure electrical conductivity in a direct current circuit. The dielectric and piezoelectric properties were measured with an impedance analyzer (HP 4294A). A high-voltage test system (Radiant Technologies RT66A unit) was used to obtain the ferroelectric hysteresis loops.
3. Results and discussion All samples achieved about 95% of theoretical density at their corresponding sintering temperature, which was necessary to obtain high resistance and withstand high electric field to pole the materials. Fig. 1 shows the X-ray diffraction diagram of the studied BiTN ceramics. All diffraction data showed the existence of a single orthorhombic phase, without a secondary phase detectable. It is in accordance with the view that Bi4 Ti3 O12 is usually considered to have a polar orthorhombic structure [3]. Fig. 2 shows the typical surface morphology and domain structures of the polished and thermally etched surfaces of BiTN samples. As Fig. 2(a)–(c) are shown, it is evident that Nb5+ addition leads to a significant change in the average size and homogeneity of the grains. Nb5+ -doping decreased gradually the grain size in a–b plane and limited the anisotropy of grain growth. The shape of grains changed from needle-like to rod-like structure as the doped content x increased. More homogeneous grains were observed especially for x = 0.20. The porosity was mainly located on grain boundaries. It is thought that the random orientation and anisotropic growth of the grains contribute to the pores at a large extent. Fig. 2(d) and (e) show the domain structures obtained under the piezoresponse mode of SFM, where the domain structures in BiTN ceramics were greatly different. In BiTN (x = 0.02) ceramics, most of domain walls were mainly 90◦ and paralleled to the c axis (see Fig. 2(d)). However, for the dopant content x = 0.08 ceramics, the domain walls mainly perpendiculared to the c axis and paralleled to a–b plane (see Fig. 2(e)). Cummins’s investigation [14] and other similar studies for Bi4 Ti3 O12 single crystal gave evidence that a number of different types of domain walls existed. Their studies indicated that 90◦ walls and walls parallel to the a–b plane are stable in the virgin crystal. In our studies, it was found that the domain walls in BiTN (x = 0.02) were mainly
Fig. 1. XRD patterns of BiTN ceramic samples.
L. Zhang et al. / Materials Science and Engineering B 116 (2005) 99–103
101
Fig. 2. SFM micrographs of polished and thermally etched surfaces of BiTN ceramics: microstructures for (a) x = 0.02, (b) x = 0.08, (c) x = 0.20 and the corresponding domain structures for (d) x = 0.02, (e) x = 0.08.
90◦ and the domain walls in BiTN (x = 0.08) ceramics mainly paralleled to a–b plane. This change of domain structures was probably due to the modifying of Nb5+ . The temperature dependence of electrical conductivity measured in a direct current circuit with BiTN ceramics was studied in the temperature rang from 160 to 600 ◦ C (see Fig. 3). Here, we gave the conductivity of pure Bi4 Ti3 O12 for comparison. It could be found that the conductivity of BiTN was obviously lower than that of the undoped Bi4 Ti3 O12 , which was accord with donor dopants decreasing the conductivity of a material with a p-type mechanism. The conductivity decreased two orders of magnitude with a small content of Nb5+ -doping (x = 0.02). When x ≥ 0.05, BiTN ce-
Fig. 3. The log of the electrical conductivity σ as a function of the temperature for BiTN ceramics.
