Dramatically enhanced non-Ohmic properties and maximum stored energy density in ceramic-metal nanocomposites: CaCu3Ti4O12/Au nanoparticles
© Tuichai et al.; licensee Springer. 2013
Received: 3 October 2013
Accepted: 16 November 2013
Published: 21 November 2013
Non-Ohmic and dielectric properties of a novel CaCu3Ti4O12/Au nanocomposite were investigated. Introduction of 2.5 vol.% Au nanoparticles in CaCu3Ti4O12 ceramics significantly reduced the loss tangent while its dielectric permittivity remained unchanged. The non-Ohmic properties of CaCu3Ti4O12/Au (2.5 vol.%) were dramatically improved. A nonlinear coefficient of ≈ 17.7 and breakdown electric field strength of 1.25 × 104 V/m were observed. The maximum stored energy density was found to be 25.8 kJ/m3, which is higher than that of pure CaCu3Ti4O12 by a factor of 8. Au addition at higher concentrations resulted in degradation of dielectric and non-Ohmic properties, which is described well by percolation theory.
Ceramic materials with high dielectric permittivity (ϵ′) have been intensively studied because of their potential for multilayer ceramic capacitor applications. The dielectric materials used in these devices must exhibit a high ϵ′ with very low loss tangent (tanδ). They also need to have a high breakdown voltage to support high-energy density storage applications. The energy density (U) performance of capacitors can be expressed as , where Eb is electric field breakdown strength . Recently, dielectric ceramics homogeneously filled with metallic particles have been of considerable scientific and technological interest. This is due to their greatly enhanced dielectric response as well as an improved tunability of ϵ′ [2–11]. Generally, ϵ′ increases rapidly in the region of the percolation threshold (PT) [4, 9]. For the Ag-Ba0.75Sr0.25TiO3 composite , the large increase in ϵ′ was suggested to result from the percolation effect. Improved tunability of Ba0.75Sr0.25TiO3 ceramics was hypothesized to be the effect of either large induced internal electric fields within the thin Ba0.75Sr0.25TiO3 layer sandwiched by electrode-like metallic Ag particles or improved densification of ceramic composites. However, Eb of a metal-ceramic composite abruptly decreased as the metallic filler concentration increased to PT .
CaCu3Ti4O12 (CCTO) is one of the most interesting ceramics because it has high ϵ′ values. CCTO polycrystalline ceramics can also exhibit non-Ohmic properties [12–20]. These two properties give CCTO potential for applications in capacitor and varistor devices, respectively. Unfortunately, high tanδ (>0.05) of CCTO ceramics is still one of the most serious problems preventing its use in applications [10, 12, 17]. The application of CCTO ceramics in varistor devices was limited by their low nonlinear coefficient (α) and Eb values. For energy storage devices, both ϵ′ and Eb need to be enhanced in order to make high performance energy-density capacitors. Therefore, investigations to systematically improve CCTO ceramics properties are very important.
In this work, CaCu3Ti4O12 powder was prepared by a solid state reaction method. First, CaCO3, CuO, and TiO2 were mixed homogeneously in ethanol for 24 h using ZrO2 balls. Second, the resulting mixture was dried and then ground into fine powders. Then, dried powder samples were calcined at 900°C for 6 h. HAuCl4, sodium citrate, and deionized water were used to prepare Au NPs by the Turkevich method . CCTO/Au nanocomposites with different Au volume fractions of 0, 0.025, 0.05, 0.1, and 0.2 (abbreviated as CCTO, CCTO/Au1, CCTO/Au2, CCTO/Au3, and CCTO/Au4 samples, respectively) were prepared. CCTO and Au NPs were mixed and pressed into pellets. Finally, the pellets were sintered in air at 1,060°C for 3 h.
