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Microstructure and dielectric properties of CCTO glass-ceramic prepared by the melt-quenching method
Ceramics International 45 (2019) 19316–19322
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Microstructure and dielectric properties of CCTO glass-ceramic prepared by the melt-quenching method
T
Tahere Nazari Aliabadi, Parvin Alizadeh∗ Department of Materials Science and Engineering, Tarbiat Modares University, Tehran, 14115-143, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: Glass Glass-ceramic CaCu3Ti4O12 Dielectric properties
In this work, CaCu3Ti4O12 (CCTO) glass-ceramic was prepared by using the melt-quenching method followed by a two-step heat treatment. The prepared glass frits with the chemical composition of 24SiO2-65PbO-3K2O7B2O3-1ZnO (wt. %) were mixed with 32, 34 and 38 wt % of CCTO including 14.03 CaO-29.66 CuO-56.31 TiO2 (wt. %) and melted at 1070-1090°C . The glass with 32 wt % of CCTO, which was XRD amorphous, was selected as the final composition. The heat treatment conditions were found to be 6 and 2 h at 468 °C and 830 °C, respectively. The faceted crystals of CCTO, two solid-solution of (Pb1-xCax)TiO3, and CaTiO(SiO4) were formed in the glassy matrix during the crystallization based on the studies related to XRD, SEM, EDS, and elemental mapping in the glass-ceramic. The measured dielectric constant of the glass was 16.3 at 1 MHz, which reached 41.2 after the heat treatment process, and the value of the tanδ increased from 4.6× 10−5 to 0.012.
1. Introduction
porosities [6,8,9,13]. Therefore, the CCTO glass-ceramic as a perovskite glass-ceramic has a high energy density and can be used in technological electronic applications such as capacitors, resonators, memory devices, and filters [15,16]. Recently, a large body of research has been conducted to improve the dielectric properties of CCTO. however, most of these studies focused on the addition of a glassy phase to CCTO ceramic in order to decrease the dielectric loss [15,17,18]. In this regard, Mohamed et al. [19] fabricated CCTO ceramics with BaO-SrO-Nb2O5-B2O3-SiO2 (BSNBS) glass additive by implementing solid-state reaction method. The dielectric constant and dielectric loss of the CCTO ceramic were reported 10482 and 0.54 at 1 MHz, respectively, and showed a decrease for the specimen with 0.10 BSNBS to ∼4000 and 0.38. In a similar study, the addition of TeO2 as a well-known glass former resulted in decreasing the dielectric loss of CCTO to 0.095 [18]. Further, Zhang et al. [20] produced CCTO glass-ceramics in K2O–Al2O3–SiO2 system. Based on the XRD patterns, the diffraction peaks of KAlSi2O5, Ca3Al2Si3O12, and Al2 (SiO4) O were detected in the as-synthesized specimens. The dielectric constant and dielectric loss of the glassceramics with 40 and 60 wt % of CCTO were 18, 0.02 and 38, 0.12, respectively. As it was already mentioned, CCTO indicates a high dielectric constant, but high dielectric loss of CCTO limits its application in capacitors. On the other hand, the low dielectric loss of glasses means producing CCTO glass to decrease the dielectric loss of the CCTO in
The fabrication of materials with high dielectric constant and low dielectric loss plays a key role in minimizing the dimensions of electronic instruments [1]. During recent years, much attention has been paid to cubic perovskite ceramics due to their desirable dielectric properties. The calcium copper titanate (CCTO) is a pseudo perovskite with a cubic unit cell and Im3 space group [2–5], which has a dielectric constant of 105 for a single crystal and 104 for bulk materials at the frequency of 1 kHz and room temperature [6–8]. It is well-known that the dielectric constant of this material in the frequency range of 102–106 Hz and a wide temperature range of 100–400 K remains relatively constant [6,9–11]. Different models have been reported for explaining the unusual properties of CCTO. The internal barrier layer capacitor (IBLC) was proposed by Sinclair et al. [12], which is known as a well-accepted model to evaluate the properties of CCTO. In this model, CCTO is composed of semi-conductive grains and insulating grain boundaries [12]. Although CCTO has a high dielectric constant, its applications have been constrained due to the high dielectric loss (higher than 0.1) at room temperature and 1 kHz and low breakdown voltage [7,12–14]. Among various materials used as dielectrics, glass-ceramics made by the melt-quenching method exhibit desirable dielectric constants, low dielectric losses, and good dielectric breakdown strengths (∼300-1200 kV.cm−1) due to the presence of a glassy phase and little amounts of
∗
Corresponding author. E-mail address: [email protected] (P. Alizadeh).
https://doi.org/10.1016/j.ceramint.2019.06.182 Received 21 May 2019; Received in revised form 15 June 2019; Accepted 18 June 2019 Available online 20 June 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 45 (2019) 19316–19322
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Table 1 The nominal composition of the specimens (wt. %). specimen
Glass frits SiO2 68 16.32 66 15.84 62 14.88
G1 G2 G3
CCTO PbO
B2O3
K2O
ZnO
44.20
4.76
2.04
0.68
42.90
4.62
1.98
0.66
40.30
4.34
1.86
0.62
CaO 32 4.49 34 4.77 38 5.33
CuO
TiO2
9.49
18.02
10.08
19.15
11.27
21.40
Fig. 3. XRD pattern of the glass-ceramic (GC1).
which determining appropriate heat treatment conditions leads to the fabrication of glass-ceramic simultaneously with low dielectric loss and high dielectric constant. Thus, the present study aimed to select a suitable composition for fabricating an air-quenched amorphous CCTO glass by using the melt-quenching method and determining the desired heat treatment conditions for converting the fabricated glass to a glassceramic with desirable dielectric properties. In this regard, the microstructure, thermal behavior, and dielectric properties of the fabricated specimens including dielectric loss and dielectric constant were considered in the present study. 2. Experimental procedure
Fig. 1. XRD patterns of G1, G2, and G3 as-quenched glasses.
2.1. Materials SiO2, PbO, H3BO3, K2CO3, and ZnO were purchased from Merck Company in order to make glass frits. Furthermore, CaCO3, CuO, and TiO2 as raw materials for the synthesis of CCTO were obtained from Merck Company. All of the used materials were utilized without more purification. 2.2. Preparation of glass-ceramic
Fig. 2. XRD patterns of the heat-treated glass at different conditions based on the results in Table 2 and DSC curve of the G1 glass (Inset).
