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Improved electrical performance of MgO-modified CaCu3Ti4O12 ceramics
Physica B: Condensed Matter 572 (2019) 98–104
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Improved electrical performance of MgO-modified CaCu3Ti4O12 ceramics *
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Xianwei Wang , Lulu Xue , Yu Zhang , Jiwei Du , Lingyun Sun , Yongchuang Shi, Wei Wang, Yifan Liang, Yixue Fu, Shuying Shang, Shaoqian Yin, Jun Shang, Yanchun Hu Laboratory of Functional Materials, College of Physics and Materials Science, Henan Normal University, Henan Key Laboratory of Photovoltaic Materials, 46 Jianshe Road, Xinxiang, 453007, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Colossal permittivity CaCu3Ti4O12 Modified Dielectric properties
Colossal permittivity material CaCu3Ti4O12 (CCTO) ceramics modified by MgO with the mass fraction of 0%, 0.5%, 1%, and 2% were synthesized using a sol-gel method followed by a conventional ceramic preparation process. The scanning electron microscope exhibits that all CCTO ceramics possess the high densities and large grain size. The results of dielectric measurement show that all the samples possess giant dielectric properties. The colossal dielectric constant of CCTO ceramics increases firstly and then decreases with the increase of mass fraction from 0% to 2% at room temperature, while the value of dielectric loss is opposite. Specifically, the sample with the mass fraction of 1% shows the highest density, largest grains and most obvious grain boundaries. 1% sample also shows better electrical properties with giant dielectric constant (~6.59 × 104 at 1 kHz) and low dielectric loss (~0.06 at 1 kHz) at room temperature. In addition, the optimal sample with the mass fraction of 1% shows a more stable dielectric constant from 20 °C to 200 °C. The electrical properties of the CCTO ceramics have achieved good performance by the addition of MgO.
1. Introduction
preparation process provides a research direction for improving properties of the CCTO ceramics. Many processes have been used to modify the microstructure of grain/grain boundary, such as powder preparation process, sintering method and doping [12–16]. The sol-gel method has been used to improve the homogeneous of the powder [12]. Previous report shows that the structure and dielectric properties are dependent on the calcining temperature of CCTO ceramics and that the optimal calcining temperature is 850 °C [13]. Different sintering method, for instance microwave sintering, rapid sintering and spark plasma sintering, were used to get the well-qualified CCTO ceramics in previous reports [13–15]. According to previous reports, a lot of doping elements, such as Li [17], Sr and Ni [16], can decrease the dielectric loss. Due to the enhanced semiconducting nature, metal oxide doping would cause the decrease of leakage current and the breakdown electric field [16,18]. In addition, many researchers seek for other strategies to enhance dielectric constant and decrease dielectric loss by modifying of the second phase, such as TiO2 [19,20], Al2O3 [21], CaTiO3 [22] and so on [23–27]. CCTO ceramics modified by TiO2 can obtain a giant dielectric constant (~6.81 × 104) with low dielectric loss (< 0.12) at room temperature [20]. Sonia et al. reported that the permittivity of CCTO could be enhanced to 81000 from 58000 at 1 kHz by altering the state of the grain boundary with Al2O3 [21]. CaTiO3, as one of the most
Recently, much attention has been paid to colossal permittivity (CP, εr > 103) materials because of their potential applications, such as high density storage devices and microelectronic devices [1–4]. CaCu3Ti4O12 (CCTO) ceramics, a kind of CP materials, have been widely studied due to its superior dielectric constant (~104) and excellent stabiles at a range of temperature from 100 to 400 K below 1 MHz [5]. Although, CCTO ceramics show application prospects in microelectronic devices and energy storage dielectric capacitors [6], the high dielectric loss hinders many practical applications. It is significantly difficult to decrease the high dielectric loss accompanied by maintaining colossal permittivity. And in order to improve the performance of colossal dielectric materials, many studies have focused on the extrinsic mechanism of the dielectric responds [7–9]. At present, the internal barrier layer capacitor (IBLC) mechanism is a more acceptable dielectric mechanism for CCTO ceramics. According to the IBLC model, the CCTO ceramics consist of n-type semiconducting grains and insulating grain boundaries. The electrical properties of grain/grain boundary could be controlled by adjusting the composition and microstructure [10,11]. Therefore, controlling the component and microstructure of grain or grain boundary by
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Corresponding author. Henan Normal University, Henan Key Laboratory of Photovoltaic Materials, 46 Jianshe Road, Xinxiang, 453007, China. E-mail address: [email protected] (X. Wang). 1 The four authors contributed equally to this work. https://doi.org/10.1016/j.physb.2019.07.048 Received 14 May 2019; Received in revised form 21 July 2019; Accepted 24 July 2019 Available online 26 July 2019 0921-4526/ © 2019 Elsevier B.V. All rights reserved.
