Effects of silica coating on the microstructures and energy storage properties of BaTiO3 ceramics

doi:10.1016/j.materresbull.2015.01.056

Materials Research Bulletin

Volume 67, July 2015, Pages 70-76

https://doi.org/10.1016/j.materresbull.2015.01.056 Get rights and content

Highlights

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BaTiO₃ cores remained up to the original grain size after sintering.
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The homogeneity of silica coating dominates contribution to breakdown strength.
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Energy storage density of BaTiO₃ ceramics increases sharply by silica coating.

Abstract

We investigated microstructures and energy storage properties of SiO₂-coated BaTiO₃ ceramics that were prepared by the so-called Stöber process. The thickness of coating layer of BaTiO₃ powders could be effectively controlled by the silica content. It could be observed that the secondary phases Ba₂TiSi₂O₈ appeared in the coated BaTiO₃ ceramics due to the interdiffusion reactions between SiO₂ and BaTiO₃ components under sintering. BaTiO₃ cores in the coated ceramics remained up to the original grain size indicating the coating layer as an inhibitor. The results showed that both breakdown strength and energy density were improved apparently. The homogeneity of silica coating in the ceramics should dominate contribution to breakdown strength, which can reduce the weak-point breakdown under high electric field. The optimized composition for BaTiO₃ ceramic coated with 2.0 wt% SiO₂ showed the maximum energy storage density of 1.2 J/cm³ with energy storage efficiency of 53.8%, which is about three times higher than that of pure BaTiO₃ (0.37 J/cm³).

Graphical abstract

Introduction

Recently, BaTiO₃-based ceramics have been widely used in the electronic ceramic industry due to their excellent dielectric and ferroelectric properties, especially as the use of dielectrics for energy storage capacitors [1], [2], [3]. However, new applications for energy storage have been driving the demand for dielectric materials which exhibit high breakdown strength (E_b) while still retaining high polarization in order to obtain high energy density [4]. Theoretically, energy storage density (γ) of the three kinds of dielectric materials (linear dielectrics, ferroelectrics, and antiferroelectrics) can be evaluated from their P–E loops, as given by the following equation [5]: $γ = \int_{0}^{D_{\max}} E d D$ where E is the applied electric field and D_max is the electric displacement (D) at the highest applied field (E_max). For the dielectrics with high relative dielectric constant, D can be replaced by the polarization (P). Accordingly, the above formula can be written as follows [5]: $γ = \int_{0}^{P_{\max}} E d P = \int_{0}^{E_{\max}} P d E$

Evidently, based on Eq. (2), the energy storage density of nonlinear dielectrics can be evaluated by integrating the area between the polarization axis and the curves of P–E loops. Obviously, both dielectric breakdown strength and polarization should be the key factors for the contribution of energy density of dielectric materials [6]. Although BaTiO₃ ceramics exhibit a very large polarization, the energy storage values have been very limited due to the large energy loss from hysteresis with large remnant polarization and the relatively low dielectric breakdown strength of ceramics when in the form of sintered pressed disks [7], [8].

In order to improve the dielectric breakdown strength, many methods have been adopted, such as controlling grain size [3], [9], [10], coating/mixing with low-melting point glass to remove the porosity [11], [12], [13], [14], [15], [16], and forming new solid solutions with other compounds to reduce the polarization loss [17]. Among these, coating is a simple but effective method and can be operated at ambient temperature. The resultant particles by coating consist of a core made of the base particles and a shell made of coating materials, therefore, known as a core–shell structure. By coating for dielectric materials, the properties of the core component, such as reactivity and dielectric stability, may be modified.

SiO₂ has been an effective coating material used in energy storage dielectrics due to high breakdown strength and low dielectric loss. Yu et al. [18], [19] have reported the energy storage properties of pure and coated BaTiO₃ homogeneous ceramics-polymer nanocomposites, indicating that the coating of SiO₂ can obviously increase energy storage efficiency of the composites. In the present paper, we fabricate BaTiO₃ nanoparticles in the ethanol/ammonia medium with the successful coating by silica shell of a few nanometers, and investigate the effects of silica coating on the microstructure and electrical properties of coated BaTiO₃ ceramics.

Section snippets

Experimental

The selected method was derived from the so-called Stöber process widely used for the synthesis of silica beads from a few tens to a few hundreds of nanometers [20], [21], [22], [23]. Here, a series of compositions can be obtained by different silica contents as follows: BaTiO₃ + x wt% SiO₂ (x = 0, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 6.0, and 8.0). The silica shell thickness can be tuned by the addition of the amount of tetraethoxysilane (TEOS). BaTiO₃ nanoparticles were purchased from Shandong Guoteng

Results and discussion

The so-called Stöber process was used to fabricate the coated BaTiO₃ powders to form coating layers of a few nanometers. Direct evidence of the coating of SiO₂ shell on the BaTiO₃ powder is provided by TEM, as shown in Fig. 1. The continuous and homogeneous coating of BaTiO₃ powder can be obtained with a series of silica thickness from 0.5 nm for 1.0 wt% SiO₂ to 12.0 nm for 8.0 wt% SiO₂, as shown in Table 1.

Fig. 2 shows the room temperature X-ray diffraction patterns of coated BaTiO₃ ceramics

Conclusions

The silica layers with a homogeneous thickness from 0.5 nm to 12.0 nm have been successfully coated on the fine-grain BaTiO₃ powders by the so-called Stöber process. It has been demonstrated that the secondary phases are formed when sintering due to the diffusion reaction between silica layer and BaTiO₃. The proper silica coating can effectively suppress the grain growth of BaTiO₃ grains which can be ascribed to the grain boundary of silica between BaTiO₃ cores. The enhancement of breakdown

Acknowledgements

The authors would like to thank the support of key programmer of Natural Science Foundation of China (No. 50932004), the International Technology Cooperation Project from the Ministry of Science and Technology of China (2011DFA52680), Natural Science Foundation of China (No. 51372191 and No. 51102189), the Fundamental Research Funds for the Central Universities (No. 2012-IV-006) and National Basic Research Program of China (973 program: 2015CB654601).

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