Preparation And Electrical Properties Of Sodium Bismuth Titanate Based Lead-free Piezoelectric Ceramics

As the development of science and technology,the application of lead-based piezoelectric sensors has been widely applied in kinds of areas.However,due to the toxic element,lead-based piezoelectrics are extremely harmful to the environment and human health.Finding lead-free piezoelectric materials,which can replace the role of lead-based materials,has been an important topic for all the researchers now.Sodium bismuth titanate(NBT)ceramic as one of perovskite lead-free piezoelectric materials has attracted much attention due to its outstanding ferroelectric properties.The research results are summarized as follow:(1)The hydrothermal method was applied to synthesize NBT nanocrystals with different morphologies: wires,plates and cubes.We analyzed the relationship of the reaction time,temperature and concentration of NaOH and the morphologies of NBT nanocrystals through the characterization of NBT nanocrystals by X-ray diffraction(XRD),scanning electron microscope(SEM)and transmission electron microscope(TEM).The results showed that elongation of reaction time,increase of reaction temperature and concentration of the NaOH can improve the crystallization of the NBT nanocrystals and affect the morphology formation in the reaction.We put up the growth mechanisms for the different morphology formation with the analysis of characterization results.(2)We then prepared NBT lead-free piezoelectric ceramics by the traditional solid-state sinter method with NBT nanocrystals of different morphologies as precursors.We characterized the ceramic structures,grain growth and ferroelectric domains with XRD and SEM and calculated the size of grains and domains.We found that the morphology of NBT nanocrystals played an important role in the grain growth and the evolution of domains during the sinter process.The mechanisms of the effect of NBT nanocrystals on the ceramics grains and domain size were discussed.(3)The piezoelectric,ferroelectric and dielectric properties of ceramics sintered with NBT nanocrystals of different morphologies were studied after the electrode and electrical poling process.With the analysis of the relationship of electrical properties and grain and domain,we believed that the NBT nanocrystals could affect the electrical properties of NBT ceramics by influencing the grain growth and domainevolution.(4)We prepared Mn-,Fe-and La-doped NBT ceramics metamaterials with high apparent piezoelectric properties by reducing the NBT-based ceramics during high temperature with graphite blocks.By reduction the joint surface of ceramics and graphite could produce oxygen vacancies during high temperature,which then generated a chemical inhomogeneity in the ceramics.The chemical inhomogeneity could induce a distortion in the lattice and a curvature in macro scale.Due to the formation of curvature,a strain gradient was introduced into the ceramics,which could improve the flexoelectric contribution to the polarization.