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High-temperature dielectrics based on 0.95(0.94Bi0.5Na0.5TiO3-0.07BiAlO3)-0.05K0.5Na0.5NbO3
Ceramics International 45 (2019) 12360–12365
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High-temperature dielectrics based on 0.95(0.94Bi0.5Na0.5TiO30.07BiAlO3)-0.05K0.5Na0.5NbO3
T
Xin Wanga, Huiqing Fanb, Pengrong Renc,∗, Kun Liud a
Laboratory of Thin Film Techniques and Optical Test, School of Photoelectrical Engineering, Xi'an Technological University, Xi'an 710032, PR China State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an 710072, PR China c Shaanxi Province Key Laboratory for Electrical Materials and Infiltration Technology, School of Materials Science and Engineering, Xi'an University of Technology, Xi'an 710048, PR China d Tianjin Long March Launch Vehicle Manufacturing Co. Ltd, Tianjin 300462, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: High temperature capacitor Bi0.5Na0.5TiO3 Temperature stability
The high temperature capacitors operated in some harsh conditions require dielectric ceramics with stable permittivity, low dielectric loss and high resistivity to temperatures above 200 °C. In this work, high temperature dielectrics based on (1-x-y)Bi0.5Na0.5TiO3-xBiAlO3-yK0.5Na0.5NbO3 ceramics were designed and prepared. The introduction of BiAlO3 leads to the reduction of dielectric loss due to the formation of defect dipoles. And the introduction of K0.5Na0.5NbO3 leads to the disruption of long-range ferroelectric order, which is helpful to increase the temperature stability of permittivity. The composition of x = 0.07 and y = 0.05 has a stable permittivity between 86.5 °C and 324.5 °C and dielectric loss below 0.02 in the temperature range of 110 °C–336 °C. Therefore, this system will be one of promising candidates of dielectrics used for high-temperature capacitors.
1. Introduction Multi-layer ceramic capacitors (MLCC) have very important applications in electronic components [1,2]. Conventional ceramics capacitors, such as X7R, X8R, X9R and so on, capable of operation below 200 °C [3,4]. However, with the demand of electronic controls and sensing systems operated in harsh environmental conditions, for example, aviation and aerospace, automotive engine, deep hole drilling in oil and gas industries, development of high-temperature capacitors is imminent [5]. The high temperature capacitors require dielectric ceramics with stable permittivity, low dielectric loss and high resistivity to temperatures above 200 °C. Therefore, high temperature dielectrics have aroused intensive interest of researchers in recent years. Bi0.5Na0.5TiO3 (BNT) is a perovskite structure and has high Curie temperature (Tc). It is rhombohedral structure (R3c) at room temperature, then turns into tetragonal structure (P4bm) at 300 °C and finally transforms to cubic structure (Pm3¯m at 540 °C [6]. In order to stabilize permittivity in a wide temperature range, many BNT-based ceramics have been developed, like Bi0.5Na0.5TiO3eBaTiO3eCaZrO3 [7], Bi0.5Na0.5TiO3eBaTiO3eNaNbO3 (BNT-BT-NN) [8] Bi0.5Na0.5TiO3eBaTiO3eK0.5Na0.5NbO3 (BNT-BT-KNN) [9], Bi0.5Na0.5TiO3eNaNbO3eCaZrO3 (BNTeNNeCZ) [10] and so on. However, although the temperature stability is improved by
∗
incorporation of K0.5Na0.5NbO3, NaNbO3 and CaZrO3, the dielectric loss of these systems is still too high to operation at high temperature. For BNT-based ceramics, bismuth evaporation is unavoidable during sintering, leading to the unbalanced charges, which are compensated by oxygen vacancies. It has been proved that even pure BNT behaves oxidized conductor due to the evaporation of bismuth [11]. As a result, BNT based ceramics have high conductivity and high dielectric loss at high temperature. Therefore, in order to reduce the dielectric loss of BNT at high temperature, there are two optional approaches: one is reducing the concentration of oxygen vacancy, the other is restraining the mobility of oxygen vacancy. Derek [12] reported that substitution of 7% mol BiAlO3 in BNT can effectively restrain the mobility of oxygen vacancies, and thus depress the dielectric loss at high temperature. The acceptor defects (AI′Ti ) and oxygen vacancies (V•• O ) can form defects • dipoles (Al′Ti − V•• O ) , and thus limit the long-range mobility of oxygen vacancies [12], which provides an effective way to reduce the dielectric loss of BNT-based ceramics at high temperature. Therefore, in our work, high temperature dielectrics based on the composition of (1-x-y)Bi0.5Na0.5TiO3-xBiAlO3-yK0.5Na0.5NbO3 (BNTBA-KNN) were designed and prepared. The introduction of BiAlO3 is to reduce the dielectric loss of BNT and K0.5Na0.5NbO3 is to improve the temperature stability of permittivity. The composition of x = 0.07 and y = 0.05 has a stable permittivity between 86.5 °C and 324.5 °C and
Corresponding author. E-mail address: [email protected] (P. Ren).
