- Email: [email protected]
Composition and temperature dependent electrical properties of BaTiO3:ZnO composites
Materials Letters 164 (2016) 303–307
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Composition and temperature dependent electrical properties of BaTiO3:ZnO composites Guang-Jian Wu a, Peng-Xiao Nie b, Ji Zhang a, Yu-Shuang Cui a, Shan-Tao Zhang a,n a National Laboratory of Solid State Microstructures and Department of Materials Science and Engineering, College of Engineering and Applied Science, Nanjing University, Nanjing 210093, China b State Key Laboratory of Mechanics and Control of Mechanical Structures, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
art ic l e i nf o
a b s t r a c t
Article history: Received 20 September 2015 Received in revised form 31 October 2015 Accepted 2 November 2015 Available online 4 November 2015
Ferroelectric/polar semiconductor BaTiO3:xZnO composites (BT:xZnO) have been prepared and investigated. The ZnO particles distribute at the BT grain boundaries to form 0–3 type composites, i.e., the isolated 0-dimensional ZnO particles are embedded in the 3-dimensional BT matrix. With increasing ZnO content, the Curie temperature keeps constant, however, both the dielectric constant and ferroelectric polarization of the composites increase, reaching the maximum at x ¼0.3, and then tend to decrease. Furthermore, the composite has improved temperature stability of ferroelectric polarization and fieldinduced strain up to 100 °C. It is believed that ZnO can induce a built-in electric field which has influence on the domain switching and thus on the observed composition- and temperature-dependent electrical properties. This work may provide some new clues for developing ferroelectric composites with high performance. & 2015 Elsevier B.V. All rights reserved.
Keywords: Ceramics BaTiO3:ZnO Composite Electrical properties
1. Introduction ABO3-type perovskite ferroelectric materials have important applications in electromechanical devices, etc. For actual applications, excellent electrical property is always desirable. Therefore many efforts have been devoted to improving the electrical property. The “domain engineering” is a considered as the basic concept for improving property of ferroelectric materials. Based on this concept, two conventional methods, forming solid solution and forming composite, have been proposed to improve electrical property [1–4]. Forming solid solution can form so-called morphotropic phase boundary (MPB) separating two coexisted different phases with enhanced electrical properties. This method is successful in Pb-based solutions like Pb(Zr0.53Ti0.47)O3 and some Pb-free solutions like Bi0.5Na0.5TiO3-BaTiO3. However, on the one hand, no Pb-free solid solutions with comparable electrical properties with Pb(Zr0.53Ti0.47)O3 have been developed to replace Pb(Zr0.53Ti0.47)O3 based on this method [1,3]. On the other hand, the enhanced electrical property of MPB compositions is generally temperature-sensitive, which is detrimental for actual applications [1,2]. Therefore, to further improve electrical property of Pb-free ferroelectrics, forming composite is worthy to be considered as an n
Corresponding author. E-mail address: [email protected] (S.-T. Zhang).
http://dx.doi.org/10.1016/j.matlet.2015.11.009 0167-577X/& 2015 Elsevier B.V. All rights reserved.
alternative way [3,4]. Up to now, forming composite has not been well explored as compared with forming solid solution. The scattered reports focus on homo-structure composites like BaTiO3:KNbO3, BaTiO3:(Pb,La)(Zr,Ti)O3, etc [4–9], and heterostructure composites like (Ba,Sr)TiO3:Ag, (Ba,Sr)TiO3:MgO, etc [10– 16]. Though both kinds of composites have considerable advantages like tunable dielectric properties [7] and enhanced magneto-electric effects [8,9], they also have some disadvantages to be further improved. The main disadvantage of homo-structure composites is that the local solid solution is easy to be formed near the boundaries of two phases, which may make the microstructure and electrical property to be processing-sensitive [5]. The disadvantages of hetero-structure composites includes: (1) Ferroelectric/metal composites generally have increased dielectric loss, and the fraction of metal must be lower than a critical value (percolation threshold), leading to limited tunable range [10–12]. (2) Ferroelectric/dielectrics composites generally have suppressed dielectric constant [13–15]. (3) In both ferroelectric/ metal and ferroelectric/dielectric composites, the ferroelectric property is generally suppressed due to decreased volume fraction of ferroelectric materials, and the metal or dielectrics has no effect on domain switching. Therefore, it is expected ferroelectric/semiconductor composite may have improved electrical property, however, there is no report on such composite. In this paper, BaTiO3:ZnO (BT:xZnO, x¼ 0.1, 0.2, 0.3, 0.4 represents the mole ratio of ZnO to BT) composites have been prepared, the composition- and temperature-dependent electrical
304
G.-J. Wu et al. / Materials Letters 164 (2016) 303–307
properties have been investigated. In this composite, tetragonal BT is a ferroelectric perovskite [1,2,17,18], and wurtzite ZnO is a polar semiconducting oxide with piezoelectricity and high resistivity [19]. The semiconductor and polar natures of ZnO may affect the electrical property of the BT:ZnO composites.