Table 2 Heat treatment conditions and crystallized phases in the heat-treated glasses. specimen
Heat treatment conditions(min, °C)
Crystallized phases
GT1 GT2 GT2ˊ
15 min at 15 min at 15 min at 840 °C 15 min at
659 °C 802 °C 802 °C and 15 min at
(Pb1-xCax)TiO3+CaTiO(SiO4) PbTi3O7 PbTi3O7+(Pb1-xCax)TiO3
870 °C
CaCu3Ti4O12
GT3
The glass frits with the chemical composition of 24 SiO2 - 65 PbO - 3 K2O - 7 B2O3 - 1 ZnO (wt. %), were prepared through water-quenching method. First, the raw materials were weighed by a high precision electronic scale. Then, the mixture of powders was ball-milled for 5 h and melted in a covered high-grade alumina crucible at 950 °C for 30 min by an electric furnace. Following this, in order to obtain glass frits, the molten glass was rapidly quenched by pouring into distilled water. After drying, the glass frits were ball-milled for 1 h in an agate mortar in order to attain a fine powder, and passed through a 100 mesh sieve. As shown in Table 1, three different weight percentages of CCTO, containing 14.03 CaO-29.66 CuO-56.31 TiO2 (wt. %), were mixed with the prepared glass frits to achieve different compositions. In addition, they were cast into a stainless steel mold after melting the batches in the air at 1070-1090 °C for 1 h in an electric furnace. The obtained airquenched glass, which was about 5 mm in thickness, annealed at 350 °C for 1 h in order to eliminate residual stresses, among which the G1 specimen was selected to be heat-treated at 468 °C for 6 h, and 830 °C for 2 h for nucleation, and growth of nucleus, respectively. 2.3. Characterization X-ray diffraction (XRD) (Philips PW3040/60) by using Cu-Kα
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Fig. 4. (a) and (b) Backscattered electron images of the melt-quenched glass, (c) elemental mappings of image b for O, Si, Pb, Ti, Cu, and K (d) EDS analysis at region 1 in Image b.
(λ = 1.5405 ˚A) and the step size of 0.02° in the range of 20 ̊ to 80 ̊ was used to study the crystallinity of the products. The specimens were prepared by their grinding to pass a 120 mesh sieve. The phase identification process was done by matching all of the peaks with standard ICDD data using X-Pert high score plus software. Differential Scanning Calorimetry (DSC) (Netzsch STA 409 PC/PG) with alumina as the reference material was used to determine the glass transition (Tg), softening point (Ts), and the crystallization temperatures of the glass with the heating rate of 10 °C. min−1 in an air flow and the temperature range of 30–1000 °C. Microstructural characterizations were conducted by using a scanning electron microscopy (SEM) (FEI Quanta 200), equipped with an energy dispersive spectroscopy (EDS) detector. In order to prepare the SEM specimens, they were polished and etched in 15% HF solution for 15-30 s. For dielectric measurements, the polished
glasses and glass-ceramics were covered with silver paste as the electrode and dried at 180 °C for 8 h. Room temperature dielectric constant and losses were measured using a precision LCR meter (GW INSTEK 8101G) in the frequency range of 200-106 Hz and an oscillation voltage of 2.0 V. 3. Results and discussion Due to the relatively low viscosity and high melt crystallization rate of the ferroelectric perovskites such as CCTO, their fabrication in the glassy form without adding a network-former is too complicated and requires a rapid quenching of the melt [21]. On the other hand, the crystallization of CCTO is problematic in some glass compositions due to their stable net structure. Thus, it is necessary to find a composition
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Fig. 5. (a) BSE micrograph of the nucleated glass, and (b) EDS analysis in the specified region.
in which the as-quenched specimens is amorphous, and the crystallization of CCTO is possible after the heat treatment [22]. Hence, in the present study, in order to obtain an amorphous structure, a glass frit with the composition of 24 SiO2 – 65 PbO -3 K2O – 7 B2O3 - 1 ZnO (wt. %) was added to CCTO. SiO2 and B2O3 were selected as a network former which increased the viscosity of the melt and prevented the crystallization. In addition, PbO was used to decrease the liquid temperature and facilitate the melting process, and improve the dielectric properties due to the high polarizability of the lead cations [21,23]. K2O can avoid evaporating PbO by decreasing the liquid temperature, and ZnO can stabilize the amorphous structure of the glass by increasing the glass relative viscosity [21,24]. This composition was obtained after examining a large number of compositions, based on the composition reported by Malek [25]. Fig. 1 illustrates the XRD patterns of the G1, G2, and G3 specimens. The broad peak observed between 2θ = 20˚-40° demonstrates the presence of the glassy phase in the specimens [26]. According to the 01-820514 and 01-070-0609 reference codes, the diffraction peaks of the TiO2 and CCTO phases in G2 and G3 indicate that the specimens are not entirely amorphous. In order to increase the stability of the glassy phase and hamper the crystallization, the amount of glass frits increased to 68 wt %. As displayed in Fig. 1, no sharp peak can be observed in the XRD pattern of G1 and the broad peak observed between 2θ = 20˚- 40° confirms the amorphous structure of this specimen. Therefore, it was selected as the final composition of the CCTO glass. DSC analysis was used to determine the thermal behavior of the glass, and accordingly the heat treatment condition for the glass to glass-ceramic conversion. DSC curve of the G1 glass and corresponding XRD patterns are shown in Fig. 2. The glass transition temperature is observed at 450 °C and an endothermic dip at 485 °C is related to the softening point of the glass [24]. After increasing the temperature, four
exothermic peaks located at 659, 802, 870 and 935 °C are observed, which can be related to the precipitation of different phases from the glassy matrix. As shown in Table 2, the G1 glass was heated at different temperatures where the peaks are observed, in order to determine crystallized phases at each of these temperatures. Two exothermic peaks centered around T1 = 659 °C are related to the formation of CaTiO (SiO4) and solid solution of (Pb1-x,Cax)TiO3 (Fig. 2-GT1). Metastable PbTi3O7 crystals are formed at T2 = 802 °C and gradually transform to equilibrium tetragonal (Pb1-xCax) TiO3 by rising the temperature (GT2 and GT2ˊ). In XRD pattern of GT2ˊ, the intensity of PbTi3O7 peaks decreased, compared to GT2, leading to the creation of the tetragonal (Pb1-xCax) TiO3 peak at 2θ = 31.58°. As shown in Fig. 2GT3, the CCTO peaks are revealed at T3 = 870 °C. The specimen was deformed at T4 = 935 °C. Table 2 indicates a summary of the crystallization process of the glass at different temperatures. The glasses are converted to the glass-ceramics through two-stage heat treatment, during which the nucleation happens in the first stage, and the growth of the nucleus within the glass occurs in the second stage [21]. The thermodynamic driving force of