https://doi.org/10.1016/j.ceramint.2019.03.161 Received 3 February 2019; Received in revised form 17 March 2019; Accepted 21 March 2019 Available online 23 March 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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dielectric loss below 0.02 between 110 °C and 336 °C. Therefore, there is a relative wide temperature range which has both stable permittivity and lower dielectric loss. 2. Experimental procedure (1-x-y)Bi0.5Na0.5TiO3-xBiAlO3-yK0.5Na0.5NbO3 (x = 0.07, y = 0.05, 0.10 and 0.15; labelled as BNT-07BA-05KNN, BNT-07BA-10KNN and BNT-07BA-15KNN) ceramics were prepared by using a solid-state reaction method with Na2CO3, Bi2O3, TiO2, Al2O3, K2CO3 and Nb2O5 (purity > 98.5%, Sinopharm Co. Ltd) as raw materials. The powders were weighted by a stoichiometric ratio and then mixed for 12 h with a rotation speed of 450 r/min by using a planetary ball milling machine with zirconia balls and ethanol as medium. Subsequently, the powders were dried and calcined at 850 °C for 4 h. After that, the calcinated powders were milled again for 12 h and dried. The dried powders were compacted into pellets in thickness of 1 mm and diameter of 10 mm and further pressed at 200 MPa for 5min using a cold isostatic press machine. Finally, the pellets were sintered at 1150 °C for 2 h. In order to make a comparison, pure Bi0.5Na0.5TiO3 (BNT), 0.93Bi0.5Na0.5TiO30.07BiAlO3 (BNT-07BA), 0.95Bi0.5Na0.5TiO3-0.05K0.5Na0.5NbO3 (BNT05KNN) were also prepared in a similar process, except that the sintering temperatures were 1175 °C, 1150 °C and 1175 °C, respectively. The phase and purity of the ceramics were studied by X-ray diffraction (XRD) (D/Max2550VBþ/PC, Rigaku, Tokyo, Japan). The microstructure such as grain size and homogeneity were investigated by using a field-emission scanning electron microscopy (FE-SEM) (JEOL6700F, Japan Electron Co., Tokyo, Japan). The FE-SEM samples were thermally etched at 1000 °C for 30 min. Silver electrodes were painted on both sides of the polished surfaces and heated at 600 °C for 30 min. Temperature dependent permittivity (ε′) and loss tangent (tanδ) were measured by a precision LCR meter (E4294A, Agilent, Santa Clara, USA) from 25 °C to 500 °C. The temperature and frequency dependent impedance spectra (IS) were determined by an impedance analyzer (Novocontrol alpha A, Novocontrol Technologies, Hund-sangen, Germany). The polarization hysteresis loops (P-E) and electric fieldinduced strain hysteresis loops (S-E) were measured using a ferroelectric test unit analyzer (TF-2000, AixACCT, Aachen, Germany) under 60 kV/cm at 1 Hz and room temperature. 3. Results and discussions Fig. 1 shows XRD patterns of BNT, BNT-07BA, BNT-05KNN and BNT-07BA-05KNN ceramics. As shown in Fig. 1(a), all the samples show single-phase perovskite structure. The substitution of BiAlO3 leads to the shift of the diffraction peak to the higher angle while substitution of K0.5Na0.5NbO3 leads to the shift of the diffraction peak to the lower angle, which can be explained as following. The radius of Ti4+ (0.605 Å) [13] is larger than that of Al3+ (0.51 Å) [14] and smaller than that of Nb5+ (0.70 Å) [15], therefore the substitution of Ti4+ by Al3+
Fig. 1. XRD patterns of: (I) BNT, (II) BNT-07BA, (III) BNT-05KNN and (IV) BNT07BA-05KNN in the 2θ range of (a) 20o-80°, (b) 39.5o-41.5° and (c) 46o-48°.
leads to the shrinkage of cell volume, and the substitution of Ti4+ by Nb5+ leads to the expansion of cell volume. The expanded diffraction region between 39.5o-41.5° and 46o-48° are shown in Fig. 1(b) and (c). Pure BNT shows a typical rhombohedral structure, with a shoulder in (110) diffraction peak and a symmetrical peak in (002) diffraction peak. It is difficult to determine the phase structure from the current diffraction peaks of (110) and (002) for BNT-07BA and BNT-05KNN. But from the results reported by Yang [12] and Kounga [16], BNT-07BA and BNT-05KNN are rhombohedral structure at room temperature. For BNT-07BA-05KNN, there is a clearly splitting in (110) and (002) diffraction peaks, suggesting that it might be located in an MPB region, with rhombohedral and tetragonal coexisting structure. SEM images of BNT, BNT-07BA, BNT-05KNN and BNT-07BA05KNN ceramics are shown in Fig. 2. All the ceramics show dense microstructure. After substitution of BiAlO3 and K0.5Na0.5NbO3, the average grain size decreases a lot. The average grain size is about 12.01, 1.11, 2.78 and 0.91 μm for BNT, BNT-07BA, BNT-05KNN and BNT07BA-05KNN, respectively. The decrease of grain size can be ascribed to two possible reasons: (1) there are some BiAlO3 and K0.5Na0.5NbO3 distributed in the grain boundary, which inhibits the mobility of grain boundary during sintering; (2) the concentration or mobility of oxygen vacancies is depressed after the substitution of BiAlO3 and K0.5Na0.5NbO3, leading to the decrease of diffusion rate during sintering. Because the second phase was not observed in the XRD pattern, we believe that the latter might be the dominant reason. XRD patterns of BNT-07BA-10KNN and BNT-07BA-15KNN were also detected and are shown in Fig. S1. From Fig. S1, it seems that BNT-07BA-10KNN and BNT-07BA-15KNN are pure phase, but from SEM images of BNT-07BA10KNN and BNT-07BA-15KNN, as shown in Fig. S2, the second phase is present in the ceramics. Therefore, dielectric properties of BNT-07BA10KNN and BNT-07BA-15KNN were not studied in our work. The temperature dependent ε′ and tanδ of BNT, BNT-07BA, BNT05KNN and BNT-07BA-05KNN ceramics measured at 0.1 kHz, 1 kHz, 10 kHz and 100 kHz are