ramics obtained lower electrical conductivity and could be poled to develop piezoelectricity. In Bi4 Ti3 O12 , hole compensation of bismuth vacancies (VBi + 3h· ) promotes p-type electronic conductivity. When Nb5+ substitutes Ti4+ in BiTN ceramics, a positive charge centers in Nb site and an electron will be created under charge neutrality restriction, which can be described as Nb·Ti + e . These electrons neutralize the influence of the holes. According to the electronic conductivity relationship σ = nqµ
(1)
where n is the number of the carriers, q the charge and µ the mobility, it was thought that the number of the hole carriers, n, was decreased. That resulted in the low conductivity in BiTN ceramics. The conductivity will decrease with donor doping to a minimum value until the concentration of the electrons equals to the hole concentration. The dielectric constants measured at room temperature at 1 kHz increased gradually from 160 to 216 with Nb5+ modifying. It indicates the influence of the bismuth vacancies. Similar to Nb5+ -doping into PZT-based piezoelectric ceramics, Nb2 O5 can be considered as a kind of soft additive in Bi4 Ti3 O12 ceramics. It will soften piezoelectric ceramics, e.g. increasing dielectric constant, dielectric loss and decreasing electrical conductivity (listed in Table 1). Fig. 4(a) shows the variation of dielectric constant as a function of the temperature at 100 kHz for various Nb5+ doped Bi4 Ti3 O12 ceramics. Tc of BiTN ceramics shifted to lower temperatures with increasing Nb5+ concentration. The reason was the result of Nb ions occupying the Ti site of
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Table 1 The resistivity, dielectric and piezoelectric properties of BiTN ceramics x
ρ(400 ◦ C) ( cm)
Tc (◦ C)
ε/ε0
tan ␦ (%)
Qm
Kp (%)
d33 (pC/N)
0.02 0.05 0.08 0.11 0.20
2.0 × 107
669 660 652 646 625
160 170 181 199 216
1.3 1.5 2.3 2.2 2.3
– 1861 2348 2492 2071
– 3.7 3.6 3.5 3.5
– 17 19 18 17
1.3 × 109 3.0 × 109 1.4 × 109 7.5 × 108
Fig. 4. (a) Dielectric constant and (b) dielectric loss tangent of BiTN ceramics as a function of the temperature at 100 kHz.
the perovskite cell in the bismuth layer-structured ceramics. From Fig. 4(b), the loss tangent of x ≥ 0.05 increased very slowly and was below 0.04 up to 400 ◦ C, which enabled the material to polarize at higher electric fields and higher temperatures. As for a small content x of 0.02, its dielectric loss increased sharply with elevated temperature due to higher conductivity. To better illustrate the temperature dependence of dielectric properties, we defined the temperature coefficient of dielectric constant as follows: Tkε = (εT − εT0 )/(εT0 × (T − T0 ))
crease of space charge density (Vo¨ ) led to the large remnant polarization [18]. In addition, the decreasing of grain size and the changing of domain structures may be the factors causing lager coercive field (see Fig. 2). Table 1 summarizes the electrical resistivity at 400 ◦ C, dielectric and piezoelectric properties of BiTN ceramics. Piezoelectric properties for x = 0.02 ceramics were not obtained because its conductivity was too high to polarize. Other composition materials were polarized and showed relatively high piezoelectric response d33 values of 17–19 pC/N.
(2)
where εT and εT0 are the dielectric constant of the samples at temperature T and room temperature T0 , respectively. Fig. 5 shows that the steady temperature coefficients of dielectric constant were very small up to 500 ◦ C with x = 0.08 and 0.11, due to the suitable Nb modification. The values of Tkε were 1.71 × 10−3 and 1.62 × 10−3 /◦ C at 500 ◦ C, respectively. For other composition materials, the temperature coefficients of dielectric constant change markedly when the temperature increased. Fig. 6 shows the ferroelectric hysteresis loops measured at around 140 kV/cm electrical fields in a silicon oil bath at 140 ◦ C. The remnant polarization and the coercive field were increased after Nb5+ doping into the Bi4 Ti3 O12 ceramics, which was similar to the investigations of some researchers [16,17]. Nb5+ substituting for Ti4+ in BiTN ceramics efficiently decreased the concentration of VO·· , which weakened the influence of domain pinning on the polarization. The de-
Fig. 5. Temperature coefficient of dielectric constant as a function of the temperature at 100 kHz for BiTN ceramics.
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Fig. 6. Ferroelectric hysteresis loops of BiTN ceramics at 140 ◦ C.
4. Conclusions
References
Donor dopant Nb2 O5 was incorporated to Bi4 Ti3 O12 layered compound and BiTN ceramics were synthesized and characterized on their electrical properties. The needle-like grains size became smaller and more homogeneous grains were observed with increasing Nb2 O5 concentration. Furthermore, it was found that Nb5+ doping could change the domain structures of BiTN ceramics. The conductivity of BiTN was decreased obviously by Nb5+ donor doping and Tc shifted to lower temperatures. Nb doping played a soft additive role in BiTN ceramics. The remnant polarization of BiTN enhanced due to Nb5+ -doping reducing the concentration of oxygen vacancies. It is difficult to improve the electrical properties of BiTN ceramics for a small Nb concentration (x = 0.02) due to its high electrical conductivity. The excellent properties were obtained for the composition x = 0.08 and 0.11. They show lower resistivity, good temperature stability of dielectric properties at elevated temperature.
