Effect of ZrO2 Doping on the Microstructure and Dielectric Properties of Ca0.97Y0.03Cu3Ti4-xZrxO12 Ceramics

doi:10.21203/rs.3.rs-955133/v1

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Effect of ZrO₂ Doping on the Microstructure and Dielectric Properties of Ca_0.97Y_0.03Cu₃Ti_4-xZr_xO₁₂ Ceramics

https://doi.org/10.21203/rs.3.rs-955133/v1

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Dielectric properties of CaCu₃Ti₄O₁₂ (CCTO) ceramics were studied by doping amount of Zr⁴⁺ donor on the basis of doping Y³⁺ donor. Ca_0.97Y_0.03Cu₃Ti_4-xZr_xO₁₂ (x=0, 0.005, 0.010, 0.015, CYCTZO) ceramics were prepared by reaction sintering method, and the phase structure, microstructure and dielectric properties of the ceramics were studied. The single phase CCTO was detected in all ceramic samples. SEM images of samples showed a high density and uniform-sized microstructure, and the average grain sizes of the ceramics was reduced with increasing content of Zr⁴⁺ donor. Interestingly, the dielectric constant (ε_r) was increased significantly while the dielectric loss tangent (tanδ) was decreased with the suitable doping amount of Zr⁴⁺. CYCTZO ceramics with x=0.010 exhibited a relatively high ε_r value of 1.02×10⁵ at 1kHz. Meanwhile, the improved grain boundaries response and dielectric properties were closely related to the strengthened grain boundaries resistance (R_gb). The present results indicate that the ε_r was improved and the tanδ was reduced by doping Zr⁴⁺ donors in Ca_0.97Y_0.03Cu₃Ti₄O₁₂(CYCTO) ceramics and explained that the electrical properties were closely related to the electrical heterogeneity of ceramic grains and grain boundaries.

Electronic Materials and Devices

Magnetics Materials and Devices

CaCu3Ti4O12 (CCTO)

dielectric constant

dielectric loss

giant dielectric properties

grain boundaries resistance

CaCu₃Ti₄O₁₂ (CCTO) ceramics, due to its giant dielectric constant (ε_r) and simple preparation method, have been widely studied [1–5]. However, the practical application of CCTO ceramics is limited due to its high dielectric loss [4–14]. Many experiments focus on the reduction of dielectric loss tangent (tanδ) in CCTO ceramics from the following three methods. Firstly, the tanδ of CCTO ceramics is reduced by decreasing the grain size. The tanδ in the low frequency range is connected to grain boundaries resistance (R_gb) [6]. The decrease in grain size means that the number of series insulating grain boundaries increases, which leads to an increase in the resistance of the ceramic [3]. Sol-gel and hydrothermal methods are to synthesize ceramics with nanoscale grains [7–9]. There are also emerging synthetic methods used to synthesize ceramics with nanoscale grains, such as accelerated sintering using electric fields [10]. However, owing to the complex process of the above methods, solid-state synthesis is widely used to synthesize CCTO ceramics. CuO is used as a raw material for the synthesis of CCTO ceramics and forms a liquid phase during the sintering process. The liquid phase, the driving force of grain growth, promotes grain growth [5]. By introducing sintering additives such as P₂O₅ [11], Bi₂O₃ [7] and halogen element [8], the formation of the liquid phase can be suppressed. Secondly, the tanδ is reduced by enhancing the heterogeneity between grains and grain boundaries. Some ions, such as Zr⁴⁺ [12–14], Al³⁺ [15], Mg²⁺ [16], Zn²⁺ [9], Sr²⁺ [17]and rare earth ions [18–20], are introduced into the lattice structure to successfully reduce the internal conductivity of the grains as well as to increase the R_gb, achieving lower the tanδ of the ceramic. In addition, smaller tanδ and larger ε_r are also obtained in CCTO ceramics by the doping of binary elements, such as Y³⁺ and Mg²⁺ [21], Zr⁴⁺ and Nb⁵⁺ [22], Al³⁺ and Ta⁵⁺ [23]. Thirdly, the dielectric properties of CCTO ceramics are influenced by the generation of intergranular phases. When the concentration of the dopant increases, the inclusion phases are segregated out in CCTO to form the intergranular phase, such as Ba²⁺ [24], SrTiO₃ [25], etc. These intergranular phases inhibit crystal growth and increase the resistance of grain boundaries. These studies not only provided experimental basis for the mechanism of giant permittivity, but also provided ideas and methods for improving the ε_r and reducing the tanδ of CCTO ceramics [7–25].