displayed in Fig. 3(a–f). Pure BNT has a dielectric peak at about 300 °C. Beyond 300 °C, temperature dependent ε′ shows large frequency dispersion, which is attributed to the high conductivity at high temperature in BNT. BNT-07BA has a broad dielectric peak in the permittivity versus temperature curve, which might be due to the compositional fluctuation in a local region after substitution of BiAlO3 in BNT. Compared to dielectric loss of pure BNT, the enhancement of dielectric loss of BNT-07BA is postponed to higher temperature (above 250 °C) for each frequency, which further confirms that defect dipoles are formed in BiAlO3-substituted BNT, depressing the dielectric loss at high temperature. At 1 kHz and 300 °C, the dielectric loss of BNT07BA is only 0.00994, while the dielectric loss of pure BNT reaches 0.24857. Dielectric spectra of BNT-05KNN and BNT-07BA-05KNN exhibit two dielectric anomalies at lower temperature (Ts) and higher temperature (Tm), which are in agreement with BNT-BT based system reported in literature [17–19]. The dielectric anomaly at Ts is probably ascribed to the transition from lower symmetry PNRs to higher symmetry ones, and the dielectric anomaly at Tm is due to the remaining high symmetry PNRs [20]. After substitution of KNN in BNT-07BA, permittivity at Tm gradually decreases and the dielectric peak at Tm is broadened, which suggests that the long-range ferroelectric order is further disrupted by substitution of KNN, and thus the relaxor phase becomes dominant in the BNT-07BA-05KNN ceramics. Compared to BNT-05KNN, dielectric loss of BNT-07BA-05KNN also reduces. For BNT-07BA-05KNN, the temperature range of dielectric loss below 0.02 is 110–336 °C, which is much wider than that of other systems. Temperature stability of permittivity is very important for capacitors. The variation of permittivity between 25 °C and 500 °C is investigated. Fig. 4 shows Δε'/ε′150 °C (Δε' = ε' – ε′150 °C) as a function of temperature for BNT, BNT-07BA, BNT-05KNN and BNT-07BA-05KNN ceramics at 1 kHz. Especially, BNT-07BA-05KNN exhibits the widest temperature range (86.5 °C–324.5 °C) according to the standard of variation of permittivity (Δε'/ε′150 °C) less than 15%. Table 1
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Fig. 2. SEM images of: (a) BNT, (b) BNT-07BA, (c) BNT-05KNN and (d) BNT-07BA-05KNN on thermal-etched surface.
summarizes dielectric properties of BNT-07BA-05KNN ceramics at 1 kHz and room temperature. Our results are also compared with other systems reported in the literature [8,21,22]. These results indicate that BNT-07BA-05KNN is a good composition for ceramic capacitors
operated at high temperature. In order to study the effects oxygen vacancy concentrations on the dielectric properties, BNT-07BA-05KNN is annealed in different atmospheres, N2, air and O2 at 700 °C for 6 h. Dielectric spectra of BNT-
Fig. 3. Temperature dependent ε′ and tanδ over 25–500 °C for (a, b) BNT, (c, d) BNT-07BA, (e, f) BNT-05KNN and (g, h) BNT-07BA-05KNN. 12362
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Fig. 4. Δε'/ε150°C (Δε' = ε' - ε150°C) as a function of temperature for BNT-xBAyKNN ceramics at 1 kHz.
07BA-05KNN annealed in different atmospheres are shown in Fig. 5. In general, permittivity and dielectric loss are less dependent on atmosphere except two different points: (1) compared to the sample annealed in air, permittivity is lower for the sample annealed in N2 while higher in O2, especially in the temperature range of two dielectric anomalies; (2) the sample annealed in O2 have relatively lower dielectric loss. The temperature range of dielectric loss below 0.02 exceeds to 374 °C for BNT-07BA-05KNN annealed in O2, which might be attributed to the reason that the oxygen vacancy concentration decreases after annealing in O2. In order to clarify the reduction of dielectric loss in BNT, BNT-07BA, BNT-05KNN and BNT-07BA-05KNN ceramics, the complex impedance spectra were measured in the temperature range of 400 °C–550 °C. Fig. 6(a) displays typical impedance spectra at 550 °C for all the samples, which illustrates a single semi-circle, indicating that the samples have an electrical-homogeneous nature. There is no spike at low frequency, which suggest that the conduction mechanism is dominated by the electronic conduction [23]. Fig. 6(b) illustrates the Arrhenius plot of BNT, BNT-07BA, BNT-05KNN and BNT-07BA-05KNN in the temperature range of 450 °C–550 °C. In this temperature range, logσ as a function of 1000/T for BNT and BNT-05KNN shows a linear behavior. The activation energy of conductivity for the BNT and BNT 05KNN is 1.16 (1) eV 1.10 (1) eV, respectively. However, there is a kink in the Arrhenius plot of BNT-07BA and BNT-07BA-05KNN, which can be divided into two regions. At lower temperature, the activation energy of conductivity is about 0.69 (1) eV and 0.79 (1) eV for BNT-07BA and BNT-07BA-05KNN. At higher temperature, the activation energy of conductivity is about 0.29 (1) eV and 0.45 (1) eV. Compared to the conductivities of BNT and BNT-05KNN, conductivities of BNT-07BA and BNT-07BA-05KNN are much lower, which are ascribed to the reason that the defect dipoles limit the mobility of oxygen vacancies. Therefore, it is rationalized that BNT-07BA and BNT-07BA-05KNN have lower dielectric loss compared with other samples. The reason for the appearance of the kink in the Arrhenius plot of BNT-07BA and BNT07BA-05KNN is not clear now. It might be related to the dissociation of defect dipoles at high temperature, which will be studied further in our further work.