[1] B. Aurivillius, Ark. Kem. 1 (1949) 463–480. [2] S.E. Cummins, L.E. Cross, J. Appl. Phys. 39 (1968) 2268–2274. [3] J.F. Dorrian, R.E. Newnham, D.K. Smith, M.I. Kay, Ferroelectrics 3 (1971) 17–27. [4] M. Villegas, C. Moure, J.F. Fernandez, P. Duran, Ceram. Int. 22 (1996) 15–22. [5] H. Watanabe, T. Kimura, T. Yamaguchi, J. Am. Ceram. Soc. 74 (1991) 139–147. [6] Y. Lu, D.T. Hoelzer, W.A. Schulze, B. Tuttle, B.G. Potter, Mater. Sci. Eng. B 39 (1996) 41–47. [7] M. Yamaguchi, K. Kawanabe, T. Nagatomo, O. Omoto, Mater. Sci. Eng. B 41 (1996) 138–142. [8] M. Villegas, A.C. Caballero, C. Moure, P. Dur´an, J.F. Fern´andez, J. Am. Ceram. Soc. 82 (1999) 2411–2416. [9] H.S. Shulman, M. Testorf, D. Damjanovic, N. Setter, J. Am. Ceram. Soc. 79 (1996) 3124–3128. [10] M. Villegas, A.C. Caballero, C. Moure, P. Dur´an, J.F. Fern´andez, J. Eur. Ceram. Soc. 19 (1999) 1183–1186. [11] M. Villegas, A.C. Caballero, J.F. Fern´andez, Ferroelectrics (2002) 267. [12] A. Megriche, L. Lebrum, M. Troccaz, Sens. Actuators 78 (1999) 88–91. [13] J. Fousek, V. Janovec, Bull. Am. Phys. Soc. 12 (1967) 902. [14] S.E. Cummins, J. Phys. Soc. Jpn. 28 (1970) 396–398. [15] H. Ogawa, M. Kimura, A. Ando, Y. Sakabe, Jpn. J. Appl. Phys. 40 (2000) 5715–5718. [16] Y. Noguchi, M. Miyayama, Appl. Phys. Lett. 78 (2001) 1903–1905. [17] Z.H. Bao, Y.Y. Yao, J.S. Zhu, Y.N. Wang, Mater. Lett. 56 (2002) 861–866. [18] L. Baudry, J. Appl. Phys. 86 (1999) 1096–1105.
Acknowledgement The author gratefully acknowledges the support of the National “973” Project (no. 2002CB613307) of China.
Short communication
Microstructure and electrical properties of niobium doped Bi4Ti3O12 layer-structured piezoelectric ceramics Lina Zhang, Ruiqing Chu, Suchuan Zhao, Guorong Li∗ , Qingrui Yin The State Key Lab of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai 200050, PR China Received 14 July 2004; accepted 12 September 2004
Abstract Layer-structured Bi4 Ti3−x Nbx O12+x/2 (BiTN) ceramics, where x = 0.02–0.20, have been prepared by the conventional sintering technique. XRD results reveal the existence of an orthorhombic structure. The grain size decreases gradually and grain growth anisotropy is limited when x increases. The domain structures of BiTN ceramics differ on x. Nb5+ donor doping decreases markedly electrical conductivity of the materials and Tc shifts gradually to lower temperatures. In addition, low dielectric losses and good temperature stability of dielectric constant are obtained in a wide temperature range. Ferroelectric hysteresis loops of the materials are also determined on x. The best properties can be found in x = 0.08 and 0.11, indicating lower electrical conductivity, good temperature stability of dielectric properties and adequate piezoelectric properties. © 2004 Elsevier B.V. All rights reserved. Keywords: Domain structure; Electrical conductivity; Temperature coefficient of dielectric constant; Piezoelectric properties; Nb5+ -doped; Bi4 Ti3 O12 ceramics
1. Introduction Lead zirconate titanate (PZT) based piezoelectric devices are employed widely in the range of 0–200 ◦ C. Recently, there was a need for sensors that can operate at higher temperatures (>400 ◦ C) without significant changes in sensing properties. The compounds of Aurivillius bismuth layer-structure ferroelectric (BLSF) are suitable candidate materials due to their high Curie temperatures, strong anisotropic characters, low dielectric losses and low aging rates [1]. Bismuth titanate (Bi4 Ti3 O12 ) belongs to the family of BLSF with a general formula (Bi2 O2 )2+ (Am−1 Bm O3m+1 )2− . It undergoes a ferroelectric to paraelectric phase transition at the Curie temperature (Tc = 675 ◦ C). At room temperature the symmetry of Bi4 Ti3 O12 is monoclinic structure (C1h = m) [2], while it can be considered as orthorhombic structure with a = 0.5448, b = 0.5411 and c = 3.283 nm ∗
Corresponding author. Tel.: +86 21 5241 2034; fax: +86 21 5241 3122. E-mail address: [email protected] (G. Li).