2. Experimental details BT was prepared by solid state reaction method with sintering temperature and sintering time of 1350 °C and 3 h. The BT powders and the commercial 25 nm ZnO particles (PlasmaChem, Germany) were weighted according to the formula of BT:xZnO with x ¼ 0.1, 0.2, 0.3, and 0.4. Each mixture was ball milled in ethanol for 24 h, dried and subsequently pressed into green disks with a diameter of 10 mm under 40 MPa. Sintering was
carried out in covered alumina crucibles at 1000–1100 °C for 0.5 h with rapid increasing and decreasing temperature rate of 10 °C/min. The crystal structures of the composites were characterized by x-ray diffraction (XRD, Rigaku Ultima III). The microstructures and element distributions were analyzed by scanning electron microscopy (SEM, FEI Quanta 200 and Zeiss Ultra 55). For electrical measurements, the circular surfaces of grounded disks were covered with silver paste, fired at 550 °C for 30 minutes. The temperature dependent dielectric constant (εr) and dielectric loss (tan δ) were measured using an impedance analyzer (HP4294A) at several frequencies. The temperature dependent polarization– electric field (P–E) hysteresis loops, current–electric field (J–E) and bipolar/unipolar strain–electric field (S–E) curves were measured by using TF analyzer 1000 (AixACCT) at 1 Hz.
Fig. 1. Typical microstructures and element distributions of the (a)–(c) BT:0.1ZnO, and (d)–(e) BT:0.3ZnO composites.
G.-J. Wu et al. / Materials Letters 164 (2016) 303–307
3. Results and discussion XRD measurements show that when ZnO is introduced into single phase BT powder, composite structure is formed since both diffraction peaks from BT and ZnO are observed. However weak diffraction peaks from third phases are detected, which may due to the weak solution between BT and ZnO, consistent with other report on BT-based composites [7,16]. The typical SEM micrographs of x¼ 0.1 and 0.3 and the corresponding distributions of Ba and Zn are shown in Fig. 1. Three microstructural features should be emphasized. (1) The introducing of ZnO lead to increased grain size and regular grain shape of BT since ZnO is a well-known sintering aid. The average grain size is 50, 80, 230, and 260 nm, respectively. (2) For x¼ 0.1, the Ba/Ti cations are homogenously distributed in whole area, whereas Zn cations are also locally, independently distributed, as shown in Fig. 1(b) and (c). The x ¼0.2 composite have the similar microstructures with that of x ¼0.1. (3) When x reaches 0.3 and 0.4, the nano-sized ZnO particles tend to aggregate, therefore, relatively larger ZnO “particles” can be seen. The results of x ¼0.3 composite are shown in Figs. 1(e) and (f). In one word, all composites have 0– 3 type microstructures. The temperature-dependent εr and tan δ are shown in Fig. 2 (a)–(d). For all composites, the peak of εr is 120 °C, indicating that ZnO has negligible effect on the Curie temperature (Tc). It is noted that Tc of pure BT is 120 °C [20], this means the introduction of ZnO has no effect on Tc. This observation can be explained in this way: in general, BT-based solid solution show shifted Tc because other cations enter into lattice and change the structure. However, this is not the case in BT-based composite. Actually, our result is consistent with other report [21]. On the
305
