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Low-temperature synthesis of CdCu3Ti4O12 powders with high dielectric permittivities
Ceramics International 45 (2019) 11899–11904
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Low-temperature synthesis of CdCu3Ti4O12 powders with high dielectric permittivities
T
Zhanhui Penga, Pengfei Liangb, Bi Chena, Yichen Xianga, Hui Penga, Di Wua, Xiaolian Chaoa,∗∗, Zupei Yanga,∗ a Key Laboratory for Macromolecular Science of Shaanxi Province, Shaanxi Key Laboratory for Advanced Energy Devices, Shaanxi Engineering Laboratory for Advanced Energy Technology, School of Materials Science and Engineering, Shaanxi Normal University, Xi'an, 710062, Shaanxi, China b School of Physics and Information Technology, Shaanxi Normal University, Xi'an, 710062, Shaanxi, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Molten salt method Low-energy sintering Perovskite phase High dielectric constant
The molten salt method was adopted in this work to synthesize CdCu3Ti4O12 precursor powders and was found capable of notably reducing the sintering temperature. It was also found that the grain size gradually increases and the grain shape changes from spherical to cubic with increasing proportions of molten salt. The optimal sintering conditions were found to be at approximately 730 °C with the addition of 20% molten salt. After further sintering at 975 °C, the obtained perovskite ceramics exhibited a high dielectric constant at low frequencies.
1. Introduction Due to the urgent requirement for the integration and miniaturization of microelectronic techniques [1–10], materials with high dielectric constants have been extensively studied recently. CaCu3Ti4O12 (CCTO) is a non–ferroelectric material because of its relatively flat high permittivity–temperature/frequency profiles. CCTO and CCTO–based ceramics have been successfully synthesized by various preparation methods, e.g., Chen et al. [7] successfully prepared CCTO materials via a standard mixed–oxide method. Considering the high-temperature sintering conditions that are adopted in the solid–state method, an oxalate co–precipitation method was introduced to prepare CCTO powders by T. R. N. Kutty et al. [8]. Li et al. [9] also successfully prepared CCTO powders with high dielectric constants using a sol–gel technique. However, these wet chemical methods are unlikely to be applied in industrial production because of their relatively high–cost materials, such as expensive metal alkoxides, use of inorganic salt solutions and complicated fabrication processes [6,8,9,11,12]. In 2013, Liu et al. [13] used a low–temperature molten salt method and successfully prepared CCTO powders and obtained a high dielectric permittivity in the resulting ceramics. The molten salt method is an efficient low–temperature sintering strategy that usually involves one or several low melting point salts as the reaction medium [13–16]. During the reaction process, low melting point salts are used as the
reaction medium, and the reactants, which have a certain degree of solubility in the salts, thus exhibit an accelerated diffusion rate. The reaction is converted into a solid–liquid reaction. The merits of this method can thus be concluded as follows: low synthetic temperatures, short reaction times, homogeneous chemical composition for the obtained powders, good crystal morphologies and high–purity phases [15]. In addition, the salts can easily be separated for recycling [16]. To date, the molten salt method has been used to fabricate powders of a wide range of dielectric materials, e.g., Hou et al. [14] reduced the sintering temperature from > 1100 °C to 700 °C for Na0.9K0.1NbO3 powders, and Yang et al. [15] reported a synthetic temperature as low as 1000 °C for (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 powders, both of whom used the molten salt method. However, there is no literature reporting on the preparation of CdCTO powders using the molten salt method, as far as we know. CdCu3Ti4O12 (abbreviated as CdCTO) has a similar crystal structure to CCTO, which is that of the perovskite structure compound ACu3Ti4O12. CdCTO has been successfully prepared and was shown to exhibit some very interesting dielectric performances, as reported [11,17–19]. Herein, the molten salt method was adopted in this work to prepare CdCTO powders at substantially lower sintering temperatures than have been used previously. The phase generation and evolution mechanism of the powders were explored and analysed using an X-ray diffractometer (XRD), differential scanning calorimeter (TG–DSC)
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Corresponding author. Key Laboratory for Macromolecular Science of Shaanxi Province, Shaanxi Key Laboratory for Advanced Energy Devices, School of Materials Science and Engineering, Shaanxi Normal University, Xi'an, 710062, Shaanxi, PR China. ∗∗ Corresponding author. E-mail addresses: [email protected] (X. Chao), [email protected] (Z. Yang). https://doi.org/10.1016/j.ceramint.2019.03.075 Received 7 December 2018; Received in revised form 28 February 2019; Accepted 11 March 2019 Available online 12 March 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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Fig. 1. The SEM images of CdCTO, CdCTO-10, CdCTO-20, CdCTO-50 precursor powders, and the elemental mapping images of CdCTO-20 sample.