this converting is the change of Gibbs free energy between the melt and crystals [27]. Nucleation temperature was determined between the glass transition temperature, Tg, and the softening point of the glass, Ts [21]. The highest point at which the specimen was without any deformation was preferred as the growth temperature. Therefore, the heat treatment conditions for nucleation and growth were determined as 6 h at 468 °C and 2 h at 830 °C, respectively. As shown in Fig. 3, which displays the XRD pattern of the heat-treated glass (GC1), the peak at 2θ = 34.40° corresponds to CCTO as the main phase. Further, some other peaks can be observed with respect to (Pb1-xCax) TiO3 solid solution and CaTiO (SiO4). XRD analysis of the glass-ceramic between 2θ = 31° to 33° was performed using a step size of 0.005 in order to ensure the formation of (Pb1-xCax) TiO3 solid solution. Based on the inset of Fig. 3, compared to the reference pattern of PbTiO3 (reference code: 01-077-2002), the peak at 2θ = 31.77° related to (101) diffraction plane shows a shift to higher angles. Furthermore, there is a shift to lower angles for the peak at 2θ = 32.55° related to (110) diffraction plane, which indicates a reduction in tetragonality (c/a ratio) due to the formation of Ca and PbTiO3 solid-solution, as well as the transformation of tetragonal PbTiO3 to pseudo-cubic. Accordingly, Pb2+ was replaced by Ca2+ within the glassy matrix because of the smaller ionic radius of Ca2+ (rCa2 += 0.99 ˚A) than that of Pb2+ (rPb2 + =1.20 ˚A) [28–30]. Fig. 4 represents the SEM, elemental mappings, and EDS results of the melt-quenched glass (G1). The microstructure of the glass reveals some precipitated needle-like crystals and a few micron regions which were separated from the matrix, while the XRD pattern indicated the formation of an amorphous structure (Fig. 4a and b). The observation may be related to the negligible amount of these crystals. Considering the elemental mapping of Fig. 4b shown in Fig. 4c, higher concentrations of O, Si, Cu, and K were detected in the matrix, compared to the separated regions which have more Pb. According to the Ti accumulation in needle-like crystals in elemental mapping for Ti in Fig. 4c and the EDS result in Fig. 4d, it can be concluded that these crystals are TiO2. The low viscosity of the separated regions which are rich in lead, facilitates their crystallization during the melt cooling [31,32]. Based on the SEM and EDS results of the nucleated glass heattreated at 468 °C for 6 h in Fig. 5 (a) and (b), needle-like crystals of TiO2 were precipitated throughout the glass. At such high temperatures, Ti ions can diffuse out of the glass network and segregate to form the crystalline phase [33]. Fig. 6 represents the SEM images and EDS of the obtained glassceramic (GC1: the heat-treated glass at 468 °C for 6 h and at 830 °C for 2 h). Fig. 6a shows an approximately homogenous microstructure of the glass-ceramic and four different phases are crystallized in the glassy matrix (Fig. 6b). Based on the XRD pattern in Fig. 3, EDX spectra
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Fig. 6. (a), (b), (c) Backscattered electron images of the glass-ceramic, (d) EDS analysis of regions 1, 2, 3 and 4 in Image b.
obtained from regions 1, 2, 3 and 4 in Fig. 6d and the elemental mapping images in Fig. 7, it is concluded that these crystals are CCTO, CaTiO (SiO4) and 2 solid solutions of (Pb1-xCax) TiO3 with 2 different Ca percentage, respectively. Based on EDS results in Fig. 6-d-3 and 6-d-4, the brighter region (region 3 in Fig. 6b) is related to (Pb1-xCax) TiO3 solid-solution with higher lead percent, which is a heavy metal. As shown in Fig. 6b and c, the precipitation of the faceted crystals of CCTO with the size distribution of 20–90 μm and (Pb1-xCax) TiO3 can be related to the high percentage of amorphous phase having low softening point due to a large amount of Pb, which minimizes the surface energy and facilitates the growth of particular faces [35]. No evidence of TiO2 needle-like crystals formed in the glass and nucleated glass are found in the obtained glass-ceramic. Therefore, those crystals were the heterogeneous nucleating agents and converted to the perovskite phases at high temperatures. Similar observations were reported by other researchers [33,34]. A large hole in Fig. 6a and a few holes around the CCTO crystals are observed in Fig. 6b, which is related to the precipitation of the high-density phases and fast growth of crystals in the glassy matrix [35]. The relative dielectric constant (ɛr) and dielectric loss (tanδ) vs. frequency of the glass and glass-ceramic are plotted in Fig. 8. The dielectric constant of a material in an alternating electric field relies on the simplicity of change in dipoles orientation by reversing the field [21]. Some polarization mechanisms such as orientation and spacecharge polarization are not adequately rapid and operated only at low frequencies [21,36]. Therefore, increasing ɛr in lower frequencies in the glass and glass-ceramic is related to the easy hopping of charge carriers under the applied electric field and the orientation and space-charge
polarization [21,36,37]. The dielectric loss of the glass was measured to be ∼4.6×10−5 at 1 MHz and this low dielectric loss is due to the large amounts of glassy phase rich in SiO2 and PbO which involves a high electrical resistance. Thus, it constrain the actual charge flow through the material [5,38,39]. The dependence of dielectric loss changes on the frequency in the glass-ceramic reveals the existence of a dipolar relaxation process at a frequency of 1 kHz. This process is probably related to the space-charge polarization [21,38]. Further, the dielectric loss increment to 0.012 at 1 MHz after the heat treatment can be related to three main factors: (i) decreasing the glass with high electrical resistance percentage and the precipitation of crystals with high dielectric loss, (ii) crystallization process leading to the smaller average distance between the hopping centers, which creates an easy conduction path for charge carriers resulting in a higher electronic conductivity, and (iii) the presence of structural defects such as pores [36,40–43]. The dielectric constant of the glass was 16.3 at 1 MHz and reached 41.2 after the heat treatment process. The increase of about 150% which was almost constant over a wide range of frequencies can be explained by the formation of high dielectric constant crystalline phases such as CCTO and (Pb1-xCax) TiO3, and the interfacial polarization caused by accumulating the charges at the interfaces. Further, the charge accommodation at the interface is resulted from the electrical resistance difference between the matrix and crystalline phases [22]. Therefore, the crystallization of the glassy phase leads to an increase in both the dielectric constant and dielectric loss.
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Fig. 7. (a) Backscattered electron image of the glass-ceramic, and (b) Elemental mappings for Ca, Cu, Ti, Si and Pb in Image a.
