Preparation,Properties And Applications Of Nanocrystalline Barium Titanate Based Materials

This dissertation mainly focuses on study of the finite-dimensional barium titanate nanoparticles(BTO NPs).An original “one-step sol” method was developed to synthesize a series of BTO NPs and films within 10 nm crystal size.Phase structure and physical properties of the BTO NPs,including the ferroelectric critical size,were systematically investigated.These NPs were successfully applied in preparing high energy-storage nanocomposite thin films and coating of nickel particles.By controlling the hydrolysis vs.crosslinking rate,“one-step sol” method,which lies between the sol-gel and precipitation method,was designed to fabricate BTO NPs with crystal size as small as 2.8 nm and their highly stable sols.Strontium titanate nanocubes and sol were also obtained by this method.Moreover,these stable sols were utilized to fabricate highly crystallized nanoparticle films under 160 °C.The results of Raman,synchrotron X-ray diffraction,pair-distribution function(PDF)analyses and X-ray photoelectron spectroscopy(XPS)confirmed that BTO NPs even as small as 2.8 nm are ferroelectric with multi-phase coexistence.And the non-centrosymmetric phases were highly related to the surface structures of the NPs.Diffused phase transition characters over a wide temperature range were observed.The non-centrosymmetric phases even persisted at temperatures as high as 400 °C.Measurements of second harmonic generation(SHG),ferroelectric response and dielectric properties further confirmed the ferroelectricity of these BTO NPs.SHG signals,which showed a non-linear polarization effect at room temperature,declined with the increase of the temperature,revealing a successive transition from the non-centrosymmetric ferroelectric phases to paraelectric ones.Dielectric measurements and the ferroelectric polarization switching behavior in response to the applied external electric field further confirmed the ferroelectricity of these BTO NPs.Several applications of the ferroelectric BTO NPs were investigated.Firstly,titanate nanoparticle thin films with excellent dielectric properties and high temperature stability were fabricated using the nanocrystal sols under160 °C;secondly,with the 8 nm BTO NP fillers,the multilayer nanocomposite films achieved large electric displacements,high breakdown strength(Eb=495kV/mm)and high energy storage densities(Ue=19.37J/cm3);finally,a dense and uniform BTO polycrystalline coating layer was obtained on nickel NPs by the modified “one-step sol” method,and this layer was proved to be effective in improving the shrinkage behavior and anti-oxidation properties of the nickel NPs during sintering.