coupled thermogravimetric analyser, and Fourier transform infrared spectroscopy (FTIR). We also observed a high low–frequency dielectric permittivity of over 10,000 for the obtained ceramics.
densities of the samples are all approximately 5.0 g/cm3, and the relative densities of the ceramics sintered at 975 °C and 985 °C are over 90%.
2. Experiments
2.2. Sample characterization
2.1. Sample preparation
The phase structures of the powders and ceramics were examined using X–ray diffraction (XRD, D/max–2550/PC Rigaku) with Cu Kα radiation. The natural surface microstructures of the samples were characterized by a scanning electron microscope (SEM, FEI, Model Quanta 200), while elemental distributions were characterized using energy dispersive X-ray spectroscopy (EDX) under a 20 kV accelerating voltage [3,11]. Raman spectra (Renishaw, Invia, 532 nm) were collected to further determine the crystallinity of the phases. The thermal behaviours of the mixed powders were explored using thermogravimetry-differential scanning calorimetry (TG–DSC, Q600SDT, TA, New Castle, DE) [11]. Fourier transform infrared spectroscopy (FTIR) was obtained from a Bruker Tensor27 FTIR spectrometer. The Archimedes method was used to obtain the bulk density.
CdCTO powders were fabricated using CdO (99%), TiO2 (99.99%), and CuO (99%) as raw materials with KCl and NaCl (99.99%) as the molten salt additives. Hot deionized water was employed as a detergent to wash off the molten salt. First, CdO (99%), TiO2 (99.99%), and CuO were mixed in stoichiometric ratios with different mass percentages of molten salts (0%, 10%, 20% and 50%, defined as CdCTO, CdCTO–10, CdCTO–20, CdCTO–50, respectively; KCl and NaCl were used in an equal molar ratio). Then, the mixed powders were milled for 10 h in ethanol with 5–6 mm agate balls as the grinding medium. The resulting slurry was dried in an oven at 60–80 °C and then calcined at 650–810 °C for 10 h. More details can be found in our previous work [15]. The samples were obtained by sintering these green pellets at 955 °C, 965 °C, 975 °C and 985 °C for 15 h [18,19]. As shown in Fig. S1, the bulk
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Fig. 2. The Raman spectra of CdCTO, CdCTO-10, CdCTO-20, CdCTO-50 precursor powders.
3. Results and discussions We first determined the optimal mass proportion of molten salts in the reaction. Fig. 1 displays the SEM images of CdCTO, CdCTO–10, CdCTO–20 and CdCTO–50 precursor powders when the sintering temperature was fixed at 730 °C. Interestingly, we found that the grain size gradually increases as the proportion of molten salt increases and that the shape of the grain gradually changes from spherical to cubic. In particular, it was found that there are some anomalously large cubic grains of dimensions of approximately 10 μm when the mass proportion of the molten salt is 50%. In addition, the elemental mapping images of the CdCTO–20 sample are displayed in Fig. 1(f–j). It was seen that all of the elements expect O are distributed inhomogeneously, indicating that the precursor powders do not have a single phase. The impure phases could include CuO/CuO2, CdTiO3, and TiO2, according to the XRD and Raman spectroscopy analysis as follows. Raman spectroscopy was used to further identify the phases of the precursor powders. As shown in Fig. 2, the Raman spectra were recorded for the CdCTO, CdCTO–10, CdCTO–20 and CdCTO–50 precursor powders that were sintered at 730 °C over the range of 200–1000 cm−1. As far as we know, the vibrational modes of CdCTO perovskite-like materials are the 4Fg+2Eg+2Ag combinations [11,18–22], which were barely seen in the CdCTO precursor powders, as shown in Fig. 2. Nevertheless, the characteristic Raman peaks of the CdCTO phase suddenly emerged in the precursor powders of CdCTO–10, CdCTO–20 and CdCTO–50 after molten salts were introduced. Six characteristic Raman bands at 263, 324, 444, 508, 576, and 764 cm−1 were detected in the CdCTO–10, CdCTO–20 and CdCTO–50 precursor powders, which can be assigned to the F1g, E1g, A1g, A2g, F3g, and F4g vibrational peaks, respectively [3]. In addition, the characteristic Raman bands of impurity phases could be identified among the Raman peaks of the CdCTO precursor powders. The peak at 238 cm−1 originates from CdO (TO) or CdTiO3 [23–25], the 324 cm−1 vibrational modes can be identified as that of either CuO (Bg) or TiO2 (Bg) [26–32], and the peaks at 392 and 508 cm−1 peaks can be identified as the B1g and A1g + B1g vibrational modes of TiO2, respectively [27,29,30]. Finally, peaks at 633 and 794 cm−1 belong to the vibration modes of CuO (Bg) and CdO (2LO), respectively [23–25]. According to the above analysis, the addition of molten salt can effectively improve the crystallization of the CdCTO phase. Eventually, considering the grain morphologies and degrees of crystallization, we selected 20% as the best molten salt ratio for further investigation.