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significant second phase, had been used to modify samples, and samples with a low dielectric loss of 0.05 and good temperature stability were obtained [22]. The second phase of metal oxides has greatly improved the electrical properties of CCTO, and it is a good way to improve the dielectric constant and reduce dielectric loss. In a previous work [28], CCTO powder prepared by a conventional mixed oxide method was directly modified by MgO nanoparticles, and both dielectric and electrical nonlinear properties of CCTO ceramics were improved by addition of 5 vol% MgO. In this work, CCTO powders prepared by sol-gel method were modified by a chemical method using Mg(NO3)2 as raw materials, and then the formed Mg(OH)2 precipitate converted into MgO by calcining. The CCTO ceramics were obtained by die pressing followed by sintering. The effect of MgO on the microstructure and dielectric properties of CCTO ceramic was studied in a wide temperature range and frequency. 2. Experimental procedures Ca(CH3COO)2·4H2O, Cu(NO3)2·3H2O and Ti(OC4H9)4 were used as raw materials in the sol-gel process. Firstly, according to the stoichiometric ratio of CaCu3Ti4O12, 0.08 mol Ti(OC4H9)4 were dissolved in 30 ml ethanol, and then 120 ml CH3COOH was immediately added into the solution to obtain solution A, followed by stirring for 30 min. 0.06mol Cu(NO3)2·3H2O were added into 80 ml ethanol to obtain a clear solution, and then the Cu(NO3)2 solution was immediately poured into solution A under stirring to form blue-green transparent solution B. 0.02 mol Ca(CH3COO)2·4H2O was added into 70 ml distilled water and stirred to form solution C. Then solution C was slowly added into solution B under stirring. The pH value of the final solution was adjusted to 3, and then stirred for 60 min. Then, the solution was heated at 85 °C for about 15 h to form blue-green dry gel, and the gel was heated at 125 °C for 8 h after it was grinded into powder. The powder was then heated at 350 °C for 3 h to form precursor powder. Then CaCu3Ti4O12 powders were obtained by calcining the precursor powder in a muffle furnace at 850 °C for 10 h. According to the mass fraction of MgO (0%, 0.5%, 1%, and 2%), different amount of Mg(NO3)2 were added into 30 mL ethanol, and then 0.5 g CaCu3Ti4O12 powders were added into the Mg(NO3)2 solution. Under magnetic stirring, the pH value was adjusted to 10 with diluted NH3.H2O. The solution were stirred for 30 min, dried at 85 °C for 6 h in a drying oven, and then calcined at 550 °C for 3 h. The powder was pressed at 200 MPa for 5 min and then sintered at 1050 °C for 15 h. The crystal structures of powders and CCTO ceramics were analyzed by x-ray diffractometer (XRD, D8 Discovery, Bruker) at 40 kV and 40 mA. The density of CCTO ceramics was measured by Archimedes M method [6] using the equation of ρ = ( M −1M ) × ρ w (M1 is the dry 2 3 weight of samples in the air, M2 is the saturated wet weight of the sample in the air, M3 is the suspension weight of the sample in distilled water, and ρ w is the density of distilled water). The microstructures of ceramics were identified by field emission scanning electron microscopy (FE-SEM, Zeiss SUPRA 40). Silver paste was painted on the both sides of ceramics and calcined at 600 °C to obtain silver electrode. Electrical properties of the samples were measured by the impedance analyzer (HP4192) in the frequency range of 102–106 Hz.
Fig. 1. XRD spectra of CCTO ceramics modified by 0%, 0.5%, 1% and 2% MgO.
ceramic samples. The lattice parameters are 7.385 Å, 7.386 Å, 7.386 Å, and 7.385 Å for CCTO ceramics modified by 0%, 0.5%, 1% and 2% MgO, respectively. The results show that the lattice parameters of CCTO ceramics modified by MgO are similar to those of pure samples. SEM images of CCTO ceramics modified with different MgO content are shown in Fig. 2. Fig. 2(a) displays pure CCTO samples with a few large grains surrounded by some smaller grains, as is also reported in previous research [20]. In Fig. 2(b–d), the grains size of the ceramics modified by MgO is bigger than that of the pure CCTO ceramics, which could be attributed to the addition of MgO. The CCTO ceramic modified by 1% MgO shows the largest grain size and the clearest grain boundary. And the grains size of CCTO ceramic modified by 2% MgO is slightly smaller than that of 1% sample shown in Fig. 2(c). The density values are 4.24 g/cm3, 4.19 g/cm3, 4.43 g/cm3, and 4.33 g/cm3 (relative density of 84%, 83%, 88%, and 86%) for CCTO ceramics modified by 0%, 0.5%, 1% and 2% MgO, respectively. The density of CCTO ceramics modified by 1% MgO is the highest (4.33 g/cm3), which corresponds to the maximum grain size. The energy dispersion spectra (EDS) results of the CCTO ceramics modified by 1% MgO are shown in the inset of Fig. 2(c). The result shows that the element ratio of Ca:Cu:Ti in grain is close to 1:3:4 (Ca:Cu:Ti = 4.78:14.24:18.59), which is consistent with the composition of CCTO. Element analysis of grain boundary reveals that Cu-rich phase is observed in grain boundary, which is consistent with previous reports [23]. Element analysis results show that Mg element is mainly distributed at the grain boundaries. It can be inferred that excess amount MgO phase prohibits grain growth, and appropriate amount of MgO has great significance in promoting grain growth.
