Original Article A novel CaCu2.8-xZnxTi4O12 system: a highperformance dielectric with nonlinear J E properties Ekaphan Swatsitang a,b , Sasitorn Putjuso c , Supinya Nijpanich d , Miskawan Sriphakdee d , Thanin Putjuso c,* a Institute of Nanomaterials Research and Innovation for Energy (IN-RIE), Khon Kaen University, Khon Kaen 40002, Thailand b Department of Physics, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand c Giant Dielectric Materials Research and Development Unit (GDM-RDU), Department of Physics and Mathematics, Faculty of Liberal Arts, Rajamangala University of Technology Rattanakosin, Wang Klai Kangwon Campus, Hua Hin, Prachuap Khiri Khan 77110, Thailand d Synchrotron Light Research Institute (Public Organization), 111 University Avenue, Muang District, Nakhon Ratchasima 30000, Thailand article info Article history: Received 1 May 2022 Accepted 2 July 2022 Available online 6 July 2022 Keywords: Zn2þ-doped CaCu2.8Ti4O12 Nonlinear J-E behaviors Chemical route Colossal dielectric properties Grain boundary abstract Colossal dielectric constants (ε0 ~11,550 e 15,535) with very low dielectric loss (tand ~0.013 e 0.033) at 30 C and 1 kHz were achieved in CaCu2.8-xZnxTi4O12 (x ¼ 0.00 e 0.10) ceramics sintered at 1100 C for 8 and 12 h. Nonlinear J-E behaviors were observed in these ceramics with an extremely high nonlinear coefficient (a) of ~10.14 in CaCu2.75Zn0.05Ti4O12 sintered for 12 h. Additionally, a breakdown electric field (Eb) was found to increase in samples with a high Zn2þ ion content. To reveal these excellent properties, the phase and microstructure of these materials were explored using XRD and FE-SEM. Elemental mapping and EDXS spectra confirmed the presence of TiO2 and CaTiO3 minor phases with Zn2þ ions in the main CaCu3Ti4O12 phase. Improved dielectric properties and nonlinear J-E behaviors are discussed based on the increased grain boundary resistance (Rgb) due to the presence of minor phases and substitution of Zn2þ ions at the Cu2þ sites of CaCu2.8-xZnxTi4O12. The XPS results confirmed that electrons could hop at the Cuþ O Cu2þ and Cu3þ O Cu2þ sites, indicating the presence of semiconducting grains in these ceramics. © 2022 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). * Corresponding author. E-mail address: [email protected] (T. Putjuso). Available online at www.sciencedirect.com journal homepage: www.elsevier.com/locate/jmrt journal of materials research and technology 2022;19:4473 e4488 https://doi.org/10.1016/j.jmrt.2022.07.016 2238-7854/© 2022 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction High-performance dielectric CaCu3Ti4O12 (CCTO)-based materials have been developed using different approaches such as doping with various transition metals (TM) in CCTO/CaTiO3 [1e4], modifying these materials with TM/rare earth (RE) elements in CCTO/TiO2 [5e11], doping and co-doping with TM in CCTO [12e15], Mg doped and non-doped Cu-deficient CaCu2.8Ti4O12 [16,17], among others [18e24]. Other than these, the colossal dielectric properties of modified materials such as TiO2 [25e27], BaTiO3 [28,29], and SrTiO3 [30,31] have been frequently reported. Presently, increasing demand for numerous kinds of high-performance electronic devices is inevitable due to the emergence of new technologies. For instance, high-performance dielectric materials are necessary for multilayer ceramic capacitor chips (MLCCs), which are widely used as passive components in high-energy density and memory devices. Compared with other dielectric materials, TM/RE doping in CCTO/CaTiO3 and CCTO/TiO2 has been recognized as the most promising way to meaningfully improve the dielectric (ε0 ) properties of such materials. In particular, the temperature range (TR) of the dielectric coefficient (Dε0 ) where it is less than ±15% (Dε0 < ± 15%) can be extended and simultaneously improve nonlinear current (J)- voltage (E) behaviors. Jumpatam and coworkers [3] reported that good nonlinear J-E properties and high-performance dielectric behaviors (ε0 ~3550, tand ~0.014 and Dε0 < ± 15% in a TR of 60 C to 160 C) of CCTO/CaTiO3 materials doped with Mg2þ could be achieved through a ball-milling process. They suggested that enhancement of nonlinear J-E and dielectric behaviors was generated by the interfacial responses at grain boundaries (GBs) of CCTO-CTO and CCTOeCCTO. Swatsitang and coworkers reported the high-performance CCTO/TiO2 materials doped with RE (Sm3þ, Pr3þ and Ho3þ) [5,7,9] and TM (Zn2þ and Mg2þ) [32,33] via a polymer pyrolysis (PP) process. In their work, impressively improved nonlinear J-E parameters and high-performance dielectric (ε0 > 9900, tand ~0.017 and Dε0 < ± 15% in a TR of 60 C to 210 C) properties were obtained. They reported that metal dopants and the presence of a TiO2 phase at GBs played a crucial role in enhancing Rgb values as well as in the improving nonlinear J-E and dielectric properties. Normally, improvement of nonlinear J-E and dielectric properties can be rationally explained using the IBLC model [2,34], corresponding to the low conductivity of GBs with the semiconducting grain (Gs) behavior [12,15,35,36]. It is reasonable that the presence of CaTiO3 and TiO2 phases could greatly enhance the electrical behaviors at GBs and improve the dielectric and nonlinear properties. Furthermore, it was found that the secondary phases of TiO2 and CaTiO3 could easily occur during the formation of a CCTO phase through a single-step preparation of Cu-nonstoichiometric CaCu3þxTi4- O12 materials [16,37e42]. Yuan et al. [39] described the increased Rgb due to the presence of CaTiO3 and TiO2 phases in a nominal CaCu3þxTi4O12 system. Moreover, the occurrence of both secondary phases could decrease tand values due to an increased Rgb in CaCu3xTi4O12 ceramics, as reported by Liu et al. [40]. Recently, we reported [16] the outstanding nonlinear J-E and dielectric behavior of CaCu3þxTi4O12 materials with incrementally changed Rgb values, caused by TiO2 and CaTiO3 phases at GBs. Moreover, many researchers successfully enhanced the giant-ε0 and low-tand values of CCTO-based materials using Zn2þ doping at Cu2þ sites [15,32,43,44]. Concurrently, we found that good nonlinear J-E and dielectric properties of CCTO-based materials could be attained by tuning the GBs density in a PP process [7,10]. Therefore, it is clearly evident that the simultaneous presence of TiO2 and CaTiO3 phases in CCTO-based materials doped with Zn2þ might enhance both the nonlinear and dielectric properties. Therefore, the aim of this work is to improve the nonlinear J-E behavior and dielectric properties of CCTO-based materials with the presence of CaTiO3 and TiO2 phases and Zn2þ doping at the Cu2þ sites in nominal compositions of CaCu2.8-xZnxTi4- O12 ceramics (where, x ¼ 0.0, 0.05 and 0.10) using a PP process. Interestingly, high-performance dielectric properties were attained with a colossal-ε0 of ~11,550, tand ~0.022 and Dε0 < ± 15% over a broad TR of 60 C to 210 C. Compared to undoped CaCu2.8Ti4.2O12 materials, the nonlinear J-E behaviors of Zn2þdoped CaCu2.8Ti4.2O12 ceramics were significantly improved. The effects of TiO2 and CaTiO3 secondary phases with Zn2þ doping on the microstructural, nonlinear J-E and dielectric behaviors are discussed. 