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3)O3 solid solution ceramics in a broad temperature range applied in X8R
Materials Science & Engineering B 238–239 (2018) 130–135
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Materials Science & Engineering B journal homepage: www.elsevier.com/locate/mseb
Excellent thermal-stability and low dielectric loss of BaTiO3-Bi(Sr2/3Nb1/3) O3 solid solution ceramics in a broad temperature range applied in X8R
T
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Xiuli Chen , Xiaoxia Li, Gaofeng Liu, Xiao Yan, Huanfu Zhou Collaborative Innovation Center for Exploration of Hidden Nonferrous Metal Deposits and Development of New Materials in Guangxi, Key Laboratory of Nonferrous Materials and New Processing Technology, Ministry of Education, School of Materials Science and Engineering, Guilin University of Technology, Guilin 541004, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Dielectric properties Phase evolution Impedance Thermal stability
(1−x)BaTiO3-xBi(Sr2/3Nb1/3)O3 [(1−x)BT-xBSN, 0.02 ≤ x ≤ 0.1] ceramics were synthesized by a traditional solid state reaction technique. The transition from tetragonal phase to pseudo cubic phase at 0.02 ≤ x ≤ 0.04 was observed in Raman spectra and X-ray diffraction patterns. As the BSN contents increased, the grain size increased. With adding Bi(Sr2/3Nb1/3)O3, the thermal stability of permittivity and dielectric loss of ceramics got a good optimization. Especially, 0.9BT-0.1BSN ceramics have small Δε/ε25°C values (≤ ± 15%) in a wide temperature range of −75 °C to 153 °C, high relative permittivity (∼1505 to 1700) and low dielectric loss (tan δ ≤ 0.02) from −88 °C to 200 °C, showing that BT-BSN ceramics are suitable for using in MLCCs of X8R. Impedance spectroscopy was analyzed the conduction and relaxation processes. The results showed that the relaxation and the conduction process in the high temperature region are thermally activated, and the oxygen vacancies are charge carriers.
1. Introduction
application. BaTiO3-Bi(Mg2/3Nb1/3)O3 have been behaved good temperaturestable dielectric in our previous work [12]. According to the above mentioned, it can be deduced that the substitution of Bi3+ for Ba and (Sr2+, Nb5+) for Ti can optimize the properties of BaTiO3, so Bi(Sr2/ 3Nb1/3)O3 was chosen to optimize properties of BaTiO3 ceramic in order to satisfy the X8R characteristics in this work. The (1−x)BaTiO3-xBi (Sr2/3Nb1/3)O3 solid solution ceramics were prepared by the solid state reaction method. Furthermore, the phase evolution, microstructure, impedance and dielectric properties of ceramics were also studied.
In recent years, the demand for multilayer ceramic capacitor (MLCC) in automotive applications has increased rapidly, such as antilock brake system, the engine electronic control unit, and programmed fuel injection [1,2]. But their working temperatures are likely to rise to about 150 °C, MLCC for the EIA (Electronic Industries Association) X7R (the change of the capacitance is less than 15% over the temperature range from −55 °C to 125 °C, Ɛ25°C = 4000–5000) cannot be employed [3]. Therefore, high temperature EIA X8R MLCCs (the temperature range from −55 °C to 150 °C, Ɛ25°C = 1000–2000) are being developed for the application of automotives [4]. Barium titanate (BaTiO3) perovskite is a main component to meet the X8R characteristic because of its excellent dielectric properties [5]. However, pure barium titanate has the noticeable changes of permittivity above the Curie temperature (Tc ∼ 130 °C), which cannot meet the temperature stability requirements in the working temperature. As an effective way to improve the material performance in electroceramics, the doping has attracted more and more attention [6,7]. So temperature-stable BaTiO3-based dielectric materials have been developed in the search for new high temperature dielectric materials [8,9], such as BaTiO3-Bi(Mg0.5Zr0.5)O3 [10], BaTiO3-Bi(Zn0.5Ti0.5)O3 [11], BaTiO3-Bi(Mg2/3Nb1/3)O3 [12], et al. These solid solutions exhibited outstanding performance over a wide range of temperature for MLCCs ⁎
2. Experimental (1−x)BaTiO3-xBi(Sr2/3Nb1/3)O3 [(1−x)BT-xBSN, 0.02 ≤ x ≤ 0.1] samples were synthesized by the traditional solid state reaction technique as described in our previous researches [12]. The samples were sintered at different temperatures for 2 h in air (1280–1380 °C). X-ray diffraction (XRD) patterns were measured at room temperature by an Xray diffractometer (XRD, Model X’Pert PRO; PANal ytical, Almelo, the Netherland) with CuKα radiation (λ = 0.15406 nm) operated at 40 kV and 40 mA with a step size of 0.02°. PanAlytical software (X’PertHighscore Plus) was analyzed the phase changes of XRD. Raman spectroscopy was carried out on a Thermo Fisher Scientific DXR using a 10 mW laser with a wave length of 532 nm. The microstructure of the
Corresponding author. E-mail address: [email protected] (X. Chen).
https://doi.org/10.1016/j.mseb.2018.12.022 Received 17 October 2017; Received in revised form 5 November 2018; Accepted 23 December 2018 0921-5107/ © 2018 Published by Elsevier B.V.
