CaCu3Ti4O12 Dielectric Composites: Synthesis, Characterization and Electrical Properties

Abstract

Novel energy materials possessing both high power density and high energy density have been constantly pursued with the development in sustainable energy industry. Dielectric capacitor is very efficient in offering high power density but its limited energy density from dielectric material impedes the further application for energy storage. CaCu3Ti4O12 (CCTO) dielectric shows considerable potential for capacitive energy storage due to its giant dielectric constant and excellent temperature independence comparing with traditional ferroelectric ceramics. However, the dielectric property of CCTO is extrinsically originated and strongly related to preparation conditions. Furthermore, the breakdown voltage of CCTO is relatively low, which is undesirable for capacitive energy storage. Therefore, this thesis research is focuses on the synthesis and characterization of CCTO materials and CCTO-polymeric (polyvinylidene fluorides, PVDF) composites, in an effort to develop a new material system for high density energy storage. A systematical in situ study of the high temperature phase evolution of CCTO solid state and sol-gel precursors was carried out via synchrotron X-ray powder diffraction. We found that the onset temperature for the CCTO phase formation is 800°C in the sol-gel precursor, lower than that in the solid state precursor (875°C). Intermediate phases were only observed in the sol-gel precursor. Both precursors are able to be treated to sub-micron sized powders by fast calcination. The phase formation sequence and mechanism during calcination are proposed. For the first time, the real time phase and lattice parameter evolution of CCTO upon sintering in ambient atmosphere is studied. It has been demonstrated that the Cu ions are not stable in CCTO structure during sintering, and lead to the formation of CuO secondary phase which precipitate at the grain boundaries. The Cu ions instability is the ultimate mechanism for the formation of the internal barrier layer capacitor structure in CCTO and therefore responsible for the giant dielectric constant. In order to synthesize homogeneous CCTO fine powders, CCTO sol-gel precursors were selected and calcined by using microwave radiation for the first time. 89.1 wt.% CCTO was achieved from the sol-gel precursor by microwave heating, using only 17 min at 950°C. In contrast, conventional calcination method required 3 h to generate 87.6 wt.% CCTO content at 1100°C. In addition, the CCTO powders prepared through 17 min microwave calcination exhibited a small particle size distribution. A lengthy hold time of 1 h by microwave sintering is required to obtain high dielectric constant (3.14×103 at 1×102 Hz) and reasonably low dielectric loss (0.161) in the sintered CCTO. The dielectric response of the sintered CCTO samples is attributed to the space charge polarization and IBLC effect. TiO2 modified CCTO has been synthesized via a facile sol-gel precipitation process followed by solid state sintering. The morphology and chemistry of grain boundaries of CCTO were significantly tailored by TiO2 modification, resulting in enhanced barrier layer capacitor effect. The formation mechanism of barrier layers based on the instability of Cu ions has been discussed. A new bimodal brick layer mechanism has been proposed. The optimized TiO2 modified CCTO sample exhibits stable dielectric permittivity which is twice as high as the unmodified one, and shows relatively low dielectric loss from 102 Hz to 105 Hz. CCTO-PVDF composites were prepared by simple melt blending and hot molding techniques. The addition of CCTO remarkably enhanced the dielectric properties and thermal conductivity of PVDF composites, while the melting point of the PVDF composites (~170°C) was almost independent of the CCTO concentration. Based on the results of dielectric constant and dielectric breakdown voltage, the PVDF composite containing 40 vol.% CCTO fillers shows the optimized capacitive energy storage potential (7.81 J/cm3). PVDF with β-SiC-CCTO hybrid fillers were also prepared. Results show that hybrid loading is preferred to achieve a reasonable combination of thermal conductivity (0.80 W∙m-1∙K-1), dielectric constant (~50) and dielectric loss (~0.07) at 103 Hz in the composite containing 40 vol.% CCTO and 10 vol.% β-SiC. The strong dipolar and interfacial polarization are account for the enhancement of the dielectric constant, while the formation of thermally conductive networks/chains by β-SiC whiskers contribute to the improved thermal conductivity. However, the introduction of β-SiC whiskers into 40 vol.%-CCTO-PVDF binary composite could reduce the breakdown voltage. Composite with 40 vol.% singular CCTO addition still shows the optimal breakdown voltage of 22.58 kV/mm among composites while pure PVDF shows a value of 40.21 kV/mm.