In the light of the internal barrier layer capacitance (IBLC) theory model, the giant dielectric properties of CCTO ceramics come from heterogeneity of insulating grain boundaries and the n-type semiconducting grains [26–28]. As a result, the ε_r of CCTO are improved by increasing semiconductivity of grains and resistance of grain boundaries. Lu Tang et al. [29] reported that ε_r was increased and tanδ was decreased by doping Y³⁺ into CCTO ceramics. The doping of the Y³⁺ donor results in a reduction in oxygen vacancies, which leads to a significant reduction in mobility at grain boundaries and consequently a reduction in grain size [29]. The decrease in grain size inevitably causes an increase in the density of grain boundaries with large potential barriers, meanwhile the Y³⁺ donor promotes the separation of the liquid phase copper from the grains and adheres to the grain boundaries, thus the impedance of the ceramics is increased [12, 29]. According to reports, single doping of Zr⁴⁺ in the B-site (Ti⁴⁺) of perovskite structure changed the electrical heterogeneity of the grains and grain boundaries, thus the dielectric properties were changed [13]. To achieve CCTO ceramics with giant ε_r and low tanδ, CCTO ceramics were studied with different contents of ZrO₂ on the basis of 3% Y³⁺ doping.

The synthesis of Ca_0.97Y_0.03Cu₃Ti_4−xZr_xO₁₂ (CYCTZO) was carried out by reaction sintering method. High purity powders including CaCO₃ (99.0%), Y₂O₃ (99.0%), CuO (99.0%), TiO₂ (99.0%), ZrO₂ (99.0%) were used as raw materials [29]. CaCO₃, Y₂O₃, CuO, TiO₂ and ZrO₂ were weighed according to the stoichiometric ratio of formula Ca_0.97Y_0.03Cu₃Ti_4−xZr_xO₁₂ (x=0, 0.005, 0.010, 0.015). The weighed powder, zirconia ceramic ball and anhydrous ethanol were put into agate ball mill, and then ground on planetary ball mill for 8h. The mixture was dried at 70℃ for 6h, and the uniform powder mixture was obtained by grinding and sieving. 3% polyvinyl alcohol binder (PVA) was added to the powder mixture, which was grinded into granules in a mortar and granulated through a 100 mesh sieve. The granular material was pressed into disc shape with diameter of 12 mm and thickness of 1 mm by uniaxial compression. After molding, the samples were kept in air at 550 ℃ for 2 hours to discharge the binder. The sample obtained was calcined in the air at 1080 ℃ for 10 hours in the crucible and cooled to room temperature in the furnace. After the surface of the sample was ground to a smooth surface. The silver oxide paste was coated on both sides of the sample, heated at 550 ℃ for 20 minutes after drying, and then formed a silver electrode after cooling.

The crystal structure and phase structure of the ceramics were analyzed by X-ray diffraction (XRD-7000X, Japan) with a CuKα (λ = 0.15406nm). The microstructure of ceramic samples was examined by scanning electron microscope (SEM, JSM-7610F, Japan). The types and contents of elements inside the grains and grain boundaries were detected by energy dispersive spectrometer (EDS, JSM-7610F, Japan). The dielectric constant, dielectric loss tangent and impedance performance of samples were measured at room temperature in the range of 10²-10⁶Hz by using a precision impedance analyzer (Agilent 4294A, Agilent Inc., American).