Materials Science & Engineering B 238–239 (2018) 130–135
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samples sintered at their optimized temperatures was observed using a scanning electron microscopy (Model JSM6380-LV SEM, JEOL, Tokyo, Japan). The silver paste is evenly applied to both sides of the sample plate, and heated up to 700 °C at a heating rate of 3 °C/min then kept for 30 min. Dielectric properties were measured with an applied voltage of 500 mV over 100 Hz-1 MHz from −120 °C to 200 °C using a precision impedance analyzer (Model E4980AL, Hewlett-Packard Co, Palo Alto, CA) at a heating rate of 2 °C/min. And the impedance was also measured with an applied voltage of 500 mV over 40 Hz-1 MHz from 640 °C to 740 °C. 3. Results and discussion Fig. 1 illustrates the XRD patterns of (1−x)BaTiO3-xBi(Sr2/3Nb1/3) O3 [(1−x)BT-xBSN, 0 ≤ x ≤ 0.1] samples sintered at their optimized temperatures. The main crystal phase of (1−x)BT-xBSN ceramics (0 ≤ x ≤ 0.1) is similar to the previous report about BaTiO3 crystal structure [13,14], indicating that BSN has assimilated into the BT lattices to form a homogeneous solid solution with perovskite phase, as shown in Fig. 1(a). As 0 ≤ x ≤ 0.02, the ceramics exhibited the tetragonal phase, as shown in Fig. 1(b). The splitting of the diffraction peak near 2θ ∼ 45° almost disappeared, which the (0 0 2) and (2 0 0) peaks combined into a single (2 0 0) peak at x ≥ 0.04, indicating that the pseudocubic or cubic structures appeared [12]. Fig. 2 gave a better demonstration for the phase evolution by carrying out the Raman spectra for (1−x)BT-xBSN (0.02 ≤ x ≤ 0.1) ceramics in the frequency range of 100–1000 cm−1 at room-temperature. In Fig. 2, it is clearly visible that all samples for (1−x)BT-xBSN (0.02 ≤ x ≤ 0.1) ceramics exhibited similar spectra, which agreed well with previous report [15]. The main spectral features for (1−x)BTxBSN ceramics are five different bands at 717, 518, 306, 270 and 170 cm−1 at x = 0.02, which are the sign of the tetragonal BaTiO3 [16,17]. The positive intensity of Raman peak around 180 cm−1 (which was observed at x = 0.02) is characteristic of the tetragonal phase [15,18]. On the contrary, the other one (apperceived for x = 0.04, 0.06, 0.08, 0.1) is indicative of the orthorhombic phase. These results are in good agreement with the results of XRD analysis, suggesting that a homogenous solid solution was formed between the BSN and BT. In addition, spectral characteristics have undergone tremendous changes in this range. A disappearance at 170 cm−1 vanishes and a new Model 3 appears in its position. Model 3 is related to the vibrations of A-O,
Fig. 1. (a) X-ray diffraction patterns of (1−x)BT-xBSN (0 ≤ x ≤ 0.1) ceramics, (b) The enlarged XRD patterns of the samples in the range of 2θ from 43° to 47°.
Fig. 2. Room temperature Raman spectra of (1−x)BT-xBSN (0.02 ≤ x ≤ 0.1) ceramics.
Fig. 3. SEM micrographs of (1−x)BT-xBSN ceramic sintered at their optimized temperatures:((a) x = 0.02, 1360 °C (b) x = 0.04, 1340 °C (c) x = 0.08, 1300 °C (d) x = 0.1, 1280 °C. 131
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Fig. 4. The grain size of (1−x)BT-xBSN (0.02 ≤ x ≤ 0.1) ceramics sintered at their optimized temperatures: ((a) x = 0.02, 1360 °C (b) x = 0.04, 1340 °C (c) x = 0.08, 1300 °C (d) x = 0.1, 1280 °C.