Fig. 3. (a) The XRD patterns of CdCTO-20 precursor powders under different calcination conditions. (b) TG–DSC spectrogram, and (c) FT–IR absorbance spectrogram for CdCTO-20 precursor powders.
We then explored the optimal calcination temperature of CdCTO with 20% molten salts. Fig. 3(a) shows the XRD patterns of the CdCTO–20 precursor powders that were obtained under different calcination conditions. The major diffraction peaks can be assigned to the CdCTO phase (JCPDS # 48–0208), which is a body–centred cubic perovskite–like structure [3,11,17–19,33]. Meanwhile, the diffraction peaks of CuO, TiO2 and other impurity phases can also be detected when the calcination temperature was low, and their characteristic peaks gradually weaken as the sintering temperature increases. A relatively pure CdCTO phase can be achieved at a sintering temperature of 730 °C or above, indicating that 730 °C could be the optimal calcination temperature. In addition, an interesting phenomenon was found whereby the dominant growth of the crystal planes of the CdCTO phase
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Fig. 4. The scheme of proposed reaction mechanism.
Fig. 5. The (a) XRD patterns, (b) element mapping, and (c) frequency dependent dielectric permittivity of CdCTO-20 ceramics.
changes from occurring in the (100) plane to occurring in the (110) plane as the sintering temperature increases. This change could be related to the role of the low melting point molten salts. Moreover, one can see that the intensity of the peaks associated with 2(CdTiO3) and 4(CuO) decreases with increasing temperature from 730 to 810 °C, which should be attributed to the formation of the perovskite phase structure of the CdCTO. The result is consistent with the analysis of the following FTIR, and TG–DSC. Next, we investigated the reaction mechanism of the molten salt method using a TG–DSC spectrogram, which is a very useful technique for analysing reaction processes. The results are shown in Fig. 3(b). The melting points of NaCl and KCl are 801 °C and 775 °C, respectively, while the eutectic melting point of KCleNaCl is approximately 640–680 °C [14,15,34]. Therefore, there is almost no weight loss until 645 °C, where a strong endothermic peak occurs. Afterwards, a small amount of weight loss could be found at approximately 730 °C, and a relatively sharp endothermic peak appeared near 770 °C, both of which should be closely related to the formation of the CdCTO phase. Then, an exothermic peak was observed at approximately 838 °C, accompanied by a dramatic loss of weight, indicating the continuous volatilization of the molten salts. The endothermic peak at 937 °C corresponds to the complete formation of the perovskite phase structure of the CdCTO ceramics. The endothermic peaks at 1007 °C should be closely linked with the eutectic melting point of TiO2+CdO, and the small amount of weight loss corresponds to the volatilization of the impurity [35–39]. To further expound the phase generation and evolution mechanisms during the molten salt sintering process, FTIR absorbance spectrograms were conducted. In general, ACu3Ti4O12 compounds with perovskitelike structures should display three typical bands of oxygen–metal. As shown in Fig. 3(c), three characteristic absorption peaks at 563, 505 and 436 cm−1 were observed and are closely linked to vTi-O−1 and vTi-O = 653–550 cm−1 [11,39–42]. In addition, Ti = 495–436 cm a weak absorption peak was observed at 3450 cm−1, which can be attributed to the OeH vibrational bands of weakly–bound water, the CeH of alkyl groups, and the alcohol in mixed powders [3,11]. Moreover, the absorption band at approximately 775 cm−1 can mainly be attributed to the contribution from vTi-O in the raw TiO2 materials, while the absorption peak at 660 cm−1 result from vM-O (M = Cd/Cu) bending [11,40]. Finally, it can be seen that three typical absorption peaks at 563, 505 and 436 cm−1 gradually appear when the sintering temperature is in the range of 730–770 °C, confirming that a relatively pure CdCTO perovskite–like structure for the powders gradually forms. The data are consistent with the analyses and data from TG–DSC and XRD. Combining the data and analyses of XRD, FTIR, and TG–DSC, the optimal CdCTO phase-formation temperature for the precursor powders was approximately 730–770 °C in the molten salt method. Furthermore, due to the presence of CO2 in the atmosphere, the absorption band at 2350 cm−1 may come from absorbed CO2, and the peak at 1635 cm−1 11902
