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Dielectric properties of (Bi0.5Nb0.5)xTi1-xO2 ceramics with colossal permittivity
Journal of Alloys and Compounds 722 (2017) 676e682
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Dielectric properties of (Bi0.5Nb0.5)xTi1-xO2 ceramics with colossal permittivity Yuechan Song, Peng Liu*, Xiaogang Zhao, Baochun Guo, Xiulei Cui College of Physics and Information Technology, Shaanxi Normal University, Xi'an, 710062, China
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a b s t r a c t
Article history: Received 17 December 2016 Received in revised form 13 June 2017 Accepted 16 June 2017
The (Bi0.5Nb0.5)xTi1-xO2 (BNTO, 0 x 0.05) ceramics were synthesized by a standard conventional solidstate reaction. The effects of sintering temperature and composition x on the crystal structure, microstructure, and dielectric properties of BNTO ceramics were investigated. A pure rutile phase is achieved as x 0.01, and secondary phase Bi1.74Ti2O6.624 is detected as x 0.025. BNTO ceramic with x ¼ 0.025 shows a high dielectric constant (155.1 k) and low dielectric loss (0.042) at 1 kHz (room temperature). Colossal permittivity is primarily related to Maxwell-Wagner effect, electron-pinned defect-dipoles or weak-binding electron. The low dielectric loss is argued to Bi ions which are doped into TiO2 lattice and the formation of Bi1.74Ti2O6.624 phase. © 2017 Elsevier B.V. All rights reserved.
Keywords: Ceramic Colossal permittivity Rutile TiO2 Dielectric
1. Introduction In recent years, the colossal permittivity (CP, ε0 >1000) materials are getting more and more attention. Nowadays, the miniaturization of electronic device is widely required. CP materials in myriad types of miniaturized devices and high-energy-density storage have a huge potential [1,2]. Several CP materials have been researched, such as dopedBaTiO3 [3], CaCu3Ti4O12 (CCTO) [4,5], doped-NiO [6], and K0.3MoO3 [7]. However, there are some problems that limit their application in practice. Doped-BaTiO3 shows a high dielectric constant, but it can be used in a narrow temperature range [8]. CCTO displays a colossal permittivity, and its dielectric loss is high and sensitive to the preparation process [9]. Therefore, high-performance materials with colossal permittivity, low dielectric loss, weak temperatureand frequency-independence should be investigated. Recently, (In0.5Nb0.5)xTi1-xO2 (INTO) ceramic is noticeable not only for its high dielectric permittivity (104) and low dielectric loss (<0.05) over a broad temperature range from 80 to 450 K, but also for the new mechanism of the origin of the colossal permittivity materials [10e19]. Hu et al. thought that electrons could be localized by appropriately constructed defect clusters and put forward a new electron-pinned defect-dipoles mechanism [10]. Then, (A0.5Nb0.5)xTi1xO2 (A ¼ Al, Ga, Er) and (AlþNb) co-doped SnO2 ceramics were * Corresponding author. E-mail address: [email protected] (P. Liu). http://dx.doi.org/10.1016/j.jallcom.2017.06.177 0925-8388/© 2017 Elsevier B.V. All rights reserved.
reported [20e23]. However, Hu et al. demonstrated that MaxwellWagner effect was responsible for the colossal permittivity and low dielectric loss in (Al0.5Nb0.5)xTi1-xO2 ceramics [20]. Dong et al. suggested that the acceptor ions with different sizes determined the formation of electron-pinned defect-dipoles, and electron-pinned defect-dipoles were not formed in (Ga0.5Nb0.5)xTi1-xO2 (GNTO) ceramics [21]. In tetragonal TiO2 rutile crystal, the ionic radius of In3þ (0.94 Å) is larger than Al3þ (0.675 Å), Ga3þ (0.76 Å) and Ti4þ (0.745 Å) ions [24e26]. Therefore, the larger ionic radius of acceptors may be conducive to the formation of defect clusters in the (A0.5Nb0.5)xTi1-xO2 ceramics [21]. Recently, Wu et al. reported (Bi0.5Nb0.5)xTi1-xO2 (BNTO) ceramics with colossal permittivity and low dielectric loss by substituting Bi3þ (1.17 Å) and Nb5þ for Ti4þ ions [27]. Considering the volatilization of Bi at elevated temperature and larger ionic radius of Bi3þ ions, whether Bi3þ ions can be doped into TiO2 lattice is questionable. It is necessary to figure out the effect of Bi3þ ions on crystal structure, microstructure, and dielectric properties of BNTO ceramics. In this work, (Bi0.5Nb0.5)xTi1-xO2 (BNTO) ceramics were synthesized by a conventional solid-state reaction. We investigated the microstructure, dielectric properties, and I-V behavior with various x values and sintering temperatures. 2. Experimental procedure The BNTO ceramics were prepared by the conventional solidstate reaction. Raw materials used in this work are rutile TiO2 (99%), Nb2O5 (99.5%), and Bi2O3 (99%). Raw materials were baked at
Y. Song et al. / Journal of Alloys and Compounds 722 (2017) 676e682