Fig. 5. (a) The complex impedance spectra of BNT-xBA-yKNN ceramics measured at 550 °C; (b) Arrhenius plot for BNT-xBA-yKNN ceramics measured in the temperature range between 450 °C to 550 °C.
Fig. 6. (a) P-E and S-E loops of BNT-xBA-yKNN ceramics under 60 kV/cm at 1 Hz and room temperature.
Fig. 7 shows the P-E and S-E loops of BNT, BNT-07BA, BNT-05KNN and BNT-07BA-05KNN ceramics under 60 kV/cm at 1 Hz and room temperature. As shown in Fig. 7(a), the P-E loop of BNT displays a well-
Table 1 Summary of dielectric properties of BNT-07BA-05KNN, BNT-07BA-05KNN annealed in O2 (BNT-07BA-05KNNeO2), 0.85(0.94Bi0.51Na0.5TiO3-0.06BaTiO3)0.15CaZrO3(B0.51NT-BT-15CZ) [21], 0.85 (0.94Bi0.5Na0.5TiO3-0.06BaTiO3)-0.15K0.5Na0.5NbO3 (BNT-BT-9KNN) [22], 0.95 (0.94Bi0.5Na0.5TiO3-0.06BaTiO3)0.05NaNbO3 (BNT-BT-0.05NN) [8] and 0.85 (0.94Bi0.5Na0.5TiO3-0.06BaTiO3)-0.15NaNbO3 (BNT-BT-0.15NN) [8]. Samples
ε' (150 °C, 1 kHz)
tanδ (150 °C 1 kHz)
T range of Δε'/ε′150 °C ≤ 15% (1 kHz)
T range of tanδ ≤ 0.02 (1 kHz)
References
BNT-07BA-05KNN BNT-07BA-05KNNeO2 B0.51NT-BT-15CZ BNT-BT-9KNN BNT-BT-0.05NN BNT-BT-0.15NN
2277 2399 882.6 ∼2800 3261 2004
0.009 0.007 ∼0.02 ∼0.01 0.01 ∼0.009
86.5–324.5 °C 81–318 °C 60–476 °C 39–343 °C 82–362 °C 35–300 °C
110–336 °C 107–373 °C 106–356 °C 100–300 °C 110–267 °Ca 71–244 °Ca
our work our work [20] [21] [8] [8]
a
Temperature range of tanδ ≤ 0.025. 12363
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Fig. 7. The hysteresis loops (P-E) and electric field-induced strain loops (S-E) of: (a) BNT, (b) BNT-07BA, (c) BNT-05KNN and (d) BNT-07BA-05KNN ceramics under the electric field of 60 kV cm-1 at 1 Hz and room temperature.
saturated loop, characteristic of normal ferroelectrics. After substitution of BiAlO3 and K0.5Na0.5NbO3, as shown in Fig. 7(b and c) the remnant polarization and electrostrain decrease, but the P-E and S-E loops show a much sharp shape, implying that the samples have lower leakage compared to BNT. Fig. 7(d) shows P-E and S-E loops of BNT-07BA05KNN. It shows a very slim loop and the electrostrain is much lower, characteristic of ferroelectric relaxors. The decrease of polarization and electrostrain further confirms that the long-range ferroelectric order is disrupted by the substitution of BiAlO3 and K0.5Na0.5NbO3 in BNT, which contributes on the improvement of stabilized permittivity. 4. Conclusions High temperature dielectrics based on the composition of (1-x-y) Bi0.5Na0.5TiO3-xBiAlO3-yK0.5Na0.5NbO3 were designed and prepared. The introduction of BiAlO3 is beneficial to the reduction of dielectric loss and the introduction of K0.5Na0.5NbO3 favors to improvement of the temperature stability of BNT-based ceramics. The composition of x = 0.07 and y = 0.05 has a stable permittivity between 86.5 °C and 324.5 °C and dielectric loss below 0.02 between 110 °C and 336 °C. Therefore, our work provides a promising candidate of dielectrics used for high-temperature capacitors. Acknowledgements This work was financially supported by National Natural Science Foundation of China (51802246), Natural Science Basic Research Plan in Shaanxi Province of China (2018JQ5110), State Key Laboratory of Solidification Processing in NWPU (SKLSP201839), Fund Program of the Scientific Activities of Selected Returned Overseas Professionals in Shaanxi Province, Special Scientific Research Plan Projects of Shaanxi Education Department (17JK0382) and Scientific and Technological Project of Yulin City (2016-16-6). Appendix A. Supplementary data Supplementary data related to this article can be found at https://
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High-temperature dielectrics based on 0.95(0.94Bi0.5Na0.5TiO30.07BiAlO3)-0.05K0.5Na0.5NbO3
T
Xin Wanga, Huiqing Fanb, Pengrong Renc,∗, Kun Liud a
Laboratory of Thin Film Techniques and Optical Test, School of Photoelectrical Engineering, Xi'an Technological University, Xi'an 710032, PR China State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an 710072, PR China c Shaanxi Province Key Laboratory for Electrical Materials and Infiltration Technology, School of Materials Science and Engineering, Xi'an University of Technology, Xi'an 710048, PR China d Tianjin Long March Launch Vehicle Manufacturing Co. Ltd, Tianjin 300462, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: High temperature capacitor Bi0.5Na0.5TiO3 Temperature stability
