Effects of Processing Conditions and Testing Atmosphere on the Electrical Properties of Calcium Copper Titanate Ceramics for Energy Storage Applications
Abstract
Large dielectric constant (ε') in CaCu3Ti4O12 (CCTO) ceramics show a potential for use in capacitive energy storage. However, large tan(δ) of CCTO is still undesirable for such applications hence finding ways to reduce the tan(δ) is very important. Dielectric properties of CCTO are very sensitive to processing conditions and are unstable at RT in air based on our new findings. The objective of the current research is to obtain a fundamental understanding of CCTO ceramics with stable large ε' and a low tan δ through processing and testing innovations. CCTO.xAl2O3 (x=0,0.5,4wt.%) powders and dense ceramics were prepared by solid state reaction and sintering. High density (>90%) is achieved for sintering at 1060°C-1115°C for 5h (S1060-S1115). The temperature dependence of complex impedance (Z*) and dielectric properties of the as-prepared (pure and doped) and thinned down CCTO samples are studied in air and in dry N2 between 23°C to 225°C. Highly irreproducible total impedance is observed in tests performed in ambient conditions at 23°C despite the sample surface conditions or doping. Stable and reproducible dielectric properties are only obtained in dry N2 due to elimination of moisture and air. Increased space charge accumulation at GBs leading to large ε' and loss was observed in air at 23°C while stable ε' with low tan δ is measured in N2. The lowest tan δ of 0.010-0.013 is obtained at 23-58°C at 1.4-6 kHz for pure S1080. The resistivity of CCTO was independent of its thickness thereby confirming that the absence of surface barrier layer contribution. Total resistance of alumina-doped CCTO increased by orders of magnitude due to highly resistive CuAl2O3 at inter-grains and Al doping into intra-grains. Very low tan δ (0.021-0.020) with a large ε' (8,815-11,090) at 23°C at 500-800 Hz for 0.5wt.% alumina doping suggests the possibility of developing CCTO as capacitors. Therefore, data on as-prepared (pure and alumina doped) and thinned down CCTO measured in air and dry N2 can be used as guidelines for developing CCTO for possible applications as capacitors. These results indicated, for the first time, that sample processing and particularly testing atmospheres control the charge carriers, resistance, and capacitance of GBs, which in turn determine the overall magnitude and reproducibility of dielectric properties of polycrystalline CCTO ceramics.
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- OSU Dissertations [11222]