obvious. So, the relative permittivity is attribution to the grain boundary effect. Impedance spectroscopy has been recognized as a powerful method. Separation technology contributes to the experimental techniques of grain, grain boundaries and interfaces [20–25]. In order to elucidate the dielectric behavior of 0.9BT-0.1BSN ceramics, the high-temperature dielectric relaxation was studied by the impedance spectroscopy. Fig. 5 shows the Nyquist plots for 0.9BT-0.1BSN ceramic at various temperatures. With the increase of temperature, the impedance decreases. It is worth noting that the sample showed an electrical inhomogeneity character by two semicircles at different measured temperatures. One is at low-frequency range, while the other is at high-frequency range. The grain effect response contributes to in the high-frequency semicircle, while the low-frequency semicircle is attributed to the grain boundary effect [26]. Fig. 6 represents the real (Z′) and imaginary (Z″) parts of the impedance for 0.9BT-0.1BSN ceramic. With increasing the frequency and temperature, the effects of the frequency and temperature on the Z′ and Z″ behaviors of the sample become clearly visible. As the frequency and temperature increase, the reduction of the Z′ and Z″ could be observed. And the values of these two parts at different temperatures are combined separately in the high frequency region, showing that the decrease in the barrier properties of the sample may be lead to the release of the space charge with rising the measured temperature. And it may be the dominant factor in the increase of ac conductivity for samples at high temperatures [27]. At lower frequency, higher impedance value is a characteristic of the space charge polarization of the materials, as demonstrated in Fig. 6(a). In Fig. 6(b), it can be seen that two peaks appeared in the imaginary part of impedance spectrum for the sample at six temperatures, corresponding to the two Debye-like semicircles. This result fully confirms that the conductivities of the materials were contributed by the grain boundary and grain. With increasing the measured temperature, the value of Z″ reaches a maximum and moves in a high frequency, widened and generalized. The widened peak hints the existence of temperature-dependent relaxation processes in the material due to the defects under high sintering temperature [28]. Fig. 6(b) shows the change in relaxation time as a function of the
Fig. 5. Nyquist plots of impedance for 0.9BT-0.1BSN ceramics at selected temperatures.
indicating the presence of Ba2+ or Bi3+ nano-sized regions [19]. Fig. 3 illustrates the SEM images of natural surface for (1−x)BTxBSN (x = 0.02, 0.04, 0.08, 0.1) ceramics sintered at their optimized temperatures. The few pores of (1−x)BT-xBSN ceramics were obtained at x = 0.02 in Fig. 3(a). As the BSN contents increased, the grain size increased slightly and pores decreased obviously, which indicates that BSN could enhance the grain growth of BT-BSN ceramics. In detail, the grain size of the (1−x)BT-xBSN (x = 0.02, 0.04, 0.08, 0.1) ceramics were showed in Fig. 4. In Fig. 4, it can be clearly observed that the uniformity of grain size was improved with increasing x values. And the average grain sizes are 0.72 µm, 0.94 µm, 1.19 µm, and 1.84 µm for BTBSN (x = 0.02, 0.04, 0.08, 0.1), respectively, indicating that the addition of BSN could enhance the grain growth of ceramics. And ceramics are divided into grain boundary and grain to deal with grain size effect. The relative permittivity at grain boundary is lower than that at grain boundary. As the grain size increased, the grain boundary effect is more 132
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Fig. 6. Frequency dependences of real part Z′ (a) and imaginary part Z″ (b) of impedance for 0.9BT-0.1BSN ceramics at selected temperatures. [Insert shows the Arrhenius plot for relaxation time. The dotted line through the data is a linear fit to Eq. (2).]
Fig. 7. Normalized imaginary parts Z″/Z″max of impedance as a function of frequency.
absolute temperature. In the relaxation system, the relaxation time (τ) can be calculated from the imaginary part (Z″) and the frequency (f) curve of the impedance [Fig. 6(b)]. The following relationship was used:
τ=
1 1 = ω 2πfmax
Fig. 8. (a) Frequency dependence of ac conductivity for 0.9BT-0.1BSN ceramics at selected temperatures. (b) Arrhenius plot of dc conductivity for the 0.9BT0.1BSN. Squares are experimental points and the dotted line is a linear fit to Eq. (4).
(1)
where fmax is the relaxation frequency. According to Eq. (1), the value of τ decreases with increasing temperature, which indicates a typical semiconductor behavior. The activation energy (Ea) of this compound
was calculated from Arrhenius relation:
τ = τ0 exp(Ea/ kB T ) 133
(2)
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Fig. 9. The optimized (0.02 ≤ x ≤ 0.1) ceramics.
sintering
temperature
of
(1−x)BT-xBSN
where τ0 is the preexponential factor, kB is the Boltzmann constant, Ea is the activation energy required for the relaxation, and T is the absolute temperature. From the slope of log τ vs 103 T−1, the activation energy of relaxation (Ea) was 1.7 eV. Fig. 7 illustrates the normalized imaginary part of the impedance Z″/Z″max as a function of the frequency. With the increase of temperature, the peaks of Z″/Z″max move to the high frequency. The change in the apparent polarization is represented by the magnitude of mismatch between the peaks of the normalized parameters [29]. As shown in Fig. 7, it is illustrated that the polarization process is caused by the local conductance of multiple carriers, which describe the presence of multiple relaxation processes in the material. It is a significant performance of short-range conductivity with different position and broad asymmetry in the peaks of normalized parameters [30]. The symmetry increases with increasing temperature, hinting that the charge carriers migrate from short distances to long distances. As the temperature increases, the reduction in material volume resistance proved the increase in long-distance conductivity, which can be easily found in the Nyquist plots. Fig. 8(a) demonstrated the frequency dependence of the ac
Fig. 11. (a) Δε/ε25°C of (1−x)BT-xBSN (x = 0.1) ceramics at 10, 100 kHz and 1 MHz from −120 °C to 200 °C, (b) Temperature dependence of relative permittivity, and dielectric loss of (1−x)BT-xBSN (x = 0.1) ceramics at 10, 100 kHz and 1 MHz from −120 °C to 200 °C.