4. Conclusion The CCTO glass was fabricated by the melt-quenching technique in a glass system of 16.32SiO2-44.2PbO-4.76B2O3-2.04K2O-0.68ZnO4.49CaO-18.02TiO2, and subsequently, heat treated for conversion to glass-ceramics. The following conclusions were drawn in the present study:
Fig. 8. Variation of dielectric constant (ɛr) and loss (tanδ) vs. frequency for glass and glass-ceramic. 19321
1 XRD pattern of the melt-quenched glass indicated the amorphous structure and SEM studies showed phase separation and a few needle-like crystals of TiO2, which were precipitated in the separated regions. 2 XRD pattern and SEM results of the glass-ceramic demonstrated the faceted crystals of CCTO and (Pb1-xCax) TiO3, as well as CaTiO (SiO4) phase in the glassy matrix. 3 Two solid-solutions of (Pb1-xCax) TiO3 were crystallized during the heat treatment, among which one was crystallized at 659 °C, and another was formed from its precursor phase, PbTi3O7, which was precipitated at 802 °C. 4 The dielectric constant of the glass and glass-ceramic were measured 16.3 and 41.2, respectively, and dielectric loss was 4.6×10−5 and
Ceramics International 45 (2019) 19316–19322
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0.012, respectively, at the frequency of 1 MHz. 5 Although the dielectric constant of the CCTO glass and glass-ceramic is lower than that of ceramic, its dielectric loss decreased significantly. In conclusion, the present study could represent the formation of CCTO glass and glass-ceramic, along with improved dielectric properties, compared to the results of the previous works. References [1] X. You, N. Chen, G. Du, Constructing three-dimensionally interwoven structures for ceramic/polymer composites to exhibit colossal dielectric constant and high mechanical strength : CaCu3Ti4O12/epoxy as an example, Compos. Part A. 105 (2018) 214–222, https://doi.org/10.1016/j.compositesa.2017.11.025. [2] Y. Qu, Y. Du, G. Fan, J. Xin, Y. Liu, P. Xie, S. You, Z. Zhang, K. Sun, R. Fan, Lowtemperature sintering Graphene/CaCu3Ti4O12 nanocomposites with tunable negative permittivity, J. Alloy. Comp. 771 (2019) 699–710, https://doi.org/10.1016/j. jallcom.2018.09.049. [3] A.K. Yadav, C.R. Gautam, A review on crystallisation behaviour of perovskite glass ceramics, Adv. Appl. Ceram. 113 (2014) 193–207, https://doi.org/10.1179/ 1743676113Y.0000000134. [4] W. Yang, S. Yu, R. Sun, R. Du, Nano- and microsize effect of CCTO fillers on the dielectric behavior of CCTO/PVDF composites, Acta Mater. 59 (2011) 5593–5602, https://doi.org/10.1016/j.actamat.2011.05.034. [5] M. Ahmadipour, M.F. Ain, Z.A. Ahmad, A short review on copper calcium titanate (CCTO) electroceramic: synthesis, dielectric properties, film deposition, and sensing application, Nano-Micro Lett. 8 (2016) 291–311, https://doi.org/10.1007/s40820016-0089-1. [6] Y. Hu, Y. Pu, P. Wang, T. Wu, Effects of Kaolinite additions on sintering behavior and dielectric properties of CaCu3Ti4O12 ceramics, J. Mater. Sci. Mater. Electron. 25 (2014) 546–551, https://doi.org/10.1007/s10854-013-1622-3. [7] S. De Almeida-didry, M. Martin, C. Autret, C. Honstettre, A. Lucas, F. Pacreau, F. Gervais, Control of grain boundary in alumina doped CCTO showing colossal permittivity by core-shell approach, J. Eur. Ceram. Soc. 38 (2018) 3182–3187, https://doi.org/10.1016/j.jeurceramsoc.2018.03.003. [8] R.W. Young, A. L, G.E. Hilmas, S.C. Zhang, Schwartz, Mechanical vs. electrical failure mechanisms in high voltage, high energy density multilayer ceramic capacitors, J. Mater. Sci. 42 (2007) 5613–5619, https://doi.org/10.1007/s10853-0061116-2. [9] J. Lee, J. Koh, Enhanced dielectric properties of Ag-doped CCTO ceramics for energy storage devices, Ceram. Int. 43 (2017) 9493–9497, https://doi.org/10.1016/j. ceramint.2017.04.130. [10] W.X. Yuan, S.K. Hark, Investigation on the origin of the giant dielectric constant in CaCu3Ti4O12 ceramics through analyzing CaCu3Ti4O12-HfO2 composites, J. Eur. Ceram. Soc. 32 (2012) 465–470, https://doi.org/10.1016/j.jeurceramsoc.2011.09. 021. [11] L. Ren, L. Yang, C. Xu, X. Zhao, R. Liao, Improvement of breakdown field and dielectric properties of CaCu3Ti4O12 ceramics by Bi and Al co-doping, J. Alloy. Comp. 768 (2018) 652–658, https://doi.org/10.1016/j.jallcom.2018.07.293. [12] X. Huang, Y. Jiang, K. Wu, CCTO giant dielectric ceramic prepared by reaction sintering, Procedia Eng. 102 (2015) 468–474, https://doi.org/10.1016/j.proeng. 2015.01.191. [13] G. Du, F. Wei, W. Li, N. Chen, Co-doping effects of A-site Y3+ and B-site Al3+ on the microstructures and dielectric properties of CaCu3Ti4O12 ceramics, J. Eur. Ceram. Soc. 37 (2017) 4653–4659, https://doi.org/10.1016/j.jeurceramsoc.2017.06.046. [14] L. Zhao, R. Xu, Y. Wei, X. Han, C. Zhai, Z. Zhang, X. Qi, B. Cui, J.L. Jones, Giant dielectric phenomenon of Ba0.5Sr0.5TiO3/CaCu3Ti4O12 multilayers due to interfacial polarization for capacitor applications Lili, J. Eur. Ceram. Soc. 39 (2019) 1116–1121, https://doi.org/10.1016/j.jeurceramsoc.2018.11.039. [15] W.X. Yuan, Z.J. Li, C.D. Wang, Relationship between Microstructural Evolution and Electric Properties of B2O3–CaCu3Ti4O12 Composite Ceramics vol 23, (2012), pp. 1552–1557, https://doi.org/10.1007/s10854-012-0627-7. [16] X. Hao, A review on the dielectric materials for high energy-storage application, J. Adv. Dielectr. 03 (2013) 1330001, https://doi.org/10.1142/S2010135X13300016. [17] S. Goswami, A. Sen, Low temperature sintering of CCTO using P2O5 as a sintering aid, Ceram. Int. 36 (2010) 1629–1631, https://doi.org/10.1016/j.ceramint.2010. 02.036. [18] W. Li, T. Zhang, S. Liu, Z. Lu, R. Xiong, Decrease in the dielectric loss of CaCu3Ti4O12 at high frequency by Ru doping, Ceram. Int. 43 (2017) 4366–4371, https://doi.org/10.1016/j.ceramint.2016.12.082. [19] J.J. Mohamed, M.F. Ab Rahman, M.F. Ain, Z.A. Ahmad, Effect of glass addition on the properties of CaCu3Ti4O12 ceramics, Mater. Sci. Forum 840 (2016) 52–56 10.
