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42351-AC10
New High Permitivity Dielectrics Based on Nanotwinned Dielectrics
In recent years, there has been a great deal of interest in dielectric materials with regards to electrical energy storage. Following many trends of current technology, it is important to achieve high capacitance densities in many applications including fuel cell applications, photovoltaics, energy harvesting applications, and military applications.
Our work has been focused on engineering the dielectric properties of CaCu3Ti4O12 (CCTO) for the purpose of maximizing the capacitance density. Due to its temperature-independent dielectric constant as well as the lack of any phase transition over a wide range of temperature, there has been considerable attention placed on this material. Most of our research on CCTO has involved the effects of doping and cation non-stoichiometry on the dielectric constant and loss tangent. The purpose of the proposed research is to develop the structure-property-process relationships to increase the capacitance density by means of cation doping schemes as well as cation and anion non-stoichiometry in CCTO.
With doping of CCTO, the substitution of the aliovalent dopant Cr2O3 on the Ti site was investigated in terms of the effects on the dielectric properties at doping levels ranging from 0.1 to 1.0%. No evidence of secondary phases was observed from XRD analysis, but both the permittivity and dielectric loss of 1% Cr2O3 doped CaCu3Ti4O12 were improved with K ≈ 19,000 and tan d ≈ 0.049 at 1 kHz. Also, 1% Cr2O3 doping was effective at maintaining the high K up to 150 V. From these results, it can be inferred that Cr2O3 doping is an efficient method to achieve a high-K and low loss.
The effects of cation non-stoichiometry were examined through varying the Cu/Ti ratio in the basic CCTO chemical formula. X-ray diffraction measurements on non-stoichiometric CCTO revealed that both Cu- and Ti-excessive CCTO compositions showed evidence of a Cu2O phase in the interior regions of a CCTO ceramic. In addition, a CuO phase was observed on the outer surface layer on all compositions. It is proposed that these phases are formed through limited reoxidation of Cu2O during cooling. In contrast, both Cu-deficient and Ti-deficient CCTO compositions showed no secondary phases. Both Cu- and Ti-excessive CCTO compositions had markedly lower dielectric constants due to the presence of the Cu2O phase. With improved dielectric constants, both Cu deficient and Ti deficient CCTO samples also showed lower tan d values at low frequencies at room temperature. The anomalous dielectric behavior of Cu deficient and Ti deficient CCTO was explained by impedance analysis indicating an enhanced boundary resistance that ultimately resulted in lower dielectric losses.
Finally, current work is focused on examining the high temperature phase equilibrium conditions during the processing of CCTO. The thermodynamics of the Cu-O system dictate that Cu2+ reduces to Cu1+ above a temperature of approximately 1073°C. If applied to the CCTO system, this would suggest a change in the Cu valence state during sintering of the ceramics. Given that the radius of Cu1+ is significantly larger than the radius of Cu2+, it is unlikely that Cu1+ would be stable in the CCTO structure. Therefore, at high temperatures a fraction of the Cu on the CCTO A-site would leave the structure, forming a second phase at high temperatures (Cu2O). Upon cooling, Cu1+ reoxidizes to Cu2+ which transforms the Cu2O to CuO. This model is supported by XRD measurements on the surface of CCTO ceramics and in the interior of the ceramic via polishing. CuO is observed on the surface regardless of the preparation method and Cu2O is observed in the interior of the ceramic. The amount of Cu2O present in the interior of the sample is dependent on the processing conditions, specifically the cooling rate. This is indicative of the diffusion of oxygen from the exterior to the interior, transforming Cu2O into CuO. Confirmation of this approach is also observed in TGA/DTA measurements, where a clear transition in oxygen content is observed from oxidizing conditions at low temperatures and reducing conditions above 1073°C.
Future work will be focused on linking the dielectric behavior and the phase equilibria through impedance spectroscopic methods. With this approach, the heterogeneous microstructure can be quantitatively analyzed and multiple contributions to the overall impedance can be separated. The ultimate objective is to develop a clear relationship between dielectric properties, stoichiometry, and processing conditions so that a high-K, low loss dielectric can be obtained.