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may correspond to the asymmetrical stretching vibration of eCOOe [11,41]. Therefore, the reaction mechanism can be illustrated in Fig. 4 as according to the above discussion. Moreover, the likely reaction equation can be expressed as follows: Afterwards, we further investigated the optimal sintering temperature for obtaining ceramics. The microstructures and bulk densities of the CdCTO–20 ceramics sintered at varying temperatures are shown in Fig. S1. A high bulk density of ∼5.34 g/cm3 and dense grain morphology can be obtained when the sintering temperature is 975 °C. Fig. 5(a) shows the XRD pattern of the obtained CdCTO–20 ceramic sintered at 975 °C, and the insert displays an SEM image of the sample surface. All of the diffraction peaks can be indexed to the body–centred cubic perovskite–like CdCTO phase (JCPDS # 48–0208) [11,17–19,33], indicating that a pure CdCu3Ti4O12 phase without a secondary phase was obtained. The obtained ceramic exhibits uniform grain sizes of approximately 15 ± 5 μm. The elemental mapping using EDX implies a homogeneous distribution of the elements, as shown in Fig. 5(b). Fig. 5(c) plots the frequency dependence of the dielectric permittivity of the CdCTO–20 ceramics. High dielectric constants (> 104) were achieved in the low-frequency range, but the dielectric constant decreases dramatically with increasing frequency. Meanwhile, the dielectric loss shows a reverse trend. It is a typical characteristic of Maxwell–Wagner polarization relaxation [3,4,6,7,9,11]. The relaxation can be simulated using a parallel resistor–capacitor element (RC element), as shown in the insert of Fig. 5(c). Therefore, the high dielectric constant should be closely related to the internal barrier layer capacitor (IBLC) effect, and two responses could describe the contributions of the grain boundary and grain, respectively. On the one hand, the frequency stability deterioration may result from the high grain resistance deteriorating the IBLC effect [43]. On the other hand, reduced generation of defects (such as oxygen vacancies) causes insufficient grain semi–conduction, which results from the reduction in the sintering temperature with the molten salt method and eventually results in the instability of the dielectric frequency properties of the obtained ceramics. 4. Conclusions We adopted a low–temperature molten salt method to synthesize CdCu3Ti4O12 precursor powders and significantly reduced the calcination temperature compared to that of other reported methods. Powers sintered at approximately 730 °C were confirmed to be of relatively pure phase for CdCu3Ti4O12 by XRD, TG–DSC, and FTIR. The obtained ceramics (sintered at 975 °C) were found to have a pure perovskite–like phase structure, homogeneous grain size and high bulk density. Remarkably, the ceramics also exhibit a high dielectric constant (> 104) at low frequency. Overall, our process for synthesizing CdCTO ceramics with the molten salt method could shed light on the low–temperature and large–scale commercial production of such materials in the future. Acknowledgements This work was supported by the National Science Foundation of China (Grant No. 51872177, 51572163, 51577111 and 51607108) and the Fundamental Research Funds for the Central Universities (Program No. 2018TS081, GK201903017, GK201802007, and 201701011). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.03.075. References [1] C. Zhao, J. Wu, Effects of secondary phases on the high-performance colossal permittivity in titanium dioxide ceramics, ACS Appl. Mater. Interfaces 10 (2018)
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Low-temperature synthesis of CdCu3Ti4O12 powders with high dielectric permittivities
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Zhanhui Penga, Pengfei Liangb, Bi Chena, Yichen Xianga, Hui Penga, Di Wua, Xiaolian Chaoa,∗∗, Zupei Yanga,∗ a Key Laboratory for Macromolecular Science of Shaanxi Province, Shaanxi Key Laboratory for Advanced Energy Devices, Shaanxi Engineering Laboratory for Advanced Energy Technology, School of Materials Science and Engineering, Shaanxi Normal University, Xi'an, 710062, Shaanxi, China b School of Physics and Information Technology, Shaanxi Normal University, Xi'an, 710062, Shaanxi, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Molten salt method Low-energy sintering Perovskite phase High dielectric constant
