Nano- and microsize effect of CCTO fillers on the dielectric behavior of CCTO/PVDF composites
Introduction
Ceramic materials with a high dielectric constant (εr), such as oxides with perovskite and related structures, are widely used in capacitors, memory devices, power systems and the automotive industry [1], [2]. High dielectric constants allow smaller capacitive components, thus offering the opportunity to decrease the size of electronic devices. One important role for ceramic materials with a high dielectric constant is as fillers in the ceramic–polymer composites [3]. These composites exhibit high εr, relatively low dielectric loss (tan δ) and good flexibility at low temperatures. Therefore, polymeric composites with ceramic fillers have attracted considerable interest and have been studied widely, because of their potential applications in the electronics industry, especially as embedded devices.
Generally, ceramic–polymer composites are good insulators, with a low dielectric constant (usually less than 10). A common way to enhance the εr of such composites is to use fillers with high permittivity, such as BaTiO3 [3], Pb(Zr, Ti)O3 [4] and NiO [5], among which BaTiO3 is the most widely used ceramic filler. The εr of ceramic–polymer composites is mainly dependent on the filler loading amount [6], [7]. However, the dielectric constant of this kind of composite is usually less than 50 even at a high ceramic loading [3].
In the past few years, much attention has been paid to a cubic perovskite ceramic–calcium copper titanate (CCTO), which has a very large dielectric permittivity (over 104). The calcium copper titanate belongs to the ACu3Ti4O12 family (where A = Ca, Cd), which has been extended to the general formula, (AC3)(B4)O12, (where A = Ca, Cd, Sr, Na or Th; B = Ti or (Ti + M5+), in which M = Ta, Sb or Nb; and C = Cu2+or Mn3+) [8]. The crystal structure of CCTO has been refined with the space group Im3 (lattice parameter a = 7.391 Å, Z = 2), which remains centrosymmetric body-centered cubic over a wide range of temperatures. CCTO was reported by Deschanvres et al. [9] as early as in 1967; however, the origins of the extraordinarily high εr are still not very clear despite being discussed intensively in the literature. Several models have been proposed to explain the unusual dielectric responses, one of which, the Maxell–Wagner (M–W) polarization (originated from internal barrier layer capacitor), is widely accepted as the principal mechanism leading to the high permittivity.
Some reports show that there is a relationship between the dielectric constant and the grain size of the CCTO ceramic. For example, Adams et al. [10] reported that, with the grain size increasing from 10 to 300 μm, the dielectric constant of the CCTO bulk material improved from 9000 to 280,000. Saji and Choe [11] reported a CCTO film on a silica substrate that had a dielectric constant of about 2000 with a grain size of 200 nm. However, Fu et al. [12] suggested that the size dependence of the dielectric property was related to the defect density in the CCTO grains. In various models [13], [14] proposing a mechanism for the large dielectric constant of CCTO, researchers assume a model of a conducting or semiconducting grain surrounded by a thin insulating grain boundary and the εr depends on the size of the grain divided by the grain boundary. In fact, the grain boundary of the CCTO crystal is also conductive or semiconductive due to O defects [12]. The dielectric constant of the nanosized CCTO is more sensitive to the defects of Ti on the Cu site and O defects due to the thinner grain boundary compared with microsized ones. Another outstanding property of the nanosized CCTO material is its exceptionally large surface area. Therefore, extraordinary properties are expected when introducing nanosized CCTO into a polymer to form a nanocomposite material.
Many studies of CCTO/polymer composites have been carried out. Arbatti et al. [15] reported that CCTO/P(VDF-TrFE) composite had a sandwich configuration, with a dielectric constant of more than 610 at 102 Hz and room temperature when the ceramic filler is 50 vol.%. Dang et al. [16] fabricated a CCTO/polyimide composite film (with 40 vol.% ceramic filler) with a dielectric permittivity of about 49 at 100 Hz and room temperature. A similar result was reported by Shri Prakash and Varma [17] with epoxy as the matrix. However, the CCTO filler of the composites mentioned above was mostly prepared by the traditional solid-state reaction with a grain size ranging from a few to tens of micrometers. Composites containing nanosized CCTO have rarely been reported.
The purpose of this study is to systematically investigate the dielectric property of a CCTO/PVDF composite, especially the effect of the CCTO’s particle dimensions on the dielectric properties of the composite. CCTO ceramic fillers with two grain sizes were introduced separately into a polyvinylidene fluoride (PVDF) matrix. One was uniform, with an average grain size of about a few hundreds of nanometers, which was synthesized via a wet chemical precursor route by our group. The other powder was microsized and prepared by the traditional solid-state reaction, and obtained from an industrial source. The microstructure and interfacial effect of the two composites were investigated. Theoretical models, including Maxwell’s, the effective medium theory and the percolation theory, were employed to explain the dielectric behavior of the composite.
Section snippets
Preparation of CCTO nanoparticles
The nano-CCTO ceramic particles (called CCTO-1) were synthesized via a wet chemical route [18]. Titanium tetrachloride (TiCl4, 99.98%) (Fuchen, China), calcium carbonate (Luoyang Chemical Reagent Co., Ltd., China) cupric chloride (Damao, China, proanalyse grade), oxalic acid (Caitong, China, analytical grade) and acetone (pure) were employed as raw materials. In the first step, a titania gel was obtained via the controlled reaction of ice-cold distilled water with titanium tetrachloride, with
Characterization of CCTO powders
Fig. 1 shows the simultaneous DSC/TGA curves of the complex precursor for CCTO-1 measured at a heating rate of 10 °C min−1 in air. The TGA curve shows a minor weight loss step between 30 and 200 °C, which is related to the losses of moisture, hydrate decomposition and trapped solvent (water and carbon dioxide). A major weight loss is observed between 250 and 400 °C, and there is almost no obvious weight loss above 400 °C. Over the temperature of 250 °C, all the weight loss was related to the
Conclusions
In conclusion, nanosized CCTO fillers possess active and “conductive” interfaces while microsized CCTO exhibits “insulating” boundaries in the PVDF matrix, which results in differences in the εr, tan δ and σ of these two groups of CCTO/PVDF composites. For composite-1 containing 40 vol.% nanosized CCTO fillers, the εr and tan δ reach 2.49 × 106 and 48, respectively, at 100 Hz and room temperature. The εr and tan δ decrease with increasing frequency and temperature, especially from 30 to 130 °C. The σ
Acknowledgements
The present research is supported by the National Natural Science Foundation of China (Nos. 50807038 and 20971089) and research funding from National S&T Major Project under Contract No. 2009ZX02038.
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