Study On Non-reducible Barium Titanate Based Energy-storage Ceramics

In this dissertation,non-reducible fine-grain BaTiO₃-based ceramics were chosen to research.We started with fabricating BaTiO₃ nano-powders through solid state reaction.Almost immediately,works of powders'microstructure,grain size,sintering technique,doping modification and so on were carried out,aiming to study the factors and mechanisms correlating with microstructures and properties of ceramics.Firstly,sand-milling technique was utilized to crash the raw BaCO₃ and TiO₂powders to less than 30nm,resulting in reducing the reaction temperature by 200°C.In order to obtain uniform nano-powders,the traditional one-step calcination was improved with two-step calcination,and BaTiO₃ nano-powders with a particle size of 75±22nm and c/a ratio of 1.0095 were prepared.And the reaction mechanism during two-step calcination was subsequently studied.Chemical solid state reaction occurred and Ba₂TiO₄intermediate phase was observed during the first step.And during the second step,mass transfer and particle growth were mainly carried out.The effects of different synthetic methods?solid-state and hydrothermal?of BaTiO₃powders on microstructures and performances of ceramics were investigated.There are a large number of internal stresses and defects?hydroxyl/proton defects,Ba vacancies and holes?in BaTiO₃ powders synthesized by hydrothermal method,resulting in higher concentration and lower activation energy of oxygen vacancy in BaTiO₃-based ceramics and thus worse reliability.What's more,one can observe that the permittivity,loss,breakdown strength,bias field stability,discharge energy density and energy storage efficiency of non-reducible BaTiO₃-based ceramics were strongly grain-size-dependent.Fine-grain ceramics exhibited the better performances on loss,breakdown strength,bias field stability,discharge energy density and energy storage efficiency except a low permittivity,which is not because of the lower contributions of leakage and ferroelectricity but because of the lower concentration and higher activation energy of oxygen vacancy in fine-grain BaTiO₃-based ceramics.Two-step sintering can not only inhibit the migration of grain boundary and thus obtain a ceramic with a smaller grain size and a narrower grain-size distribution,lower contributions of leakage and ferroelectricity and lower concentration of oxygen vacancy,it can also improve the insulation,reliability,breakdown strength and bias field stability.In addition,introducing proper amounts of SiO₂ glass phase and/or high-breakdown-strength Al₂O₃ could not refine grain size but also reduce the loss and improve the insulation,reliability,breakdown strength,bias field stability,discharge energy density and energy storage efficiency.Finally,chemical coating method was also utilized to modify 0.9BaTiO₃-0.1(Bi_0.5Na_0.5)TiO₃ powders aiming to achieve wider temperature stability.Nb elements diffused into the interior of the grains resulting in forming“core-shell”structure,which could improve the dielectric properties,insulation and temperature stability.The influences of sintering temperature on element diffusion,microstructures,dielectric properties and temperature stability were further investigated.