3.2. Dielectric properties The dependences of dielectric constant and dielectric loss on frequency (~ 102–106 Hz, at room temperature) for CCTO ceramics are showed in Fig. 3. As shown in Fig. 3(a), the dielectric constant of all the samples are higher than 104 over a wide frequency range (102–106 Hz). The dielectric constant decreases gradually with frequency increase and shows a significant decline at around 105 Hz due to Debye-relaxation [23,29]. The permittivity increases firstly and then decreases with the increase of MgO amount. Dielectric constant value is the highest for CCTO ceramics modified by 1% MgO. While, the tendency of dielectric loss is opposite, as displayed in Fig. 3(b). According to Fig. 3(b), it can be observed that the tan δ values of samples modified with 0.5% and 1% MgO are lower than that of the pure ceramic, which indicates that the tan δ value can be decreased by modifying with MgO. In the inset of
3. Results and discussion 3.1. Structural characteristics XRD patterns of all sintered CCTO ceramics modified with different MgO concentrations (w = 0%, 0.5%, 1%, 2%) are presented in Fig. 1. The spectra reveal that ceramics show single-phase cubic crystal structure, and the secondary phase diffractions are not detected in Fig. 1. There is also no clear characteristic diffraction of MgO in patterns, and this may be attributed to the low content of Mg in the 99
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Fig. 2. SEM images of CCTO ceramics modified by (a) x = 0%, (b) x = 0.5%, (c) x = 1% and (d) x = 2% MgO. Insets: EDS results of CCTO ceramics modified by 1% MgO.
Fig. 3, there is a fast increase at 105 Hz, which corresponds to the rapid decline of permittivity. The results show that dielectric constant of the sample modified with 1% MgO reaches the maximum value (about 6.95 × 104 at 103 Hz), while the value of tan δ is 0.06, which is almost equivalent to the pure CCTO at 103 Hz. Previous researches reveal that dielectric constant is affected by the microstructure and grain size [21]. We can draw a conclusion that modification of MgO has a significant effect on dielectric properties of CCTO ceramics. Therefore, modifying CCTO ceramics with MgO is a significant way to improve the dielectric constant and decrease the dielectric loss at room temperature. Fig. 4 reveals the dependences of dielectric constant on frequency at different temperatures varied from 20 °C to 200 °C. In Fig. 4, all the samples show large dielectric constants, and the dielectric constants gradually increase with the increase of temperature at low frequencies. With the increase of frequency, the dielectric constant decreases gradually, which is due to the decrease of frequency response of polarization mechanism to external electric field. In Fig. 4(a–b), the dielectric constant values of 0% and 0.5% MgO-modified CCTO ceramics show a large variation at different frequencies. According to Fig. 4(c) and (d), the dielectric constant values of 1% and 2% MgO-modified CCTO ceramics change slightly. Moreover, the dielectric constant of 1% MgO-modified CCTO ceramics is larger than that of 2% samples. In
Fig. 4(a–d), dielectric constant values for all samples at different temperatures decline rapidly with the increase of frequency at around 105 Hz due to Dybey-relaxation [12]. In Fig. 4(a–c), another relaxation in the medium frequencies of 102–105 Hz moves to higher frequencies as the temperature increases (> 20 °C), and the dielectric response could be explained by the interfacial polarization, which was also reported in previous work [30]. According to the previous work [30], this phenomenon is caused by non-Ohmic contact of sample-electrode interface, and a strong dielectric response is observed in the low-frequency range with the increase of temperature. As depicted in Fig. 4, it can be deduced that CCTO ceramics modified by 1% MgO possess a larger dielectric constant and a better stability on frequency. Fig. 5 shows the dependences of dielectric loss on frequency at different temperatures varied from 20 °C to 200 °C. As shown in Fig. 5(a–d), dielectric loss of CCTO ceramics increases rapidly with increasing temperature, which can be interpreted as the increase of conductivity at high temperatures. In the inset of Fig. 5, we can also observe a dielectric loss relaxation peak because of Debye-relaxation, and it shifts to higher frequency with the increase of temperature. The dependence of dielectric loss relaxation peak on temperature is consistent with the change of dielectric constant, and this was also observed in previous work [31].
Fig. 3. Frequency dependence of (a) dielectric constant and (b) dielectric loss of CCTO ceramics measured at room temperature. 100
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Fig. 4. The dependences of dielectric constant on frequency for CCTO ceramic modified by (a) x = 0%, (b) x = 0.5%, (c) x = 1%, (d) x = 2% MgO at different temperatures (20 °C–200 °C).
frequency (< 104 Hz). With the increase of the frequency, the rapidgrowing region of dielectric constant shifts to higher temperature, which is similar to previous works [12]. Fig. 6(b) shows that the dielectric constant of CCTO ceramics modified by 0.5% MgO possesses the similar trend with the pure ceramics. As displayed in Fig. 6(c–d), a plateau region of permittivity can be seen at higher temperature above 120 °C for 0.4 and 1 kHz, as observed in many studies [19]. The dielectric constants of CCTO ceramics modified with 1% and 2% MgO
Fig. 6 shows the temperature dependence of dielectric constant and dielectric loss at some typical frequencies (0.4–300 kHz). According to Fig. 6(a–d), CCTO ceramics possess high dielectric constant at low frequency and high temperature. In all samples, the dielectric constant increases with the increase of temperature at low frequencies and changes slightly at high frequencies. In Fig. 6(a), the permittivity of the pure CCTO ceramics shows an obvious step-like growth, which increases rapidly with the increase of temperature (60 °C–120 °C) at low
Fig. 5. The dependences of dielectric loss on frequency for CCTO ceramic modified by (a) x = 0%, (b) x = 0.5%, (c) x = 1%, (d) x = 2% MgO at different temperatures (20 °C–200 °C). 101
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Fig. 6. The dependences of dielectric constant and dielectric loss (insets) on temperature for CCTO ceramic modified by (a) x = 0%, (b) x = 0.5%, (c) x = 1%, (d) x = 2% MgO at some typical frequencies.