conductivity (σac) for 0.9BT-0.1BSN ceramic. The σac increases with increasing frequency and temperature. With increasing the temperature, it can be seen that the dispersion of the conductivity appeared in low frequency. When the σac decreases a certain value in low frequency, it becomes independent of frequency. This phenomenon is attributed to the long-range translational hopping of charge carriers that is conducive to dc conductivity (σdc). So, the direct-current conductivity (σdc) can be generated by extrapolating this part towards lower frequency. Moreover, it could be expressed through the universal dielectric response law [31]:
σac = σdc + Aωn
(3)
where σdc is the dc conductivity, ω is the angular frequency of ac field. The A and n (0 < n < 1) are two temperature dependent adjusting constants. The temperature dependence of σdc was obtained by the
Fig. 10. Temperature dependence of relative permittivity and dielectric loss for (1−x)BT-xBSN (0.02 ≤ x ≤ 0.1) ceramics measured at 10, 100 kHz and 1 MHz. 134
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Arrhenius law:
σde = σ0 exp(−Edc / kB T )
(4)
[2]
where σ0 is the pre-exponential factor, Edc is the activation energy of conduction and will be calculated from the slope of log(σ) vs 1000/T. The value is 1.56 eV, and it is very close to the activation energy of relaxation (Ea), indicating that these two processes may be caused by the same factor of charge carriers, oxygen vacancies, as shown in the inserted of Fig. 6(b). According to Steinsvik’s investigation [32], ABO3 perovskite structure activation energy changes with the concentration of oxygen vacancies, the motion of oxygen vacancies gives rise to activation energy and is considered as one of the mobile charge carriers in perovskite ferroelectrics [33]. And the charge compensation follows the reaction with [34]: VO = VO″ + 2e′ or VO′ = VO″ + e′. The double charge oxygen vacancy is deemed to most mobile charge and makes an important impact on conduction. And the direct-current conductivity is generated by the long-range movement of the double ionization oxygen vacancy [18]. The optimized sintering temperature of (1−x)BT-xBSN (0.02 ≤ x ≤ 0.1) ceramics was shown in Fig. 9. It can be clearly seen that the optimized sintering temperature of the sample material was decreased with the increase of BSN content. This result indicates that the addition of BSN could reduce the sintering temperature of BaTiO3 based dielectric ceramic samples. Fig. 10 demonstrates the temperature dependences of the relative permittivity(εr) and dielectric loss (tan δ) of (1−x)BT-xBSN (0.02 ≤ x ≤ 0.1) ceramics in the measured temperature range from −120 °C to 200 °C at frequencies of 10, 100 kHz and 1 MHz. With increasing the BSN contents, the peak values of εr decreased from ∼5539 (x = 0.02, 10 kHz) to ∼1700 (x = 0.1, 10 kHz), the Δε/ε25°C and tan δ of ceramics got the ideal optimization. The outstanding performances with high εr (∼1505 to 1700), small Δε/ε25 °C values ( ± 15%) over a broad temperature range from −75 °C to 153 °C and tan δ ≤ 0.02 from −88 °C to 200 °C were obtained at x = 0.1. Therefore, the Δε/ε25°C, εr and tan δ of 0.9BT-0.1BSN ceramics at 10, 100 kHz and 1 MHz from −120 °C to 200 °C are illustrated in Fig. 11. These outstanding performances of 0.9BT-0.1BSN were observed in Fig. 11, hinting that BT-BSN ceramics are suitable for using in MLCCs of X8R.
[3]
[4]
[5]
[6]
[7]
[8] [9] [10]
[11]
[12]
[13] [14] [15]
[16] [17] [18] [19] [20]
[21] [22]
4. Conclusions
[23]
(1−x)BT-xBSN (0.02 ≤ x ≤ 0.1) lead-free ceramics have been investigated using the conventional solid-state processing techniques. The structural transformation of the system from tetragonal to pseudo cubic phase occurred at 0.02 ≤ x ≤ 0.04, which was observed in Raman spectra and X-ray diffraction patterns. As the BSN contents increased, the significant reduction in the peak εm values with the broadening of tetragonal-cubic transition peaks was observed. 0.9BT-0.1BSN ceramics, showing the outstanding dielectric performances with small Δε/ ε25°C values ( ± 15%) in temperature range of −75 °C to 153 °C, high εr (∼1505–1700) and low tan δ ≤ 0.02 from −88 °C to 200 °C. All results showed that the relaxation and the conduction process in the high temperature region are thermally activated, and the oxygen vacancies are charge carriers.
[24]
[25]
[26] [27] [28]
[29]
[30]
Acknowledgments
[31]
This work was supported by Natural Science Foundation of China (Nos. 11664008 and 61761015), Natural Science Foundation of Guangxi (Nos. 2017GXNSFFA198011 and 2017GXNSFDA198027), Research Start-up Funds Doctor of Guilin University of Technology (Nos. 002401003281 and 002401003282).