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Balakumar, F.J. Kumar, C. Subramanian, P. Ramasamy, Growth and characterization of Pb1-xCaxTiO3 single crystals by self-fux technique, Mater. Sci. Commun. 57 (1999) 281–284. [31] S. Fujino, C. Hwang, K. Morinaga, Density, surface tension, and viscosity of PbOB2O3-SiO2, J. Am. Ceram. Soc. 16 (2004) 10–16. [32] B. Yang, J.W.P. Schmelzer, B. Zhao, Y. Gao, C. Schick, Glass transition and primary crystallization of Al86Ni6Y4.5Co2La1.5 metallic glass at heating rates spanning over six orders of magnitude Bin, Scr. Mater. 162 (2019) 146–150, https://doi.org/10. 1016/j.scriptamat.2018.11.014. [33] K. Yao, L. Zhang, X. Yao, W. Zhu, Growth, structure, and morphology of lead titanate crystallites in the sol-gel derived glass-ceramics, Mater. Sci. Eng. B. 41 (1996) 322–328, https://doi.org/10.1016/S0921-5107(96)01899-5. [34] J. Marillet, D. Bourret, Crystallization of lead titanate PbTiO3 in gel-derived solder glasses, J. Non-Cryst. Solids 148 (1992) 266–270. [35] J. Zhao, J. Liu, G. 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Microstructure and dielectric properties of CCTO glass-ceramic prepared by the melt-quenching method
T
Tahere Nazari Aliabadi, Parvin Alizadeh∗ Department of Materials Science and Engineering, Tarbiat Modares University, Tehran, 14115-143, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: Glass Glass-ceramic CaCu3Ti4O12 Dielectric properties
In this work, CaCu3Ti4O12 (CCTO) glass-ceramic was prepared by using the melt-quenching method followed by a two-step heat treatment. The prepared glass frits with the chemical composition of 24SiO2-65PbO-3K2O7B2O3-1ZnO (wt. %) were mixed with 32, 34 and 38 wt % of CCTO including 14.03 CaO-29.66 CuO-56.31 TiO2 (wt. %) and melted at 1070-1090°C . The glass with 32 wt % of CCTO, which was XRD amorphous, was selected as the final composition. The heat treatment conditions were found to be 6 and 2 h at 468 °C and 830 °C, respectively. The faceted crystals of CCTO, two solid-solution of (Pb1-xCax)TiO3, and CaTiO(SiO4) were formed in the glassy matrix during the crystallization based on the studies related to XRD, SEM, EDS, and elemental mapping in the glass-ceramic. The measured dielectric constant of the glass was 16.3 at 1 MHz, which reached 41.2 after the heat treatment process, and the value of the tanδ increased from 4.6× 10−5 to 0.012.
1. Introduction
porosities [6,8,9,13]. Therefore, the CCTO glass-ceramic as a perovskite glass-ceramic has a high energy density and can be used in technological electronic applications such as capacitors, resonators, memory devices, and filters [15,16]. Recently, a large body of research has been conducted to improve the dielectric properties of CCTO. however, most of these studies focused on the addition of a glassy phase to CCTO ceramic in order to decrease the dielectric loss [15,17,18]. In this regard, Mohamed et al. [19] fabricated CCTO ceramics with BaO-SrO-Nb2O5-B2O3-SiO2 (BSNBS) glass additive by implementing solid-state reaction method. The dielectric constant and dielectric loss of the CCTO ceramic were reported 10482 and 0.54 at 1 MHz, respectively, and showed a decrease for the specimen with 0.10 BSNBS to ∼4000 and 0.38. In a similar study, the addition of TeO2 as a well-known glass former resulted in decreasing the dielectric loss of CCTO to 0.095 [18]. Further, Zhang et al. [20] produced CCTO glass-ceramics in K2O–Al2O3–SiO2 system. Based on the XRD patterns, the diffraction peaks of KAlSi2O5, Ca3Al2Si3O12, and Al2 (SiO4) O were detected in the as-synthesized specimens. The dielectric constant and dielectric loss of the glassceramics with 40 and 60 wt % of CCTO were 18, 0.02 and 38, 0.12, respectively. As it was already mentioned, CCTO indicates a high dielectric constant, but high dielectric loss of CCTO limits its application in capacitors. On the other hand, the low dielectric loss of glasses means producing CCTO glass to decrease the dielectric loss of the CCTO in
The fabrication of materials with high dielectric constant and low dielectric loss plays a key role in minimizing the dimensions of electronic instruments [1]. During recent years, much attention has been paid to cubic perovskite ceramics due to their desirable dielectric properties. The calcium copper titanate (CCTO) is a pseudo perovskite with a cubic unit cell and Im3 space group [2–5], which has a dielectric constant of 105 for a single crystal and 104 for bulk materials at the frequency of 1 kHz and room temperature [6–8]. It is well-known that the dielectric constant of this material in the frequency range of 102–106 Hz and a wide temperature range of 100–400 K remains relatively constant [6,9–11]. Different models have been reported for explaining the unusual properties of CCTO. The internal barrier layer capacitor (IBLC) was proposed by Sinclair et al. [12], which is known as a well-accepted model to evaluate the properties of CCTO. In this model, CCTO is composed of semi-conductive grains and insulating grain boundaries [12]. Although CCTO has a high dielectric constant, its applications have been constrained due to the high dielectric loss (higher than 0.1) at room temperature and 1 kHz and low breakdown voltage [7,12–14]. Among various materials used as dielectrics, glass-ceramics made by the melt-quenching method exhibit desirable dielectric constants, low dielectric losses, and good dielectric breakdown strengths (∼300-1200 kV.cm−1) due to the presence of a glassy phase and little amounts of
∗
Corresponding author. E-mail address: [email protected] (P. Alizadeh).
https://doi.org/10.1016/j.ceramint.2019.06.182 Received 21 May 2019; Received in revised form 15 June 2019; Accepted 18 June 2019 Available online 20 June 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 45 (2019) 19316–19322
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Table 1 The nominal composition of the specimens (wt. %). specimen
Glass frits SiO2 68 16.32 66 15.84 62 14.88
G1 G2 G3
CCTO PbO
B2O3
K2O
ZnO
44.20
4.76
2.04
0.68
42.90
4.62
1.98
0.66
40.30
4.34
1.86
0.62
CaO 32 4.49 34 4.77 38 5.33
CuO
TiO2
9.49
18.02
10.08
19.15
11.27
21.40
Fig. 3. XRD pattern of the glass-ceramic (GC1).
which determining appropriate heat treatment conditions leads to the fabrication of glass-ceramic simultaneously with low dielectric loss and high dielectric constant. Thus, the present study aimed to select a suitable composition for fabricating an air-quenched amorphous CCTO glass by using the melt-quenching method and determining the desired heat treatment conditions for converting the fabricated glass to a glassceramic with desirable dielectric properties. In this regard, the microstructure, thermal behavior, and dielectric properties of the fabricated specimens including dielectric loss and dielectric constant were considered in the present study. 2. Experimental procedure
Fig. 1. XRD patterns of G1, G2, and G3 as-quenched glasses.
2.1. Materials SiO2, PbO, H3BO3, K2CO3, and ZnO were purchased from Merck Company in order to make glass frits. Furthermore, CaCO3, CuO, and TiO2 as raw materials for the synthesis of CCTO were obtained from Merck Company. All of the used materials were utilized without more purification. 2.2. Preparation of glass-ceramic
Fig. 2. XRD patterns of the heat-treated glass at different conditions based on the results in Table 2 and DSC curve of the G1 glass (Inset).