other hand, the maximum dielectric constant tends to increase with increasing x, reaching maximum of 2200 for x ¼0.3 composite, then decreases. The increased εr is reasonable since the charges from semiconducting ZnO can result in a local field, as will be discussed below. Actually, similar results have been observed in SrBi2Nb2O9:Ag composites [11]. A possible reason for the slightly decreased dielectric constant is that when x reaches 0.4, the volume fraction of BT decreases, thus the main contribution from ferroelectric matrix to dielectric constant decreases. At room temperature, all the BT:xZnO composites show ferroelectric P–E hysteresis loops and butterfly shaped bipolar S–E curves, as plotted in Fig. 3, confirming the ferroelectric nature of these composites [22]. Both the ferroelectric polarization and strain values increase with increasing ZnO content, reaching the maximum at x ¼0.3, then decrease. Such results are not observed in ferroelectric/dielectric or ferroelectric/metal composites where the ferroelectric property generally is weakened. The increased polarization and strain is due to that when external filed is applied, the charges from ZnO will be distributed orderly along the BT grain boundaries. When the external poling filed is removed, these ordered charges will lead to a local built-in electric filed, which is parallel to external filed [23]. The polar nature of ZnO may be further helpful for enhancing and keeping this local builtin field. This local built-in field will prevent both the backswitching of aligned domains and the relaxation of strained lattice, so leading to enhanced polarization, strain values, and the contributions to dielectric constant. However, when x ¼0.4, the volume fraction of ferroelectric BT matrix decreases significantly, so the domain contributions to electrical property decreases. In addition, based on the J–E curves, it is seen that the introduction of ZnO can increase the resistivity, as confirmed by the decreased
Fig. 2. Temperature dependent dielectric constant and dielectric loss of BT:xZnO composites, (a) x ¼0.1, (b) x¼ 0.2, (c) x ¼0.3, and (d) x¼ 0.4.
306
G.-J. Wu et al. / Materials Letters 164 (2016) 303–307
Fig. 3. Composition dependent P–E loops, J–E and bipolar S–E curves of BT:xZnO composites, (a) x¼ 0.1, (b) x ¼0.2, (c) x¼ 0.3, and (d) x ¼0.4.
Fig. 4. Temperature dependent P–E loops and bipolar and unipolar S-E curves of BT:0.3ZnO composites, (a) 25 °C, (b) 50 °C, (c) 75 °C, and (d) 100 °C.
G.-J. Wu et al. / Materials Letters 164 (2016) 303–307
current density with increasing ZnO content. The P–E hysteresis loops, bipolar and unipolar S–E curves of x ¼0.3 composite were measured at RT (25 °C), 50 °C, 75 °C, and 100 °C and shown in Fig. 4. With temperature increases, the polarization decreases slightly from 13.6 μC/cm2 at 25 °C to 12.4 μC/cm2 at 75 °C, which is due to thermal-weakened ferroelectric nature. Obvious contribution from leakage to P–E is observed at 100 °C. On the other hand, both the bipolar and unipolar strain values decrease with increasing temperature, the unipolar strain values are 0.086%, 0.083%, 0.082%, and 0.072% at 25 °C, 50 °C, 75 °C, and 100 °C temperatures, respectively. Clearly, electrical properties of the composite have good temperature stability.
4. Conclusions In summary, BT:xZnO composites were prepared and investigated. The ZnO has not connected to other ZnO to from 1-dimensional ZnO lines or 2-dimensional planes, i.e., the ZnO particles are isolated to form 0-dimensional particles. However, BT grains connect each other to form 3-dimensional matrix. In this way, the isolated 0-dimensional ZnO particles are embedded into 3-dimensional BT matrix to form 0–3 type composites. The introduction of ZnO has no effect on Tc but significant effect on electrical properties. When xo 0.4, the charges stemming from ZnO form a local field to suppress the back-switching of aligned domains and the relaxation of strains, thus the ferroelectric polarization, strain, and dielectric constant increase with increasing ZnO content. When x ¼0.4, the volume fraction of ferroelectric BT matrix decreases, so the electrical properties are suppressed. The electrical property of the composites shows good temperature stability. We believe our results can provide some useful information on develop ferroelectric composite with improved electrical property.