change slightly with the increase of the temperatures, which indicates that dielectric constants of the two samples show low dependence on temperature. What's more, the permittivity of CCTO ceramics modified with 1% MgO is higher than that of CCTO ceramics modified with 2% MgO at the same temperatures and frequencies. In the inset of Fig. 6(a–d), the dielectric loss of all samples increases with the temperature increasing, especially for the low frequency dielectric loss. With the increase of frequency, the fast-growing region shifts to higher temperature, which has been reported in previous study [19]. The results show that dielectric constant of CCTO ceramics modified by 1% MgO exhibit a large dielectric constant in a wide temperature range, and the dielectric constant is almost independent on temperature. 3.3. Impedance spectra analysis Fig. 7 shows the impedance spectra of CCTO ceramics modified by different amount of MgO (0%, 0.5%, 1%, and 2%) at room temperature. And the insets show a circuit model and the local magnification of the spectrum in the high frequency range. Resistance data could be obtained by fitting the impedance spectra using the equivalent circuit model shown in the inset of Fig. 7. According to the impedance spectra, it shows an incomplete semicircular arc, which is the contribution of the parallel of a grain boundary resistance and a constant phase element (CPE). On the Z′ axis, the left nonzero intercept of the horizontal axis at high frequencies corresponds to the grain resistance (Rg). The right intercept of the horizontal axis is equal to the bulk resistance of the ceramics, which is the sum of grain resistance and grain boundary resistance (Rgb). The low Rg and high Rgb (Rg ≪ Rgb) are observed in the impedance spectra of CCTO ceramics. Based on the IBLC model [32], the result of Rg ≪ Rgb means that the grains of the CCTO ceramics are semiconducting and the grain boundaries are insulating in the CCTO ceramics. In all CCTO ceramic samples, it can be observed that the R value increases slightly, and grain boundary resistance decreases firstly and then increases with the increase of MgO concentration. The grain boundary resistance of CCTO ceramics modified with 1% MgO is the smallest (3.37 × 105 Ω), and it is contributed to the improvement of dielectric properties. The results show that the adding of MgO with
Fig. 7. The impedance spectra for CCTO ceramics modified by different amount of MgO at room temperature. Insets: The local magnification of the spectra and an equivalent circuit model.
proper amount in CCTO ceramics can reduce insulation of grain boundary resistance and improve grain resistance. Fig. 8(a–d) shows impedance spectra of all samples at different temperatures (20–200 °C). According to the previous work [33], the experimental data at the high temperatures (> 20 °C) are fitted by using another equivalent circuit model in the inset of Fig. 8 (a). In the circuit model, Rg, Rgb and Rel represent the resistance of grain, grain boundary and electrode. In addition, CPEgb and CPEel is constant phase element of grain boundary and electrode, respectively. According to Fig. 8, the impedance complex spectra simulation of all samples at the high temperature consists of two semicircular arcs, which are the contribution of the grain boundary and the sample-electrode interface, respectively. The nonzero intercept on the left of the Z′ axis corresponds to grain resistance, intercept on the right part of the arc at medium frequency is grain boundary resistance and right part of arc at low 102
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Fig. 8. The impedance spectra of CCTO ceramics modified with (a) x = 0%, (b) x = 0.5%, (c) x = 1% and (d) x = 2% MgO at different temperatures. Inset in (a): the equivalent circuit model.
frequency is associated with resistance of electrode. The resistance values of all samples are fitted with the equivalent circuit model, and the Rgb value at 80 °C is 8650 Ω, 9154 Ω, 2202 Ω, and 102790 Ω for the four samples, respectively, while Re value is 17052 Ω, 19277 Ω, 7994 Ω, 8995 Ω, respectively. According to the fitting data of the samples modified with 0%, 0.5%, 1% MgO, the total resistance is controlled by the Rgb and Re. It has been reported in previous studies that the contact between the sample and the electrode can contribute to the low-frequency dielectric response of CCTO ceramics [34]. In Fig. 4(a–c), a large increase of low-frequency ε could be attributed to the sampleelectrode contact and low Rgb, which has been reported in other work [33].
4. Conclusions In conclusion, CaCu3Ti4O12 ceramics modified by MgO (0%, 0.5%, 1%, and 2%) were prepared by the conventional sol-gel method. Content of MgO has a significant influence on the microstructure and electrical properties of CCTO ceramics. In CCTO ceramics modified with MgO, the density, grain size, dielectric constant and frequency and temperature stability have been improved. CCTO ceramics modified by 1% MgO possess dielectric constant of 6.59 × 104 and lower dielectric loss of 0.06 (at 103 Hz and room temperature). At different temperatures (20–200 °C) and frequencies (102–106 Hz), the dielectric constant of CCTO ceramics modified with 1% MgO is higher and it also shows a better stability on frequency and temperature.
Conflict of interest form They have no conflict of interest.