2. Experimental details CaCu2.8-xZnxTi4O12 (x ¼ 0.00, 0.05 and 0.10) powders were fabricated via a PP process. The purities of all chemicals are greater than 99.95%, i.e., Zn)NO3(2.6H2O, Cu(NO3)2.3H2O, TiC16H28O6, Ca(NO3)2.4H2O, CH2 ¼ CHCOOH and N2H8S2O4. They were purchased from SigmaeAldrich. A schematic with complete details of the PP process, describing the powder preparation of CaCu2.8-xZnxTi4O12 (x ¼ 0.00, 0.05 and 0.10) was given in our earlier research [9,10]. To prepare ceramic samples, the obtained powders were pressed at 1.5 106 Pa into pellets of a thickness (d) ~1.15 mm and a radius (r) ~4.8 mm. After that they were separately sintered for 8 and 12 h at 1100 C in air. The ceramics sintered for 8 h were designated as 1100e8 (CC2TO-8, 05Zn-8 and 10Zn-8) and those for 12 h as 1100e12 (CC2TO-12, 05Zn-12 and 10Zn-12). Energy-dispersive X-ray spectroscopy (EDXS) and fieldemission scanning electron microscopy (FE-SEM) (FEI, Helios, Nano lab) were used to study the elemental distribution of O, Ti, Cu, Ca and Zn in the ceramics and their backscattered surface morphology. The components and phase structure of these ceramics were investigated using X-ray diffraction (XRD) (AG-D8, Advance, Bio-spin, Bruker) coupled with a Rietveld refinement profile fit (RRF) scheme. The RRF scheme had been extensively reported in previous work [45]. Complete details, describing the determination process of RRF essential parameters such as R (Rexp, Rwp, and Rp) values and goodness of fit (GOF) were given elsewhere [46]. X-ray photoelectron spectroscopy (XPS; PHI5000, ULVAC, PHI) was used to explore the various transition valence states (Cuþ, Cu2þ and Cu3þ) of Cu ions in selected samples. A Hewlett Packard 4194 A gain phase analyzer was used to measure the capacitance (C) and tand values of all ceramics in the current study over broad temperature (60 C 210 C) and frequency (100 Hz 1 MHz) ranges. The ε0 values were calculated from ε' ¼ Cd/ε0A, where A is the surface area (A ¼ pr 2 ) of a pellet. The 4474 journal of materials research and technology 2022;19:4473 e4488
nonlinear J-E behaviors were explored at 30 C with a Keithley Model 247 high voltage unit. J-E plots were examined to obtain the nonlinear parameters of ceramics (Eb and a) [8,10]. 3. Results and discussion The RRF of XRD patterns for all 1100e8 and 1100e12 ceramics are respectively presented in Fig. 1 (a)e(c) and (d) ‒ (f). All of the ceramic's main peaks are consistent with the standard CaCu3Ti4O12 (ICSD-95714). Additionally, the secondary phases of CaTiO3 (ICSD-06-2149) and TiO2 (ICSD-26715) were detected. From the RRF patterns, the CaCu3Ti4O12: TiO2: CaTiO3 ratios were calculated and these results were presented in Table 1. The XRD peaks of all ceramics match the bcc structure with space group Im3 (204) of CCTO ceramics. All GOF parameters and R factors are acceptable and the values are in the ranges of ~1.3e1.7 and ~3.4e5.8%, respectively. Moreover, the lattice constant (a) of all ceramics corresponds with a ¼ 7.390 (8) A (standard CCTO), as reported in the literature [47,48]. Notably, Fig. 1 e RRF results for XRD patterns of (a) CC2TO-8, (b) 05Zn-8, (c) 10Zn-8, (d) CC2TO-12, (e) 05Zn-12 and (f) 10Zn-12 ceramics. journal of materials research and technology 2022;19:4473 e4488 4475
the a value does not significantly change with an increased level of Zn2þ dopant due to the slightly different ionic radii of Cu2þ (87 pm) and Zn2þ (88 pm) ions. The calculated a value, resulting from Zn2þ doping in this work, is similar to those reported for other oxide materials [44,49,50]. Additionally, the average crystallite size (Dav) was calculated using Dav ¼ kl= b cos q [ 51,52 ], where b and q are respectively the FWHM and diffraction angle, while l is the X-ray wavelength (0.15406 nm). The obtained Dav values of all ceramics were found to vary over a 2.58e2.88 nm range. Increasing sintering time from 8 to 12 h slightly increased the Dav value, as illustrated in Table 1. Moreover, the microstrain values (ε) of these ceramics were calculated as described in earlier published literature [51,52]. As seen in Table 1, the ε values varied slightly depending on the level of Zn doping and sintering time. Furthermore, the experimental and theoretical densities (Dexp and Dxrd) of all ceramics were determined and found to vary slightly with Zn content and sintering time, as revealed in Table 1. Additionally, the porosity P (%) was computed using Pð%Þ ¼ 100 ðDxrd D expÞ=Dxrd [51,52]. The obtained P (%) data, as shown in Table 1, were found to decrease with increasing Zn content and sintering time, i.e., from 19.21 to 15.89 and 10.50 to 6.97 for 1100e8 and 1100e12 ceramics, respectively. It is well established that the nonlinear J-E behaviors and dielectric properties of CCTO-based materials are significantly dependent on their microstructure, as previously reported in the literature [6,23]. Therefore, the microstructure and composite phases of these ceramics at various Zn2þ doping level and different sintering conditions were meticulously studied [15,49,53]. Backscattered FE-SEM images of the refined surfaces of 1100e8 and 1100e12 ceramics are respectively revealed in Fig. 2 (a)e(c) and (d)-(f). As seen in these figures, the matrix of all ceramics is composed of small and large grains with pores. Some of very small grains are observed to embed in the CCTO matrix, while some of them are randomly distributed on the grains and most of them on GBs. This observed texture might have been caused by the unequal growth rates of CaCu2.8-xZnxTi4O12 grains and the formation of TiO2 and CaTiO3 phases during the sintering process [1,2,16,54]. The determined average grain sizes (AGS) from FESEM images in Fig. 2 (a)e(c) were found to be 5.687 ± 2.421, 7.201 ± 2.234 and 7.947 ± 2.726 mm for CC2TO-8, 05Zn-8 and 10Zn-8 ceramics, respectively. Moreover, increase sintering time can result in a slight increase of AGS of 6.519 ± 2.542, 7.623 ± 2.366 and 7.754 ± 1.892 mm for CC2TO-12, 05Zn-12 and 10Zn-12 ceramics, respectively. These data support the hypothesis that sintering time causes an increased AGS of the ceramics [55,56]. Slight increase of AGS in this work are consistent with those reported for the Ca(Cu1-xZnx)3Ti4O12 system [50]. To verify the presence and distribution of CaTiO3 and TiO2 in CaCu2.8-xZnxTi4O12 ceramics, elemental mapping and EDXS spectra were performed and analyzed. The backscattered FESEM images of undoped (CC2TO-12) and doped (10Zn-12) ceramics are shown in Fig. 3 (a) and (d), respectively. Sub-images of Fig. 3 (a) display the mapping of Ca, Ti, Cu and O elements, corresponding to the backscattered FE-SEM images of the CC2TO-12 ceramic shown in Fig. 3 (a). In these sub-images, the presence CaTiO3 and TiO2 phases were mostly found at GBs shown as the light blue region (marked point (þ1)) and the red region (marked point (þ2)), respectively. This confirms that Cu was not detected at these two marked points. Moreover, the EDXS results of marked point (þ1), shown in Fig. 3(b), revealed a slight deviation in the ratio of Ca (20.0 A t%): Cu (19.5 A t%): Ti (40.1 A t%): O (20.3 A t%), which is equivalent to the presence of a CaTiO3 phase. Additionally, the ratio of Ca (3.9 A t%): Cu (20.0 A t%): Ti (57.8 A t%): O (18.3 A t%) calculated from the EDXS data of marked point (þ2), displayed in Fig. 3(c), indicates the presence of a TiO2 phase. These results suggest that the red and light blue regions correspond to TiO2 and CaTiO3 phases, respectively. Furthermore, as seen in Fig. 3(d), the TiO2 phase at GBs (marked point (þ3)) of 10Zn-12 was found to be at a higher level than that observed in the CC2TO12 ceramic, corresponding to a significant decrease of the CaTiO3 phase at this point. As a result, it might be suggested that the incorporation of Ti atoms to form a pure CCTO phase in 10Zn-12 occurred to a lesser degree than in the CC2TO-12 ceramic [16,37,54]. Additionally, Ca and Cu were not detected at marked point (þ3), whereas many light blue regions were observed at GBs. Furthermore, as seen in the sub-image of Fig. 3(d), the mapping of elemental Zn shows a homogenous dispersion, confirming that no Zn-based oxide was formed at either the GBs or Gs of the 10Zn-12 ceramic. Additionally, the high At% of Ti and O in the Ca: Cu: Ti: O ratio of 1.4: 5.5: 51.4: 41.7 at marked point (þ3), calculated from the EDXS spectra in Table 1 e The important parameters derived from the RRF results for XRD patterns of 1100e8 and 1100e12 ceramics. Ceramics 1100e8 1100e12 CC2TO-8 05Zn-8 10Zn-8 CC2TO-12 05Zn-12 10Zn-12 a (A) 7.39336 (6) 7.39358 (9) 7.39345 (8) 7.39345 (7) 7.39414 (7) 7.39354 (7) Rp (profile %) 3.67886 4.59661 4.24067 3.69344 4.00054 4.11049 Rwp (weighted profile %) 4.71378 5.83000 5.47373 4.89045 5.16314 5.36816 Rexp (expected %) 3.59453 4.56704 4.38613 3.60861 4.39435 4.68345 GOF (Goodness of fit) 1.71970 1.62955 1.55741 1.73661 1.38051 1.31377 Dxrd (g.cm3 ) 4.9742 5.0047 4.9938 4.9826 5.0108 4.9978 Dexp (g.cm3 ) 4.0184 4.0766 4.2002 4.4594 4.5562 4.6490 Е 0.0065 0.0059 0.0087 0.0063 0.0065 0.0079 P (%) 19.21 18.54 15.89 10.50 9.07 6.97 Dav (nm) 2.69 2.58 2.72 2.88 2.87 2.79 CCTO: TiO2: CTO 92:5.6:2.4 94:4.8:1.5 92:5.6:2.1 90.9:6.6:2.5 94:4.3:1.9 93:5.2:2.1 4476 journal of materials research and technology 2022;19:4473 e4488
Fig. 3(e), confirms the presence of a TiO2 phase. Moreover, as seen in Fig. 3(f), the determined At% ratio Ca: Cu: Ti: O of 1.4: 5.5: 38.4: 41.7 at marked point (þ4) in Fig. 3(d) was close to the 1: 3: 4: 12 stoichiometric formula of the CCTO ceramic. Fig. 2 e Backscattered FE-SEM images of the refined surfaces of (a) CC2TO-8, (b) 05Zn-8, (c) 10Zn-8, (d) CC2TO-12, (e) 05Zn-12 and (f) 10Zn-12 ceramics. journal of materials research and technology 2022;19:4473 e4488 4477
Fig. 3 e Backscattered FE-SEM images with elemental mapping of (a) CC2TO-12 and (d)10Zn-12 ceramics (b, c) And (e, f) show the EDXS spectra detected at (þ1, þ2) and (þ3, þ4) points, respectively. 4478 journal of materials research and technology 2022;19:4473 e4488
Fig. 4(a) and its inset display the plots of frequency dependence of ε0 and tand for 1100e8 ceramics, respectively. In Fig. 4(a), the ε0 behavior of all ceramics is slightly dependent on frequency in the range of 102 e 2 105 Hz, but rapidly drops at frequencies higher than ~ 2 105 Hz. The ε0 drop above ~2 105 Hz of these ceramics is related to the effects of interfacial MaxwelleWagner (MW) polarization [57e59], based on a decremental change of interfacial polarization. In fact, this behavior results from the inability of charge carriers to follow changes in the applied alternating electric field. Consequently, tand values greatly increased, as seen in the inset of Fig. 4(a). It can be seen in this inset that the increased tand values at frequencies less than 103 Hz might be due to free charge carriers moving across the GBs in long-range motion [60]. The great increase in tand at frequencies above 104 Hz was due to the relaxation-dielectric of the primary polarization caused by polarization at the interfaces of CCTOeCCTO grains [60,61]. Moreover, another dielectric relaxation in a frequency range 103 e104 Hz was correlated with the polarization relaxation of the active interfacial response between the grains of CCTO-CaTiO3 and/or CCTOTiO2 [62]. The ε0 and tand values of the ceramics were estimated from these figures. ε0 and tand values at 30 C and 1 kHz were listed in Table 2. Remarkably, all 1100e8 ceramics display ε0 values higher than 12,900 and tand less than 0.018. These two exceptional values are beneficial for capacitor applications. It is clear from inspection of Table 2 that the ε0 value of the 05Zn-8 ceramic increased by ~1.03 times, whereas that of 10Zn-8 ceramic decreased by ~1.05 times, compared with the CC2TO-8 ceramic. Furthermore, these ε0 values are relatively constant over the frequency range from 102 to 2 105 Hz, whereas tand values are less than 0.05 over the range of 102 5 105 Hz. In the case of 1100e12 ceramics, their ε0 and tand values are respectively presented in Fig. 4 (b) and its inset. The ε0 values of all 1100e12 ceramics are rather constant over the frequency range of 102 3 105 Hz, whereas their tand values are less than 0.05 over the range from 4 102 to 9 105 Hz. The ε0 and tand values at 1 kHz and 30 C of the 1100e12 ceramics were listed in Table 2. Compared to the CC2TO-12 ceramic, the ε0 value of the 05Zn-12 ceramic increased by ~21.7%, whereas that of 10Zn-12 ceramic decreased by ~9.3%. Notably, these ε0 values are quite satisfactory (higher than 104 ) for capacitor applications. At 30 C and 1 kHz, the tand values of 1100e12 ceramics were in the range of ~0.022e0.033. The lowest tand value, ~0.022, was obtained in the 10Zn-12 ceramic. It is notable that, compared to other CCTO based ceramics [12,13,63], the 1100e8 and 1100e12 ceramics have excellent dielectric properties (tand Fig. 4 e Frequency dependence of ε0 and tand (their insets) of (a)1100-8 and (b)1100e12 ceramics. Temperature dependence of ε0 of (c) 1100e8 and (d)1100e12 ceramics. Their insets display histograms where tand <0.05. journal of materials research and technology 2022;19:4473 e4488 4479