307
Acknowledgments This work was supported by the 973 Program (2013CB632900), the National Natural Science Foundation of China (U1432112) and “Deng Feng B” of Nanjing University.
References [1] S.J. Zhang, R. Xia, R.T. Shrout, J. Electroceram. 19 (2007) 251. [2] J. Rödel, W. Jo, K. Seifert, E.M. Anton, T. Granzow, D. Damjanovic, J. Am. Ceram. Soc. 92 (2009) 1153. [3] S. Zhang, F. Li, X. Jiang, J. Kim, J. Luo, X. Geng, Prog. Mater. Sci. 68 (2015) 1. [4] C. Groh, D.J. Franzbach, W. Jo, K.G. Webber, J. Kling, L.A. Schmitt, H.J. Kleebe, S. J. Jeong, J.S. Lee, J. Rödel, Adv. Funct. Mater. 24 (2014) 356. [5] I. Fujii, S. Shimizu, K. Yamashita, K. Nakashima, N. Kumada, C. Moriyoshi, Y. Kuroiwa, Y. Fujikawa, D. Tanaka, M. Furukawa, S. Wada, Appl. Phys. Lett. 99 (2011) 202902. [6] C. Groh, W. Jo, J. Rödel, J. Appl. Phys. 115 (2014) 234107. [7] N. Zhang, L.X. Li, J.X. Chen, J.Y. Yu, Mater. Lett. 138 (2015) 228. [8] A.S. Priya, I.B.S. Banu, S. Anwar, Mater. Lett. 142 (2015) 42. [9] R. Koferstein, T. Walther, D. Hesse, D. Ebbinghaus, J. Alloy. Compd. 638 (2015) 141. [10] V. Bobnar, M. Hrovat, J. Holc, M. Kosec, Appl. Phys. Lett. 92 (2008) 182911. [11] J. Huang, Y. Cao, M. Hong, P.Y. Du, Appl. Phys. Lett. 92 (2008) 022911. [12] C. Wang, Q.F. Fang, Z.G. Zhu, Appl. Phys. Lett. 80 (2002) 3578. [13] Q. Zhang, J.W. Zhai, L. Kong, X. Yao, Appl. Phys. Lett. 97 (2010) 182903. [14] L. Tang, J. Wang, J.W. Zhai, L. Kong, X. Yao, Appl. Phys. Lett. 102 (2013) 142907. [15] J. Lesseur, D. Bernard, U.C. Chung, C. Estournès, M. Maglione, C. Elissalde, J. Eur. Ceram. Soc. 35 (2015) 337. [16] D. Xu, W.L. Li, L.D. Wang, W. Wang, W.P. Cao, W.D. Fei, Acta Mater. 79 (2014) 84. [17] J. Rödel, K.G. Webber, R. Dittmer, W. Jo, M. Kimura, D. Damjanovic, J. Eur. Ceram. Soc. 35 (2015) 1659. [18] H.B. Zhang, P.W. Xu, E. Patterson, J.D. Zang, S.L. Jiang, J. Rödel, J. Eur. Ceram. Soc. 35 (2015) 2501. [19] Z.L. Wang, Adv. Mater. 24 (2012) 4632. [20] Q. Yuan, Y. Pu, B. Cao, Mater. Lett. 86 (2012) 115. [21] Y.H. Huang, Y.J. Wu, W.J. Qiu, J. Li, X.M. Chen, J. Eur. Ceram. Soc. 35 (2015) 1469. [22] G. Viola, R. McKinnon, V. Koval, A. Adomkevicius, S. Dunn, H. Yan, J. Phys. Chem. C 118 (2014) 8564. [23] J. Zhang, Z. Pan, P.X. Nie, Y.S. Cui, B. Yang, J. Chen, S.T. Zhang, Appl. Phys. Lett. 106 (2015) 232904.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Composition and temperature dependent electrical properties of BaTiO3:ZnO composites Guang-Jian Wu a, Peng-Xiao Nie b, Ji Zhang a, Yu-Shuang Cui a, Shan-Tao Zhang a,n a National Laboratory of Solid State Microstructures and Department of Materials Science and Engineering, College of Engineering and Applied Science, Nanjing University, Nanjing 210093, China b State Key Laboratory of Mechanics and Control of Mechanical Structures, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
art ic l e i nf o
a b s t r a c t
Article history: Received 20 September 2015 Received in revised form 31 October 2015 Accepted 2 November 2015 Available online 4 November 2015
Ferroelectric/polar semiconductor BaTiO3:xZnO composites (BT:xZnO) have been prepared and investigated. The ZnO particles distribute at the BT grain boundaries to form 0–3 type composites, i.e., the isolated 0-dimensional ZnO particles are embedded in the 3-dimensional BT matrix. With increasing ZnO content, the Curie temperature keeps constant, however, both the dielectric constant and ferroelectric polarization of the composites increase, reaching the maximum at x ¼0.3, and then tend to decrease. Furthermore, the composite has improved temperature stability of ferroelectric polarization and fieldinduced strain up to 100 °C. It is believed that ZnO can induce a built-in electric field which has influence on the domain switching and thus on the observed composition- and temperature-dependent electrical properties. This work may provide some new clues for developing ferroelectric composites with high performance. & 2015 Elsevier B.V. All rights reserved.