473 K for 24 h and weighed accurately according to their chemical compositions, and then ball mixed for 24 h with ethanol as medium. The mixed powders were calcined at 1323 K for 2 h in the air. After the calcined powders second ball-milled, powders were mixed with 5 wt% of PVA as binder. Disks of 10 mm in diameter and approximate 1 mm in thickness were obtained under uniaxial pressure of 267 MPa. Disks were pretreated at 873 K for 2 h to expel the binder, and then sintered at 1523e1673 K for 4 h with a raising and cooling rate of 3 K/min. Silver pasted on both sides of the sintered samples and annealed at 923 K for 30 min. The phases were characterized by X-ray diffraction (XRD, Rigaku D/Max-2400, Japan). The microstructure and element distribution were obtained by scanning electron microscopy (SEM, Fei Nova 650, Germany) with energy-dispersive X-ray analysis (EDX). The relative density was calculated by bulk density and theoretical density. Bulk densities were measured by an electronic balance (XT220, Switzerland) based on the Archimedes principle. The calculation of the theoretical density took into account the second phase, volatilization of Bi, and the oxygen vacancies which were caused by Bi [28]. The volatilization of Bi is obtained by the weight of ceramics
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before and after sintering. The valence states of elements were performed by X-ray photoelectron spectroscopy (XPS, Kratos analytical Ltd/AXIS ULTRA, Japan). Oxygen vacancies and Ti3þ ions contents were calculated by the area ratio of Gaussian peaks. And the dielectric properties of the samples were measured by Agilent E4980A with a temperature controller vibration sample magnetometer (VSM, England) ranging from 10 to 300 K and a Novocontrol broadband dielectric spectrometer with an alpha-A high performance frequency analyzer in the frequency range of 1e3 M Hz over the temperature range of 113e550 K (BDS40, Germany). The nonlinear I-V characteristics for samples were measured at room temperature with a radiant precision workstation ferroelectric testing system (Radiant Technologies Inc, Albuquerque, NM). 3. Results and discussion The XRD patterns of BNTO ceramics with various compositions x and sintering temperatures are shown in Fig. 1. Fig. 1(a) shows BNTO samples sintered at 1573 K as a function of x. For the powders samples, the pure TiO2 rutile phase (PDF#21-1276) is observed in
Fig. 1. (a) XRD patterns of BNTO (x ¼ 0, 0.005, 0.01, 0.025 and 0.05) ceramics sintered at 1573 K for 4 h. (b) An enlarged view of the peak (110) of BNTO ceramics sintered at 1573 K for 4 h. (c) XRD patterns of BNTO ceramics with x ¼ 0.025 sintered at different temperatures. (d) An enlarged view of the peak (110) for x ¼ 0.025 sintered at different temperatures.
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Table 1 Relative density, average grain size, volatilization of Bi, lattice parameters, and dielectric properties of BNTO ceramics with various compositions and sintering temperatures. Composition
Sintering temperature (K)
Relative density
Volatilization of Bi (wt.%)
a (Å)
c (Å)
Average grain size (mm)
ε0 1 kHz
tand 1 kHz
x ¼ 0.005
1523 1573 1623 1673 1523 1573 1623 1673 1523 1573 1623 1673 1523 1573 1623 1673
95% 94% 94% 92% 94% 93% 92% 93% 94% 95% 94% 94% 95% 94% 94% 94%
0.09% 0.15% 0.19% 0.22% 0.30% 0.35% 0.43% 0.48% 0.93% 1.02% 1.10% 1.24% 2.05% 2.22% 2.35% 2.37%
4.592 4.596 4.588 4.583 4.591 4.598 4.595 4.592 4.600 4.601 4.601 4.597 4.597 4.604 4.601 4.601
2.960 2.961 2.959 2.958 2.959 2.962 2.960 2.958 2.961 2.962 2.962 2.961 2.961 2.964 2.961 2.961
2.5 2.6 3.4 5.1 2.9 3.5 5.1 5.8 6.3 7.6 11.1 14.5 6.7 13.9 14.5 27.0
10.3 k 12.3 k 51.9 k 44.9 k 14.4 k 63.4 k 45.9 k 45.5 k 69 k 155.1 k 155.5 k 192 k 73.4 k 158.9 k 98 k 115.7 k
1.049 0.535 0.181 0.261 0.790 0.527 0.318 0.617 0.068 0.042 0.060 0.217 0.133 0.107 0.251 59.53
x ¼ 0.01
x ¼ 0.025
x ¼ 0.05
the samples with x ¼ 0.005, 0.01 and 0.025. As x ¼ 0.05, the Bi1.74Ti2O6.624 secondary phase (PDF#89-4732) appears, which agrees with reference [27]. However, the Bi1.74Ti2O6.624 secondary phase is detected on the surface of sample with x ¼ 0.025, but it is not found on the polished surface of sample with x ¼ 0.025 (the top XRD pattern in Fig. 1(a)). Fig. 1(b) shows the enlarged view of the peak (110) of Fig. 1(a). It is very clear that the (110) diffraction peak slightly shifts toward to lower angle, indicating lattice expansion with increasing x. By consideration of coordination number crystal radius of Ti4þ (VI, 0.745 Å), Bi3þ (VI, 1.17 Å) and Nb5þ (VI, 0.78 Å) [24e26], the lattice expansion is attributed to Nb5þ and/or Bi3þ entering the TiO2 lattice. Furthermore, the (110) peaks become broaden with composition x, which is associated with the dopants
in TiO2 matrix [29]. Fig. 1(c) shows the XRD patterns of BNTO ceramic powders with x ¼ 0.025 sintered at different temperatures, and Fig. 1(d) is the enlarged view of the peak (110). Bi1.74Ti2O6.624 phase is observed at 1523 K and disappears with increase of sintering temperatures. (110) peaks slightly shift toward to higher angle with sintering temperatures due to lattice shrinkage. Since Bi2O3 has a low melting point (Tm(Bi2O3) ¼ 1099 K) [30], the lattice shrinkage should be ascribed to the volatilization of Bi3þ from lattice matrix (Table 1). Thus, combined with the results of Fig. 1(b), we supposed that some Bi3þ ions substituted for Ti4þ sites and entered the TiO2 lattice. Fig. 2 shows the SEM images of the BNTO ceramics. As shown in Fig. 2(a)-(d) and Table 1, the average grain size increases from 2.6 to
Fig. 2. SEM images of BNTO sintered at 1573 K: (a) x ¼ 0.005, (b) x ¼ 0.01, (c) x ¼ 0.025, (d) x ¼ 0.05. SEM images for x ¼ 0.025 sintered at different temperatures: (e) 1523 K, (f) 1623 K, (g) 1673 K. (h) Element mapping for x ¼ 0.025 sintered at 1573 K.