The high temperature capacitors operated in some harsh conditions require dielectric ceramics with stable permittivity, low dielectric loss and high resistivity to temperatures above 200 °C. In this work, high temperature dielectrics based on (1-x-y)Bi0.5Na0.5TiO3-xBiAlO3-yK0.5Na0.5NbO3 ceramics were designed and prepared. The introduction of BiAlO3 leads to the reduction of dielectric loss due to the formation of defect dipoles. And the introduction of K0.5Na0.5NbO3 leads to the disruption of long-range ferroelectric order, which is helpful to increase the temperature stability of permittivity. The composition of x = 0.07 and y = 0.05 has a stable permittivity between 86.5 °C and 324.5 °C and dielectric loss below 0.02 in the temperature range of 110 °C–336 °C. Therefore, this system will be one of promising candidates of dielectrics used for high-temperature capacitors.
1. Introduction Multi-layer ceramic capacitors (MLCC) have very important applications in electronic components [1,2]. Conventional ceramics capacitors, such as X7R, X8R, X9R and so on, capable of operation below 200 °C [3,4]. However, with the demand of electronic controls and sensing systems operated in harsh environmental conditions, for example, aviation and aerospace, automotive engine, deep hole drilling in oil and gas industries, development of high-temperature capacitors is imminent [5]. The high temperature capacitors require dielectric ceramics with stable permittivity, low dielectric loss and high resistivity to temperatures above 200 °C. Therefore, high temperature dielectrics have aroused intensive interest of researchers in recent years. Bi0.5Na0.5TiO3 (BNT) is a perovskite structure and has high Curie temperature (Tc). It is rhombohedral structure (R3c) at room temperature, then turns into tetragonal structure (P4bm) at 300 °C and finally transforms to cubic structure (Pm3¯m at 540 °C [6]. In order to stabilize permittivity in a wide temperature range, many BNT-based ceramics have been developed, like Bi0.5Na0.5TiO3eBaTiO3eCaZrO3 [7], Bi0.5Na0.5TiO3eBaTiO3eNaNbO3 (BNT-BT-NN) [8] Bi0.5Na0.5TiO3eBaTiO3eK0.5Na0.5NbO3 (BNT-BT-KNN) [9], Bi0.5Na0.5TiO3eNaNbO3eCaZrO3 (BNTeNNeCZ) [10] and so on. However, although the temperature stability is improved by
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incorporation of K0.5Na0.5NbO3, NaNbO3 and CaZrO3, the dielectric loss of these systems is still too high to operation at high temperature. For BNT-based ceramics, bismuth evaporation is unavoidable during sintering, leading to the unbalanced charges, which are compensated by oxygen vacancies. It has been proved that even pure BNT behaves oxidized conductor due to the evaporation of bismuth [11]. As a result, BNT based ceramics have high conductivity and high dielectric loss at high temperature. Therefore, in order to reduce the dielectric loss of BNT at high temperature, there are two optional approaches: one is reducing the concentration of oxygen vacancy, the other is restraining the mobility of oxygen vacancy. Derek [12] reported that substitution of 7% mol BiAlO3 in BNT can effectively restrain the mobility of oxygen vacancies, and thus depress the dielectric loss at high temperature. The acceptor defects (AI′Ti ) and oxygen vacancies (V•• O ) can form defects • dipoles (Al′Ti − V•• O ) , and thus limit the long-range mobility of oxygen vacancies [12], which provides an effective way to reduce the dielectric loss of BNT-based ceramics at high temperature. Therefore, in our work, high temperature dielectrics based on the composition of (1-x-y)Bi0.5Na0.5TiO3-xBiAlO3-yK0.5Na0.5NbO3 (BNTBA-KNN) were designed and prepared. The introduction of BiAlO3 is to reduce the dielectric loss of BNT and K0.5Na0.5NbO3 is to improve the temperature stability of permittivity. The composition of x = 0.07 and y = 0.05 has a stable permittivity between 86.5 °C and 324.5 °C and
Corresponding author. E-mail address: [email protected] (P. Ren).
