No abstract

Barium strontium titanate glass-ceramics were successfully produced with one major crystalline phase when Al 2 O 3 was added to the melt. A dielectric constant of 1000 and a breakdown strength of 800 kV/cm was achieved; however the energy density was only measured to be 0.3-0.9 J/cm 3 . These energy density values were lower than anticipated due to the presence of dendrites and pores in the microstructure. Using BaF 2 as a refining agent improved the microstructure and doubled the energy density for BST 80/20 samples. However, no refining agent reduced the increasing amount of hysteresis that developed with increasing applied electric field. This phenomenon is believed to be due to interfacial polarization.

Barium strontium titanate (BST) has been targeted as one potential ferroelectric glass–ceramic for high‐energy density dielectric materials. Previous testing has shown that the dielectric constant of these materials was as high as 1000 and the dielectric breakdown strength up to 800 kV/cm. This did not, however, result in exceptional energy density (∼0.90 J/cc). In order to increase overall energy density refining agents can be added to the melt, but the nucleation and growth of the ceramic particles can also play a role. Therefore, in this study the crystallization kinetics were studied to more fully understand how BST phase forms so that the optimal energy density could be obtained. It was determined that the activation energy of the crystallization of BST 70/30 glass–ceramic is approximately 430 kJ/mol which is close to the dissociation energy of Si–O bonds. The Avrami parameter was found to be ∼3 meaning that three‐dimensional growth is dominant and the mechanism of growth was interface controlled.

The bimodal distribution of grain size is frequently observed in calcium copper titanate (CaCu 3 Ti 4 O 12 or CCTO) ceramics that exhibit high dielectric constant. In this study, we propose an analytical model composed of grains of two different sizes to estimate the dielectric behavior of CCTO based on microstructural features. Important assumptions are (1) CCTO is a barrier layer dielectric and (2) the insulating layer thickness is the same in both large and small grains. Explicit expressions for the effective dielectric constant of serial, parallel, and logarithmic mixture models are given. When compared with experimental results, the present bimodal model appears to be acceptable for describing the dielectric behavior of CCTO.