X-ray diffraction (XRD; Philips PW3040, Philips, Eindhoven, The Netherlands) was used to characterize the phase formation of sintered CCTO/Au nanocomposites. Scanning electron microscopy (SEM; LEO 1450VP, LEO Electron Microscopy Ltd, Cambridge, UK) coupled with energy-dispersive X-ray spectrometry (EDS) were used to characterize the microstructure of these materials. Transmission electron microscopy (TEM) (FEI Tecnai G2, FEI, Hillsboro, OR, USA) was used to reveal Au NPs. The polished surfaces of sintered CCTO/Au samples were coated with Au sputtered electrode. Dielectric properties were measured using an Agilent 4294A Precision Impedance Analyzer (Agilent Technologies, Santa Clara, CA, USA) over the frequency range from 102 to 107 Hz with an oscillation voltage of 0.5 V.
Results and discussion
Large increases in ϵ′ of percolating composites are generally attributed to formation of microcapacitor networks in the composites and/or Maxwell-Wagner polarization [4, 9, 22]. For pure CCTO ceramics, the giant dielectric response is normally associated with the mean grain size [16, 17, 25]. Although there is a small amount of relatively large grains (5 to 10 μm) in the microstructure of CCTO/Au3 and CCTO/Au4 (data not presented), the large observed enhancement of ϵ′ is likely due to the percolation effect.
The CCTO/Au1 sample exhibited the best non-Ohmic properties among all samples. These values are comparable to those observed in CaCu3Ti3.8Sn0.2O12 ceramic . There are many factors that are potentially responsible for strong improvement of non-Ohmic properties. It was found that the non-Ohmic properties of CCTO ceramics could effectively be improved by fabricating composite systems of CCTO/CTO [28, 29]. As shown in Figure 1, the observed CTO phase in all of the CCTO/Au composites tended to increase with increasing Au content. However, the non-Ohmic properties of CCTO/Au strongly degraded as the Au filler concentration increased. Thus, the excellent non-Ohmic properties of the CCTO/Au1 sample are not mainly caused by a CTO phase. For CCTO polycrystalline ceramics, the non-Ohmic behavior is due to the existence of Schottky barriers at the GBs . Thus, the existence of metallic Au NPs at the GBs of CCTO ceramics may contribute the formation of Schottky barriers at GBs. However, the mechanism by which Au NPs contribute to enhancement of non-Ohmic properties is still unclear.
It is worth noting that improved nonlinear properties of the CCTO/Au1 sample may also be related to modification of microstructure. Although the introduction of metallic particles in a ceramic matrix with concentration near the PT can dramatically enhance the dielectric response, a large increase in the conduction of charge carriers was observed simultaneously, leading to decreases in Eb and energy density. The maximum stored energy densities of all the samples were calculated and found to be 3.11, 25.8, 26.0, 1.39, and 0.54 kJ/m3 for the CCTO, CCTO/Au1, CCTO/Au2, CCTO/Au3, and CCTO/Au4 samples, respectively. Notably, introduction of Au NPs into CCTO ceramics in small concentrations, between 2.5 and 5.0 vol.%, caused a strong increase in the maximum stored energy density as well as their non-Ohmic properties.
In conclusion, the investigation of non-Ohmic and dielectric properties of CCTO/Au revealed that addition of Au NPs to CCTO in the concentration of 2.5 vol.% can decrease tanδ, while ϵ′ was unaltered. The non-Ohmic properties of this composition were also successfully improved showing α ≈ 17.7 and Eb ≈ 1.25 × 104 V/cm. The maximum stored energy density of CCTO ceramics were significantly enhanced by introducing of Au NPs in concentrations of 2.5 to 5.0 vol.%. The dielectric and non-Ohmic properties as well as energy density were degraded when Au NP concentrations were greater. The mechanisms of dielectric response and non-Ohmic properties can be well described by using the percolation theory.
This work was financially supported by the Nanotechnology Center (NANOTEC), NSTDA, Ministry of Science and Technology, Thailand, through its program of Center of Excellence Network. WT extends his gratitude to the Thailand Graduate Institute of Science and Technology (TGIST) for his Master of Science Degree scholarship.
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