The molten salt method was adopted in this work to synthesize CdCu3Ti4O12 precursor powders and was found capable of notably reducing the sintering temperature. It was also found that the grain size gradually increases and the grain shape changes from spherical to cubic with increasing proportions of molten salt. The optimal sintering conditions were found to be at approximately 730 °C with the addition of 20% molten salt. After further sintering at 975 °C, the obtained perovskite ceramics exhibited a high dielectric constant at low frequencies.
1. Introduction Due to the urgent requirement for the integration and miniaturization of microelectronic techniques [1–10], materials with high dielectric constants have been extensively studied recently. CaCu3Ti4O12 (CCTO) is a non–ferroelectric material because of its relatively flat high permittivity–temperature/frequency profiles. CCTO and CCTO–based ceramics have been successfully synthesized by various preparation methods, e.g., Chen et al. [7] successfully prepared CCTO materials via a standard mixed–oxide method. Considering the high-temperature sintering conditions that are adopted in the solid–state method, an oxalate co–precipitation method was introduced to prepare CCTO powders by T. R. N. Kutty et al. [8]. Li et al. [9] also successfully prepared CCTO powders with high dielectric constants using a sol–gel technique. However, these wet chemical methods are unlikely to be applied in industrial production because of their relatively high–cost materials, such as expensive metal alkoxides, use of inorganic salt solutions and complicated fabrication processes [6,8,9,11,12]. In 2013, Liu et al. [13] used a low–temperature molten salt method and successfully prepared CCTO powders and obtained a high dielectric permittivity in the resulting ceramics. The molten salt method is an efficient low–temperature sintering strategy that usually involves one or several low melting point salts as the reaction medium [13–16]. During the reaction process, low melting point salts are used as the
reaction medium, and the reactants, which have a certain degree of solubility in the salts, thus exhibit an accelerated diffusion rate. The reaction is converted into a solid–liquid reaction. The merits of this method can thus be concluded as follows: low synthetic temperatures, short reaction times, homogeneous chemical composition for the obtained powders, good crystal morphologies and high–purity phases [15]. In addition, the salts can easily be separated for recycling [16]. To date, the molten salt method has been used to fabricate powders of a wide range of dielectric materials, e.g., Hou et al. [14] reduced the sintering temperature from > 1100 °C to 700 °C for Na0.9K0.1NbO3 powders, and Yang et al. [15] reported a synthetic temperature as low as 1000 °C for (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 powders, both of whom used the molten salt method. However, there is no literature reporting on the preparation of CdCTO powders using the molten salt method, as far as we know. CdCu3Ti4O12 (abbreviated as CdCTO) has a similar crystal structure to CCTO, which is that of the perovskite structure compound ACu3Ti4O12. CdCTO has been successfully prepared and was shown to exhibit some very interesting dielectric performances, as reported [11,17–19]. Herein, the molten salt method was adopted in this work to prepare CdCTO powders at substantially lower sintering temperatures than have been used previously. The phase generation and evolution mechanism of the powders were explored and analysed using an X-ray diffractometer (XRD), differential scanning calorimeter (TG–DSC)
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Corresponding author. Key Laboratory for Macromolecular Science of Shaanxi Province, Shaanxi Key Laboratory for Advanced Energy Devices, School of Materials Science and Engineering, Shaanxi Normal University, Xi'an, 710062, Shaanxi, PR China. ∗∗ Corresponding author. E-mail addresses: [email protected] (X. Chao), [email protected] (Z. Yang). https://doi.org/10.1016/j.ceramint.2019.03.075 Received 7 December 2018; Received in revised form 28 February 2019; Accepted 11 March 2019 Available online 12 March 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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Fig. 1. The SEM images of CdCTO, CdCTO-10, CdCTO-20, CdCTO-50 precursor powders, and the elemental mapping images of CdCTO-20 sample.
