Bimodal distribution of grains

The increasing demand for sustainable energy has steered research in energy storage materials. The choice of materials for such applications needs to be made by keeping in mind the econometrics of the energy market and the environmental impact. Ceramic capacitors, with their shorter relaxation time and high power density [1][2] and [3], turn out to be strong competitors against batteries. Moreover, they are more economically feasible, and possess better thermal and mechanical properties.

There is strong demand to develop new materials with a temperature independent dielectric constant that is superior to those currently available. Lead free perovskite-like oxide CaCu3Ti4O12 (CCTO) has attracted extensive interest with materials researchers[4] and [5].

Hence, the study of the correlation of microstructure and electrical properties to characterize the dielectric property of CCTO based ceramics has elicited considerable research interest to optimize the grain/grain boundary layer capacitance, additionally improving our understanding of how to process CCTO ceramics to optimize the internal barrier layer capacitance effects [6] and [7].

Dielectric properties of CCTO materials were found to be very sensitive to the microstructure and processing conditions, such as sintering temperature and dwell time[8][9][10] and [11]. Our earlier work [10] confirmed that CCTO ceramics show uniform and homogeneous grain growth (dodecahedron shaped grains) for all samples sintered up to 1100 °C, 2 h.

This issue's cover of Materials Today, shows the microstructure of the fractured surface of CCTO ceramics, processed at 1125 °C for 2 hours under isothermal conditions in an air atmosphere. The surface morphology was performed by a FEI Quanta 200 FEG field emission scanning electron microscope (FESEM) of the fractured surfaces. CCTO was prepared by the sol–gel method; details of which are given elsewhere [11]. The figure represents a bimodal distribution of CaCu3Ti4O12 ceramic. The difference in the structure of the grains, that is a distinction between the fine grain structure and the larger polygonal grains close to 100 μm, can be attributed to the significant less dwell time available for the grain growth procedure.

The effect of the sintering cycle is evident, with the picture depicting the disparity in the grain size: this may be explained on the basis of the fact that the grain growth is directly related to the amount of time the grains are exposed at the higher temperature. Further, the activation energy calculated (0.53 eV) for the process relates it to the mechanism of solution of second phase by boundary diffusion [11]. This mechanism occurs when one of the components of the formulation has a melting point close to the sintering temperature, which is cupric oxide in this case.

The mechanism of diffusion supports the microstructure formed, given that one of the components is in the solution phase and the time given for the formation of grains is too low. The diffusion of the components into the grain has been extremely heterogeneous as well, as suggested by Energy Dispersive X-Ray Analysis (EDS) data [11]. The excess of Cu at the grain boundary, with the Cu/Ca ratio being 2.58 and 3.02 in the grain and grain boundary respectively aligns with the diffusion mechanism. The excess oxygen concentration, higher by 4.33% at the grain boundary, also supports the aforementioned procedure.

Given that the difference in the size of grains and the presence of grain boundaries significantly affect the dielectric constant of the material, the smaller size of the grains in this sample, compared to the samples processed for longer dwell times [11], affects the dielectric constant severely. As observed, the presence of a large number of fine grains leads to the formations of a greater number of barrier layers – grain boundaries – which cause a decrease in the net dielectric constant of the material. CCTO shows high room temperature dielectric constant (?r ∼ 66,900) with low dielectric loss (tan δ ∼ 0.22) at 1 kHz frequency for samples sintered at 1125 °C/2 h [11]. Based on this promising outcome, this material can be used for a high-energy density lead-free ceramic for energy storage applications. At the same time the high dielectric behavior at room temperature due to the optimization of sintering process will boost the research on new CCTO based ceramics.

Image reproduced from Ceramics International, Volume 41, Issue 9, Part B, November 2015, Pages 12386–12392.

Further reading

[1] S. Kwon, et al. IEEE Electr. Insul. Mag., 27 (2011), pp. 43–55

[2] K. Yao, et al. IEEE Trans. Ultrason. Ferroelectr. Freq. Control, 58 (2011), pp. 1968–1974

[3] B. Chu, et al. Science, 313 (2006), pp. 334–336

[4] M.A. Subramanian, et al. J. Solid State Chem., 151 (2000), pp. 323–325

[5] D. Fu, et al. Chem. Mater., 20 (2008), pp. 1694–1698

[6] E. Andreja, et al. J. Am. Ceram. Soc., 94 (2011), pp. 3900–3906

[7] C.P.L. Rubinger, et al. J. Appl. Phys., 110 (2011), p. 074102

[8] P. Thongbai, et al. J. Phys. Condens. Matter, 19 (2007), p. 236208

[9] J.L. Zhang, et al. Appl. Phys. Lett., 87 (2005), p. 142901

[10] P.K. Patel, K.L. Yadav Mater. Res. Exp., 1 (2014), p. 015037

[11] P.K. Patel, K.L. Yadav Ceram. Int., 41 (2015), pp. 12386–12392

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DOI: 10.1016/j.mattod.2015.12.001