https://doi.org/10.1016/j.ceramint.2019.03.161 Received 3 February 2019; Received in revised form 17 March 2019; Accepted 21 March 2019 Available online 23 March 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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dielectric loss below 0.02 between 110 °C and 336 °C. Therefore, there is a relative wide temperature range which has both stable permittivity and lower dielectric loss. 2. Experimental procedure (1-x-y)Bi0.5Na0.5TiO3-xBiAlO3-yK0.5Na0.5NbO3 (x = 0.07, y = 0.05, 0.10 and 0.15; labelled as BNT-07BA-05KNN, BNT-07BA-10KNN and BNT-07BA-15KNN) ceramics were prepared by using a solid-state reaction method with Na2CO3, Bi2O3, TiO2, Al2O3, K2CO3 and Nb2O5 (purity > 98.5%, Sinopharm Co. Ltd) as raw materials. The powders were weighted by a stoichiometric ratio and then mixed for 12 h with a rotation speed of 450 r/min by using a planetary ball milling machine with zirconia balls and ethanol as medium. Subsequently, the powders were dried and calcined at 850 °C for 4 h. After that, the calcinated powders were milled again for 12 h and dried. The dried powders were compacted into pellets in thickness of 1 mm and diameter of 10 mm and further pressed at 200 MPa for 5min using a cold isostatic press machine. Finally, the pellets were sintered at 1150 °C for 2 h. In order to make a comparison, pure Bi0.5Na0.5TiO3 (BNT), 0.93Bi0.5Na0.5TiO30.07BiAlO3 (BNT-07BA), 0.95Bi0.5Na0.5TiO3-0.05K0.5Na0.5NbO3 (BNT05KNN) were also prepared in a similar process, except that the sintering temperatures were 1175 °C, 1150 °C and 1175 °C, respectively. The phase and purity of the ceramics were studied by X-ray diffraction (XRD) (D/Max2550VBþ/PC, Rigaku, Tokyo, Japan). The microstructure such as grain size and homogeneity were investigated by using a field-emission scanning electron microscopy (FE-SEM) (JEOL6700F, Japan Electron Co., Tokyo, Japan). The FE-SEM samples were thermally etched at 1000 °C for 30 min. Silver electrodes were painted on both sides of the polished surfaces and heated at 600 °C for 30 min. Temperature dependent permittivity (ε′) and loss tangent (tanδ) were measured by a precision LCR meter (E4294A, Agilent, Santa Clara, USA) from 25 °C to 500 °C. The temperature and frequency dependent impedance spectra (IS) were determined by an impedance analyzer (Novocontrol alpha A, Novocontrol Technologies, Hund-sangen, Germany). The polarization hysteresis loops (P-E) and electric fieldinduced strain hysteresis loops (S-E) were measured using a ferroelectric test unit analyzer (TF-2000, AixACCT, Aachen, Germany) under 60 kV/cm at 1 Hz and room temperature. 3. Results and discussions Fig. 1 shows XRD patterns of BNT, BNT-07BA, BNT-05KNN and BNT-07BA-05KNN ceramics. As shown in Fig. 1(a), all the samples show single-phase perovskite structure. The substitution of BiAlO3 leads to the shift of the diffraction peak to the higher angle while substitution of K0.5Na0.5NbO3 leads to the shift of the diffraction peak to the lower angle, which can be explained as following. The radius of Ti4+ (0.605 Å) [13] is larger than that of Al3+ (0.51 Å) [14] and smaller than that of Nb5+ (0.70 Å) [15], therefore the substitution of Ti4+ by Al3+
Fig. 1. XRD patterns of: (I) BNT, (II) BNT-07BA, (III) BNT-05KNN and (IV) BNT07BA-05KNN in the 2θ range of (a) 20o-80°, (b) 39.5o-41.5° and (c) 46o-48°.
leads to the shrinkage of cell volume, and the substitution of Ti4+ by Nb5+ leads to the expansion of cell volume. The expanded diffraction region between 39.5o-41.5° and 46o-48° are shown in Fig. 1(b) and (c). Pure BNT shows a typical rhombohedral structure, with a shoulder in (110) diffraction peak and a symmetrical peak in (002) diffraction peak. It is difficult to determine the phase structure from the current diffraction peaks of (110) and (002) for BNT-07BA and BNT-05KNN. But from the results reported by Yang [12] and Kounga [16], BNT-07BA and BNT-05KNN are rhombohedral structure at room temperature. For BNT-07BA-05KNN, there is a clearly splitting in (110) and (002) diffraction peaks, suggesting that it might be located in an MPB region, with rhombohedral and tetragonal coexisting structure. SEM images of BNT, BNT-07BA, BNT-05KNN and BNT-07BA05KNN ceramics are shown in Fig. 2. All the ceramics show dense microstructure. After substitution of BiAlO3 and K0.5Na0.5NbO3, the average grain size decreases a lot. The average grain size is about 12.01, 1.11, 2.78 and 0.91 μm for BNT, BNT-07BA, BNT-05KNN and BNT07BA-05KNN, respectively. The decrease of grain size can be ascribed to two possible reasons: (1) there are some BiAlO3 and K0.5Na0.5NbO3 distributed in the grain boundary, which inhibits the mobility of grain boundary during sintering; (2) the concentration or mobility of oxygen vacancies is depressed after the substitution of BiAlO3 and K0.5Na0.5NbO3, leading to the decrease of diffusion rate during sintering. Because the second phase was not observed in the XRD pattern, we believe that the latter might be the dominant reason. XRD patterns of BNT-07BA-10KNN and BNT-07BA-15KNN were also detected and are shown in Fig. S1. From Fig. S1, it seems that BNT-07BA-10KNN and BNT-07BA-15KNN are pure phase, but from SEM images of BNT-07BA10KNN and BNT-07BA-15KNN, as shown in Fig. S2, the second phase is present in the ceramics. Therefore, dielectric properties of BNT-07BA10KNN and BNT-07BA-15KNN were not studied in our work. The temperature dependent ε′ and tanδ of BNT, BNT-07BA, BNT05KNN and BNT-07BA-05KNN ceramics measured at 0.1 kHz, 1 kHz, 10 kHz and 100 kHz are