~0.013e0.033 and ε0 ~11,550e15,535). Moreover, a very low tand value (~0.018) for the 05Zn-8 ceramic is less than those reported for CCTO ceramics doped with Zn [49] (Zn and Al) codoped CCTO [64] (Zn and La) co-doped CCTO [53], and Zn doped YCTO [44] ceramics. Thus, it can be concluded that these 1100e8 and 1100e12 ceramics with very low-tand and colossal ε0 can be used in the commercial manufacture of capacitors. To explore the temperature dependence of ε0 for all ceramics, plots of their ε0 values as a function of temperature (65 ‒ 180 C) are given in Fig. 4(c) and (d) for 1100e8 and 1100e12 ceramics, respectively. The ε0 values of 1100e8 ceramics are quite stable over the TR of -60 ‒ 100 º C and gradually increase at temperatures above 100 C. However, ε0 values of 1100e12 ceramics are nearly constant over a TR of -60 ‒ 120 C, and then they slightly increase when the temperature is greater than 120 C. Therefore, it is suggested that the temperature stability of ε0 can be improved by increasing the sintering time from 8 to 12 h. As shown in the insets of Fig. 4(c) and (d), tand values of 1100e8 and 1100e12 ceramics are less than 0.05 (tand <0.05) over a TR of -60 ‒ 80 C. Remarkably, the TR for tand <0.05 of these ceramics is wider than those of other Zn-doped CCTO ceramics reported in literature [15,43,49,50,53,64]. Additionally, the effects of Zn2þ doping and sintering time on the dielectric parameters of CaCu2.8-xZnxTi4O12 ceramics can be more comprehensively viewed from the bar plots of ε0 and tand values for 1100e8 and 1100e12 ceramics shown in Fig. 5(a) and (b), respectively. These plots show that all tand and ε0 values significantly depend on Zn2þ doping level and sintering time. Although the very high ε0 of CaCu2.8-xZnxTi4O12 ceramics was considerably altered regardless of the Zn2þ doping or sintering time, their marvelous dielectric properties are still adequate for commercial applications (tand <0.035 and ε0 > 104 ). Changes in the values of ε0 and tand might be due to the electrical response inside Gs and at GBs, resulting from Table 2 e Dielectric properties (ε′ and tand) at 30 C and 1 kHz, average grain size (AGS), Egb, Eg, Rgb and Rg at 100 C, TR where Dε′<±15%, nonlinear parameters (a and Eb) at 30 C and barrier height (4B) of all ceramics. Ceramics ε0 tand AGS (mm) Egb (eV) Eg (eV) Rgb (kU.cm) Rg (U.cm) TR (C) of Dε0 < ±15% a Eb (V.cm1 ) 4B (eV) CC2TO-8 13,521 0.013 5.687 ± 2.421 0.608 0.0362 548 61.8 ‒ 60 to 120 8.72 3151.2 0.879 05Zn-8 13,925 0.018 7.201 ± 2.234 0.607 0.0390 838 87.7 ‒ 60 to 140 8.49 4214.2 0.885 10Zn-8 12,934 0.015 7.947 ± 2.726 0.604 0.0422 934 128.9 ‒ 60 to 140 8.93 4259.8 0.884 CC2TO-12 12,736 0.033 6.519 ± 2.542 0.601 0.0506 568 46.0 ‒ 60 to 140 6.64 3078.0 0.815 05Zn-12 15,535 0.026 7.623 ± 2.366 0.602 0.0458 668 43.7 ‒ 60 to 150 10.14 4116.3 0.880 10Zn-12 11,550 0.022 7.754 ± 1.892 0.605 0.0432 764 62.8 ‒ 60 to 210 9.75 4951.4 0.879 CCTO-SnO2 [81] 51,443 0.061 ~50 3.77 485.7 0.52 Eu-CCTO [82] ~900a ~0.5a ~1 2.56 6410 0.59 CCTO-1h [83] ~5000a ~0.25a ~16 5.8725 437,025 0.47 CCTO-3hQ [84] 17,887 0.089 ~23 5.1125 72,025 0.59 25 ¼ at 25 C. a Estimated numerical values from diagrams in reference cites. Fig. 5 e Bar plots of (a) ε′ and (b) tand values for 1100e8 and 1100e12 ceramics. 4480 journal of materials research and technology 2022;19:4473 e4488
the presence of TiO2 and CaTiO3 phases coupled with Zn2þ doping effects. As can be seen in Fig. 5(a) and (b), for 1100e8 ceramics, increased Zn2þ doping levels give rise to an increase in both ε0 and tand for 05Zn-8 ceramic and slight decreases in the 10Zn-8 ceramic. For 1100e12 ceramics, the ε0 values increase in 05Zn-12 and reduce for 10Zn-12 ceramics. Notably, the tand values of 1100e12 ceramics tend to decrease with increased Zn dopant levels. Enhancement of the ε0 with the reduction of tand values of CaCu2.8-xZnxTi4O12 ceramics in the current study is comparable to the results observed in other ceramics [15,49,64]. Generally, the tand of commercial dielectric materials must be lower than 0.05 and the colossal ε0 for Dε0 < ± 15% should be applicable over a wide TR. The TR of these CaCu2.8-xZnxTi4O12 ceramics where Dε0 < ± 15% was examined. Dε0 (%) is generally calculated as, Dε0 ð%Þ ¼ 100 ε0 ðTÞ ε0 ð30Þ ε0 ð30Þ [7,9. TR values where Dε0 < ± 15% of all ceramics are summarized in Table 2. A comparative study of Dε0 (%) at different sintering times for each pair of (CC2TO-8, CC2TO-12) (05Zn-8, 05Zn-12) and (10Zn-8,10Zn-12) ceramics is presented in Fig. 6(a), (b) and (c), respectively. As can be seen in these figures, the Dε0 (%) values of 1100e8 ceramics are higher than those of 1100e12 ceramics. It is suggested that in the low temperature range of 60 40 C, the observed Dε0 (%) was very closely related to the relaxation-dielectric parameters of the frozen charge carriers [65]. Alternatively, the Dε0 (%) peak at hightemperatures (above ~120 º C) was due to the relaxationdielectric associated with polarization at the interfaces between grains of CCTOeCCTO [60,61]. Furthermore, another Dε0 (%) peak was observed at moderate temperatures in the range of ~ 40 35 C. This relaxation peak may have been due to the polarization at the active interfaces between grains of CCTO-CaTiO3 and/or CCTO-TiO2 [62], as demonstrated by the XRD results (Fig. 1) and FE-SEM images with elemental mapping and EDXS analysis (Figs. 2 and 3). From the histograms in Fig. 6(d) and the data tabulated in Table 2, the TR where Dε0 < ± 15% is clearly narrower for all 1100e8 ceramics than for the 1100e12 ceramics. Moreover, referring to the EIA standards, CC2TO-8 (05Zn-8, 10Zn-8 and CC2TO-12), 05Zn-12 and 10Zn12 ceramics are found to be suitable for incorporation into X5R, X7R, X8R and X9R capacitors, respectively. Generally, the capacitors codes, X (5, 7, 8 and 9)R, are assigned for materials with Dε0 < ± 15% in a TR, starting from 55 C and having upper temperature limits of 85 C, 125 C, 150 C and 200 C, respectively [66]. Interestingly, high-performance dielectrics with a broad TR where Dε0 < ± 15% suitable for use in X9R capacitor can be accomplished in 10Zn-12 ceramic. It is proposed that the enhanced dielectric response in the 10Zn-12 ceramic might have originated from the increased Rgb values caused by modifying the electrical interfacial response Fig. 6 e Variation of the dielectric coefficient (Dε′) as a function of temperature (¡65 to 210 C), comparing for (a) CC2TO-8 and CC2TO-12, (b) 05Zn-8 and 05Zn-12, and (c) 10Zn-8 and 10Zn-12 ceramics. (d) Histograms for all ceramics, displaying a good TR where Dε′ < ± 15%. journal of materials research and technology 2022;19:4473 e4488 4481
between the grains of CCTO-CaTiO3, CCTO-TiO2 and CCTOeCCTO, as a result of Zn2þ doping. In this work, CaTiO3 and TiO2 phases were observed in all CaCu2.8-xZnxTi4O12 ceramics, as established by their XRD results and FE-SEM images with elemental mapping and EDXS analysis. It is suggested that the heterogeneous microstructure of these ceramics could provide an active electrical interface between the grains of CCTO-TiO2, CCTO-CaTiO3 and CCTOeCCTO. The electrical interfacial response between the grains of CCTO-CTO and CCTOeCCTO in CCTO ceramic was confirmed by electrostatic force microscopy (EFM), as reported by Ramı´rez et al. [62]. Moreover, the electrical interfacial response reported in Sr2þdoped CCTO/CTO ceramics [60] could have resulted in the improvement of their dielectric properties. Regarding to our work, it is obvious that the dielectric properties of 10Zn-12 ceramic are better than those of CCTO/CTO-composites [1e4,67], CCTO-based compounds [12,13,68] and doped/codoped TiO2-based oxides [25e27]. Moreover, the dielectric parameters of 10Zn-12 ceramic are comparable to those of BaTiO3 based X8R ceramics [29,69,70]. The nonlinear J-E behaviors of these ceramics are shown in Fig. 7 (a). The important nonlinear parameters (a and Eb) of all ceramics were evaluated from these J-E plots, details of which were described elsewhere [1,13,22,23], and the obtained data were recorded in Table 2. Notably, the a and Eb values range from 6.64 to 10.14 and 3078.0e4951.4 V cm1 , respectively. The Fig. 7 e (A) J-E plots at 30 C of all ceramics in the current study, (b)e(g) show the plots of ln (J/AT2 ) vs. E1/2 for each ceramic, (h) histograms, illustrating the comparison of Eb, 4B and a for 1100e8 (light blue) and 1100e12 (red) ceramics. 4482 journal of materials research and technology 2022;19:4473 e4488