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Fig. 3. Temperature dependence of the dielectric properties of BNTO ceramics sintered at 1573 K: (a) x ¼ 0.005 (20e550 K), (b) x ¼ 0.025 (10e550 K). Frequency dependence of the dielectric properties of BNTO (1573 K) measured at different temperatures: (c) x ¼ 0.005, (d) x ¼ 0.025.
13.9 mm with compositions x ranging from 0.005 to 0.05 for the ceramics sintered at 1573 K. As shown in Fig. 2(c), (e)-(g) and Table 1, for x ¼ 0.025, the average grain size increases from 6.3 to 14.5 mm with sintering temperatures from 1523 K to 1673 K. It can be seen that the secondary phase is located at trigeminal grain boundaries at 1523 K (Fig. 2(e)) and 1573 K (Fig. 2(c)), and then observed at grain boundaries at 1623 K (Fig. 2(f)). Fig. 2(g) shows that the secondary phase exhibits the melting state at 1673 K. EDX results are shown in Fig. 2(h). Elements mapping of Bi and Ti atoms illustrate that this phase is full of Bi and Ti atoms. Combined with XRD patterns, it should be Bi1.74Ti2O6.624 phase which is favorable for grain growth. EDX results show that Nb has a slight aggregation in Bi1.74Ti2O6.624 phase. Nb, Bi and Ti atoms homogeneously distribute in grains, and O atoms are lacking at grain boundaries. Table 1 displays relative density, volatilization of Bi, lattice parameters, average grain size and dielectric properties of BNTO ceramics with various x values and sintering temperatures. It can be seen from Table 1, the volatilization of Bi and average grain size increase as sintering temperature increases from 1523 K to 1673 K. The lattice parameters increase, and then decline after reaching a maximum at 1573 K. These indicate that higher sintering temperature is not only conducive to Bi3þ and Nb5þ ions entering the lattice but also will make more Bi volatilize. However, the relative density doesn't change significantly with sintering temperature. This may be due to the fact that new pores are produced because of the volatilization of Bi at higher temperature. As shown in Table 1, the dielectric constant (ε0 ) and dielectric loss (tan d) depend on x values and sintering temperatures. As sintering temperature increases, the dielectric loss first decreases and then increases, especially for x ¼ 0.05. Combined with SEM and XRD results, we think that Bi ions which enter the lattice and the formation of Bi1.74Ti2O6.624 phase are favorable for the decrease of dielectric loss.
BNTO ceramic with x ¼ 0.025 sintered at 1573 K shows a lower dielectric loss (0.042) than previous report [27]. Basically, the volatilization of Bi, average grain size, dielectric constant, and lattice parameters increase with x value increasing from 0.005 to 0.05. Fig. 3 shows the dielectric properties for BNTO ceramics with x ¼ 0.005 and 0.025 sintered at 1573 K as a function of temperature (10e550 K) and frequency (20-2M Hz), respectively. As shown in Fig. 3(a) and (b), thermally activated peaks R1, R2 and R3 can be identified in x ¼ 0.005 and 0.025. The thermally activated peak can
Fig. 4. Current density-electric field characteristics for BNTO ceramics with x ¼ 0.005, 0.01 and 0.025 sintered at 1573 K. The solid line is the fitting data. The table is fitting parameters.
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be described by the Arrhenius law:
f ¼ f1 exp½ Ea =ðKB TÞ where f1 is the maximum imaginary of dielectric response, Ea is the activation energy, and KB is the Boltzmann constant. As shown in the inset of Fig. 3(a), the activation energies of R1 and R3 for x ¼ 0.005 were calculated to be 0.025 eV and 0.3006 eV, respectively. For x ¼ 0.025, activation energies were calculated to be 0.0285 eV and 0.63 eV. As x ¼ 0.005, the pre-exponential factors f0 are 7.17 107 Hz (R1) and 5.65 107 Hz (R3); as x ¼ 0.025, the preexponential factors f0 are 4.92 108 Hz (R1) and 1.50 1016 Hz (R3). By consideration of activation energies, R1 may be caused by
electron-pinned defect-dipoles (INTO) or weak-binding electron (TiO2 crystal) [10,31]. As shown in inset of Fig. 3(b), R2 of x ¼ 0.025 should be fitted by two Arrhenius type nearest-neighbor-hopping models, which is similar to that reported in GNTO, (YbþNb) codoped rutile TiO2 and TiO2 ceramics [21,31,32]. Activation energies of R2 for x ¼ 0.025 were calculated to be 0.0839 eV (low temperature) and 0.1248 eV (high temperature); the preexponential factors f0 are 3.28 105 Hz (low temperature) and 5.52 106 Hz (high temperature). In BNTO ceramics, Nb5þ ions generate electrons and form Ti3þ ions, and oxygen vacancies are produced by Bi3þ ions. Oxygen vacancies are common defects in the oxides [33]. The complex clusters among Bi0Ti , NbTi , Ti0Ti , V O , as well 0 0 0 0 as Bi0Ti /NbTi , Bi0Ti /V )Bi , Bi /V )Ti , Nb Ti /TiTi , O O Ti Ti Ti
Fig. 5. (a) Impedance analysis for BNTO ceramics with x ¼ 0.005, 0.01, 0.025 and 0.05 sintered at 1573 K, the inset is equivalent electric circuit and an enlarged IS view for the high frequency data. The solid line is the fitting data; the table is fitting parameters. (b) The real part of ac conductivity of BNTO ceramics as a function of frequency.