displayed in Fig. 3(a–f). Pure BNT has a dielectric peak at about 300 °C. Beyond 300 °C, temperature dependent ε′ shows large frequency dispersion, which is attributed to the high conductivity at high temperature in BNT. BNT-07BA has a broad dielectric peak in the permittivity versus temperature curve, which might be due to the compositional fluctuation in a local region after substitution of BiAlO3 in BNT. Compared to dielectric loss of pure BNT, the enhancement of dielectric loss of BNT-07BA is postponed to higher temperature (above 250 °C) for each frequency, which further confirms that defect dipoles are formed in BiAlO3-substituted BNT, depressing the dielectric loss at high temperature. At 1 kHz and 300 °C, the dielectric loss of BNT07BA is only 0.00994, while the dielectric loss of pure BNT reaches 0.24857. Dielectric spectra of BNT-05KNN and BNT-07BA-05KNN exhibit two dielectric anomalies at lower temperature (Ts) and higher temperature (Tm), which are in agreement with BNT-BT based system reported in literature [17–19]. The dielectric anomaly at Ts is probably ascribed to the transition from lower symmetry PNRs to higher symmetry ones, and the dielectric anomaly at Tm is due to the remaining high symmetry PNRs [20]. After substitution of KNN in BNT-07BA, permittivity at Tm gradually decreases and the dielectric peak at Tm is broadened, which suggests that the long-range ferroelectric order is further disrupted by substitution of KNN, and thus the relaxor phase becomes dominant in the BNT-07BA-05KNN ceramics. Compared to BNT-05KNN, dielectric loss of BNT-07BA-05KNN also reduces. For BNT-07BA-05KNN, the temperature range of dielectric loss below 0.02 is 110–336 °C, which is much wider than that of other systems. Temperature stability of permittivity is very important for capacitors. The variation of permittivity between 25 °C and 500 °C is investigated. Fig. 4 shows Δε'/ε′150 °C (Δε' = ε' – ε′150 °C) as a function of temperature for BNT, BNT-07BA, BNT-05KNN and BNT-07BA-05KNN ceramics at 1 kHz. Especially, BNT-07BA-05KNN exhibits the widest temperature range (86.5 °C–324.5 °C) according to the standard of variation of permittivity (Δε'/ε′150 °C) less than 15%. Table 1
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Fig. 2. SEM images of: (a) BNT, (b) BNT-07BA, (c) BNT-05KNN and (d) BNT-07BA-05KNN on thermal-etched surface.
summarizes dielectric properties of BNT-07BA-05KNN ceramics at 1 kHz and room temperature. Our results are also compared with other systems reported in the literature [8,21,22]. These results indicate that BNT-07BA-05KNN is a good composition for ceramic capacitors
operated at high temperature. In order to study the effects oxygen vacancy concentrations on the dielectric properties, BNT-07BA-05KNN is annealed in different atmospheres, N2, air and O2 at 700 °C for 6 h. Dielectric spectra of BNT-
Fig. 3. Temperature dependent ε′ and tanδ over 25–500 °C for (a, b) BNT, (c, d) BNT-07BA, (e, f) BNT-05KNN and (g, h) BNT-07BA-05KNN. 12362
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Fig. 4. Δε'/ε150°C (Δε' = ε' - ε150°C) as a function of temperature for BNT-xBAyKNN ceramics at 1 kHz.
07BA-05KNN annealed in different atmospheres are shown in Fig. 5. In general, permittivity and dielectric loss are less dependent on atmosphere except two different points: (1) compared to the sample annealed in air, permittivity is lower for the sample annealed in N2 while higher in O2, especially in the temperature range of two dielectric anomalies; (2) the sample annealed in O2 have relatively lower dielectric loss. The temperature range of dielectric loss below 0.02 exceeds to 374 °C for BNT-07BA-05KNN annealed in O2, which might be attributed to the reason that the oxygen vacancy concentration decreases after annealing in O2. In order to clarify the reduction of dielectric loss in BNT, BNT-07BA, BNT-05KNN and BNT-07BA-05KNN ceramics, the complex impedance spectra were measured in the temperature range of 400 °C–550 °C. Fig. 6(a) displays typical impedance spectra at 550 °C for all the samples, which illustrates a single semi-circle, indicating that the samples have an electrical-homogeneous nature. There is no spike at low frequency, which suggest that the conduction mechanism is dominated by the electronic conduction [23]. Fig. 6(b) illustrates the Arrhenius plot of BNT, BNT-07BA, BNT-05KNN and BNT-07BA-05KNN in the temperature range of 450 °C–550 °C. In this temperature range, logσ as a function of 1000/T for BNT and BNT-05KNN shows a linear behavior. The activation energy of conductivity for the BNT and BNT 05KNN is 1.16 (1) eV 1.10 (1) eV, respectively. However, there is a kink in the Arrhenius plot of BNT-07BA and BNT-07BA-05KNN, which can be divided into two regions. At lower temperature, the activation energy of conductivity is about 0.69 (1) eV and 0.79 (1) eV for BNT-07BA and BNT-07BA-05KNN. At higher temperature, the activation energy of conductivity is about 0.29 (1) eV and 0.45 (1) eV. Compared to the conductivities of BNT and BNT-05KNN, conductivities of BNT-07BA and BNT-07BA-05KNN are much lower, which are ascribed to the reason that the defect dipoles limit the mobility of oxygen vacancies. Therefore, it is rationalized that BNT-07BA and BNT-07BA-05KNN have lower dielectric loss compared with other samples. The reason for the appearance of the kink in the Arrhenius plot of BNT-07BA and BNT07BA-05KNN is not clear now. It might be related to the dissociation of defect dipoles at high temperature, which will be studied further in our further work.