Figure 1 shows the XRD patterns of different doping amount of CYCTZO ceramics in the 2θ range of 20 to 80 at room temperature. XRD curves were basically consistent with the peak shape of single-phase CCTO (JCPDS 75-2188), and no impurity phase was detected [29]. Each diffraction peak in the XRD pattern of CYCTZO ceramics perfectly matched with a body-centered cubic perovskite-like structure [22]. The figure on the right side of Figure 1 shows diffraction peaks (220) pattern when different amounts of ZrO₂ were doped. With the increase of ZrO₂, the diffraction peaks (220) shifted to a large angle. As doping amount from 0.010 to 0.015, diffraction peaks (220) shifted to a small angle [22]. This phenomenon suggestes that Zr⁴⁺ entered the lattice structure, which caused lattice shrinkage [29].

Figure 2 is the SEM image of CYCTZO ceramic sample at 1080℃ for 10h. As shown in Fig. 2, all samples had clear grains, and the grain size of the samples was relatively small. The average grain size of the samples was 3.05 µm without doping Zr⁴⁺. Smaller fine grains were adhered to larger coarse ones, which was similar to the pinning effect of the second phase [22]. For the doped CYCTZO ceramics with x=0.005, 0.010 and 0.015, the average grain size was 2.16µm, 2.13µm and 2.11µm respectively. Compared to the grain size interval (~1.20-5.78µm) of the undoped sample, grain size interval (~1.16-4.96µm) of sample with x=0.005 was narrower. The uniformity of the ceramic grains was improved by doping Zr⁴⁺. The reduction of grain sizes in CYCTZO ceramics may be related to the inhibition of the formation of liquid phase during sintering and consequently hinders the movement of grain boundaries [30, 31]. In addition, the lattice strain energy is diminished as a result of the mismatch of ionic radii between the dopants of Zr⁴⁺ and host Ti⁴⁺ in CCTO, resulting in the smaller grain size [22]. This is also confirmed by the deviation of diffraction peaks (220) in the XRD pattern.

The EDS spectra of CYCTZO ceramics with x = 0.005 and x = 0.015 are shown in Fig. 3 Pt-1 and pt-2 respectively represent grains and grain boundaries grain boundary. The spectra showed the changes of element type and content in the grains and grain boundaries of the samples doped with Zr⁴⁺. The content of copper elements in the grain was higher than that in the grain boundaries for both, which was due to Y³⁺ promoting the liquefaction of Cu in the grain and precipitation at the grain boundaries. Zr elements appear in the grains and the grain boundaries, and the content of Zr elements was similar for doping concentration x=0.005. When the doping content increases to x=0.015, the Zr⁴⁺ content at the grain boundaries was obviously higher than that inside the grain.

The ε_r and tanδ of CYCTZO ceramics as a function of frequency in the range of 10²-10⁶ Hz are shown in Fig. 4. The ε_r was decreased slowly in the range of 10²-10⁵ Hz, and decreased sharply in the range of 10⁵-10⁶ Hz. The rapid decrease in the ε_r of ceramic samples at high frequencies could be explained by typical Debye-related behavior [22]. For doping amount of Zr⁴⁺ from x=0 to x=0.010, the ε_r value of CYCTZO ceramic was gradually increased. The highest ε_r value was 1.02×10⁵ at a frequency of 1KHz for doping content x=0.010. Compared with Y³⁺ single-doped [29] and Zr⁴⁺ single-doped [32], (Y³⁺, Zr⁴⁺) co-doped CCTO ceramics had a larger ε_r value. It was noted that the ε_r of the ceramic sample was dropped drastically and was lower than that of the Zr⁴⁺ non-doped sample. The result shows that the ε_r of ceramics could be raised by doping Zr⁴⁺ donor of suitable amount. To further explore the influence of Zr⁴⁺ donor on the dielectric properties of ceramics, it is also necessary to investigate the change of tanδ value. Fig. 4b showed the frequency dependence of tanδ with different doping amount. Without doping Zr⁴⁺, the ceramics had lower tanδ value at low and medium frequencies. For the ceramic samples with the doping amount x=0, 0.005, 0.010, overall trend of tanδ curves was almost same. In the frequency range of 10²-10³ Hz, the tanδ value was showed a slow increase trend as the doping content increases. In the frequency range of about 10³-10⁵Hz, the tanδ value of CYCTZO ceramics with the doping content x=0.005 and x=0.10 were lower than the tanδ of undoped Zr⁴⁺ ceramics. The ceramic sample with doping amount x=0.005 had the lowest tanδ value of 0.078 in this experiment at 6×10³Hz. The tanδ value of CYCTZO ceramics was successfully reduced by doping appropriate content of Zr⁴⁺.