Acknowledgements This work has been supported by the Special Scientific Research Foundation in Henan Normal University (No. 20180543), the Key Scientific Research Foundation in Henan Province (No. 19B430005), and the National University Student Innovation Program (No. 20160098), the National Natural Science Foundation of China (Nos. 51402091, 51601059, 11847136), Foundation of Henan Educational Committee (No. 19A 140010), and Science and Technology Research Project of Henan Province (No.182102210375). 103
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Physica B: Condensed Matter journal homepage: www.elsevier.com/locate/physb
Improved electrical performance of MgO-modified CaCu3Ti4O12 ceramics *
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Xianwei Wang , Lulu Xue , Yu Zhang , Jiwei Du , Lingyun Sun , Yongchuang Shi, Wei Wang, Yifan Liang, Yixue Fu, Shuying Shang, Shaoqian Yin, Jun Shang, Yanchun Hu Laboratory of Functional Materials, College of Physics and Materials Science, Henan Normal University, Henan Key Laboratory of Photovoltaic Materials, 46 Jianshe Road, Xinxiang, 453007, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Colossal permittivity CaCu3Ti4O12 Modified Dielectric properties
Colossal permittivity material CaCu3Ti4O12 (CCTO) ceramics modified by MgO with the mass fraction of 0%, 0.5%, 1%, and 2% were synthesized using a sol-gel method followed by a conventional ceramic preparation process. The scanning electron microscope exhibits that all CCTO ceramics possess the high densities and large grain size. The results of dielectric measurement show that all the samples possess giant dielectric properties. The colossal dielectric constant of CCTO ceramics increases firstly and then decreases with the increase of mass fraction from 0% to 2% at room temperature, while the value of dielectric loss is opposite. Specifically, the sample with the mass fraction of 1% shows the highest density, largest grains and most obvious grain boundaries. 1% sample also shows better electrical properties with giant dielectric constant (~6.59 × 104 at 1 kHz) and low dielectric loss (~0.06 at 1 kHz) at room temperature. In addition, the optimal sample with the mass fraction of 1% shows a more stable dielectric constant from 20 °C to 200 °C. The electrical properties of the CCTO ceramics have achieved good performance by the addition of MgO.
1. Introduction
preparation process provides a research direction for improving properties of the CCTO ceramics. Many processes have been used to modify the microstructure of grain/grain boundary, such as powder preparation process, sintering method and doping [12–16]. The sol-gel method has been used to improve the homogeneous of the powder [12]. Previous report shows that the structure and dielectric properties are dependent on the calcining temperature of CCTO ceramics and that the optimal calcining temperature is 850 °C [13]. Different sintering method, for instance microwave sintering, rapid sintering and spark plasma sintering, were used to get the well-qualified CCTO ceramics in previous reports [13–15]. According to previous reports, a lot of doping elements, such as Li [17], Sr and Ni [16], can decrease the dielectric loss. Due to the enhanced semiconducting nature, metal oxide doping would cause the decrease of leakage current and the breakdown electric field [16,18]. In addition, many researchers seek for other strategies to enhance dielectric constant and decrease dielectric loss by modifying of the second phase, such as TiO2 [19,20], Al2O3 [21], CaTiO3 [22] and so on [23–27]. CCTO ceramics modified by TiO2 can obtain a giant dielectric constant (~6.81 × 104) with low dielectric loss (< 0.12) at room temperature [20]. Sonia et al. reported that the permittivity of CCTO could be enhanced to 81000 from 58000 at 1 kHz by altering the state of the grain boundary with Al2O3 [21]. CaTiO3, as one of the most
Recently, much attention has been paid to colossal permittivity (CP, εr > 103) materials because of their potential applications, such as high density storage devices and microelectronic devices [1–4]. CaCu3Ti4O12 (CCTO) ceramics, a kind of CP materials, have been widely studied due to its superior dielectric constant (~104) and excellent stabiles at a range of temperature from 100 to 400 K below 1 MHz [5]. Although, CCTO ceramics show application prospects in microelectronic devices and energy storage dielectric capacitors [6], the high dielectric loss hinders many practical applications. It is significantly difficult to decrease the high dielectric loss accompanied by maintaining colossal permittivity. And in order to improve the performance of colossal dielectric materials, many studies have focused on the extrinsic mechanism of the dielectric responds [7–9]. At present, the internal barrier layer capacitor (IBLC) mechanism is a more acceptable dielectric mechanism for CCTO ceramics. According to the IBLC model, the CCTO ceramics consist of n-type semiconducting grains and insulating grain boundaries. The electrical properties of grain/grain boundary could be controlled by adjusting the composition and microstructure [10,11]. Therefore, controlling the component and microstructure of grain or grain boundary by
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Corresponding author. Henan Normal University, Henan Key Laboratory of Photovoltaic Materials, 46 Jianshe Road, Xinxiang, 453007, China. E-mail address: [email protected] (X. Wang). 1 The four authors contributed equally to this work. https://doi.org/10.1016/j.physb.2019.07.048 Received 14 May 2019; Received in revised form 21 July 2019; Accepted 24 July 2019 Available online 26 July 2019 0921-4526/ © 2019 Elsevier B.V. All rights reserved.