coupled thermogravimetric analyser, and Fourier transform infrared spectroscopy (FTIR). We also observed a high low–frequency dielectric permittivity of over 10,000 for the obtained ceramics.
densities of the samples are all approximately 5.0 g/cm3, and the relative densities of the ceramics sintered at 975 °C and 985 °C are over 90%.
2. Experiments
2.2. Sample characterization
2.1. Sample preparation
The phase structures of the powders and ceramics were examined using X–ray diffraction (XRD, D/max–2550/PC Rigaku) with Cu Kα radiation. The natural surface microstructures of the samples were characterized by a scanning electron microscope (SEM, FEI, Model Quanta 200), while elemental distributions were characterized using energy dispersive X-ray spectroscopy (EDX) under a 20 kV accelerating voltage [3,11]. Raman spectra (Renishaw, Invia, 532 nm) were collected to further determine the crystallinity of the phases. The thermal behaviours of the mixed powders were explored using thermogravimetry-differential scanning calorimetry (TG–DSC, Q600SDT, TA, New Castle, DE) [11]. Fourier transform infrared spectroscopy (FTIR) was obtained from a Bruker Tensor27 FTIR spectrometer. The Archimedes method was used to obtain the bulk density.
CdCTO powders were fabricated using CdO (99%), TiO2 (99.99%), and CuO (99%) as raw materials with KCl and NaCl (99.99%) as the molten salt additives. Hot deionized water was employed as a detergent to wash off the molten salt. First, CdO (99%), TiO2 (99.99%), and CuO were mixed in stoichiometric ratios with different mass percentages of molten salts (0%, 10%, 20% and 50%, defined as CdCTO, CdCTO–10, CdCTO–20, CdCTO–50, respectively; KCl and NaCl were used in an equal molar ratio). Then, the mixed powders were milled for 10 h in ethanol with 5–6 mm agate balls as the grinding medium. The resulting slurry was dried in an oven at 60–80 °C and then calcined at 650–810 °C for 10 h. More details can be found in our previous work [15]. The samples were obtained by sintering these green pellets at 955 °C, 965 °C, 975 °C and 985 °C for 15 h [18,19]. As shown in Fig. S1, the bulk
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Fig. 2. The Raman spectra of CdCTO, CdCTO-10, CdCTO-20, CdCTO-50 precursor powders.
3. Results and discussions We first determined the optimal mass proportion of molten salts in the reaction. Fig. 1 displays the SEM images of CdCTO, CdCTO–10, CdCTO–20 and CdCTO–50 precursor powders when the sintering temperature was fixed at 730 °C. Interestingly, we found that the grain size gradually increases as the proportion of molten salt increases and that the shape of the grain gradually changes from spherical to cubic. In particular, it was found that there are some anomalously large cubic grains of dimensions of approximately 10 μm when the mass proportion of the molten salt is 50%. In addition, the elemental mapping images of the CdCTO–20 sample are displayed in Fig. 1(f–j). It was seen that all of the elements expect O are distributed inhomogeneously, indicating that the precursor powders do not have a single phase. The impure phases could include CuO/CuO2, CdTiO3, and TiO2, according to the XRD and Raman spectroscopy analysis as follows. Raman spectroscopy was used to further identify the phases of the precursor powders. As shown in Fig. 2, the Raman spectra were recorded for the CdCTO, CdCTO–10, CdCTO–20 and CdCTO–50 precursor powders that were sintered at 730 °C over the range of 200–1000 cm−1. As far as we know, the vibrational modes of CdCTO perovskite-like materials are the 4Fg+2Eg+2Ag combinations [11,18–22], which were barely seen in the CdCTO precursor powders, as shown in Fig. 2. Nevertheless, the characteristic Raman peaks of the CdCTO phase suddenly emerged in the precursor powders of CdCTO–10, CdCTO–20 and CdCTO–50 after molten salts were introduced. Six characteristic Raman bands at 263, 324, 444, 508, 576, and 764 cm−1 were detected in the CdCTO–10, CdCTO–20 and CdCTO–50 precursor powders, which can be assigned to the F1g, E1g, A1g, A2g, F3g, and F4g vibrational peaks, respectively [3]. In addition, the characteristic Raman bands of impurity phases could be identified among the Raman peaks of the CdCTO precursor powders. The peak at 238 cm−1 originates from CdO (TO) or CdTiO3 [23–25], the 324 cm−1 vibrational modes can be identified as that of either CuO (Bg) or TiO2 (Bg) [26–32], and the peaks at 392 and 508 cm−1 peaks can be identified as the B1g and A1g + B1g vibrational modes of TiO2, respectively [27,29,30]. Finally, peaks at 633 and 794 cm−1 belong to the vibration modes of CuO (Bg) and CdO (2LO), respectively [23–25]. According to the above analysis, the addition of molten salt can effectively improve the crystallization of the CdCTO phase. Eventually, considering the grain morphologies and degrees of crystallization, we selected 20% as the best molten salt ratio for further investigation.