Table 2 Heat treatment conditions and crystallized phases in the heat-treated glasses. specimen
Heat treatment conditions(min, °C)
Crystallized phases
GT1 GT2 GT2ˊ
15 min at 15 min at 15 min at 840 °C 15 min at
659 °C 802 °C 802 °C and 15 min at
(Pb1-xCax)TiO3+CaTiO(SiO4) PbTi3O7 PbTi3O7+(Pb1-xCax)TiO3
870 °C
CaCu3Ti4O12
GT3
The glass frits with the chemical composition of 24 SiO2 - 65 PbO - 3 K2O - 7 B2O3 - 1 ZnO (wt. %), were prepared through water-quenching method. First, the raw materials were weighed by a high precision electronic scale. Then, the mixture of powders was ball-milled for 5 h and melted in a covered high-grade alumina crucible at 950 °C for 30 min by an electric furnace. Following this, in order to obtain glass frits, the molten glass was rapidly quenched by pouring into distilled water. After drying, the glass frits were ball-milled for 1 h in an agate mortar in order to attain a fine powder, and passed through a 100 mesh sieve. As shown in Table 1, three different weight percentages of CCTO, containing 14.03 CaO-29.66 CuO-56.31 TiO2 (wt. %), were mixed with the prepared glass frits to achieve different compositions. In addition, they were cast into a stainless steel mold after melting the batches in the air at 1070-1090 °C for 1 h in an electric furnace. The obtained airquenched glass, which was about 5 mm in thickness, annealed at 350 °C for 1 h in order to eliminate residual stresses, among which the G1 specimen was selected to be heat-treated at 468 °C for 6 h, and 830 °C for 2 h for nucleation, and growth of nucleus, respectively. 2.3. Characterization X-ray diffraction (XRD) (Philips PW3040/60) by using Cu-Kα
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Fig. 4. (a) and (b) Backscattered electron images of the melt-quenched glass, (c) elemental mappings of image b for O, Si, Pb, Ti, Cu, and K (d) EDS analysis at region 1 in Image b.
(λ = 1.5405 ˚A) and the step size of 0.02° in the range of 20 ̊ to 80 ̊ was used to study the crystallinity of the products. The specimens were prepared by their grinding to pass a 120 mesh sieve. The phase identification process was done by matching all of the peaks with standard ICDD data using X-Pert high score plus software. Differential Scanning Calorimetry (DSC) (Netzsch STA 409 PC/PG) with alumina as the reference material was used to determine the glass transition (Tg), softening point (Ts), and the crystallization temperatures of the glass with the heating rate of 10 °C. min−1 in an air flow and the temperature range of 30–1000 °C. Microstructural characterizations were conducted by using a scanning electron microscopy (SEM) (FEI Quanta 200), equipped with an energy dispersive spectroscopy (EDS) detector. In order to prepare the SEM specimens, they were polished and etched in 15% HF solution for 15-30 s. For dielectric measurements, the polished
glasses and glass-ceramics were covered with silver paste as the electrode and dried at 180 °C for 8 h. Room temperature dielectric constant and losses were measured using a precision LCR meter (GW INSTEK 8101G) in the frequency range of 200-106 Hz and an oscillation voltage of 2.0 V. 3. Results and discussion Due to the relatively low viscosity and high melt crystallization rate of the ferroelectric perovskites such as CCTO, their fabrication in the glassy form without adding a network-former is too complicated and requires a rapid quenching of the melt [21]. On the other hand, the crystallization of CCTO is problematic in some glass compositions due to their stable net structure. Thus, it is necessary to find a composition
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Fig. 5. (a) BSE micrograph of the nucleated glass, and (b) EDS analysis in the specified region.
in which the as-quenched specimens is amorphous, and the crystallization of CCTO is possible after the heat treatment [22]. Hence, in the present study, in order to obtain an amorphous structure, a glass frit with the composition of 24 SiO2 – 65 PbO -3 K2O – 7 B2O3 - 1 ZnO (wt. %) was added to CCTO. SiO2 and B2O3 were selected as a network former which increased the viscosity of the melt and prevented the crystallization. In addition, PbO was used to decrease the liquid temperature and facilitate the melting process, and improve the dielectric properties due to the high polarizability of the lead cations [21,23]. K2O can avoid evaporating PbO by decreasing the liquid temperature, and ZnO can stabilize the amorphous structure of the glass by increasing the glass relative viscosity [21,24]. This composition was obtained after examining a large number of compositions, based on the composition reported by Malek [25]. Fig. 1 illustrates the XRD patterns of the G1, G2, and G3 specimens. The broad peak observed between 2θ = 20˚-40° demonstrates the presence of the glassy phase in the specimens [26]. According to the 01-820514 and 01-070-0609 reference codes, the diffraction peaks of the TiO2 and CCTO phases in G2 and G3 indicate that the specimens are not entirely amorphous. In order to increase the stability of the glassy phase and hamper the crystallization, the amount of glass frits increased to 68 wt %. As displayed in Fig. 1, no sharp peak can be observed in the XRD pattern of G1 and the broad peak observed between 2θ = 20˚- 40° confirms the amorphous structure of this specimen. Therefore, it was selected as the final composition of the CCTO glass. DSC analysis was used to determine the thermal behavior of the glass, and accordingly the heat treatment condition for the glass to glass-ceramic conversion. DSC curve of the G1 glass and corresponding XRD patterns are shown in Fig. 2. The glass transition temperature is observed at 450 °C and an endothermic dip at 485 °C is related to the softening point of the glass [24]. After increasing the temperature, four
exothermic peaks located at 659, 802, 870 and 935 °C are observed, which can be related to the precipitation of different phases from the glassy matrix. As shown in Table 2, the G1 glass was heated at different temperatures where the peaks are observed, in order to determine crystallized phases at each of these temperatures. Two exothermic peaks centered around T1 = 659 °C are related to the formation of CaTiO (SiO4) and solid solution of (Pb1-x,Cax)TiO3 (Fig. 2-GT1). Metastable PbTi3O7 crystals are formed at T2 = 802 °C and gradually transform to equilibrium tetragonal (Pb1-xCax) TiO3 by rising the temperature (GT2 and GT2ˊ). In XRD pattern of GT2ˊ, the intensity of PbTi3O7 peaks decreased, compared to GT2, leading to the creation of the tetragonal (Pb1-xCax) TiO3 peak at 2θ = 31.58°. As shown in Fig. 2GT3, the CCTO peaks are revealed at T3 = 870 °C. The specimen was deformed at T4 = 935 °C. Table 2 indicates a summary of the crystallization process of the glass at different temperatures. The glasses are converted to the glass-ceramics through two-stage heat treatment, during which the nucleation happens in the first stage, and the growth of the nucleus within the glass occurs in the second stage [21]. The thermodynamic driving force of