maximum a (10.14) and Eb (4951.4 V cm1 ) values were obtained in 05Zn-12 and 10Zn-12 ceramics, respectively. These a and Eb values are comparable to those of other ceramics reported in the literature [1,2,5,19,23,37,45]. Additionally, a comparison of the nonlinear (a and Eb) and dielectric (tand and ε0 ) parameters of all CaCu2.8-xZnxTi4O12 ceramics with those of other related-CCTO ceramics is provided in Table 2. The highest Eb (4951.4 V cm1 ) is consistent with the broadest TR, where Dε0 < ± 15% in the 10Zn-12 ceramic. Additionally, both Eb and a values are significantly enhanced with increasing Zn dopant content. Enhancement of these two values might have originated from the improved Rgb values of the ceramics [1,5,14]. Notably, the enhanced nonlinear J-E properties of these ceramics could have arisen from the increase of their Rgb values, caused by the improved interfacial responses among grains of CCTO-CaTiO3, CCTO-TiO2 and CCTOeCCTO, as results of Zn2þ doping and associated porosity effects. These behaviors are comparable with those reported in the literature [65,71e74]. This indicates that the nonlinear J-E behaviors of CaCu2.8Ti4O12 ceramics can be improved by Zn doping. Therefore, it can be hypothesized that Zn doping at the Cu sites of CaCu2.8Ti4O12 ceramics plays an important role in improving the nonlinear J-E parameters. Additionally, the potential barrier heights (4B) of all ceramics were considered for understanding of the electrical behavior at GBs. Generally, 4B values are calculated using the equation lnðJ =AT2Þ ¼ ð b kBTÞE1=2 4B kBT [14,23. Accordingly, the ln (J/AT2 ) vs. E1/2 plots of all ceramics are shown in Fig. 7(b)e(g). All linear data fit parameters of each ceramic were summarized in the insets of these figures. Using the relationship, y intercept ¼ 4B kBT, 4B values were determined and were tabulated in Table 2. As illustrated in this table, these ceramics display 4B values in the range of 0.815e0.885 eV, which are higher than those reported for other CCTO based materials [14,45,64]. Additionally, the density-energy storage (w) of each ceramic was obtained using a simple relationship, w ¼ 0.5ε0εrEb 2 [5,23]. The w values of CC2TO-8, 05Zn-8, 10Zn-8, CC2TO-12, 05Zn-12 and 10Zn-12 ceramics were found to be 0.066, 0.121, 0.115, 0.059, 0.129 and 0.139 mJ/m3 . Notably, the w values were enhanced in all Zndoped ceramics. Moreover, these enhancements could expand their application for use in manufacturing electronic devices [23,75]. Additionally, increased Zn doping could result in higher 4B values for ceramics sintered for 8 and 12 h. Moreover, the effects of Zn content and sintering time on these parameters (4B, a and Eb) are shown by the histograms in Fig. 7 (h). It can be seen that the variation of Zn level and sintering time affects the Eb and a values moderately for Eb, but with more fluctuation in a. However, the 4B value is not likely affected by these factors. Thus, it is remarkable that both the a and Eb values are more sensitive to the Zn content and sintering time than 4B. The electrical behaviors at Gs and GBs of the ceramics, affected by Zn doping and sintering time through improvement of GB-resistances (Rgb) and G-resistances (Rg), were further studied using Z* plots (Z0 vs. -Z00) [12,13]. Generally, the complex impedance (Z*) of the heterogeneous electrical structure of CCTO ceramics can be represented as a parallel and series arrangement of a capacitor (C) and a resistor (R) [76e78], as defined by the equation, Z* ¼ Rg 1þðiuRgCgÞ þ Rgb 1þðiuRgbCgbÞ . The first term of this equation represents grainsemiconduction, comprised of a capacitor-resistor of grain (CgRg). The second term is for the capacitor-resistor of the GB (CgbRgb). Using this equation, the obtained Z* plots at high and low frequencies are generally used for determination of Rg and Rgb values, respectively [12,13,79]. In this study, only low frequency-semicircular arcs were observed. Therefore, the experimental data can be well fitted using a single parallel CR circuit, given by Z* ¼ Rgb 1þðiuRgbCgbÞ a [6,12e14,19e21,34], where u and a are the angular frequency of an applied electric field and a constant, respectively. Consequently, the Rg and Rgb values of all ceramics can be predicted, as shown in Fig. 8(a) and (b). In these figures, the low frequencysemicircular arcs of 1100e8 and 1100e12 ceramics increase with the Zn dopant level. The Rgb and Rg values of all ceramics at 100 C are determined and summarized in Table 2. It was found that Rgb values of the 1100e8 and 1100e12 ceramics are in the range of 548 e 934 kU cm, correlating to very low tand values in the range of 0.013 e 0.033. These Rgb and tand values are comparable with those of the Rgb (~99.1 kU cm) and tand (~0.074) of CaCu3-xZnxTi4O12 ceramics [49], the Rgb (~530 kU cm) and tand (~0.033) of Y2/3Cu3-xZnxTi4O12 ceramics [44], and the Rgb (~179 kU cm) and tand (~0.071) of La/Zn codoped CCTO ceramics [53]. Moreover, enhanced Rg values of 1100e12 ceramics correspond to the changes in their ε0 values, whereas those of 1100e8 ceramics are not consistent. Generally, Rg values of dielectric materials are inversely related to their ε0 values [12,13,63]. Furthermore, G-conductivity (sg) and GB-conductivity (sgb) were considered to gain a clearer understanding of the electrical behavior of these ceramics at Gs and GBs. The sg and sgb values were calculated using the relationships, sg a Rg 1 and sgb a Rgb 1 . Fig. 8(c) and its inset display Rgb and Rg values of the 10Zn-12 ceramic measured at various temperatures from 100 C to 150 C and 10 C to 110 C, respectively. It can be seen that both Rgb and Rg values significantly decrease with increasing temperature. Figure 8 (d) displays the plots of ln sgb & 1000/T (upper) and ln sg & 1000/T (lower), following equation (1), for the1100-8 and 1100e12 ceramics. ln sgb ðsgÞ ¼ Egb ðEgÞ 1000kB ½ 1000 T þ ln s0 (1) where Egb (Eg) and T are the activation energies for sgb (sg) and the absolute temperature, respectively. s0 is a pre-exponential parameter and kB is the Boltzmann constant, which is equal to 1.3806 1023 m2 kg/s2 .K. Egb and Eg were calculated using the slope of each plot and Eq. (1). The obtained data were listed in Table 2. It was found that a slight change in Egb values of 1100e8 and 1100e12 ceramics was inversely correlated with their tand values. Notably, Egb values of CC2TO-8 (~0.608 eV) and 10Zn-12 (~0.605 eV) are consistent with their lowest tand values, which are comparable with those of other CCTO based materials doped with Zn [43,53,64]. Additionally, gradually increasing sintering time from 8 to 12 h caused incremental changes in Eg values, correlating with the decreased ε0 values of CC2TO-12 (compared to CC2TO-8) and 10Zn-12 (compared to 10Zn-8) ceramics. For example, compared to the 10Zn-12 ceramic, the Eg value of the 10Zn-8 ceramic increased by ~2.4%, correlating to a decreased ε0 value of ~10.7%. However, journal of materials research and technology 2022;19:4473 e4488 4483