0921-5107/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2004.09.007
from X-ray and neutron diffraction patterns [3]. The crystal structure is characterized by three perovskite-like unit cells sandwiched between (Bi2 O2 )2+ layers along the c-axis. Bi4 Ti3 O12 single crystal has two independent spontaneous polarizations with a major component of 50 C/cm2 in the a–b plane [2]. The piezoelectric coefficient is also very high in this plane. These characteristics suggest that Bi4 Ti3 O12 is a good material for high-temperature ferroelectric and piezoelectric applications. Moreover, it is considered to be a promising, environmentally friendly material alternative to PZT. Effort has been devoted to Bi4 Ti3 O12 ceramics and thin films fabricated by various methods [4–7]. The applications have been limited because of its high electrical conductivity, which is maximum in the a–b plane. It is difficult to polarize Bi4 Ti3 O12 to obtain high piezoelectric activity. It is well known that donor dopants decrease the conductivity of Bi4 Ti3 O12 material with a p-type mechanism. There have been correlative investigations about modifying electrical conductivity of Bi4 Ti3 O12 by Nb5+ , W6+ substituting Ti4+ [8–12].
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L. Zhang et al. / Materials Science and Engineering B 116 (2005) 99–103
According to the theory of Fousek and Janovec [13], there were 18 possibilities of domain-wall structures in Bi4 Ti3 O12 . These complex domain-wall structures may lead to a considerable contribution to the piezoelectric properties. Cummins [2,14] studied the domain walls in Bi4 Ti3 O12 single crystal by optical method and electron microscopy, respectively, and many kinds of domain-wall structures had been verified. But there is no report on the domain structures of Bi4 Ti3 O12 ceramics. Excellent temperature stability of piezoelectric materials is one of important factors for high temperature applications. Ogawa et al. [15] has investigated the temperature dependence of resonance frequency of the grain-oriented CaBi4 Ti4 O15 ceramics. In generally, if dielectric properties change remarkably with elevated temperature, the poled ceramics would be depolarized and the piezoelectric properties will be worse. Therefore, analyzing the dependence of dielectric constant versus temperature is an important subject. In the present study, microstructure, electrical conductivity, dielectric and piezoelectric properties of Nb5+ -doped Bi4 Ti3 O12 prepared by the conventional sintering behavior have been studied, in particular ferroelectric domain structures and temperature stability of dielectric properties were discussed.
2. Experimental Bi4 Ti3−x Nbx O12+x/2 (x = 0.02, 0.05, 0.08, 0.11 and 0.20) (BiTN) ceramics were prepared by the conventional sintering technique. Bi2 O3 , TiO2 and Nb2 O5 reagents were thoroughly ball-milled and calcined at 850 ◦ C for 2 h in alumina crucibles. Then the powders were ground again to gain fine granularity. The dried powders were sieved and added with about 8 wt.% of a 5% PVA binder. After that, compacted disks of 12 mm diameter and 1.5 mm thickness were obtained by uniaxially pressed at 150 MPa. These disks were sintered with a heating rate of 2.5 ◦ C min−1 at 1000–1150 ◦ C for 1 h in air. The sintered disks were ground to 0.5 mm in thickness. Silver electrodes were pasted on both sides of the samples and fired at 740 ◦ C for 20 min. The samples were polarized in a silicon oil bath at 140 ◦ C in a field of 60–100 kV/cm for 20 min. The bulk density was measured using the Archimedes method. X-ray diffraction (RAX-10, Rigaku, Japan) using Cu K␣ radiation was conducted to determine the phase of sintered samples. The microstructures and ferroelectric domain structures were examined on polished and thermally etched surfaces by a commercial scanning force microscopy (SFM) instrument (Seiko SPA 300/SPI 3800). A high resistance meter (KEITHLEY 6517A) was used to measure electrical conductivity in a direct current circuit. The dielectric and piezoelectric properties were measured with an impedance analyzer (HP 4294A). A high-voltage test system (Radiant Technologies RT66A unit) was used to obtain the ferroelectric hysteresis loops.