Fig. 5. (a) The complex impedance spectra of BNT-xBA-yKNN ceramics measured at 550 °C; (b) Arrhenius plot for BNT-xBA-yKNN ceramics measured in the temperature range between 450 °C to 550 °C.
Fig. 6. (a) P-E and S-E loops of BNT-xBA-yKNN ceramics under 60 kV/cm at 1 Hz and room temperature.
Fig. 7 shows the P-E and S-E loops of BNT, BNT-07BA, BNT-05KNN and BNT-07BA-05KNN ceramics under 60 kV/cm at 1 Hz and room temperature. As shown in Fig. 7(a), the P-E loop of BNT displays a well-
Table 1 Summary of dielectric properties of BNT-07BA-05KNN, BNT-07BA-05KNN annealed in O2 (BNT-07BA-05KNNeO2), 0.85(0.94Bi0.51Na0.5TiO3-0.06BaTiO3)0.15CaZrO3(B0.51NT-BT-15CZ) [21], 0.85 (0.94Bi0.5Na0.5TiO3-0.06BaTiO3)-0.15K0.5Na0.5NbO3 (BNT-BT-9KNN) [22], 0.95 (0.94Bi0.5Na0.5TiO3-0.06BaTiO3)0.05NaNbO3 (BNT-BT-0.05NN) [8] and 0.85 (0.94Bi0.5Na0.5TiO3-0.06BaTiO3)-0.15NaNbO3 (BNT-BT-0.15NN) [8]. Samples
ε' (150 °C, 1 kHz)
tanδ (150 °C 1 kHz)
T range of Δε'/ε′150 °C ≤ 15% (1 kHz)
T range of tanδ ≤ 0.02 (1 kHz)
References
BNT-07BA-05KNN BNT-07BA-05KNNeO2 B0.51NT-BT-15CZ BNT-BT-9KNN BNT-BT-0.05NN BNT-BT-0.15NN
2277 2399 882.6 ∼2800 3261 2004
0.009 0.007 ∼0.02 ∼0.01 0.01 ∼0.009
86.5–324.5 °C 81–318 °C 60–476 °C 39–343 °C 82–362 °C 35–300 °C
110–336 °C 107–373 °C 106–356 °C 100–300 °C 110–267 °Ca 71–244 °Ca
our work our work [20] [21] [8] [8]
a
Temperature range of tanδ ≤ 0.025. 12363
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Fig. 7. The hysteresis loops (P-E) and electric field-induced strain loops (S-E) of: (a) BNT, (b) BNT-07BA, (c) BNT-05KNN and (d) BNT-07BA-05KNN ceramics under the electric field of 60 kV cm-1 at 1 Hz and room temperature.
saturated loop, characteristic of normal ferroelectrics. After substitution of BiAlO3 and K0.5Na0.5NbO3, as shown in Fig. 7(b and c) the remnant polarization and electrostrain decrease, but the P-E and S-E loops show a much sharp shape, implying that the samples have lower leakage compared to BNT. Fig. 7(d) shows P-E and S-E loops of BNT-07BA05KNN. It shows a very slim loop and the electrostrain is much lower, characteristic of ferroelectric relaxors. The decrease of polarization and electrostrain further confirms that the long-range ferroelectric order is disrupted by the substitution of BiAlO3 and K0.5Na0.5NbO3 in BNT, which contributes on the improvement of stabilized permittivity. 4. Conclusions High temperature dielectrics based on the composition of (1-x-y) Bi0.5Na0.5TiO3-xBiAlO3-yK0.5Na0.5NbO3 were designed and prepared. The introduction of BiAlO3 is beneficial to the reduction of dielectric loss and the introduction of K0.5Na0.5NbO3 favors to improvement of the temperature stability of BNT-based ceramics. The composition of x = 0.07 and y = 0.05 has a stable permittivity between 86.5 °C and 324.5 °C and dielectric loss below 0.02 between 110 °C and 336 °C. Therefore, our work provides a promising candidate of dielectrics used for high-temperature capacitors. Acknowledgements This work was financially supported by National Natural Science Foundation of China (51802246), Natural Science Basic Research Plan in Shaanxi Province of China (2018JQ5110), State Key Laboratory of Solidification Processing in NWPU (SKLSP201839), Fund Program of the Scientific Activities of Selected Returned Overseas Professionals in Shaanxi Province, Special Scientific Research Plan Projects of Shaanxi Education Department (17JK0382) and Scientific and Technological Project of Yulin City (2016-16-6). Appendix A. Supplementary data Supplementary data related to this article can be found at https://
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