such a correlation was not found for the 05Zn-8 and 05Zn-12 ceramics. Additionally, the Eg values obtained in this work are comparable with those of other CCTO based materials [12,13,16,55]. The incremental changes in Eg values might have been due to the variation of the electrical response inside the Gs, resulting from the effects of Zn doping and the nonuniform presence of TiO2 and CaTiO3 phases in the ceramics. It is established that electron hopping between various sites of such as Cuþ O Cu2þ, Cu3þ O Cu2þ and Ti3þ O Ti4þ is the primary feature that is generally used to indicate the semiconducting-grain nature of CCTO based materials [48,80]. Therefore, in this work, the XPS technique was employed to explore the oxidation states of Cu in CC2TO8, CC2TO-12, 05Zn-12 and 10Zn-12 ceramics. As seen in Fig. 9(a)e(d), the XPS results show three different Cu2p3/2 spectra of Cu ions of various oxidation states, i.e., Cuþ, Cu2þ, and Cu3þ in these ceramics. The binding energy peak positions for the Cuþ, Cu2þ, and Cu3þ states of CC2TO-8, CC2TO-12, 05Zn-12 and 10Zn-12 ceramics are shown in Fig. 9(a)e(d), respectively. As illustrated in these figures, the ratios of Cuþ:Cu2þ:Cu3þare 20.35%:64.22%:15.43%, 23.14%:57.74%: 19.12%, 11.39%:67.19%:21.42%, and 13.10%:64.20%:22.70%, respectively, for CC2TO-8, CC2TO-12, 05Zn-12 and 10Zn-12 ceramics. These values are similar to those observed in other dielectric ceramics [35,49,79]. Moreover, to clearly observe the changes of Cuþ and Cu3þ states, all of the Cuþ:Cu2þ:Cu3þ ratios were normalized for Cu2þ and found to be 0.32:1:0.24, 0.40:1:0.33, 0.17:1:0.32, and 0.20:1:0.35 for CC2TO-8, CC2TO-12, 05Zn-12 and 10Zn-12 ceramics, respectively. As can be seen from these ratios, variation of Cuþ and Cu3þ states in all 1100e12 ceramics is consistent with variation of their ε0 values. The XPS results for Ti3þ and Ti4þ were not determined. However, from these results, it can be proposed that the semiconducting-grain state of these ceFig. 8 e Z* plots at 100 C with insets showing enlarged views at high-frequencies for (a) 1100e8 and (b) 1100e12 ceramics. (c) Z* plots at various temperatures (100 e 150 C) with inset showing a nonzero intercept at high frequency for the 10Zn-12 ceramic. (d) Plots of ln sgb vs. 1000/T (upper) and ln sg & 1000/T (lower) for all of 1100e8 and 1100e12 ceramics. 4484 journal of materials research and technology 2022;19:4473 e4488
ramics originated from electron hopping between Cuþ O Cu2þ and Cu3þ O Cu2þ sites. 4. Conclusions Very good dielectric properties (tand ~ 0.013 e 0.033 and ε0 ~ 11,550 e 15,535) were achieved in a novel CaCu2.8-xZnxTi4 O12 (x ¼ 0.00 e 0.10) ceramic system. The temperature stability of ε0 over a wide TR, from 60 to 210 C, where Dε0 < ±15% was achieved in CaCu2.7Zn0.1Ti4O12 ceramic sintered at 1100 C for 12 h, suggests that this material can be used in the manufacture of X9R capacitors. Compared to the undoped CaCu2.8Ti4.2O12 ceramic, the nonlinear J-E parameters of Zn2þdoped samples were greatly enhanced. Incremental changes in the dielectric constant of these ceramics were described based on electron hopping between Cuþ O Cu2þ and Cu3þ O Cu2þ sites as a result of Zn doping effects. The very low tand was due to increased Rgb values resulting from the formation of TiO2 and CaTiO3 phases at GBs. Moreover, the electrical responses inside Gs and at GBs are supported by the IBLC Schottky barrier model. It is suggested that the highperformance electrical properties of ceramics achieved by this facile technique could guide the practical design of hightemperature capacitor devices. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was financially supported by Rajamangala University of Technology Rattanakosin, Wang Klai Kangwon Campus, Hua Hin, Prachuap Khiri khan, Thailand (A19/2565). Fig. 9 e XPS spectra of Cu2p3/2 lines of (a) CC2TO-8, (b) CC2TO-12, (c) 05Zn-12 and (d) 10Zn-12 ceramics. journal of materials research and technology 2022;19:4473 e4488 4485
This research was also funded by the Research and Graduate Studies of Khon Kaen University, Thailand. We are grateful for their financial support. references [1] Cotrim G, Cortes JA, Moreno H, Freitas SM, Rezende MVS, Hein LRO, et al. Tunable capacitor-varistor response of CaCu3Ti4O12/CaTiO3 ceramic composites with SnO2 addition. Mater Char 2020;170:110699. [2] Maleki Shahraki M, Daeijavad H, Emami AH, Abdollahi M, Karimi A. An engineering design based on nano/micro-sized composite for CaTiO3/CaCu3Ti4O12 materials and its dielectric and non-Ohmic properties. Ceram Int 2019;45:21676e83. [3] Jumpatam J, Putasaeng B, Yamwong T, Thongbai P, Maensiri S. A novel strategy to enhance dielectric performance and non-Ohmic properties in Ca2Cu2xMgxTi4O12. J Eur Ceram Soc 2014;34:2941e50. [4] Jumpatam J, Putasaeng B, Yamwong T, Thongbai P, Maensiri S. A novel route to greatly enhanced dielectric permittivity with reduce loss tangent in CaCu3xZnxTi4O12/ CaTiO3 composites. J Am Ceram Soc 2014;97:2368e71. [5] Swatsitang E, Nijpanich S, Putjuso S, Putjuso T. Effect of sintering temperature and Sm3þ doping on the dielectric properties and non-Ohmic behavior of Ca1-1.5xSmxCu3Ti4.2O12 (x ¼ 0.05 and 0.10) ceramics. Results Phys 2021;30:104896. [6] Sripakdee C, Putjuso S, Putjuso T. Improvement of temperature stability, dielectric properties and nonlinear current-electric field characteristic of CaCu3Ti4.2xSnxO12 ceramics. J Electron Mater 2020;49:3555e65. [7] Swatsitang E, Prompa K, Putjuso T. A novel strategy to improve the thermal stability of dielectric properties and reduce the dielectric loss tangent of Ca1-1.5xPrxCu3Ti4O12/TiO2 ceramics. Ceram Int 2019;45:14733e41. [8] Swatsitang E, Prompa K, Putjuso T. Temperature stability of the dielectric properties of Zr4þ-doped CaCu3Ti4.2O12 ceramics for X9R capacitor applications. J Alloys Compd 2019;789:231e9. [9] Sripakdee C, Prompa K, Swatsitang E, Putjuso T. Very highperformance dielectric and non-ohmic properties of novel X8R type Ca1-1.5xHoxCu3Ti4O12/TiO2 ceramics. J Alloys Compd 2019;779:521e30. [10] Swatsitang E, Prompa K, Putjuso T. Very high temperature stability and excellent dielectric properties of a novel X9Rtype Ca1-xSrxCu3Ti4O12/TiO2 nanocomposite synthesized by a polymer pyrolysis technique. Appl Surf Sci 2019;478:197e205. [11] Ouyang X, Habib M, Cao P, Wei S, Huang Z, Zhang W, et al. Enhanced extrinsic dielectric response of TiO2 modified CaCu3Ti4O12 ceramics. Ceram Int 