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significant second phase, had been used to modify samples, and samples with a low dielectric loss of 0.05 and good temperature stability were obtained [22]. The second phase of metal oxides has greatly improved the electrical properties of CCTO, and it is a good way to improve the dielectric constant and reduce dielectric loss. In a previous work [28], CCTO powder prepared by a conventional mixed oxide method was directly modified by MgO nanoparticles, and both dielectric and electrical nonlinear properties of CCTO ceramics were improved by addition of 5 vol% MgO. In this work, CCTO powders prepared by sol-gel method were modified by a chemical method using Mg(NO3)2 as raw materials, and then the formed Mg(OH)2 precipitate converted into MgO by calcining. The CCTO ceramics were obtained by die pressing followed by sintering. The effect of MgO on the microstructure and dielectric properties of CCTO ceramic was studied in a wide temperature range and frequency. 2. Experimental procedures Ca(CH3COO)2·4H2O, Cu(NO3)2·3H2O and Ti(OC4H9)4 were used as raw materials in the sol-gel process. Firstly, according to the stoichiometric ratio of CaCu3Ti4O12, 0.08 mol Ti(OC4H9)4 were dissolved in 30 ml ethanol, and then 120 ml CH3COOH was immediately added into the solution to obtain solution A, followed by stirring for 30 min. 0.06mol Cu(NO3)2·3H2O were added into 80 ml ethanol to obtain a clear solution, and then the Cu(NO3)2 solution was immediately poured into solution A under stirring to form blue-green transparent solution B. 0.02 mol Ca(CH3COO)2·4H2O was added into 70 ml distilled water and stirred to form solution C. Then solution C was slowly added into solution B under stirring. The pH value of the final solution was adjusted to 3, and then stirred for 60 min. Then, the solution was heated at 85 °C for about 15 h to form blue-green dry gel, and the gel was heated at 125 °C for 8 h after it was grinded into powder. The powder was then heated at 350 °C for 3 h to form precursor powder. Then CaCu3Ti4O12 powders were obtained by calcining the precursor powder in a muffle furnace at 850 °C for 10 h. According to the mass fraction of MgO (0%, 0.5%, 1%, and 2%), different amount of Mg(NO3)2 were added into 30 mL ethanol, and then 0.5 g CaCu3Ti4O12 powders were added into the Mg(NO3)2 solution. Under magnetic stirring, the pH value was adjusted to 10 with diluted NH3.H2O. The solution were stirred for 30 min, dried at 85 °C for 6 h in a drying oven, and then calcined at 550 °C for 3 h. The powder was pressed at 200 MPa for 5 min and then sintered at 1050 °C for 15 h. The crystal structures of powders and CCTO ceramics were analyzed by x-ray diffractometer (XRD, D8 Discovery, Bruker) at 40 kV and 40 mA. The density of CCTO ceramics was measured by Archimedes M method [6] using the equation of ρ = ( M −1M ) × ρ w (M1 is the dry 2 3 weight of samples in the air, M2 is the saturated wet weight of the sample in the air, M3 is the suspension weight of the sample in distilled water, and ρ w is the density of distilled water). The microstructures of ceramics were identified by field emission scanning electron microscopy (FE-SEM, Zeiss SUPRA 40). Silver paste was painted on the both sides of ceramics and calcined at 600 °C to obtain silver electrode. Electrical properties of the samples were measured by the impedance analyzer (HP4192) in the frequency range of 102–106 Hz.
Fig. 1. XRD spectra of CCTO ceramics modified by 0%, 0.5%, 1% and 2% MgO.
ceramic samples. The lattice parameters are 7.385 Å, 7.386 Å, 7.386 Å, and 7.385 Å for CCTO ceramics modified by 0%, 0.5%, 1% and 2% MgO, respectively. The results show that the lattice parameters of CCTO ceramics modified by MgO are similar to those of pure samples. SEM images of CCTO ceramics modified with different MgO content are shown in Fig. 2. Fig. 2(a) displays pure CCTO samples with a few large grains surrounded by some smaller grains, as is also reported in previous research [20]. In Fig. 2(b–d), the grains size of the ceramics modified by MgO is bigger than that of the pure CCTO ceramics, which could be attributed to the addition of MgO. The CCTO ceramic modified by 1% MgO shows the largest grain size and the clearest grain boundary. And the grains size of CCTO ceramic modified by 2% MgO is slightly smaller than that of 1% sample shown in Fig. 2(c). The density values are 4.24 g/cm3, 4.19 g/cm3, 4.43 g/cm3, and 4.33 g/cm3 (relative density of 84%, 83%, 88%, and 86%) for CCTO ceramics modified by 0%, 0.5%, 1% and 2% MgO, respectively. The density of CCTO ceramics modified by 1% MgO is the highest (4.33 g/cm3), which corresponds to the maximum grain size. The energy dispersion spectra (EDS) results of the CCTO ceramics modified by 1% MgO are shown in the inset of Fig. 2(c). The result shows that the element ratio of Ca:Cu:Ti in grain is close to 1:3:4 (Ca:Cu:Ti = 4.78:14.24:18.59), which is consistent with the composition of CCTO. Element analysis of grain boundary reveals that Cu-rich phase is observed in grain boundary, which is consistent with previous reports [23]. Element analysis results show that Mg element is mainly distributed at the grain boundaries. It can be inferred that excess amount MgO phase prohibits grain growth, and appropriate amount of MgO has great significance in promoting grain growth.