[32] [33] [34]
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Materials Science & Engineering B journal homepage: www.elsevier.com/locate/mseb
Excellent thermal-stability and low dielectric loss of BaTiO3-Bi(Sr2/3Nb1/3) O3 solid solution ceramics in a broad temperature range applied in X8R
T
⁎
Xiuli Chen , Xiaoxia Li, Gaofeng Liu, Xiao Yan, Huanfu Zhou Collaborative Innovation Center for Exploration of Hidden Nonferrous Metal Deposits and Development of New Materials in Guangxi, Key Laboratory of Nonferrous Materials and New Processing Technology, Ministry of Education, School of Materials Science and Engineering, Guilin University of Technology, Guilin 541004, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Dielectric properties Phase evolution Impedance Thermal stability
(1−x)BaTiO3-xBi(Sr2/3Nb1/3)O3 [(1−x)BT-xBSN, 0.02 ≤ x ≤ 0.1] ceramics were synthesized by a traditional solid state reaction technique. The transition from tetragonal phase to pseudo cubic phase at 0.02 ≤ x ≤ 0.04 was observed in Raman spectra and X-ray diffraction patterns. As the BSN contents increased, the grain size increased. With adding Bi(Sr2/3Nb1/3)O3, the thermal stability of permittivity and dielectric loss of ceramics got a good optimization. Especially, 0.9BT-0.1BSN ceramics have small Δε/ε25°C values (≤ ± 15%) in a wide temperature range of −75 °C to 153 °C, high relative permittivity (∼1505 to 1700) and low dielectric loss (tan δ ≤ 0.02) from −88 °C to 200 °C, showing that BT-BSN ceramics are suitable for using in MLCCs of X8R. Impedance spectroscopy was analyzed the conduction and relaxation processes. The results showed that the relaxation and the conduction process in the high temperature region are thermally activated, and the oxygen vacancies are charge carriers.
1. Introduction
application. BaTiO3-Bi(Mg2/3Nb1/3)O3 have been behaved good temperaturestable dielectric in our previous work [12]. According to the above mentioned, it can be deduced that the substitution of Bi3+ for Ba and (Sr2+, Nb5+) for Ti can optimize the properties of BaTiO3, so Bi(Sr2/ 3Nb1/3)O3 was chosen to optimize properties of BaTiO3 ceramic in order to satisfy the X8R characteristics in this work. The (1−x)BaTiO3-xBi (Sr2/3Nb1/3)O3 solid solution ceramics were prepared by the solid state reaction method. Furthermore, the phase evolution, microstructure, impedance and dielectric properties of ceramics were also studied.
In recent years, the demand for multilayer ceramic capacitor (MLCC) in automotive applications has increased rapidly, such as antilock brake system, the engine electronic control unit, and programmed fuel injection [1,2]. But their working temperatures are likely to rise to about 150 °C, MLCC for the EIA (Electronic Industries Association) X7R (the change of the capacitance is less than 15% over the temperature range from −55 °C to 125 °C, Ɛ25°C = 4000–5000) cannot be employed [3]. Therefore, high temperature EIA X8R MLCCs (the temperature range from −55 °C to 150 °C, Ɛ25°C = 1000–2000) are being developed for the application of automotives [4]. Barium titanate (BaTiO3) perovskite is a main component to meet the X8R characteristic because of its excellent dielectric properties [5]. However, pure barium titanate has the noticeable changes of permittivity above the Curie temperature (Tc ∼ 130 °C), which cannot meet the temperature stability requirements in the working temperature. As an effective way to improve the material performance in electroceramics, the doping has attracted more and more attention [6,7]. So temperature-stable BaTiO3-based dielectric materials have been developed in the search for new high temperature dielectric materials [8,9], such as BaTiO3-Bi(Mg0.5Zr0.5)O3 [10], BaTiO3-Bi(Zn0.5Ti0.5)O3 [11], BaTiO3-Bi(Mg2/3Nb1/3)O3 [12], et al. These solid solutions exhibited outstanding performance over a wide range of temperature for MLCCs ⁎
2. Experimental (1−x)BaTiO3-xBi(Sr2/3Nb1/3)O3 [(1−x)BT-xBSN, 0.02 ≤ x ≤ 0.1] samples were synthesized by the traditional solid state reaction technique as described in our previous researches [12]. The samples were sintered at different temperatures for 2 h in air (1280–1380 °C). X-ray diffraction (XRD) patterns were measured at room temperature by an Xray diffractometer (XRD, Model X’Pert PRO; PANal ytical, Almelo, the Netherland) with CuKα radiation (λ = 0.15406 nm) operated at 40 kV and 40 mA with a step size of 0.02°. PanAlytical software (X’PertHighscore Plus) was analyzed the phase changes of XRD. Raman spectroscopy was carried out on a Thermo Fisher Scientific DXR using a 10 mW laser with a wave length of 532 nm. The microstructure of the
Corresponding author. E-mail address: [email protected] (X. Chen).
https://doi.org/10.1016/j.mseb.2018.12.022 Received 17 October 2017; Received in revised form 5 November 2018; Accepted 23 December 2018 0921-5107/ © 2018 Published by Elsevier B.V.