3. Results and discussion All samples achieved about 95% of theoretical density at their corresponding sintering temperature, which was necessary to obtain high resistance and withstand high electric field to pole the materials. Fig. 1 shows the X-ray diffraction diagram of the studied BiTN ceramics. All diffraction data showed the existence of a single orthorhombic phase, without a secondary phase detectable. It is in accordance with the view that Bi4 Ti3 O12 is usually considered to have a polar orthorhombic structure [3]. Fig. 2 shows the typical surface morphology and domain structures of the polished and thermally etched surfaces of BiTN samples. As Fig. 2(a)–(c) are shown, it is evident that Nb5+ addition leads to a significant change in the average size and homogeneity of the grains. Nb5+ -doping decreased gradually the grain size in a–b plane and limited the anisotropy of grain growth. The shape of grains changed from needle-like to rod-like structure as the doped content x increased. More homogeneous grains were observed especially for x = 0.20. The porosity was mainly located on grain boundaries. It is thought that the random orientation and anisotropic growth of the grains contribute to the pores at a large extent. Fig. 2(d) and (e) show the domain structures obtained under the piezoresponse mode of SFM, where the domain structures in BiTN ceramics were greatly different. In BiTN (x = 0.02) ceramics, most of domain walls were mainly 90◦ and paralleled to the c axis (see Fig. 2(d)). However, for the dopant content x = 0.08 ceramics, the domain walls mainly perpendiculared to the c axis and paralleled to a–b plane (see Fig. 2(e)). Cummins’s investigation [14] and other similar studies for Bi4 Ti3 O12 single crystal gave evidence that a number of different types of domain walls existed. Their studies indicated that 90◦ walls and walls parallel to the a–b plane are stable in the virgin crystal. In our studies, it was found that the domain walls in BiTN (x = 0.02) were mainly
Fig. 1. XRD patterns of BiTN ceramic samples.
L. Zhang et al. / Materials Science and Engineering B 116 (2005) 99–103
101
Fig. 2. SFM micrographs of polished and thermally etched surfaces of BiTN ceramics: microstructures for (a) x = 0.02, (b) x = 0.08, (c) x = 0.20 and the corresponding domain structures for (d) x = 0.02, (e) x = 0.08.
90◦ and the domain walls in BiTN (x = 0.08) ceramics mainly paralleled to a–b plane. This change of domain structures was probably due to the modifying of Nb5+ . The temperature dependence of electrical conductivity measured in a direct current circuit with BiTN ceramics was studied in the temperature rang from 160 to 600 ◦ C (see Fig. 3). Here, we gave the conductivity of pure Bi4 Ti3 O12 for comparison. It could be found that the conductivity of BiTN was obviously lower than that of the undoped Bi4 Ti3 O12 , which was accord with donor dopants decreasing the conductivity of a material with a p-type mechanism. The conductivity decreased two orders of magnitude with a small content of Nb5+ -doping (x = 0.02). When x ≥ 0.05, BiTN ce-
Fig. 3. The log of the electrical conductivity σ as a function of the temperature for BiTN ceramics.
ramics obtained lower electrical conductivity and could be poled to develop piezoelectricity. In Bi4 Ti3 O12 , hole compensation of bismuth vacancies (VBi + 3h· ) promotes p-type electronic conductivity. When Nb5+ substitutes Ti4+ in BiTN ceramics, a positive charge centers in Nb site and an electron will be created under charge neutrality restriction, which can be described as Nb·Ti + e . These electrons neutralize the influence of the holes. According to the electronic conductivity relationship σ = nqµ
(1)
where n is the number of the carriers, q the charge and µ the mobility, it was thought that the number of the hole carriers, n, was decreased. That resulted in the low conductivity in BiTN ceramics. The conductivity will decrease with donor doping to a minimum value until the concentration of the electrons equals to the hole concentration. The dielectric constants measured at room temperature at 1 kHz increased gradually from 160 to 216 with Nb5+ modifying. It indicates the influence of the bismuth vacancies. Similar to Nb5+ -doping into PZT-based piezoelectric ceramics, Nb2 O5 can be considered as a kind of soft additive in Bi4 Ti3 O12 ceramics. It will soften piezoelectric ceramics, e.g. increasing dielectric constant, dielectric loss and decreasing electrical conductivity (listed in Table 1). Fig. 4(a) shows the variation of dielectric constant as a function of the temperature at 100 kHz for various Nb5+ doped Bi4 Ti3 O12 ceramics. Tc of BiTN ceramics shifted to lower temperatures with increasing Nb5+ concentration. The reason was the result of Nb ions occupying the Ti site of
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Table 1 The resistivity, dielectric and piezoelectric properties of BiTN ceramics x
ρ(400 ◦ C) ( cm)
Tc (◦ C)
ε/ε0
tan ␦ (%)
Qm
Kp (%)
d33 (pC/N)
0.02 0.05 0.08 0.11 0.20
2.0 × 107
669 660 652 646 625
160 170 181 199 216
1.3 1.5 2.3 2.2 2.3
– 1861 2348 2492 2071
– 3.7 3.6 3.5 3.5
– 17 19 18 17
1.3 × 109 3.0 × 109 1.4 × 109 7.5 × 108
Fig. 4. (a) Dielectric constant and (b) dielectric loss tangent of BiTN ceramics as a function of the temperature at 100 kHz.