2015;41:13447e54. [12] Mao P, Wang J, Xiao P, Zhang L, Kang F, Gong H. Colossal dielectric response and relaxation behavior in novel system of Zr4þ and Nb5þ co-substituted CaCu3Ti4O12 ceramics. Ceram Int 2021;47:111e20. [13] Cortes JA, Moreno H, Orrego S, Bezzon VDN, Ramı ´rez MA. Dielectric and non-ohmic analysis of Sr2þ influences on CaCu3Ti4O12-based ceramic composites. Mater Res Bull 2021;134:111071. [14] Lin L, Liu Y, Zhang J, Li Z, Lei Z, Li Y, et al. Enhancement of breakdown electric field and dielectric properties of CaCu3Ti4O12 ceramics by Sr doping. Mater Chem Phys 2020;244:122722. [15] Xu Z, Chen Y, Cheng N. Enhanced dielectric properties of Ca0.9Sr0.1Cu3-yZnyTi4-xBxO12 ceramics by modifying grain boundary response. Mater Chem Phys 2021;261:124207. [16] Swatsitang E, Kumnorkaew P, Putjuso T. Thermal stability improvement of dielectric properties and non-ohmic characteristic of CaCu3þxTi4O12 ceramics via a Cunonstoichiometric approach. Ceram Int 2021;47:24149e62. [17] Swatsitang E, Putjuso S, Nijpanich S, Putjuso T. Modification of Cu-deficient CaCu2.8Ti4O12 ceramics via Mg2þ substitution at Cu sites for improved dielectric properties and thermal stability. J Alloys Compd 2022;902:163643. [18] Peng Z, Zhou X, Wang J, Zhu J, Liang P, Chao X, et al. Origin of colossal permittivity and low dielectric loss in Na1/3Cd1/3Y1/ 3Cu3Ti4O12 ceramics. Ceram Int 2020;46:11154e9. [19] Tang Z, Wu K, Li J, Huang S. Optimized dual-function varistor-capacitor ceramics of core-shell structured xBi2/ 3Cu3Ti4O12/(1-x)CaCu3Ti4O12 composites. J Eur Ceram Soc 2020;40:3437e44. [20] Zhao N, Liang P, Wu D, Chao X, Yang Z. Temperature stability and low dielectric loss of lithium-doped CdCu3Ti4O12 ceramics for X9R capacitor applications. Ceram Int 2019;45:22991e7. [21] Zhao L, Xu R, Wei Y, Han X, Zhai C, Zhang Z, et al. Giant dielectric phenomenon of Ba0.5Sr0.5TiO3/CaCu3Ti4O12 multilayers due to interfacial polarization for capacitor applications. J Eur Ceram Soc 2019;39:1116e21. [22] Peng Z, Wang J, Zhang F, Xu S, Lei X, Liang P, et al. High energy storage and colossal permittivity CdCu3Ti4O12 oxide ceramics. Ceram Int 2022;48:4255e60. [23] Peng Z, Wang J, Zhou X, Zhu J, Lei X, Liang P, et al. Grain engineering inducing high energy storage in CdCu3Ti4O12 ceramics. Ceram Int 2020;46:14425e30. [24] Peng Z, Liang P, Chen X, Yang Z, Chao X. High thermal stability and excellent dielectric properties of a novel X8Rtype CdCu3Ti4O12 ceramics through a sol-gel technique. Mater Res Bull 2018;98:340e8. [25] Maleki Shahraki M, Mahmoudi P, Karimi A. Air quenching as a reliable technique to obtain colossal dielectric constant with low loss in (In, Nb)-co doped TiO2. Mater Lett 2020;267:127513. [26] Peng P, Chen C, Cui B, Li J, Xu D, Tang B. Influence of the electric field on flash-sintered (Zr þ Ta) co-doped TiO2 colossal permittivity ceramics. Ceram Int 2022;48:6016e23. [27] Fan J, Chen Y, Long Z, He G, Hu Z. Grain-boundary engineering inducing thermal stability, low dielectric loss and high energy storage in TaþHo co-doped TiO2 ceramics. Ceram Int 2022;48:21584e92. [28] Pan Y, Bai X, Feng J, Huang L, Li G, Chen Y. Influences of holmium substitution on the phase structure and piezoelectric properties of BiFeO3-BaTiO3-based ceramics. J Alloys Compd 2022;918:165582. [29] Liu R, Chen Z, Lu Z, Wang X. Effects of sintering temperature and Bi2O3, Y2O3 and MgO co-doping on the dielectric properties of X8R BaTiO3-based ceramics. Ceram Int 2022;48:2377e84. [30] Zhong B, Long Z, Yang C, Li Y, Wei X. Colossal dielectric permittivity in co-doping SrTiO3 ceramics by Nb and Mg. Ceram Int 2020;46:20565e9. [31] Bin C, Hou X, Yang H, Liao L, Xie Y, Wei H, et al. Flexible leadfree film capacitor based on BiMg0.5Ti0.5O3-SrTiO3 for highperformance energy storage. Chem Eng J 2022;445:136728. [32] Prompa K, Swatsitang E, Putjuso T. Ultra-stable X9R type CaCu3-xZnxTi4.1O12 ceramics. Ceram Int 2018;44:20739e48. [33] Swatsitang E, Prompa K, Putjuso T. Very high thermal stability with excellent dielectric, and non-ohmics properties of Mg-doped CaCu3Ti4.2O12 ceramics. J Mater Sci Mater Electron 2018;29:12639e51. [34] Felix AA, Saska LA, Bezzon VDN, Cilense M, Ramirez MA. Enhanced electrical behavior in Ca1-xSrxCu3Ti4O12 ceramics. Ceram Int 2019;45:14305e11. 4486 journal of materials research and technology 2022;19:4473 e4488
[35] Boonlakhorn J, Chanlek N, Manyam J, Srepusharawoot P, Thongbai P. Simultaneous two-step enhanced permittivity and reduced loss tangent in Mg/Ge-Doped CaCu3Ti4O12 ceramics. J Alloys Compd 2021;877:160322. [36] Boonlakhorn J, Putasaeng B, Kidkhunthod P, Manyam J, Krongsuk S, Srepusharawoot P, et al. First-principles calculations and experimental study of enhanced nonlinear and dielectric properties of Sn4þ-doped CaCu2.95Mg0.05Ti4O12 ceramics. J Eur Ceram Soc 2021;41:5176e83. [37] Wang J, Lu Z, Chen Z. The novel effects of Cu-deficient on the dielectric properties and voltageecurrent nonlinearity in CaCu3Ti4O12 ceramics. Mater Sci Eng B 2019;243:1e7. [38] Abu MJ, Mohamed JJ, Ain MF, Ahmad ZA. Phase structure, microstructure and broadband dielectric response of Cu nonstoichiometry CaCu3Ti4O12 ceramic. J Alloys Compd 2016;683:579e89. [39] Yuan WX, Wu Q, Liu C, Luo Z, Li Z. Effect of phase purity on dielectric properties of CaCu3þxTi4O12 ceramics. Solid State Sci 2013;24:58e61. [40] Liu L, Fan H, Chen X, Fang P. Electrical properties and microstructural characteristics of nonstoichiometric CaCu3xTi4O12 ceramics. J Alloys Compd 2009;469:529e34. [41] Kim KM, Lee JH, Lee KM, Kim DY, Riu DH, Lee SB. Microstructural evolution and dielectric properties of Cudeficient and Cu-excess CaCu3Ti4O12 ceramics. Mater Res Bull 2008;43:284e91. [42] Fang TT, Mei LT. Evidence of Cu deficiency: a key point for the understanding of the mystery of the giant dielectric constant in CaCu3Ti4O12. J Am Ceram Soc 2007;90:638e40. [43] Pan W, Cao M, Wang H, Jan A, Hao H, Yao Z, et al. Giant dielectric response in (Nb þ Zn) co-doped strontium titanate ceramics tailored by atmosphere. Scripta Mater 2019;170:166e71. [44] Wang X, Liang P, Peng Z, Peng H, Xiang Y, Chao X, et al. Significantly enhanced breakdown electric field in Zn-doped Y2/3Cu3Ti4O12 ceramics. J Alloys Compd 2019;778:391e7. [45] Grzebielucka EC, Leandro Monteiro JFH, de Souza ECF, Ferreira Borges CP, de Andrade AVC, Cordoncillo E, et al. Improvement in varistor properties of CaCu3Ti4O12 ceramics by chromium addition. J Mater Sci Technol 2020;41:12e20. [46] Dwibedy D, Sahoo AK, Panigrahi MR. Structural, microstructural and optical properties of perovskite CCNbxTi1x O (x ¼ 0.02) thin film explored for photovoltaic applications. Optik 2021;243:167496. [47] Subramanian MA, Li D, Duan N, Reisner BA, Sleight AW. High dielectric constant in ACu3Ti4O12 and ACu3Ti3FeO12 phases. J Solid State Chem 2000;151:323e5. [48] Lin H, Xu W, Zhang H, Chen C, Zhou Y, Yi Z. Origin of high dielectric performance in fine grain-sized CaCu3Ti4O12 materials. J Eur Ceram Soc 2020;40:1957e66. [49] Boonlakhorn J, Chanlek N, Manyam J, Krongsuk S, Thongbai P, Srepusharawoot