Keywords: Ceramics BaTiO3:ZnO Composite Electrical properties
1. Introduction ABO3-type perovskite ferroelectric materials have important applications in electromechanical devices, etc. For actual applications, excellent electrical property is always desirable. Therefore many efforts have been devoted to improving the electrical property. The “domain engineering” is a considered as the basic concept for improving property of ferroelectric materials. Based on this concept, two conventional methods, forming solid solution and forming composite, have been proposed to improve electrical property [1–4]. Forming solid solution can form so-called morphotropic phase boundary (MPB) separating two coexisted different phases with enhanced electrical properties. This method is successful in Pb-based solutions like Pb(Zr0.53Ti0.47)O3 and some Pb-free solutions like Bi0.5Na0.5TiO3-BaTiO3. However, on the one hand, no Pb-free solid solutions with comparable electrical properties with Pb(Zr0.53Ti0.47)O3 have been developed to replace Pb(Zr0.53Ti0.47)O3 based on this method [1,3]. On the other hand, the enhanced electrical property of MPB compositions is generally temperature-sensitive, which is detrimental for actual applications [1,2]. Therefore, to further improve electrical property of Pb-free ferroelectrics, forming composite is worthy to be considered as an n
Corresponding author. E-mail address: [email protected] (S.-T. Zhang).
http://dx.doi.org/10.1016/j.matlet.2015.11.009 0167-577X/& 2015 Elsevier B.V. All rights reserved.
alternative way [3,4]. Up to now, forming composite has not been well explored as compared with forming solid solution. The scattered reports focus on homo-structure composites like BaTiO3:KNbO3, BaTiO3:(Pb,La)(Zr,Ti)O3, etc [4–9], and heterostructure composites like (Ba,Sr)TiO3:Ag, (Ba,Sr)TiO3:MgO, etc [10– 16]. Though both kinds of composites have considerable advantages like tunable dielectric properties [7] and enhanced magneto-electric effects [8,9], they also have some disadvantages to be further improved. The main disadvantage of homo-structure composites is that the local solid solution is easy to be formed near the boundaries of two phases, which may make the microstructure and electrical property to be processing-sensitive [5]. The disadvantages of hetero-structure composites includes: (1) Ferroelectric/metal composites generally have increased dielectric loss, and the fraction of metal must be lower than a critical value (percolation threshold), leading to limited tunable range [10–12]. (2) Ferroelectric/dielectrics composites generally have suppressed dielectric constant [13–15]. (3) In both ferroelectric/ metal and ferroelectric/dielectric composites, the ferroelectric property is generally suppressed due to decreased volume fraction of ferroelectric materials, and the metal or dielectrics has no effect on domain switching. Therefore, it is expected ferroelectric/semiconductor composite may have improved electrical property, however, there is no report on such composite. In this paper, BaTiO3:ZnO (BT:xZnO, x¼ 0.1, 0.2, 0.3, 0.4 represents the mole ratio of ZnO to BT) composites have been prepared, the composition- and temperature-dependent electrical
304
G.-J. Wu et al. / Materials Letters 164 (2016) 303–307
properties have been investigated. In this composite, tetragonal BT is a ferroelectric perovskite [1,2,17,18], and wurtzite ZnO is a polar semiconducting oxide with piezoelectricity and high resistivity [19]. The semiconductor and polar natures of ZnO may affect the electrical property of the BT:ZnO composites.