Based on the IBLC model, the value of εr is inversely proportional to the grain size [33]. However, CYCTZO ceramic with a doping amount x=0.010 had a smaller grain size and maximum ε_r. As the amount of doping increases, the crystal grain size was decreased and the ε_r was increased, which was contrary to IBLC model. As we all know, the giant dielectric effect of CCTO ceramics is considered to be caused by the external polarization of the n-type semiconductor and insulating grain boundaries [34, 35]. A larger number of free electrons usually enhances the conductivity of n-type semiconducting grains [22]. The grade of the grain semiconductor was improved by doping Zr⁴⁺. Therefore the ε_r of the CYCTZO ceramics with doping amount increases from x=0 to x=0.010 is raised. The ε_r was decreased as the Zr⁴⁺ content continues to increase, due to the decrease of the carrier concentration inside the crystal grain and the change of the conduction mechanism. The carriers inside the CCTO ceramic grains are mainly provided by oxygen vacancies and the charge transfers between Cu²⁺→Cu⁺ and Ti⁴⁺→Ti³⁺ [36–38]. Increasing Zr⁴⁺ in CCTO ceramics would lead to the decrease of oxygen vacancies [22, 39]:

The impedance spectrum of CYCTZO ceramics at room temperature is shown in Fig. 5 The non-zero intercept on the Z' axis and the diameter of the semicircle formed by the impedance line at low frequency repre sent the grain resistance (R_g) and the R_gb of the ceramic sample, respectively [34, 35]. The impedance diagram showed that R_gb was decreased with increasing Zr⁴⁺ doping content. According to the EDS results, the decrease of R_gb is related to the increase of Zr element content at the grain boundaries. It is easy to find that the trend of R_gb is consistent with trend of the tanδ in the low frequency region in Fig. 4(b). Interestingly, in the med-frequency region, the tanδ values of CYCTZO ceramics with doping content x=0.005 and x=0.010 were lower than that of undoped Zr⁴⁺. Not only the ε_r was increased, but also the tanδ value was reduced doping appropriate amount of Zr⁴⁺.

In conclusion, CYCTZO ceramics were successfully synthesized by reaction sintering method at 1080℃. The phase composition, microstructure and dielectric properties of the ceramics were studied. The experimental results show that the donor Zr⁴⁺ doping has little effect on Ca_0.97Y_0.03Cu₃Ti₄O₁₂ (CYCTO) ceramic phase composition, but reduces the crystal grain size and improves the dielectric properties of the ceramic. For Zr⁴⁺ donor doping amount x=0.005, CYCTZO ceramic had larger ε_r value of 1.02×10⁵ at 1KHz and the lowest tanδ value of 0.078. In addition, the highest ε_r value of 1.03×10⁵ at 1KHz and smaller tanδ value of 0.08 were obtained for ceramic doped with Zr⁴⁺ amount of 0.010. Our results further strengthen the argument for optimizing the dielectric properties of ceramics by adjusting the properties of grains and grain boundaries.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (52072329)， the Opening Project of State Key Laboratory of High Performance Ceramics and Superfine Microstructures (KLIFMD202001, KLIFMD202105).

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Effect of ZrO₂ Doping on the Microstructure and Dielectric Properties of Ca_0.97Y_0.03Cu₃Ti_4-xZr_xO₁₂ Ceramics

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