Fig. 3. (a) The XRD patterns of CdCTO-20 precursor powders under different calcination conditions. (b) TG–DSC spectrogram, and (c) FT–IR absorbance spectrogram for CdCTO-20 precursor powders.
We then explored the optimal calcination temperature of CdCTO with 20% molten salts. Fig. 3(a) shows the XRD patterns of the CdCTO–20 precursor powders that were obtained under different calcination conditions. The major diffraction peaks can be assigned to the CdCTO phase (JCPDS # 48–0208), which is a body–centred cubic perovskite–like structure [3,11,17–19,33]. Meanwhile, the diffraction peaks of CuO, TiO2 and other impurity phases can also be detected when the calcination temperature was low, and their characteristic peaks gradually weaken as the sintering temperature increases. A relatively pure CdCTO phase can be achieved at a sintering temperature of 730 °C or above, indicating that 730 °C could be the optimal calcination temperature. In addition, an interesting phenomenon was found whereby the dominant growth of the crystal planes of the CdCTO phase
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Fig. 4. The scheme of proposed reaction mechanism.
Fig. 5. The (a) XRD patterns, (b) element mapping, and (c) frequency dependent dielectric permittivity of CdCTO-20 ceramics.
changes from occurring in the (100) plane to occurring in the (110) plane as the sintering temperature increases. This change could be related to the role of the low melting point molten salts. Moreover, one can see that the intensity of the peaks associated with 2(CdTiO3) and 4(CuO) decreases with increasing temperature from 730 to 810 °C, which should be attributed to the formation of the perovskite phase structure of the CdCTO. The result is consistent with the analysis of the following FTIR, and TG–DSC. Next, we investigated the reaction mechanism of the molten salt method using a TG–DSC spectrogram, which is a very useful technique for analysing reaction processes. The results are shown in Fig. 3(b). The melting points of NaCl and KCl are 801 °C and 775 °C, respectively, while the eutectic melting point of KCleNaCl is approximately 640–680 °C [14,15,34]. Therefore, there is almost no weight loss until 645 °C, where a strong endothermic peak occurs. Afterwards, a small amount of weight loss could be found at approximately 730 °C, and a relatively sharp endothermic peak appeared near 770 °C, both of which should be closely related to the formation of the CdCTO phase. Then, an exothermic peak was observed at approximately 838 °C, accompanied by a dramatic loss of weight, indicating the continuous volatilization of the molten salts. The endothermic peak at 937 °C corresponds to the complete formation of the perovskite phase structure of the CdCTO ceramics. The endothermic peaks at 1007 °C should be closely linked with the eutectic melting point of TiO2+CdO, and the small amount of weight loss corresponds to the volatilization of the impurity [35–39]. To further expound the phase generation and evolution mechanisms during the molten salt sintering process, FTIR absorbance spectrograms were conducted. In general, ACu3Ti4O12 compounds with perovskitelike structures should display three typical bands of oxygen–metal. As shown in Fig. 3(c), three characteristic absorption peaks at 563, 505 and 436 cm−1 were observed and are closely linked to vTi-O−1 and vTi-O = 653–550 cm−1 [11,39–42]. In addition, Ti = 495–436 cm a weak absorption peak was observed at 3450 cm−1, which can be attributed to the OeH vibrational bands of weakly–bound water, the CeH of alkyl groups, and the alcohol in mixed powders [3,11]. Moreover, the absorption band at approximately 775 cm−1 can mainly be attributed to the contribution from vTi-O in the raw TiO2 materials, while the absorption peak at 660 cm−1 result from vM-O (M = Cd/Cu) bending [11,40]. Finally, it can be seen that three typical absorption peaks at 563, 505 and 436 cm−1 gradually appear when the sintering temperature is in the range of 730–770 °C, confirming that a relatively pure CdCTO perovskite–like structure for the powders gradually forms. The data are consistent with the analyses and data from TG–DSC and XRD. Combining the data and analyses of XRD, FTIR, and TG–DSC, the optimal CdCTO phase-formation temperature for the precursor powders was approximately 730–770 °C in the molten salt method. Furthermore, due to the presence of CO2 in the atmosphere, the absorption band at 2350 cm−1 may come from absorbed CO2, and the peak at 1635 cm−1 11902