Y. Song et al. / Journal of Alloys and Compounds 722 (2017) 676e682 0 Ti0Ti /V O )TiTi will be formed in BNTO ceramics [27]. Thus, R2 should be attributed to electron hopping. Fig. 3(c)-(d) show frequency dependence of dielectric properties for BNTO with x ¼ 0.005 and 0.025 measured at selected temperatures, respectively. BNTO with x ¼ 0.025 shows a better frequency independent dielectric constant than x ¼ 0.005 sample at temperature range from 120 to 300 K. To convince the nature of R3, Fig. 4 demonstrates the current density-electric field characteristics of BNTO ceramics measured at room temperature. Nonlinear I-V behaviors are observed for BNTO ceramics. It is found that the breakdown voltage (1 mA/cm2) of BNTO ceramics with x ¼ 0.005, 0.01, and 0.025 sintered at 1573 K are 1000 V/cm, 120 V/cm, and 45 V/cm, respectively. The breakdown voltage of BNTO with x ¼ 0.005 is comparable to those previously reported in INTO ceramics [11,13,16]. The nonlinear currentvoltage curves are fitted with the following general equation for varistor [34],
J ¼ K Ea where K is a constant related to the electrical resistivity of the material, a is the nonlinear coefficient value. The nonlinear coefficient value of BNTO ceramics with x ¼ 0.005, 0.01, and 0.025 sintered at 1573 K are 3.9, 2.5, and 4.3, respectively. Thus, the breakdown voltage can be tuned from 45 to 1000 V/cm by varying x values. Fig. 5(a) shows the complex impedance plots of BNTO ceramics (1573 K) measured at room temperature. The complex impedance can be described by an equivalent circuit. Two parallel RC elements
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make up this circuit, where Rg and Rgb (Rgb > Rg) are the resistance of grain and grain boundary, respectively. Cg and Cgb (Cgb > Cg) are the capacitance respectively for grain and grain boundary [16]. The grain and grain boundary arc are obtained in Fig. 5(a) and inset. The grain resistance for x ¼ 0.005, 0.01, 0.025, and 0.05 are 6.7 kU cm, 960 U cm, 490 U cm, and 56 U cm, respectively. Fig. 5(b) shows the frequency dependence of the real part of the RT ac conductivity spectra for BNTO ceramics. The conductivity at high frequency is dominated by the grain conductance [11]. Grain conductivity increases with x values. Therefore, R3 should be argued to MaxwellWagner effect [11,16]. The contribution of R3 to the static permittivity is 87.8% and 28.3% respectively for x ¼ 0.005 and 0.025, measured at room temperature. Fig. 6 shows XPS spectrum of O 1s and Ti 2p for BNTO ceramics sintered at 1573 K. All the data were calibrated by C 1s 284.6 eV. Fig. 6(a) and (b) show O 1s and Ti 2p data, respectively. After deducting the Shirley background, the fitting results are shown in Fig. 6(c)-(d) and the inset table. As shown in Fig. 6(c), two peaks appear in O 1s spectrum corresponding to the tested results: oxygen vacancy and lattice oxygen [35,36]. Fig. 6(d) demonstrates that Ti3þ ions are present in BNTO ceramics. The fitting results shown in the inset table exhibit that the oxygen vacancies and Ti3þ ions content increase with x values from 29.8% to 37.7% and from 33.1% to 38.0%, respectively. These results indicate that more electrons will be produced. Meanwhile, the variation of dielectric constant, breakdown voltage, and conductivity with x values confirm these results. All results show that defect concentration of BNTO ceramics can be adjusted by doping under the same sintering process. Simultaneously, a higher defect concentration than expected also
Fig. 6. The XPS spectra of BNTO ceramics with x ¼ 0.005, 0.01, 0.025 and 0.05 sintered at 1573 K: (a) O 1s, (b) Ti 2p, (c) O 1s with x ¼ 0.005 and 0.025, (d) Ti 2p with x ¼ 0.005 and 0.025. The inset table is the fitting results of O 1s and Ti 2p data.
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indicates that the sintering process in BNTO material system has a great effect on the performance of BNTO ceramics. In the future, in order to obtain the sample with colossal permittivity, low dielectric loss, and high breakdown voltage, we will modify the resistance and size of grain and grain boundaries. 4. Conclusions In summary, the microstructure, dielectric properties, impedance spectroscopy, and nonlinear I-V behavior of BNTO ceramics were systematically investigated. Sintering temperature and compositions x have huge influence on dielectric properties of BNTO ceramics. Colossal permittivity is mainly related to MaxwellWagner effect, electron-pinned defect-dipoles or weak-binding electron. Existence of Bi1.74Ti2O6.624 phase and Bi ions which are doped into the lattice matrix contribute to low dielectric loss. BNTO with x ¼ 0.005 shows a relatively high breakdown strength 1000 V/ cm. Oxygen vacancies and Ti3þ ions content increase with x values. BNTO with x ¼ 0.025 exhibits a high permittivity (155.1 k) and low dielectric loss (0.042) at 1 kHz measured at room temperature. Acknowledgments This work was supported by National Science Foundation of China (Grant nos. 51572162) and Fundamental Research Funds for the Central Universities (2016TS036). References [1] C.C. Homes, T. Vogt, S.M. Shapiro, S. Wakimoto, A.P. Ramirez, Science 293 (2001) 673e676. [2] S. Krohns, P. Lunkenheimer, S. Meissner, A. Reller, B. Gleich, A. Rathgeber, T. Gaugler, H.U. Buhl, D.C. Sinclair, A. Loidl, Nat. Mater. 10 (2011) 899e901. [3] M.T. Buscaglia, M. Viviani, V. Buscaglia, L. Mitoseriu, A. Testino, P. Nanni, Zhao, Zhe, M. Nygren, C. Harnagea, D. Piazza, C. Galassi, Phys. Rev. B 73 (2006) 064114e064123. [4] C.C. Wang, L.W. Zhang, Appl. Phys. Lett. 88 (2006) 042906e042908. [5] M. Li, A. Feteira, D.C. Sinclair, A.R. West, Appl. Phys. Lett. 88 (2006) 232903e232905. [6] J. Wu, C.W. Nan, Y. Lin, D. Yuan, Phys. Rev. Lett. 89 (2002) 217601e217604. [7] R.J. Cava, R.M. Fleming, P. Littlewood, E.A. Rietman, L.F. Schneemeyer, Phys. Rev. B 30 (1984) 3228e3239.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Dielectric properties of (Bi0.5Nb0.5)xTi1-xO2 ceramics with colossal permittivity Yuechan Song, Peng Liu*, Xiaogang Zhao, Baochun Guo, Xiulei Cui College of Physics and Information Technology, Shaanxi Normal University, Xi'an, 710062, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 December 2016 Received in revised form 13 June 2017 Accepted 16 June 2017
The (Bi0.5Nb0.5)xTi1-xO2 (BNTO, 0 x 0.05) ceramics were synthesized by a standard conventional solidstate reaction. The effects of sintering temperature and composition x on the crystal structure, microstructure, and dielectric properties of BNTO ceramics were investigated. A pure rutile phase is achieved as x 0.01, and secondary phase Bi1.74Ti2O6.624 is detected as x 0.025. BNTO ceramic with x ¼ 0.025 shows a high dielectric constant (155.1 k) and low dielectric loss (0.042) at 1 kHz (room temperature). Colossal permittivity is primarily related to Maxwell-Wagner effect, electron-pinned defect-dipoles or weak-binding electron. The low dielectric loss is argued to Bi ions which are doped into TiO2 lattice and the formation of Bi1.74Ti2O6.624 phase. © 2017 Elsevier B.V. All rights reserved.
