| The effects of compositional modifications on phase development, dielectric property and energy storage performance of (Pb0.925-xLa0.05Bax)[Zr0.52(SnyTiz)0.48]O3 (PLZST, x=0,2,4,6,8,10; y/z=39/9, 35/13,32.5/15.5) ceramics synthesized by solid state reaction method were investigated. We employed highly accelerated lifetime testing (HALT), impedance spectroscopy (IS) coupled with thermally stimulated depolarization currents (TSDC) to identify the failure mechanism.The effect of barium content on phase development, dielectric property and energy storage performance of (Pb0.925-xLa0.05Bax)(Zr0.52Sn0.39Ti0.09)O3 ceramics was investigated. X-ray diffraction patterns and scanning electron microscope micrographs illustrated that the pyrochlore phase was effectively suppressed by the introduction of barium in the PLZST ceramics. When the amount of barium content was increased from 0 to 10 mol%, the maximum dielectric constants increased from 900 to 1200, and the temperature at the maximum dielectric constants decreased approximately by 4.8℃/mol%, which may be related to the increase of tetragonality in the PLZST ceramics. Both charged and discharged energy storage density increase sharply at first, and then decrease slightly with increasing barium content. The maximum energy storage density of 0.7 J/cm3 is obtained at x=0.06.All the samples exhibit high energy storage efficiency, which is attributed to the slanted hysteresis loops of samples. The study of cyclic charge-discharge showed that barium doped PLZST ceramic capacitor can withstand up to 10000 cycles with about 8% energy density loss.The effect of Sn/Ti ratio on the dielectric and energy storage properties of (Pb0.925La0.05Ba)[Zr0.52(Sn,Tiz)0.48]03 ceramics was studied. As the Sn/Ti ratio decreased, dielectric constant increased and the transition temperature shifted to lower temperature. The maximum polarization increased, while the switching field decreased when the Sn/Ti ratio decreased. Both charged energy density and discharged energy density increased with decreasing Sn/Ti ratio. The released energy density of the PLZST ceramic with the Sn/Ti ratio of 32.5/15.5 was reduced by 27% after 10000 cycles, while only 1% energy density loss was observed for the PLZST ceramic with the Sn/Ti ratio of 39/9.The time dependent leakage current observed in HALT indicates that failure mechanism in PLZST ceramics might be better described by a two-stage dielectric degradation that initiated with a slow dielectric degradation, characterized by a gradual leakage current increase against time, followed by a catastrophic breakdown. The IS and TSDC study indicate that in the low temperature region, oxygen vacancies Vo·· would move around the impurity center Vpb", which results in the formation of defect dipole complexes Vpb"-Vo··, whereas this defect dipole complex would break up as the temperature is increased, and the oxygen vacancy would migrate across grain boundaries and start accumulating near the cathode. The migration of oxygen vacancies not only gives rise to a gradual increase of leakage current against time, but also changes the initial stoichiometry of PLZST ceramics and causes the intrinsic defects and the local low-resistance. The substitution of Pb2+ by Ba2+ will efficiently restricts the generation of oxygen vacancy and the defect dipole complexes Vpb"-Vo··. Therefore, the improved dielectric degradation of the Ba-doped PLZST ceramics is attributed to a lower defect dipole complexes concentration.The burnout conditions in multilayer ceramic capacitor (MLCC) have been investigated to optimize the binder removal condition and to control the microstructure during the burnout process, which is very important in the fabrication of MLCC. The burnout temperature could be selected as 120,250, and 350℃ based on the result of DSC and TG analysis of green MLCC. The first peak at 120℃ is ascribed to the elimination of excess solvent, and the exothermic peaks at 250℃ and 350℃ are believed to be due to the decomposition of binders. In addition, a slowly heating rate was necessary to prevent the delamination in the burnout process of MLCC. The isothermal holding time at burnout temperature was also effective in removing the residual organics and in developing the final microstructure. Besides, the delamination and cracks were effectively suppressed by the introduction of advanced drying oven and applying Al2O3 capping layer on the surface of the MLCC. |
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