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may correspond to the asymmetrical stretching vibration of eCOOe [11,41]. Therefore, the reaction mechanism can be illustrated in Fig. 4 as according to the above discussion. Moreover, the likely reaction equation can be expressed as follows: Afterwards, we further investigated the optimal sintering temperature for obtaining ceramics. The microstructures and bulk densities of the CdCTO–20 ceramics sintered at varying temperatures are shown in Fig. S1. A high bulk density of ∼5.34 g/cm3 and dense grain morphology can be obtained when the sintering temperature is 975 °C. Fig. 5(a) shows the XRD pattern of the obtained CdCTO–20 ceramic sintered at 975 °C, and the insert displays an SEM image of the sample surface. All of the diffraction peaks can be indexed to the body–centred cubic perovskite–like CdCTO phase (JCPDS # 48–0208) [11,17–19,33], indicating that a pure CdCu3Ti4O12 phase without a secondary phase was obtained. The obtained ceramic exhibits uniform grain sizes of approximately 15 ± 5 μm. The elemental mapping using EDX implies a homogeneous distribution of the elements, as shown in Fig. 5(b). Fig. 5(c) plots the frequency dependence of the dielectric permittivity of the CdCTO–20 ceramics. High dielectric constants (> 104) were achieved in the low-frequency range, but the dielectric constant decreases dramatically with increasing frequency. Meanwhile, the dielectric loss shows a reverse trend. It is a typical characteristic of Maxwell–Wagner polarization relaxation [3,4,6,7,9,11]. The relaxation can be simulated using a parallel resistor–capacitor element (RC element), as shown in the insert of Fig. 5(c). Therefore, the high dielectric constant should be closely related to the internal barrier layer capacitor (IBLC) effect, and two responses could describe the contributions of the grain boundary and grain, respectively. On the one hand, the frequency stability deterioration may result from the high grain resistance deteriorating the IBLC effect [43]. On the other hand, reduced generation of defects (such as oxygen vacancies) causes insufficient grain semi–conduction, which results from the reduction in the sintering temperature with the molten salt method and eventually results in the instability of the dielectric frequency properties of the obtained ceramics. 4. Conclusions We adopted a low–temperature molten salt method to synthesize CdCu3Ti4O12 precursor powders and significantly reduced the calcination temperature compared to that of other reported methods. Powers sintered at approximately 730 °C were confirmed to be of relatively pure phase for CdCu3Ti4O12 by XRD, TG–DSC, and FTIR. The obtained ceramics (sintered at 975 °C) were found to have a pure perovskite–like phase structure, homogeneous grain size and high bulk density. Remarkably, the ceramics also exhibit a high dielectric constant (> 104) at low frequency. Overall, our process for synthesizing CdCTO ceramics with the molten salt method could shed light on the low–temperature and large–scale commercial production of such materials in the future. Acknowledgements This work was supported by the National Science Foundation of China (Grant No. 51872177, 51572163, 51577111 and 51607108) and the Fundamental Research Funds for the Central Universities (Program No. 2018TS081, GK201903017, GK201802007, and 201701011). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.03.075. References [1] C. Zhao, J. Wu, Effects of secondary phases on the high-performance colossal permittivity in titanium dioxide ceramics, ACS Appl. Mater. Interfaces 10 (2018)
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