2. Experimental details BT was prepared by solid state reaction method with sintering temperature and sintering time of 1350 °C and 3 h. The BT powders and the commercial 25 nm ZnO particles (PlasmaChem, Germany) were weighted according to the formula of BT:xZnO with x ¼ 0.1, 0.2, 0.3, and 0.4. Each mixture was ball milled in ethanol for 24 h, dried and subsequently pressed into green disks with a diameter of 10 mm under 40 MPa. Sintering was
carried out in covered alumina crucibles at 1000–1100 °C for 0.5 h with rapid increasing and decreasing temperature rate of 10 °C/min. The crystal structures of the composites were characterized by x-ray diffraction (XRD, Rigaku Ultima III). The microstructures and element distributions were analyzed by scanning electron microscopy (SEM, FEI Quanta 200 and Zeiss Ultra 55). For electrical measurements, the circular surfaces of grounded disks were covered with silver paste, fired at 550 °C for 30 minutes. The temperature dependent dielectric constant (εr) and dielectric loss (tan δ) were measured using an impedance analyzer (HP4294A) at several frequencies. The temperature dependent polarization– electric field (P–E) hysteresis loops, current–electric field (J–E) and bipolar/unipolar strain–electric field (S–E) curves were measured by using TF analyzer 1000 (AixACCT) at 1 Hz.
Fig. 1. Typical microstructures and element distributions of the (a)–(c) BT:0.1ZnO, and (d)–(e) BT:0.3ZnO composites.
G.-J. Wu et al. / Materials Letters 164 (2016) 303–307
3. Results and discussion XRD measurements show that when ZnO is introduced into single phase BT powder, composite structure is formed since both diffraction peaks from BT and ZnO are observed. However weak diffraction peaks from third phases are detected, which may due to the weak solution between BT and ZnO, consistent with other report on BT-based composites [7,16]. The typical SEM micrographs of x¼ 0.1 and 0.3 and the corresponding distributions of Ba and Zn are shown in Fig. 1. Three microstructural features should be emphasized. (1) The introducing of ZnO lead to increased grain size and regular grain shape of BT since ZnO is a well-known sintering aid. The average grain size is 50, 80, 230, and 260 nm, respectively. (2) For x¼ 0.1, the Ba/Ti cations are homogenously distributed in whole area, whereas Zn cations are also locally, independently distributed, as shown in Fig. 1(b) and (c). The x ¼0.2 composite have the similar microstructures with that of x ¼0.1. (3) When x reaches 0.3 and 0.4, the nano-sized ZnO particles tend to aggregate, therefore, relatively larger ZnO “particles” can be seen. The results of x ¼0.3 composite are shown in Figs. 1(e) and (f). In one word, all composites have 0– 3 type microstructures. The temperature-dependent εr and tan δ are shown in Fig. 2 (a)–(d). For all composites, the peak of εr is 120 °C, indicating that ZnO has negligible effect on the Curie temperature (Tc). It is noted that Tc of pure BT is 120 °C [20], this means the introduction of ZnO has no effect on Tc. This observation can be explained in this way: in general, BT-based solid solution show shifted Tc because other cations enter into lattice and change the structure. However, this is not the case in BT-based composite. Actually, our result is consistent with other report [21]. On the
305