Keywords: Ceramic Colossal permittivity Rutile TiO2 Dielectric
1. Introduction In recent years, the colossal permittivity (CP, ε0 >1000) materials are getting more and more attention. Nowadays, the miniaturization of electronic device is widely required. CP materials in myriad types of miniaturized devices and high-energy-density storage have a huge potential [1,2]. Several CP materials have been researched, such as dopedBaTiO3 [3], CaCu3Ti4O12 (CCTO) [4,5], doped-NiO [6], and K0.3MoO3 [7]. However, there are some problems that limit their application in practice. Doped-BaTiO3 shows a high dielectric constant, but it can be used in a narrow temperature range [8]. CCTO displays a colossal permittivity, and its dielectric loss is high and sensitive to the preparation process [9]. Therefore, high-performance materials with colossal permittivity, low dielectric loss, weak temperatureand frequency-independence should be investigated. Recently, (In0.5Nb0.5)xTi1-xO2 (INTO) ceramic is noticeable not only for its high dielectric permittivity (104) and low dielectric loss (<0.05) over a broad temperature range from 80 to 450 K, but also for the new mechanism of the origin of the colossal permittivity materials [10e19]. Hu et al. thought that electrons could be localized by appropriately constructed defect clusters and put forward a new electron-pinned defect-dipoles mechanism [10]. Then, (A0.5Nb0.5)xTi1xO2 (A ¼ Al, Ga, Er) and (AlþNb) co-doped SnO2 ceramics were * Corresponding author. E-mail address: [email protected] (P. Liu). http://dx.doi.org/10.1016/j.jallcom.2017.06.177 0925-8388/© 2017 Elsevier B.V. All rights reserved.
reported [20e23]. However, Hu et al. demonstrated that MaxwellWagner effect was responsible for the colossal permittivity and low dielectric loss in (Al0.5Nb0.5)xTi1-xO2 ceramics [20]. Dong et al. suggested that the acceptor ions with different sizes determined the formation of electron-pinned defect-dipoles, and electron-pinned defect-dipoles were not formed in (Ga0.5Nb0.5)xTi1-xO2 (GNTO) ceramics [21]. In tetragonal TiO2 rutile crystal, the ionic radius of In3þ (0.94 Å) is larger than Al3þ (0.675 Å), Ga3þ (0.76 Å) and Ti4þ (0.745 Å) ions [24e26]. Therefore, the larger ionic radius of acceptors may be conducive to the formation of defect clusters in the (A0.5Nb0.5)xTi1-xO2 ceramics [21]. Recently, Wu et al. reported (Bi0.5Nb0.5)xTi1-xO2 (BNTO) ceramics with colossal permittivity and low dielectric loss by substituting Bi3þ (1.17 Å) and Nb5þ for Ti4þ ions [27]. Considering the volatilization of Bi at elevated temperature and larger ionic radius of Bi3þ ions, whether Bi3þ ions can be doped into TiO2 lattice is questionable. It is necessary to figure out the effect of Bi3þ ions on crystal structure, microstructure, and dielectric properties of BNTO ceramics. In this work, (Bi0.5Nb0.5)xTi1-xO2 (BNTO) ceramics were synthesized by a conventional solid-state reaction. We investigated the microstructure, dielectric properties, and I-V behavior with various x values and sintering temperatures. 2. Experimental procedure The BNTO ceramics were prepared by the conventional solidstate reaction. Raw materials used in this work are rutile TiO2 (99%), Nb2O5 (99.5%), and Bi2O3 (99%). Raw materials were baked at
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473 K for 24 h and weighed accurately according to their chemical compositions, and then ball mixed for 24 h with ethanol as medium. The mixed powders were calcined at 1323 K for 2 h in the air. After the calcined powders second ball-milled, powders were mixed with 5 wt% of PVA as binder. Disks of 10 mm in diameter and approximate 1 mm in thickness were obtained under uniaxial pressure of 267 MPa. Disks were pretreated at 873 K for 2 h to expel the binder, and then sintered at 1523e1673 K for 4 h with a raising and cooling rate of 3 K/min. Silver pasted on both sides of the sintered samples and annealed at 923 K for 30 min. The phases were characterized by X-ray diffraction (XRD, Rigaku D/Max-2400, Japan). The microstructure and element distribution were obtained by scanning electron microscopy (SEM, Fei Nova 650, Germany) with energy-dispersive X-ray analysis (EDX). The relative density was calculated by bulk density and theoretical density. Bulk densities were measured by an electronic balance (XT220, Switzerland) based on the Archimedes principle. The calculation of the theoretical density took into account the second phase, volatilization of Bi, and the oxygen vacancies which were caused by Bi [28]. The volatilization of Bi is obtained by the weight of ceramics
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before and after sintering. The valence states of elements were performed by X-ray photoelectron spectroscopy (XPS, Kratos analytical Ltd/AXIS ULTRA, Japan). Oxygen vacancies and Ti3þ ions contents were calculated by the area ratio of Gaussian peaks. And the dielectric properties of the samples were measured by Agilent E4980A with a temperature controller vibration sample magnetometer (VSM, England) ranging from 10 to 300 K and a Novocontrol broadband dielectric spectrometer with an alpha-A high performance frequency analyzer in the frequency range of 1e3 M Hz over the temperature range of 113e550 K (BDS40, Germany). The nonlinear I-V characteristics for samples were measured at room temperature with a radiant precision workstation ferroelectric testing system (Radiant Technologies Inc, Albuquerque, NM). 3. Results and discussion The XRD patterns of BNTO ceramics with various compositions x and sintering temperatures are shown in Fig. 1. Fig. 1(a) shows BNTO samples sintered at 1573 K as a function of x. For the powders samples, the pure TiO2 rutile phase (PDF#21-1276) is observed in
Fig. 1. (a) XRD patterns of BNTO (x ¼ 0, 0.005, 0.01, 0.025 and 0.05) ceramics sintered at 1573 K for 4 h. (b) An enlarged view of the peak (110) of BNTO ceramics sintered at 1573 K for 4 h. (c) XRD patterns of BNTO ceramics with x ¼ 0.025 sintered at different temperatures. (d) An enlarged view of the peak (110) for x ¼ 0.025 sintered at different temperatures.