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samples sintered at their optimized temperatures was observed using a scanning electron microscopy (Model JSM6380-LV SEM, JEOL, Tokyo, Japan). The silver paste is evenly applied to both sides of the sample plate, and heated up to 700 °C at a heating rate of 3 °C/min then kept for 30 min. Dielectric properties were measured with an applied voltage of 500 mV over 100 Hz-1 MHz from −120 °C to 200 °C using a precision impedance analyzer (Model E4980AL, Hewlett-Packard Co, Palo Alto, CA) at a heating rate of 2 °C/min. And the impedance was also measured with an applied voltage of 500 mV over 40 Hz-1 MHz from 640 °C to 740 °C. 3. Results and discussion Fig. 1 illustrates the XRD patterns of (1−x)BaTiO3-xBi(Sr2/3Nb1/3) O3 [(1−x)BT-xBSN, 0 ≤ x ≤ 0.1] samples sintered at their optimized temperatures. The main crystal phase of (1−x)BT-xBSN ceramics (0 ≤ x ≤ 0.1) is similar to the previous report about BaTiO3 crystal structure [13,14], indicating that BSN has assimilated into the BT lattices to form a homogeneous solid solution with perovskite phase, as shown in Fig. 1(a). As 0 ≤ x ≤ 0.02, the ceramics exhibited the tetragonal phase, as shown in Fig. 1(b). The splitting of the diffraction peak near 2θ ∼ 45° almost disappeared, which the (0 0 2) and (2 0 0) peaks combined into a single (2 0 0) peak at x ≥ 0.04, indicating that the pseudocubic or cubic structures appeared [12]. Fig. 2 gave a better demonstration for the phase evolution by carrying out the Raman spectra for (1−x)BT-xBSN (0.02 ≤ x ≤ 0.1) ceramics in the frequency range of 100–1000 cm−1 at room-temperature. In Fig. 2, it is clearly visible that all samples for (1−x)BT-xBSN (0.02 ≤ x ≤ 0.1) ceramics exhibited similar spectra, which agreed well with previous report [15]. The main spectral features for (1−x)BTxBSN ceramics are five different bands at 717, 518, 306, 270 and 170 cm−1 at x = 0.02, which are the sign of the tetragonal BaTiO3 [16,17]. The positive intensity of Raman peak around 180 cm−1 (which was observed at x = 0.02) is characteristic of the tetragonal phase [15,18]. On the contrary, the other one (apperceived for x = 0.04, 0.06, 0.08, 0.1) is indicative of the orthorhombic phase. These results are in good agreement with the results of XRD analysis, suggesting that a homogenous solid solution was formed between the BSN and BT. In addition, spectral characteristics have undergone tremendous changes in this range. A disappearance at 170 cm−1 vanishes and a new Model 3 appears in its position. Model 3 is related to the vibrations of A-O,
Fig. 1. (a) X-ray diffraction patterns of (1−x)BT-xBSN (0 ≤ x ≤ 0.1) ceramics, (b) The enlarged XRD patterns of the samples in the range of 2θ from 43° to 47°.
Fig. 2. Room temperature Raman spectra of (1−x)BT-xBSN (0.02 ≤ x ≤ 0.1) ceramics.
Fig. 3. SEM micrographs of (1−x)BT-xBSN ceramic sintered at their optimized temperatures:((a) x = 0.02, 1360 °C (b) x = 0.04, 1340 °C (c) x = 0.08, 1300 °C (d) x = 0.1, 1280 °C. 131
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Fig. 4. The grain size of (1−x)BT-xBSN (0.02 ≤ x ≤ 0.1) ceramics sintered at their optimized temperatures: ((a) x = 0.02, 1360 °C (b) x = 0.04, 1340 °C (c) x = 0.08, 1300 °C (d) x = 0.1, 1280 °C.