other hand, the maximum dielectric constant tends to increase with increasing x, reaching maximum of 2200 for x ¼0.3 composite, then decreases. The increased εr is reasonable since the charges from semiconducting ZnO can result in a local field, as will be discussed below. Actually, similar results have been observed in SrBi2Nb2O9:Ag composites [11]. A possible reason for the slightly decreased dielectric constant is that when x reaches 0.4, the volume fraction of BT decreases, thus the main contribution from ferroelectric matrix to dielectric constant decreases. At room temperature, all the BT:xZnO composites show ferroelectric P–E hysteresis loops and butterfly shaped bipolar S–E curves, as plotted in Fig. 3, confirming the ferroelectric nature of these composites [22]. Both the ferroelectric polarization and strain values increase with increasing ZnO content, reaching the maximum at x ¼0.3, then decrease. Such results are not observed in ferroelectric/dielectric or ferroelectric/metal composites where the ferroelectric property generally is weakened. The increased polarization and strain is due to that when external filed is applied, the charges from ZnO will be distributed orderly along the BT grain boundaries. When the external poling filed is removed, these ordered charges will lead to a local built-in electric filed, which is parallel to external filed [23]. The polar nature of ZnO may be further helpful for enhancing and keeping this local builtin field. This local built-in field will prevent both the backswitching of aligned domains and the relaxation of strained lattice, so leading to enhanced polarization, strain values, and the contributions to dielectric constant. However, when x ¼0.4, the volume fraction of ferroelectric BT matrix decreases significantly, so the domain contributions to electrical property decreases. In addition, based on the J–E curves, it is seen that the introduction of ZnO can increase the resistivity, as confirmed by the decreased
Fig. 2. Temperature dependent dielectric constant and dielectric loss of BT:xZnO composites, (a) x ¼0.1, (b) x¼ 0.2, (c) x ¼0.3, and (d) x¼ 0.4.
306
G.-J. Wu et al. / Materials Letters 164 (2016) 303–307
Fig. 3. Composition dependent P–E loops, J–E and bipolar S–E curves of BT:xZnO composites, (a) x¼ 0.1, (b) x ¼0.2, (c) x¼ 0.3, and (d) x ¼0.4.
Fig. 4. Temperature dependent P–E loops and bipolar and unipolar S-E curves of BT:0.3ZnO composites, (a) 25 °C, (b) 50 °C, (c) 75 °C, and (d) 100 °C.
G.-J. Wu et al. / Materials Letters 164 (2016) 303–307
current density with increasing ZnO content. The P–E hysteresis loops, bipolar and unipolar S–E curves of x ¼0.3 composite were measured at RT (25 °C), 50 °C, 75 °C, and 100 °C and shown in Fig. 4. With temperature increases, the polarization decreases slightly from 13.6 μC/cm2 at 25 °C to 12.4 μC/cm2 at 75 °C, which is due to thermal-weakened ferroelectric nature. Obvious contribution from leakage to P–E is observed at 100 °C. On the other hand, both the bipolar and unipolar strain values decrease with increasing temperature, the unipolar strain values are 0.086%, 0.083%, 0.082%, and 0.072% at 25 °C, 50 °C, 75 °C, and 100 °C temperatures, respectively. Clearly, electrical properties of the composite have good temperature stability.
4. Conclusions In summary, BT:xZnO composites were prepared and investigated. The ZnO has not connected to other ZnO to from 1-dimensional ZnO lines or 2-dimensional planes, i.e., the ZnO particles are isolated to form 0-dimensional particles. However, BT grains connect each other to form 3-dimensional matrix. In this way, the isolated 0-dimensional ZnO particles are embedded into 3-dimensional BT matrix to form 0–3 type composites. The introduction of ZnO has no effect on Tc but significant effect on electrical properties. When xo 0.4, the charges stemming from ZnO form a local field to suppress the back-switching of aligned domains and the relaxation of strains, thus the ferroelectric polarization, strain, and dielectric constant increase with increasing ZnO content. When x ¼0.4, the volume fraction of ferroelectric BT matrix decreases, so the electrical properties are suppressed. The electrical property of the composites shows good temperature stability. We believe our results can provide some useful information on develop ferroelectric composite with improved electrical property.