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Table 1 Relative density, average grain size, volatilization of Bi, lattice parameters, and dielectric properties of BNTO ceramics with various compositions and sintering temperatures. Composition
Sintering temperature (K)
Relative density
Volatilization of Bi (wt.%)
a (Å)
c (Å)
Average grain size (mm)
ε0 1 kHz
tand 1 kHz
x ¼ 0.005
1523 1573 1623 1673 1523 1573 1623 1673 1523 1573 1623 1673 1523 1573 1623 1673
95% 94% 94% 92% 94% 93% 92% 93% 94% 95% 94% 94% 95% 94% 94% 94%
0.09% 0.15% 0.19% 0.22% 0.30% 0.35% 0.43% 0.48% 0.93% 1.02% 1.10% 1.24% 2.05% 2.22% 2.35% 2.37%
4.592 4.596 4.588 4.583 4.591 4.598 4.595 4.592 4.600 4.601 4.601 4.597 4.597 4.604 4.601 4.601
2.960 2.961 2.959 2.958 2.959 2.962 2.960 2.958 2.961 2.962 2.962 2.961 2.961 2.964 2.961 2.961
2.5 2.6 3.4 5.1 2.9 3.5 5.1 5.8 6.3 7.6 11.1 14.5 6.7 13.9 14.5 27.0
10.3 k 12.3 k 51.9 k 44.9 k 14.4 k 63.4 k 45.9 k 45.5 k 69 k 155.1 k 155.5 k 192 k 73.4 k 158.9 k 98 k 115.7 k
1.049 0.535 0.181 0.261 0.790 0.527 0.318 0.617 0.068 0.042 0.060 0.217 0.133 0.107 0.251 59.53
x ¼ 0.01
x ¼ 0.025
x ¼ 0.05
the samples with x ¼ 0.005, 0.01 and 0.025. As x ¼ 0.05, the Bi1.74Ti2O6.624 secondary phase (PDF#89-4732) appears, which agrees with reference [27]. However, the Bi1.74Ti2O6.624 secondary phase is detected on the surface of sample with x ¼ 0.025, but it is not found on the polished surface of sample with x ¼ 0.025 (the top XRD pattern in Fig. 1(a)). Fig. 1(b) shows the enlarged view of the peak (110) of Fig. 1(a). It is very clear that the (110) diffraction peak slightly shifts toward to lower angle, indicating lattice expansion with increasing x. By consideration of coordination number crystal radius of Ti4þ (VI, 0.745 Å), Bi3þ (VI, 1.17 Å) and Nb5þ (VI, 0.78 Å) [24e26], the lattice expansion is attributed to Nb5þ and/or Bi3þ entering the TiO2 lattice. Furthermore, the (110) peaks become broaden with composition x, which is associated with the dopants
in TiO2 matrix [29]. Fig. 1(c) shows the XRD patterns of BNTO ceramic powders with x ¼ 0.025 sintered at different temperatures, and Fig. 1(d) is the enlarged view of the peak (110). Bi1.74Ti2O6.624 phase is observed at 1523 K and disappears with increase of sintering temperatures. (110) peaks slightly shift toward to higher angle with sintering temperatures due to lattice shrinkage. Since Bi2O3 has a low melting point (Tm(Bi2O3) ¼ 1099 K) [30], the lattice shrinkage should be ascribed to the volatilization of Bi3þ from lattice matrix (Table 1). Thus, combined with the results of Fig. 1(b), we supposed that some Bi3þ ions substituted for Ti4þ sites and entered the TiO2 lattice. Fig. 2 shows the SEM images of the BNTO ceramics. As shown in Fig. 2(a)-(d) and Table 1, the average grain size increases from 2.6 to
Fig. 2. SEM images of BNTO sintered at 1573 K: (a) x ¼ 0.005, (b) x ¼ 0.01, (c) x ¼ 0.025, (d) x ¼ 0.05. SEM images for x ¼ 0.025 sintered at different temperatures: (e) 1523 K, (f) 1623 K, (g) 1673 K. (h) Element mapping for x ¼ 0.025 sintered at 1573 K.