P. Structural and dielectric properties, and nonlinear electrical response of the CaCu3- xZnxTi4O12 ceramics: experimental and computational studies. Ceram Int 2021;47:22390e6. [50] Ni L, Chen XM. Enhancement of giant dielectric response in CaCu3Ti4O12 ceramics by Zn substitution. J Am Ceram Soc 2010;93:184e9. [51] Gholizadeh A, Beyranvand M. Investigation on the structural, magnetic, dielectric and impedance analysis of Mg0.3- xBaxCu0.2Zn0.5Fe2O4 nanoparticles. Phys B Condens Matter 2020;584:412079. [52] Kumar NS, Suvarna RP, Naidu KCB. Multiferroic nature of microwave-processed and sol-gel synthesized NanoPb1- xCoxTiO3 (x ¼ 0.2e0.8) ceramics. Cryst Res Technol 2018;53:1800139. [53] Rani S, Ahlawat N, Punia R, Sangwan KM, Khandelwal P. Dielectric and impedance studies of La and Zn co-doped complex perovskite CaCu3Ti4O12 ceramic. Ceram Int 2018;44:23125e36. [54] Hao W, Xu P, Wang M, Yang S, Yupeng W, Wu H, et al. Microstructure and dielectrical properties of CaCu3Ti4O12/ TiO2 composite ceramics with core-shell structure. J Alloys Compd 2018;740:1159e64. [55] Jumpatam J, Putasaeng B, Chanlek N, Manyam J, Srepusharawoot P, Krongsuk S, et al. Influence of Sn and F dopants on giant dielectric response and Schottky potential barrier at grain boundaries of CCTO ceramics. Ceram Int 2021;47:27908e15. [56] De Almeida-Didry S, Nomel MM, Autret C, Honstettre C, Lucas A, Pacreau F, et al. Control of grain boundary in alumina doped CCTO showing colossal permittivity by coreshell approach. J Eur Ceram Soc 2018;38:3182e7. [57] Kumar NS, Suvarna RP, Krishna Reddy KR, Babu TA, Ramesh S, Shiva Reddy BV, et al. Tetragonal structure and dielectric behaviour of rare-earth substituted La0.8Co0.16- xEu0.04GdxTiO3 (x ¼ 0.04e0.16) nanorods. Mater Chem Phys 2022;278:125598. [58] Babu TA, Ramesh KV, Badapanda T, Ramesh S, Naidu KCB, Sastry DL. Structural and electrical studies of excessively Sm2O3 substituted soft PZT nanoceramics. Ceram Int 2021;47:31294e301. [59] Dastagiri S, Lakshmaiah MV, Chandra Babu Naidu K, Suresh Kumar N, Khan A. Induced dielectric behavior in high dense AlxLa1-xTiO3 (x ¼ 0.2e0.8) nanospheres. J Mater Sci Mater Electron 2019;30:20253e64. [60] Jumpatam J, Putasaeng B, Chanlek N, Thongbai P. Influences of Sr2þ doping on microstructure, giant dielectric behavior, and non-ohmic properties of CaCu3Ti4O12/CaTiO3 ceramic composites. Molecules 2021;26:1994. [61] Liu J, Duan CG, Yin WG, Mei WN, Smith RW, Hardy JR. Large dielectric constant and Maxwell-Wagner relaxation in Bi2/ 3Cu3Ti4O12. Phys Rev B 2004;70:144106. [62] Ramı´rez MA, Bueno PR, Tararam R, Cavalheiro AA, Longo E, Varela JA. Evaluation of the effect of the stoichiometric ratio of Ca/Cu on the electrical and microstructural properties of the CaCu3Ti4O12 polycrystalline system. J Phys D Appl Phys 2009;42:185503. [63] Zhuk NA, Sekushin NA, Krzhizhanovskaya MG, Belyy VA, Korolev RI. Electrical properties of Ni-doped CaCu3Ti4O12 ceramics. Solid State Ionics 2021;364:115633. [64] Xu C, Zhao X, Ren L, Sun J, Yang L, Guo J, et al. Enhanced electrical properties of CaCu3Ti4O12 ceramics by spark plasma sintering: role of Zn and Al co-doping. J Alloys Compd 2019;792:1079e87. [65] Liu L, Ren S, Liu J, Han F, Zhang J, Peng B, et al. Localized polarons and conductive charge carriers: understanding CaCu3Ti4O12 over a broad temperature range. Phys Rev B 2019;99:094110. [66] Clarke DR. Varistor ceramics. J Am Ceram Soc 1999;82:485e502. [67] Cortes JA, Cotrim G, Orrego S, Sim oes AZ, Ramı ~ ´rez MA. Dielectric and non-ohmic properties of Ca2Cu2Ti4-xSnxO12 (0.0 x 4.0) multiphasic ceramic composites. J Alloys Compd 2018;735:140e9. [68] Kumar V, Pandey S, Kumar A, Verma MK, Singh S, Rai VS, et al. Investigation of dielectric, magnetic and impedance spectroscopic properties of CaCu3-XMnXTi4-XMnXO12 (X ¼ 0.10) nano-ceramic synthesized through semi-wet route. J Mater Res Technol 2020;9:12936e45. [69] Hu W, Chen Z, Lu Z, Wang X, Fu X. Effect of Bi2O3 and Ho2O3 co-doping on the dielectric properties and temperature reliability of X8R BaTiO3-based ceramics. Ceram Int 2011;47:24982e7. [70] Li L, Li J, Zhang N, Zheng H, Cai Z, Lu T. Effects for different particle size ZnO-B2O3-Bi2O3 additive on the dielectric journal of materials research and technology 2022;19:4473 e4488 4487
properties and microstructure of BaTiO3-based X8R ceramics. Ceram Int 2018;44:894e9. [71] Li G, Chen Z, Sun X, Liu L, Fang L, Elouadi B. Electrical properties of AC3B4O12-type perovskite ceramics with different cation vacancies. Mater Res Bull 2015;65:260e5. [72] Liu L, Shi D, Zheng S, Huang Y, Wu S, Li Y, et al. Polaron relaxation and non-ohmic behavior in CaCu3Ti4O12 ceramics with different cooling methods. Mater Chem Phys 2013;139:844e50. [73] Liu L, Fan H, Fang P, Chen X. Solegel derived CaCu3Ti4O12 ceramics: synthesis, characterization and electrical properties. Mater Res Bull 2008;43:1800e7. [74] Liu L, Fan H, Fang P, Jin L. Electrical heterogeneity in CaCu3Ti4O12 ceramics fabricated by solegel method. Solid State Commun 2007;142:573e6. [75] Tang Z, Huang Y, Wu K, Li J. Significantly enhanced breakdown field in Ca1-xSrxCu3Ti4O12 ceramics by tailoring donor densities. J Eur Ceram Soc 2018;38:1569e75. [76] Powles JG. Cole-Cole plots as they should be. J Mol Liq 1993;56:35e47. [77] Raghuram N, Subba Rao T, Chandra Babu Naidu K. Electrical and impedance spectroscopy properties of hydrothermally synthesized Ba0.2Sr0.8-yLayFe12O19 (y ¼ 0.2e0.8) nanorods. Ceram Int 2020;46:5894e906. [78] Venkata Shiva Reddy B, Srinivas K, Suresh Kumar N, Chandra Babu Naidu K, Ramesh S. Nanorods like microstructure, photocatalytic activity and ac-electrical properties of (1-x) (Al0.2La0.8TiO3) þ (x) (BaTiO3) (x ¼ 0.2, 0.4, 0.6 & 0.8) nanocomposites. Chem Phys Lett 2020;752:137552. [79] Moreno H, Cortes JA, Praxedes FM, Freitas SM, Rezende MVS, Simoes AZ, et al. Tunable photoluminescence of CaCu ~ 3Ti4O12 based ceramics modified with tungsten. J Alloys Compd 2021;850:156652. [80] Kumar V, Kumar A, Verma MK, Singh S, Pandey S, Rai VS, et al. Investigation of dielectric and electrochemical behavior of CaCu3xMnxTi4O12 (x ¼ 0, 1) ceramic synthesized through semi-wet route. Mater Chem Phys 2020;245:122804. [81] Liu L, Huang Y, Li Y, Shi D, Zheng S, Wu S, et al. Dielectric and non-Ohmic properties of CaCu3Ti4O12 ceramics modified with NiO, SnO2, SiO2, and Al2O3 additives. J Mater Sci 2012;47:2294e9. [82] Liu L, Fang L, Huang Y, Li Y, Shi D, Zheng S, et al. Dielectric and nonlinear currentevoltage characteristics of rareeearth doped CaCu3Ti4O12 ceramics. J Appl Phys 2011;110:094101. [83] Huang Y, Liu L, Shi D, Wu S, Zheng S, Fang L, et al. Giant dielectric permittivity and non-linear electrical behavior in CaCu3Ti4O12 varistors from the molten-salt synthesized powder. Ceram Int 2013;39:6063e8. [84] Huang Y, Shi D, Li Y, Li G, Wang Q, Liu L, et al. Effect of holding time on the dielectric properties and non-ohmic behavior of CaCu3Ti4O12 capacitor-varistors. J Mater Sci Mater Electron 2013;24:1994e9. 4488 journal of materials research and technology 2022;19:4473 e4488