3.2. Dielectric properties The dependences of dielectric constant and dielectric loss on frequency (~ 102–106 Hz, at room temperature) for CCTO ceramics are showed in Fig. 3. As shown in Fig. 3(a), the dielectric constant of all the samples are higher than 104 over a wide frequency range (102–106 Hz). The dielectric constant decreases gradually with frequency increase and shows a significant decline at around 105 Hz due to Debye-relaxation [23,29]. The permittivity increases firstly and then decreases with the increase of MgO amount. Dielectric constant value is the highest for CCTO ceramics modified by 1% MgO. While, the tendency of dielectric loss is opposite, as displayed in Fig. 3(b). According to Fig. 3(b), it can be observed that the tan δ values of samples modified with 0.5% and 1% MgO are lower than that of the pure ceramic, which indicates that the tan δ value can be decreased by modifying with MgO. In the inset of
3. Results and discussion 3.1. Structural characteristics XRD patterns of all sintered CCTO ceramics modified with different MgO concentrations (w = 0%, 0.5%, 1%, 2%) are presented in Fig. 1. The spectra reveal that ceramics show single-phase cubic crystal structure, and the secondary phase diffractions are not detected in Fig. 1. There is also no clear characteristic diffraction of MgO in patterns, and this may be attributed to the low content of Mg in the 99
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Fig. 2. SEM images of CCTO ceramics modified by (a) x = 0%, (b) x = 0.5%, (c) x = 1% and (d) x = 2% MgO. Insets: EDS results of CCTO ceramics modified by 1% MgO.
Fig. 3, there is a fast increase at 105 Hz, which corresponds to the rapid decline of permittivity. The results show that dielectric constant of the sample modified with 1% MgO reaches the maximum value (about 6.95 × 104 at 103 Hz), while the value of tan δ is 0.06, which is almost equivalent to the pure CCTO at 103 Hz. Previous researches reveal that dielectric constant is affected by the microstructure and grain size [21]. We can draw a conclusion that modification of MgO has a significant effect on dielectric properties of CCTO ceramics. Therefore, modifying CCTO ceramics with MgO is a significant way to improve the dielectric constant and decrease the dielectric loss at room temperature. Fig. 4 reveals the dependences of dielectric constant on frequency at different temperatures varied from 20 °C to 200 °C. In Fig. 4, all the samples show large dielectric constants, and the dielectric constants gradually increase with the increase of temperature at low frequencies. With the increase of frequency, the dielectric constant decreases gradually, which is due to the decrease of frequency response of polarization mechanism to external electric field. In Fig. 4(a–b), the dielectric constant values of 0% and 0.5% MgO-modified CCTO ceramics show a large variation at different frequencies. According to Fig. 4(c) and (d), the dielectric constant values of 1% and 2% MgO-modified CCTO ceramics change slightly. Moreover, the dielectric constant of 1% MgO-modified CCTO ceramics is larger than that of 2% samples. In
Fig. 4(a–d), dielectric constant values for all samples at different temperatures decline rapidly with the increase of frequency at around 105 Hz due to Dybey-relaxation [12]. In Fig. 4(a–c), another relaxation in the medium frequencies of 102–105 Hz moves to higher frequencies as the temperature increases (> 20 °C), and the dielectric response could be explained by the interfacial polarization, which was also reported in previous work [30]. According to the previous work [30], this phenomenon is caused by non-Ohmic contact of sample-electrode interface, and a strong dielectric response is observed in the low-frequency range with the increase of temperature. As depicted in Fig. 4, it can be deduced that CCTO ceramics modified by 1% MgO possess a larger dielectric constant and a better stability on frequency. Fig. 5 shows the dependences of dielectric loss on frequency at different temperatures varied from 20 °C to 200 °C. As shown in Fig. 5(a–d), dielectric loss of CCTO ceramics increases rapidly with increasing temperature, which can be interpreted as the increase of conductivity at high temperatures. In the inset of Fig. 5, we can also observe a dielectric loss relaxation peak because of Debye-relaxation, and it shifts to higher frequency with the increase of temperature. The dependence of dielectric loss relaxation peak on temperature is consistent with the change of dielectric constant, and this was also observed in previous work [31].
Fig. 3. Frequency dependence of (a) dielectric constant and (b) dielectric loss of CCTO ceramics measured at room temperature. 100
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Fig. 4. The dependences of dielectric constant on frequency for CCTO ceramic modified by (a) x = 0%, (b) x = 0.5%, (c) x = 1%, (d) x = 2% MgO at different temperatures (20 °C–200 °C).
frequency (< 104 Hz). With the increase of the frequency, the rapidgrowing region of dielectric constant shifts to higher temperature, which is similar to previous works [12]. Fig. 6(b) shows that the dielectric constant of CCTO ceramics modified by 0.5% MgO possesses the similar trend with the pure ceramics. As displayed in Fig. 6(c–d), a plateau region of permittivity can be seen at higher temperature above 120 °C for 0.4 and 1 kHz, as observed in many studies [19]. The dielectric constants of CCTO ceramics modified with 1% and 2% MgO
Fig. 6 shows the temperature dependence of dielectric constant and dielectric loss at some typical frequencies (0.4–300 kHz). According to Fig. 6(a–d), CCTO ceramics possess high dielectric constant at low frequency and high temperature. In all samples, the dielectric constant increases with the increase of temperature at low frequencies and changes slightly at high frequencies. In Fig. 6(a), the permittivity of the pure CCTO ceramics shows an obvious step-like growth, which increases rapidly with the increase of temperature (60 °C–120 °C) at low
Fig. 5. The dependences of dielectric loss on frequency for CCTO ceramic modified by (a) x = 0%, (b) x = 0.5%, (c) x = 1%, (d) x = 2% MgO at different temperatures (20 °C–200 °C). 101
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Fig. 6. The dependences of dielectric constant and dielectric loss (insets) on temperature for CCTO ceramic modified by (a) x = 0%, (b) x = 0.5%, (c) x = 1%, (d) x = 2% MgO at some typical frequencies.