this converting is the change of Gibbs free energy between the melt and crystals [27]. Nucleation temperature was determined between the glass transition temperature, Tg, and the softening point of the glass, Ts [21]. The highest point at which the specimen was without any deformation was preferred as the growth temperature. Therefore, the heat treatment conditions for nucleation and growth were determined as 6 h at 468 °C and 2 h at 830 °C, respectively. As shown in Fig. 3, which displays the XRD pattern of the heat-treated glass (GC1), the peak at 2θ = 34.40° corresponds to CCTO as the main phase. Further, some other peaks can be observed with respect to (Pb1-xCax) TiO3 solid solution and CaTiO (SiO4). XRD analysis of the glass-ceramic between 2θ = 31° to 33° was performed using a step size of 0.005 in order to ensure the formation of (Pb1-xCax) TiO3 solid solution. Based on the inset of Fig. 3, compared to the reference pattern of PbTiO3 (reference code: 01-077-2002), the peak at 2θ = 31.77° related to (101) diffraction plane shows a shift to higher angles. Furthermore, there is a shift to lower angles for the peak at 2θ = 32.55° related to (110) diffraction plane, which indicates a reduction in tetragonality (c/a ratio) due to the formation of Ca and PbTiO3 solid-solution, as well as the transformation of tetragonal PbTiO3 to pseudo-cubic. Accordingly, Pb2+ was replaced by Ca2+ within the glassy matrix because of the smaller ionic radius of Ca2+ (rCa2 += 0.99 ˚A) than that of Pb2+ (rPb2 + =1.20 ˚A) [28–30]. Fig. 4 represents the SEM, elemental mappings, and EDS results of the melt-quenched glass (G1). The microstructure of the glass reveals some precipitated needle-like crystals and a few micron regions which were separated from the matrix, while the XRD pattern indicated the formation of an amorphous structure (Fig. 4a and b). The observation may be related to the negligible amount of these crystals. Considering the elemental mapping of Fig. 4b shown in Fig. 4c, higher concentrations of O, Si, Cu, and K were detected in the matrix, compared to the separated regions which have more Pb. According to the Ti accumulation in needle-like crystals in elemental mapping for Ti in Fig. 4c and the EDS result in Fig. 4d, it can be concluded that these crystals are TiO2. The low viscosity of the separated regions which are rich in lead, facilitates their crystallization during the melt cooling [31,32]. Based on the SEM and EDS results of the nucleated glass heattreated at 468 °C for 6 h in Fig. 5 (a) and (b), needle-like crystals of TiO2 were precipitated throughout the glass. At such high temperatures, Ti ions can diffuse out of the glass network and segregate to form the crystalline phase [33]. Fig. 6 represents the SEM images and EDS of the obtained glassceramic (GC1: the heat-treated glass at 468 °C for 6 h and at 830 °C for 2 h). Fig. 6a shows an approximately homogenous microstructure of the glass-ceramic and four different phases are crystallized in the glassy matrix (Fig. 6b). Based on the XRD pattern in Fig. 3, EDX spectra
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Fig. 6. (a), (b), (c) Backscattered electron images of the glass-ceramic, (d) EDS analysis of regions 1, 2, 3 and 4 in Image b.
obtained from regions 1, 2, 3 and 4 in Fig. 6d and the elemental mapping images in Fig. 7, it is concluded that these crystals are CCTO, CaTiO (SiO4) and 2 solid solutions of (Pb1-xCax) TiO3 with 2 different Ca percentage, respectively. Based on EDS results in Fig. 6-d-3 and 6-d-4, the brighter region (region 3 in Fig. 6b) is related to (Pb1-xCax) TiO3 solid-solution with higher lead percent, which is a heavy metal. As shown in Fig. 6b and c, the precipitation of the faceted crystals of CCTO with the size distribution of 20–90 μm and (Pb1-xCax) TiO3 can be related to the high percentage of amorphous phase having low softening point due to a large amount of Pb, which minimizes the surface energy and facilitates the growth of particular faces [35]. No evidence of TiO2 needle-like crystals formed in the glass and nucleated glass are found in the obtained glass-ceramic. Therefore, those crystals were the heterogeneous nucleating agents and converted to the perovskite phases at high temperatures. Similar observations were reported by other researchers [33,34]. A large hole in Fig. 6a and a few holes around the CCTO crystals are observed in Fig. 6b, which is related to the precipitation of the high-density phases and fast growth of crystals in the glassy matrix [35]. The relative dielectric constant (ɛr) and dielectric loss (tanδ) vs. frequency of the glass and glass-ceramic are plotted in Fig. 8. The dielectric constant of a material in an alternating electric field relies on the simplicity of change in dipoles orientation by reversing the field [21]. Some polarization mechanisms such as orientation and spacecharge polarization are not adequately rapid and operated only at low frequencies [21,36]. Therefore, increasing ɛr in lower frequencies in the glass and glass-ceramic is related to the easy hopping of charge carriers under the applied electric field and the orientation and space-charge
polarization [21,36,37]. The dielectric loss of the glass was measured to be ∼4.6×10−5 at 1 MHz and this low dielectric loss is due to the large amounts of glassy phase rich in SiO2 and PbO which involves a high electrical resistance. Thus, it constrain the actual charge flow through the material [5,38,39]. The dependence of dielectric loss changes on the frequency in the glass-ceramic reveals the existence of a dipolar relaxation process at a frequency of 1 kHz. This process is probably related to the space-charge polarization [21,38]. Further, the dielectric loss increment to 0.012 at 1 MHz after the heat treatment can be related to three main factors: (i) decreasing the glass with high electrical resistance percentage and the precipitation of crystals with high dielectric loss, (ii) crystallization process leading to the smaller average distance between the hopping centers, which creates an easy conduction path for charge carriers resulting in a higher electronic conductivity, and (iii) the presence of structural defects such as pores [36,40–43]. The dielectric constant of the glass was 16.3 at 1 MHz and reached 41.2 after the heat treatment process. The increase of about 150% which was almost constant over a wide range of frequencies can be explained by the formation of high dielectric constant crystalline phases such as CCTO and (Pb1-xCax) TiO3, and the interfacial polarization caused by accumulating the charges at the interfaces. Further, the charge accommodation at the interface is resulted from the electrical resistance difference between the matrix and crystalline phases [22]. Therefore, the crystallization of the glassy phase leads to an increase in both the dielectric constant and dielectric loss.
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Fig. 7. (a) Backscattered electron image of the glass-ceramic, and (b) Elemental mappings for Ca, Cu, Ti, Si and Pb in Image a.