the perovskite cell in the bismuth layer-structured ceramics. From Fig. 4(b), the loss tangent of x ≥ 0.05 increased very slowly and was below 0.04 up to 400 ◦ C, which enabled the material to polarize at higher electric fields and higher temperatures. As for a small content x of 0.02, its dielectric loss increased sharply with elevated temperature due to higher conductivity. To better illustrate the temperature dependence of dielectric properties, we defined the temperature coefficient of dielectric constant as follows: Tkε = (εT − εT0 )/(εT0 × (T − T0 ))
crease of space charge density (Vo¨ ) led to the large remnant polarization [18]. In addition, the decreasing of grain size and the changing of domain structures may be the factors causing lager coercive field (see Fig. 2). Table 1 summarizes the electrical resistivity at 400 ◦ C, dielectric and piezoelectric properties of BiTN ceramics. Piezoelectric properties for x = 0.02 ceramics were not obtained because its conductivity was too high to polarize. Other composition materials were polarized and showed relatively high piezoelectric response d33 values of 17–19 pC/N.
(2)
where εT and εT0 are the dielectric constant of the samples at temperature T and room temperature T0 , respectively. Fig. 5 shows that the steady temperature coefficients of dielectric constant were very small up to 500 ◦ C with x = 0.08 and 0.11, due to the suitable Nb modification. The values of Tkε were 1.71 × 10−3 and 1.62 × 10−3 /◦ C at 500 ◦ C, respectively. For other composition materials, the temperature coefficients of dielectric constant change markedly when the temperature increased. Fig. 6 shows the ferroelectric hysteresis loops measured at around 140 kV/cm electrical fields in a silicon oil bath at 140 ◦ C. The remnant polarization and the coercive field were increased after Nb5+ doping into the Bi4 Ti3 O12 ceramics, which was similar to the investigations of some researchers [16,17]. Nb5+ substituting for Ti4+ in BiTN ceramics efficiently decreased the concentration of VO·· , which weakened the influence of domain pinning on the polarization. The de-
Fig. 5. Temperature coefficient of dielectric constant as a function of the temperature at 100 kHz for BiTN ceramics.
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Fig. 6. Ferroelectric hysteresis loops of BiTN ceramics at 140 ◦ C.
4. Conclusions
References
Donor dopant Nb2 O5 was incorporated to Bi4 Ti3 O12 layered compound and BiTN ceramics were synthesized and characterized on their electrical properties. The needle-like grains size became smaller and more homogeneous grains were observed with increasing Nb2 O5 concentration. Furthermore, it was found that Nb5+ doping could change the domain structures of BiTN ceramics. The conductivity of BiTN was decreased obviously by Nb5+ donor doping and Tc shifted to lower temperatures. Nb doping played a soft additive role in BiTN ceramics. The remnant polarization of BiTN enhanced due to Nb5+ -doping reducing the concentration of oxygen vacancies. It is difficult to improve the electrical properties of BiTN ceramics for a small Nb concentration (x = 0.02) due to its high electrical conductivity. The excellent properties were obtained for the composition x = 0.08 and 0.11. They show lower resistivity, good temperature stability of dielectric properties at elevated temperature.
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Acknowledgement The author gratefully acknowledges the support of the National “973” Project (no. 2002CB613307) of China.