change slightly with the increase of the temperatures, which indicates that dielectric constants of the two samples show low dependence on temperature. What's more, the permittivity of CCTO ceramics modified with 1% MgO is higher than that of CCTO ceramics modified with 2% MgO at the same temperatures and frequencies. In the inset of Fig. 6(a–d), the dielectric loss of all samples increases with the temperature increasing, especially for the low frequency dielectric loss. With the increase of frequency, the fast-growing region shifts to higher temperature, which has been reported in previous study [19]. The results show that dielectric constant of CCTO ceramics modified by 1% MgO exhibit a large dielectric constant in a wide temperature range, and the dielectric constant is almost independent on temperature. 3.3. Impedance spectra analysis Fig. 7 shows the impedance spectra of CCTO ceramics modified by different amount of MgO (0%, 0.5%, 1%, and 2%) at room temperature. And the insets show a circuit model and the local magnification of the spectrum in the high frequency range. Resistance data could be obtained by fitting the impedance spectra using the equivalent circuit model shown in the inset of Fig. 7. According to the impedance spectra, it shows an incomplete semicircular arc, which is the contribution of the parallel of a grain boundary resistance and a constant phase element (CPE). On the Z′ axis, the left nonzero intercept of the horizontal axis at high frequencies corresponds to the grain resistance (Rg). The right intercept of the horizontal axis is equal to the bulk resistance of the ceramics, which is the sum of grain resistance and grain boundary resistance (Rgb). The low Rg and high Rgb (Rg ≪ Rgb) are observed in the impedance spectra of CCTO ceramics. Based on the IBLC model [32], the result of Rg ≪ Rgb means that the grains of the CCTO ceramics are semiconducting and the grain boundaries are insulating in the CCTO ceramics. In all CCTO ceramic samples, it can be observed that the R value increases slightly, and grain boundary resistance decreases firstly and then increases with the increase of MgO concentration. The grain boundary resistance of CCTO ceramics modified with 1% MgO is the smallest (3.37 × 105 Ω), and it is contributed to the improvement of dielectric properties. The results show that the adding of MgO with
Fig. 7. The impedance spectra for CCTO ceramics modified by different amount of MgO at room temperature. Insets: The local magnification of the spectra and an equivalent circuit model.
proper amount in CCTO ceramics can reduce insulation of grain boundary resistance and improve grain resistance. Fig. 8(a–d) shows impedance spectra of all samples at different temperatures (20–200 °C). According to the previous work [33], the experimental data at the high temperatures (> 20 °C) are fitted by using another equivalent circuit model in the inset of Fig. 8 (a). In the circuit model, Rg, Rgb and Rel represent the resistance of grain, grain boundary and electrode. In addition, CPEgb and CPEel is constant phase element of grain boundary and electrode, respectively. According to Fig. 8, the impedance complex spectra simulation of all samples at the high temperature consists of two semicircular arcs, which are the contribution of the grain boundary and the sample-electrode interface, respectively. The nonzero intercept on the left of the Z′ axis corresponds to grain resistance, intercept on the right part of the arc at medium frequency is grain boundary resistance and right part of arc at low 102
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Fig. 8. The impedance spectra of CCTO ceramics modified with (a) x = 0%, (b) x = 0.5%, (c) x = 1% and (d) x = 2% MgO at different temperatures. Inset in (a): the equivalent circuit model.
frequency is associated with resistance of electrode. The resistance values of all samples are fitted with the equivalent circuit model, and the Rgb value at 80 °C is 8650 Ω, 9154 Ω, 2202 Ω, and 102790 Ω for the four samples, respectively, while Re value is 17052 Ω, 19277 Ω, 7994 Ω, 8995 Ω, respectively. According to the fitting data of the samples modified with 0%, 0.5%, 1% MgO, the total resistance is controlled by the Rgb and Re. It has been reported in previous studies that the contact between the sample and the electrode can contribute to the low-frequency dielectric response of CCTO ceramics [34]. In Fig. 4(a–c), a large increase of low-frequency ε could be attributed to the sampleelectrode contact and low Rgb, which has been reported in other work [33].
4. Conclusions In conclusion, CaCu3Ti4O12 ceramics modified by MgO (0%, 0.5%, 1%, and 2%) were prepared by the conventional sol-gel method. Content of MgO has a significant influence on the microstructure and electrical properties of CCTO ceramics. In CCTO ceramics modified with MgO, the density, grain size, dielectric constant and frequency and temperature stability have been improved. CCTO ceramics modified by 1% MgO possess dielectric constant of 6.59 × 104 and lower dielectric loss of 0.06 (at 103 Hz and room temperature). At different temperatures (20–200 °C) and frequencies (102–106 Hz), the dielectric constant of CCTO ceramics modified with 1% MgO is higher and it also shows a better stability on frequency and temperature.
Conflict of interest form They have no conflict of interest.
Acknowledgements This work has been supported by the Special Scientific Research Foundation in Henan Normal University (No. 20180543), the Key Scientific Research Foundation in Henan Province (No. 19B430005), and the National University Student Innovation Program (No. 20160098), the National Natural Science Foundation of China (Nos. 51402091, 51601059, 11847136), Foundation of Henan Educational Committee (No. 19A 140010), and Science and Technology Research Project of Henan Province (No.182102210375). 103
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