obvious. So, the relative permittivity is attribution to the grain boundary effect. Impedance spectroscopy has been recognized as a powerful method. Separation technology contributes to the experimental techniques of grain, grain boundaries and interfaces [20–25]. In order to elucidate the dielectric behavior of 0.9BT-0.1BSN ceramics, the high-temperature dielectric relaxation was studied by the impedance spectroscopy. Fig. 5 shows the Nyquist plots for 0.9BT-0.1BSN ceramic at various temperatures. With the increase of temperature, the impedance decreases. It is worth noting that the sample showed an electrical inhomogeneity character by two semicircles at different measured temperatures. One is at low-frequency range, while the other is at high-frequency range. The grain effect response contributes to in the high-frequency semicircle, while the low-frequency semicircle is attributed to the grain boundary effect [26]. Fig. 6 represents the real (Z′) and imaginary (Z″) parts of the impedance for 0.9BT-0.1BSN ceramic. With increasing the frequency and temperature, the effects of the frequency and temperature on the Z′ and Z″ behaviors of the sample become clearly visible. As the frequency and temperature increase, the reduction of the Z′ and Z″ could be observed. And the values of these two parts at different temperatures are combined separately in the high frequency region, showing that the decrease in the barrier properties of the sample may be lead to the release of the space charge with rising the measured temperature. And it may be the dominant factor in the increase of ac conductivity for samples at high temperatures [27]. At lower frequency, higher impedance value is a characteristic of the space charge polarization of the materials, as demonstrated in Fig. 6(a). In Fig. 6(b), it can be seen that two peaks appeared in the imaginary part of impedance spectrum for the sample at six temperatures, corresponding to the two Debye-like semicircles. This result fully confirms that the conductivities of the materials were contributed by the grain boundary and grain. With increasing the measured temperature, the value of Z″ reaches a maximum and moves in a high frequency, widened and generalized. The widened peak hints the existence of temperature-dependent relaxation processes in the material due to the defects under high sintering temperature [28]. Fig. 6(b) shows the change in relaxation time as a function of the
Fig. 5. Nyquist plots of impedance for 0.9BT-0.1BSN ceramics at selected temperatures.
indicating the presence of Ba2+ or Bi3+ nano-sized regions [19]. Fig. 3 illustrates the SEM images of natural surface for (1−x)BTxBSN (x = 0.02, 0.04, 0.08, 0.1) ceramics sintered at their optimized temperatures. The few pores of (1−x)BT-xBSN ceramics were obtained at x = 0.02 in Fig. 3(a). As the BSN contents increased, the grain size increased slightly and pores decreased obviously, which indicates that BSN could enhance the grain growth of BT-BSN ceramics. In detail, the grain size of the (1−x)BT-xBSN (x = 0.02, 0.04, 0.08, 0.1) ceramics were showed in Fig. 4. In Fig. 4, it can be clearly observed that the uniformity of grain size was improved with increasing x values. And the average grain sizes are 0.72 µm, 0.94 µm, 1.19 µm, and 1.84 µm for BTBSN (x = 0.02, 0.04, 0.08, 0.1), respectively, indicating that the addition of BSN could enhance the grain growth of ceramics. And ceramics are divided into grain boundary and grain to deal with grain size effect. The relative permittivity at grain boundary is lower than that at grain boundary. As the grain size increased, the grain boundary effect is more 132
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Fig. 6. Frequency dependences of real part Z′ (a) and imaginary part Z″ (b) of impedance for 0.9BT-0.1BSN ceramics at selected temperatures. [Insert shows the Arrhenius plot for relaxation time. The dotted line through the data is a linear fit to Eq. (2).]
Fig. 7. Normalized imaginary parts Z″/Z″max of impedance as a function of frequency.
absolute temperature. In the relaxation system, the relaxation time (τ) can be calculated from the imaginary part (Z″) and the frequency (f) curve of the impedance [Fig. 6(b)]. The following relationship was used:
τ=
1 1 = ω 2πfmax
Fig. 8. (a) Frequency dependence of ac conductivity for 0.9BT-0.1BSN ceramics at selected temperatures. (b) Arrhenius plot of dc conductivity for the 0.9BT0.1BSN. Squares are experimental points and the dotted line is a linear fit to Eq. (4).
(1)
where fmax is the relaxation frequency. According to Eq. (1), the value of τ decreases with increasing temperature, which indicates a typical semiconductor behavior. The activation energy (Ea) of this compound
was calculated from Arrhenius relation:
τ = τ0 exp(Ea/ kB T ) 133
(2)
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Fig. 9. The optimized (0.02 ≤ x ≤ 0.1) ceramics.
sintering
temperature
of
(1−x)BT-xBSN
where τ0 is the preexponential factor, kB is the Boltzmann constant, Ea is the activation energy required for the relaxation, and T is the absolute temperature. From the slope of log τ vs 103 T−1, the activation energy of relaxation (Ea) was 1.7 eV. Fig. 7 illustrates the normalized imaginary part of the impedance Z″/Z″max as a function of the frequency. With the increase of temperature, the peaks of Z″/Z″max move to the high frequency. The change in the apparent polarization is represented by the magnitude of mismatch between the peaks of the normalized parameters [29]. As shown in Fig. 7, it is illustrated that the polarization process is caused by the local conductance of multiple carriers, which describe the presence of multiple relaxation processes in the material. It is a significant performance of short-range conductivity with different position and broad asymmetry in the peaks of normalized parameters [30]. The symmetry increases with increasing temperature, hinting that the charge carriers migrate from short distances to long distances. As the temperature increases, the reduction in material volume resistance proved the increase in long-distance conductivity, which can be easily found in the Nyquist plots. Fig. 8(a) demonstrated the frequency dependence of the ac
Fig. 11. (a) Δε/ε25°C of (1−x)BT-xBSN (x = 0.1) ceramics at 10, 100 kHz and 1 MHz from −120 °C to 200 °C, (b) Temperature dependence of relative permittivity, and dielectric loss of (1−x)BT-xBSN (x = 0.1) ceramics at 10, 100 kHz and 1 MHz from −120 °C to 200 °C.