4. Conclusion The CCTO glass was fabricated by the melt-quenching technique in a glass system of 16.32SiO2-44.2PbO-4.76B2O3-2.04K2O-0.68ZnO4.49CaO-18.02TiO2, and subsequently, heat treated for conversion to glass-ceramics. The following conclusions were drawn in the present study:
Fig. 8. Variation of dielectric constant (ɛr) and loss (tanδ) vs. frequency for glass and glass-ceramic. 19321
1 XRD pattern of the melt-quenched glass indicated the amorphous structure and SEM studies showed phase separation and a few needle-like crystals of TiO2, which were precipitated in the separated regions. 2 XRD pattern and SEM results of the glass-ceramic demonstrated the faceted crystals of CCTO and (Pb1-xCax) TiO3, as well as CaTiO (SiO4) phase in the glassy matrix. 3 Two solid-solutions of (Pb1-xCax) TiO3 were crystallized during the heat treatment, among which one was crystallized at 659 °C, and another was formed from its precursor phase, PbTi3O7, which was precipitated at 802 °C. 4 The dielectric constant of the glass and glass-ceramic were measured 16.3 and 41.2, respectively, and dielectric loss was 4.6×10−5 and
Ceramics International 45 (2019) 19316–19322
T. Nazari Aliabadi and P. Alizadeh
0.012, respectively, at the frequency of 1 MHz. 5 Although the dielectric constant of the CCTO glass and glass-ceramic is lower than that of ceramic, its dielectric loss decreased significantly. In conclusion, the present study could represent the formation of CCTO glass and glass-ceramic, along with improved dielectric properties, compared to the results of the previous works. References [1] X. You, N. Chen, G. Du, Constructing three-dimensionally interwoven structures for ceramic/polymer composites to exhibit colossal dielectric constant and high mechanical strength : CaCu3Ti4O12/epoxy as an example, Compos. Part A. 105 (2018) 214–222, https://doi.org/10.1016/j.compositesa.2017.11.025. [2] Y. Qu, Y. Du, G. Fan, J. Xin, Y. Liu, P. Xie, S. You, Z. Zhang, K. Sun, R. Fan, Lowtemperature sintering Graphene/CaCu3Ti4O12 nanocomposites with tunable negative permittivity, J. Alloy. Comp. 771 (2019) 699–710, https://doi.org/10.1016/j. jallcom.2018.09.049. [3] A.K. Yadav, C.R. Gautam, A review on crystallisation behaviour of perovskite glass ceramics, Adv. Appl. Ceram. 113 (2014) 193–207, https://doi.org/10.1179/ 1743676113Y.0000000134. [4] W. Yang, S. Yu, R. Sun, R. Du, Nano- and microsize effect of CCTO fillers on the dielectric behavior of CCTO/PVDF composites, Acta Mater. 59 (2011) 5593–5602, https://doi.org/10.1016/j.actamat.2011.05.034. [5] M. Ahmadipour, M.F. Ain, Z.A. Ahmad, A short review on copper calcium titanate (CCTO) electroceramic: synthesis, dielectric properties, film deposition, and sensing application, Nano-Micro Lett. 8 (2016) 291–311, https://doi.org/10.1007/s40820016-0089-1. [6] Y. Hu, Y. Pu, P. Wang, T. Wu, Effects of Kaolinite additions on sintering behavior and dielectric properties of CaCu3Ti4O12 ceramics, J. Mater. Sci. Mater. Electron. 25 (2014) 546–551, https://doi.org/10.1007/s10854-013-1622-3. [7] S. De Almeida-didry, M. Martin, C. Autret, C. Honstettre, A. Lucas, F. Pacreau, F. Gervais, Control of grain boundary in alumina doped CCTO showing colossal permittivity by core-shell approach, J. Eur. Ceram. Soc. 38 (2018) 3182–3187, https://doi.org/10.1016/j.jeurceramsoc.2018.03.003. [8] R.W. Young, A. L, G.E. Hilmas, S.C. Zhang, Schwartz, Mechanical vs. electrical failure mechanisms in high voltage, high energy density multilayer ceramic capacitors, J. Mater. Sci. 42 (2007) 5613–5619, https://doi.org/10.1007/s10853-0061116-2. [9] J. Lee, J. Koh, Enhanced dielectric properties of Ag-doped CCTO ceramics for energy storage devices, Ceram. Int. 43 (2017) 9493–9497, https://doi.org/10.1016/j. ceramint.2017.04.130. [10] W.X. Yuan, S.K. Hark, Investigation on the origin of the giant dielectric constant in CaCu3Ti4O12 ceramics through analyzing CaCu3Ti4O12-HfO2 composites, J. Eur. Ceram. Soc. 32 (2012) 465–470, https://doi.org/10.1016/j.jeurceramsoc.2011.09. 021. [11] L. Ren, L. Yang, C. Xu, X. Zhao, R. Liao, Improvement of breakdown field and dielectric properties of CaCu3Ti4O12 ceramics by Bi and Al co-doping, J. Alloy. Comp. 768 (2018) 652–658, https://doi.org/10.1016/j.jallcom.2018.07.293. [12] X. Huang, Y. Jiang, K. Wu, CCTO giant dielectric ceramic prepared by reaction sintering, Procedia Eng. 102 (2015) 468–474, https://doi.org/10.1016/j.proeng. 2015.01.191. [13] G. Du, F. Wei, W. Li, N. Chen, Co-doping effects of A-site Y3+ and B-site Al3+ on the microstructures and dielectric properties of CaCu3Ti4O12 ceramics, J. Eur. Ceram. Soc. 37 (2017) 4653–4659, https://doi.org/10.1016/j.jeurceramsoc.2017.06.046. [14] L. Zhao, R. Xu, Y. Wei, X. Han, C. Zhai, Z. Zhang, X. Qi, B. Cui, J.L. Jones, Giant dielectric phenomenon of Ba0.5Sr0.5TiO3/CaCu3Ti4O12 multilayers due to interfacial polarization for capacitor applications Lili, J. Eur. Ceram. Soc. 39 (2019) 1116–1121, https://doi.org/10.1016/j.jeurceramsoc.2018.11.039. [15] W.X. Yuan, Z.J. Li, C.D. Wang, Relationship between Microstructural Evolution and Electric Properties of B2O3–CaCu3Ti4O12 Composite Ceramics vol 23, (2012), pp. 1552–1557, https://doi.org/10.1007/s10854-012-0627-7. [16] X. Hao, A review on the dielectric materials for high energy-storage application, J. Adv. Dielectr. 03 (2013) 1330001, https://doi.org/10.1142/S2010135X13300016. [17] S. Goswami, A. Sen, Low temperature sintering of CCTO using P2O5 as a sintering aid, Ceram. Int. 36 (2010) 1629–1631, https://doi.org/10.1016/j.ceramint.2010. 02.036. [18] W. Li, T. Zhang, S. Liu, Z. Lu, R. Xiong, Decrease in the dielectric loss of CaCu3Ti4O12 at high frequency by Ru doping, Ceram. Int. 43 (2017) 4366–4371, https://doi.org/10.1016/j.ceramint.2016.12.082. [19] J.J. Mohamed, M.F. Ab Rahman, M.F. Ain, Z.A. Ahmad, Effect of glass addition on the properties of CaCu3Ti4O12 ceramics, Mater. Sci. Forum 840 (2016) 52–56 10.
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