307
Acknowledgments This work was supported by the 973 Program (2013CB632900), the National Natural Science Foundation of China (U1432112) and “Deng Feng B” of Nanjing University.
References [1] S.J. Zhang, R. Xia, R.T. Shrout, J. Electroceram. 19 (2007) 251. [2] J. Rödel, W. Jo, K. Seifert, E.M. Anton, T. Granzow, D. Damjanovic, J. Am. Ceram. Soc. 92 (2009) 1153. [3] S. Zhang, F. Li, X. Jiang, J. Kim, J. Luo, X. Geng, Prog. Mater. Sci. 68 (2015) 1. [4] C. Groh, D.J. Franzbach, W. Jo, K.G. Webber, J. Kling, L.A. Schmitt, H.J. Kleebe, S. J. Jeong, J.S. Lee, J. Rödel, Adv. Funct. Mater. 24 (2014) 356. [5] I. Fujii, S. Shimizu, K. Yamashita, K. Nakashima, N. Kumada, C. Moriyoshi, Y. Kuroiwa, Y. Fujikawa, D. Tanaka, M. Furukawa, S. Wada, Appl. Phys. Lett. 99 (2011) 202902. [6] C. Groh, W. Jo, J. Rödel, J. Appl. Phys. 115 (2014) 234107. [7] N. Zhang, L.X. Li, J.X. Chen, J.Y. Yu, Mater. Lett. 138 (2015) 228. [8] A.S. Priya, I.B.S. Banu, S. Anwar, Mater. Lett. 142 (2015) 42. [9] R. Koferstein, T. Walther, D. Hesse, D. Ebbinghaus, J. Alloy. Compd. 638 (2015) 141. [10] V. Bobnar, M. Hrovat, J. Holc, M. Kosec, Appl. Phys. Lett. 92 (2008) 182911. [11] J. Huang, Y. Cao, M. Hong, P.Y. Du, Appl. Phys. Lett. 92 (2008) 022911. [12] C. Wang, Q.F. Fang, Z.G. Zhu, Appl. Phys. Lett. 80 (2002) 3578. [13] Q. Zhang, J.W. Zhai, L. Kong, X. Yao, Appl. Phys. Lett. 97 (2010) 182903. [14] L. Tang, J. Wang, J.W. Zhai, L. Kong, X. Yao, Appl. Phys. Lett. 102 (2013) 142907. [15] J. Lesseur, D. Bernard, U.C. Chung, C. Estournès, M. Maglione, C. Elissalde, J. Eur. Ceram. Soc. 35 (2015) 337. [16] D. Xu, W.L. Li, L.D. Wang, W. Wang, W.P. Cao, W.D. Fei, Acta Mater. 79 (2014) 84. [17] J. Rödel, K.G. Webber, R. Dittmer, W. Jo, M. Kimura, D. Damjanovic, J. Eur. Ceram. Soc. 35 (2015) 1659. [18] H.B. Zhang, P.W. Xu, E. Patterson, J.D. Zang, S.L. Jiang, J. Rödel, J. Eur. Ceram. Soc. 35 (2015) 2501. [19] Z.L. Wang, Adv. Mater. 24 (2012) 4632. [20] Q. Yuan, Y. Pu, B. Cao, Mater. Lett. 86 (2012) 115. [21] Y.H. Huang, Y.J. Wu, W.J. Qiu, J. Li, X.M. Chen, J. Eur. Ceram. Soc. 35 (2015) 1469. [22] G. Viola, R. McKinnon, V. Koval, A. Adomkevicius, S. Dunn, H. Yan, J. Phys. Chem. C 118 (2014) 8564. [23] J. Zhang, Z. Pan, P.X. Nie, Y.S. Cui, B. Yang, J. Chen, S.T. Zhang, Appl. Phys. Lett. 106 (2015) 232904.