conductivity (σac) for 0.9BT-0.1BSN ceramic. The σac increases with increasing frequency and temperature. With increasing the temperature, it can be seen that the dispersion of the conductivity appeared in low frequency. When the σac decreases a certain value in low frequency, it becomes independent of frequency. This phenomenon is attributed to the long-range translational hopping of charge carriers that is conducive to dc conductivity (σdc). So, the direct-current conductivity (σdc) can be generated by extrapolating this part towards lower frequency. Moreover, it could be expressed through the universal dielectric response law [31]:
σac = σdc + Aωn
(3)
where σdc is the dc conductivity, ω is the angular frequency of ac field. The A and n (0 < n < 1) are two temperature dependent adjusting constants. The temperature dependence of σdc was obtained by the
Fig. 10. Temperature dependence of relative permittivity and dielectric loss for (1−x)BT-xBSN (0.02 ≤ x ≤ 0.1) ceramics measured at 10, 100 kHz and 1 MHz. 134
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Arrhenius law:
σde = σ0 exp(−Edc / kB T )
(4)
[2]
where σ0 is the pre-exponential factor, Edc is the activation energy of conduction and will be calculated from the slope of log(σ) vs 1000/T. The value is 1.56 eV, and it is very close to the activation energy of relaxation (Ea), indicating that these two processes may be caused by the same factor of charge carriers, oxygen vacancies, as shown in the inserted of Fig. 6(b). According to Steinsvik’s investigation [32], ABO3 perovskite structure activation energy changes with the concentration of oxygen vacancies, the motion of oxygen vacancies gives rise to activation energy and is considered as one of the mobile charge carriers in perovskite ferroelectrics [33]. And the charge compensation follows the reaction with [34]: VO = VO″ + 2e′ or VO′ = VO″ + e′. The double charge oxygen vacancy is deemed to most mobile charge and makes an important impact on conduction. And the direct-current conductivity is generated by the long-range movement of the double ionization oxygen vacancy [18]. The optimized sintering temperature of (1−x)BT-xBSN (0.02 ≤ x ≤ 0.1) ceramics was shown in Fig. 9. It can be clearly seen that the optimized sintering temperature of the sample material was decreased with the increase of BSN content. This result indicates that the addition of BSN could reduce the sintering temperature of BaTiO3 based dielectric ceramic samples. Fig. 10 demonstrates the temperature dependences of the relative permittivity(εr) and dielectric loss (tan δ) of (1−x)BT-xBSN (0.02 ≤ x ≤ 0.1) ceramics in the measured temperature range from −120 °C to 200 °C at frequencies of 10, 100 kHz and 1 MHz. With increasing the BSN contents, the peak values of εr decreased from ∼5539 (x = 0.02, 10 kHz) to ∼1700 (x = 0.1, 10 kHz), the Δε/ε25°C and tan δ of ceramics got the ideal optimization. The outstanding performances with high εr (∼1505 to 1700), small Δε/ε25 °C values ( ± 15%) over a broad temperature range from −75 °C to 153 °C and tan δ ≤ 0.02 from −88 °C to 200 °C were obtained at x = 0.1. Therefore, the Δε/ε25°C, εr and tan δ of 0.9BT-0.1BSN ceramics at 10, 100 kHz and 1 MHz from −120 °C to 200 °C are illustrated in Fig. 11. These outstanding performances of 0.9BT-0.1BSN were observed in Fig. 11, hinting that BT-BSN ceramics are suitable for using in MLCCs of X8R.
[3]
[4]
[5]
[6]
[7]
[8] [9] [10]
[11]
[12]
[13] [14] [15]
[16] [17] [18] [19] [20]
[21] [22]
4. Conclusions
[23]
(1−x)BT-xBSN (0.02 ≤ x ≤ 0.1) lead-free ceramics have been investigated using the conventional solid-state processing techniques. The structural transformation of the system from tetragonal to pseudo cubic phase occurred at 0.02 ≤ x ≤ 0.04, which was observed in Raman spectra and X-ray diffraction patterns. As the BSN contents increased, the significant reduction in the peak εm values with the broadening of tetragonal-cubic transition peaks was observed. 0.9BT-0.1BSN ceramics, showing the outstanding dielectric performances with small Δε/ ε25°C values ( ± 15%) in temperature range of −75 °C to 153 °C, high εr (∼1505–1700) and low tan δ ≤ 0.02 from −88 °C to 200 °C. All results showed that the relaxation and the conduction process in the high temperature region are thermally activated, and the oxygen vacancies are charge carriers.
[24]
[25]
[26] [27] [28]
[29]
[30]
Acknowledgments
[31]
This work was supported by Natural Science Foundation of China (Nos. 11664008 and 61761015), Natural Science Foundation of Guangxi (Nos. 2017GXNSFFA198011 and 2017GXNSFDA198027), Research Start-up Funds Doctor of Guilin University of Technology (Nos. 002401003281 and 002401003282).
[32] [33] [34]
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