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Fig. 3. Temperature dependence of the dielectric properties of BNTO ceramics sintered at 1573 K: (a) x ¼ 0.005 (20e550 K), (b) x ¼ 0.025 (10e550 K). Frequency dependence of the dielectric properties of BNTO (1573 K) measured at different temperatures: (c) x ¼ 0.005, (d) x ¼ 0.025.
13.9 mm with compositions x ranging from 0.005 to 0.05 for the ceramics sintered at 1573 K. As shown in Fig. 2(c), (e)-(g) and Table 1, for x ¼ 0.025, the average grain size increases from 6.3 to 14.5 mm with sintering temperatures from 1523 K to 1673 K. It can be seen that the secondary phase is located at trigeminal grain boundaries at 1523 K (Fig. 2(e)) and 1573 K (Fig. 2(c)), and then observed at grain boundaries at 1623 K (Fig. 2(f)). Fig. 2(g) shows that the secondary phase exhibits the melting state at 1673 K. EDX results are shown in Fig. 2(h). Elements mapping of Bi and Ti atoms illustrate that this phase is full of Bi and Ti atoms. Combined with XRD patterns, it should be Bi1.74Ti2O6.624 phase which is favorable for grain growth. EDX results show that Nb has a slight aggregation in Bi1.74Ti2O6.624 phase. Nb, Bi and Ti atoms homogeneously distribute in grains, and O atoms are lacking at grain boundaries. Table 1 displays relative density, volatilization of Bi, lattice parameters, average grain size and dielectric properties of BNTO ceramics with various x values and sintering temperatures. It can be seen from Table 1, the volatilization of Bi and average grain size increase as sintering temperature increases from 1523 K to 1673 K. The lattice parameters increase, and then decline after reaching a maximum at 1573 K. These indicate that higher sintering temperature is not only conducive to Bi3þ and Nb5þ ions entering the lattice but also will make more Bi volatilize. However, the relative density doesn't change significantly with sintering temperature. This may be due to the fact that new pores are produced because of the volatilization of Bi at higher temperature. As shown in Table 1, the dielectric constant (ε0 ) and dielectric loss (tan d) depend on x values and sintering temperatures. As sintering temperature increases, the dielectric loss first decreases and then increases, especially for x ¼ 0.05. Combined with SEM and XRD results, we think that Bi ions which enter the lattice and the formation of Bi1.74Ti2O6.624 phase are favorable for the decrease of dielectric loss.
BNTO ceramic with x ¼ 0.025 sintered at 1573 K shows a lower dielectric loss (0.042) than previous report [27]. Basically, the volatilization of Bi, average grain size, dielectric constant, and lattice parameters increase with x value increasing from 0.005 to 0.05. Fig. 3 shows the dielectric properties for BNTO ceramics with x ¼ 0.005 and 0.025 sintered at 1573 K as a function of temperature (10e550 K) and frequency (20-2M Hz), respectively. As shown in Fig. 3(a) and (b), thermally activated peaks R1, R2 and R3 can be identified in x ¼ 0.005 and 0.025. The thermally activated peak can
Fig. 4. Current density-electric field characteristics for BNTO ceramics with x ¼ 0.005, 0.01 and 0.025 sintered at 1573 K. The solid line is the fitting data. The table is fitting parameters.
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be described by the Arrhenius law:
f ¼ f1 exp½ Ea =ðKB TÞ where f1 is the maximum imaginary of dielectric response, Ea is the activation energy, and KB is the Boltzmann constant. As shown in the inset of Fig. 3(a), the activation energies of R1 and R3 for x ¼ 0.005 were calculated to be 0.025 eV and 0.3006 eV, respectively. For x ¼ 0.025, activation energies were calculated to be 0.0285 eV and 0.63 eV. As x ¼ 0.005, the pre-exponential factors f0 are 7.17 107 Hz (R1) and 5.65 107 Hz (R3); as x ¼ 0.025, the preexponential factors f0 are 4.92 108 Hz (R1) and 1.50 1016 Hz (R3). By consideration of activation energies, R1 may be caused by
electron-pinned defect-dipoles (INTO) or weak-binding electron (TiO2 crystal) [10,31]. As shown in inset of Fig. 3(b), R2 of x ¼ 0.025 should be fitted by two Arrhenius type nearest-neighbor-hopping models, which is similar to that reported in GNTO, (YbþNb) codoped rutile TiO2 and TiO2 ceramics [21,31,32]. Activation energies of R2 for x ¼ 0.025 were calculated to be 0.0839 eV (low temperature) and 0.1248 eV (high temperature); the preexponential factors f0 are 3.28 105 Hz (low temperature) and 5.52 106 Hz (high temperature). In BNTO ceramics, Nb5þ ions generate electrons and form Ti3þ ions, and oxygen vacancies are produced by Bi3þ ions. Oxygen vacancies are common defects in the oxides [33]. The complex clusters among Bi0Ti , NbTi , Ti0Ti , V O , as well 0 0 0 0 as Bi0Ti /NbTi , Bi0Ti /V )Bi , Bi /V )Ti , Nb Ti /TiTi , O O Ti Ti Ti
Fig. 5. (a) Impedance analysis for BNTO ceramics with x ¼ 0.005, 0.01, 0.025 and 0.05 sintered at 1573 K, the inset is equivalent electric circuit and an enlarged IS view for the high frequency data. The solid line is the fitting data; the table is fitting parameters. (b) The real part of ac conductivity of BNTO ceramics as a function of frequency.
Y. Song et al. / Journal of Alloys and Compounds 722 (2017) 676e682 0 Ti0Ti /V O )TiTi will be formed in BNTO ceramics [27]. Thus, R2 should be attributed to electron hopping. Fig. 3(c)-(d) show frequency dependence of dielectric properties for BNTO with x ¼ 0.005 and 0.025 measured at selected temperatures, respectively. BNTO with x ¼ 0.025 shows a better frequency independent dielectric constant than x ¼ 0.005 sample at temperature range from 120 to 300 K. To convince the nature of R3, Fig. 4 demonstrates the current density-electric field characteristics of BNTO ceramics measured at room temperature. Nonlinear I-V behaviors are observed for BNTO ceramics. It is found that the breakdown voltage (1 mA/cm2) of BNTO ceramics with x ¼ 0.005, 0.01, and 0.025 sintered at 1573 K are 1000 V/cm, 120 V/cm, and 45 V/cm, respectively. The breakdown voltage of BNTO with x ¼ 0.005 is comparable to those previously reported in INTO ceramics [11,13,16]. The nonlinear currentvoltage curves are fitted with the following general equation for varistor [34],
J ¼ K Ea where K is a constant related to the electrical resistivity of the material, a is the nonlinear coefficient value. The nonlinear coefficient value of BNTO ceramics with x ¼ 0.005, 0.01, and 0.025 sintered at 1573 K are 3.9, 2.5, and 4.3, respectively. Thus, the breakdown voltage can be tuned from 45 to 1000 V/cm by varying x values. Fig. 5(a) shows the complex impedance plots of BNTO ceramics (1573 K) measured at room temperature. The complex impedance can be described by an equivalent circuit. Two parallel RC elements
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make up this circuit, where Rg and Rgb (Rgb > Rg) are the resistance of grain and grain boundary, respectively. Cg and Cgb (Cgb > Cg) are the capacitance respectively for grain and grain boundary [16]. The grain and grain boundary arc are obtained in Fig. 5(a) and inset. The grain resistance for x ¼ 0.005, 0.01, 0.025, and 0.05 are 6.7 kU cm, 960 U cm, 490 U cm, and 56 U cm, respectively. Fig. 5(b) shows the frequency dependence of the real part of the RT ac conductivity spectra for BNTO ceramics. The conductivity at high frequency is dominated by the grain conductance [11]. Grain conductivity increases with x values. Therefore, R3 should be argued to MaxwellWagner effect [11,16]. The contribution of R3 to the static permittivity is 87.8% and 28.3% respectively for x ¼ 0.005 and 0.025, measured at room temperature. Fig. 6 shows XPS spectrum of O 1s and Ti 2p for BNTO ceramics sintered at 1573 K. All the data were calibrated by C 1s 284.6 eV. Fig. 6(a) and (b) show O 1s and Ti 2p data, respectively. After deducting the Shirley background, the fitting results are shown in Fig. 6(c)-(d) and the inset table. As shown in Fig. 6(c), two peaks appear in O 1s spectrum corresponding to the tested results: oxygen vacancy and lattice oxygen [35,36]. Fig. 6(d) demonstrates that Ti3þ ions are present in BNTO ceramics. The fitting results shown in the inset table exhibit that the oxygen vacancies and Ti3þ ions content increase with x values from 29.8% to 37.7% and from 33.1% to 38.0%, respectively. These results indicate that more electrons will be produced. Meanwhile, the variation of dielectric constant, breakdown voltage, and conductivity with x values confirm these results. All results show that defect concentration of BNTO ceramics can be adjusted by doping under the same sintering process. Simultaneously, a higher defect concentration than expected also
Fig. 6. The XPS spectra of BNTO ceramics with x ¼ 0.005, 0.01, 0.025 and 0.05 sintered at 1573 K: (a) O 1s, (b) Ti 2p, (c) O 1s with x ¼ 0.005 and 0.025, (d) Ti 2p with x ¼ 0.005 and 0.025. The inset table is the fitting results of O 1s and Ti 2p data.
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indicates that the sintering process in BNTO material system has a great effect on the performance of BNTO ceramics. In the future, in order to obtain the sample with colossal permittivity, low dielectric loss, and high breakdown voltage, we will modify the resistance and size of grain and grain boundaries. 4. Conclusions In summary, the microstructure, dielectric properties, impedance spectroscopy, and nonlinear I-V behavior of BNTO ceramics were systematically investigated. Sintering temperature and compositions x have huge influence on dielectric properties of BNTO ceramics. Colossal permittivity is mainly related to MaxwellWagner effect, electron-pinned defect-dipoles or weak-binding electron. Existence of Bi1.74Ti2O6.624 phase and Bi ions which are doped into the lattice matrix contribute to low dielectric loss. BNTO with x ¼ 0.005 shows a relatively high breakdown strength 1000 V/ cm. Oxygen vacancies and Ti3þ ions content increase with x values. BNTO with x ¼ 0.025 exhibits a high permittivity (155.1 k) and low dielectric loss (0.042) at 1 kHz measured at room temperature. Acknowledgments This work was supported by National Science Foundation of China (Grant nos. 51572162) and Fundamental Research Funds for the Central Universities (2016TS036). References [1] C.C. Homes, T. Vogt, S.M. Shapiro, S. Wakimoto, A.P. Ramirez, Science 293 (2001) 673e676. [2] S. Krohns, P. Lunkenheimer, S. Meissner, A. Reller, B. Gleich, A. Rathgeber, T. Gaugler, H.U. Buhl, D.C. Sinclair, A. Loidl, Nat. Mater. 10 (2011) 899e901. [3] M.T. Buscaglia, M. Viviani, V. Buscaglia, L. Mitoseriu, A. Testino, P. Nanni, Zhao, Zhe, M. Nygren, C. Harnagea, D. Piazza, C. Galassi, Phys. Rev. B 73 (2006) 064114e064123. [4] C.C. Wang, L.W. Zhang, Appl. Phys. Lett. 88 (2006) 042906e042908. [5] M. Li, A. Feteira, D.C. Sinclair, A.R. West, Appl. Phys. Lett. 88 (2006) 232903e232905. [6] J. Wu, C.W. Nan, Y. Lin, D. Yuan, Phys. Rev. Lett. 89 (2002) 217601e217604. [7] R.J. Cava, R.M. Fleming, P. Littlewood, E.A. Rietman, L.F. Schneemeyer, Phys. Rev. B 30 (1984) 3228e3239.
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