Preparation And Properties Of Bi_0.5Na_0.5TiO₃-based Lead-free Energy Storage Ceramics

In the past few decades,dielectric ceramics are widely used in all kinds of capacitors due to their high power density and fast charge and discharge rates.Lead-free ferroelectric ceramics have been extensively studied from the viewpoint of human health and environmental friendliness.As a ferroelectric material with the perovskite structure（ABO₃）,Bi_0.5Na_0.5Ti O₃is one of promising lead-free ferroelectrics because of their large remnant polarization and high Curie temperature.However,the high coercive filed and low electrical breakdown strength limit its application as energy storage medium.In general,doping elements at the A-site or/and B-site in BNT latticewill lead to the disorder of chemical composition and the change of Ti-oxygen octahedron,which is regarded as an extremely effective method to enhance its energy storage properties.In this thesis,BNT-based ceramic nanopowders were prepared by a hydrothermal method.The effects of ion doping（Sr,Ba and Ca ions）and sintering temperature on the microstructure,dielectric properties and energy storage properties of BNT-based ceramics were systematically studied.The main research contents and results are as follows:（1）BNT ceramic powder was synthesized by a hydrothermal method.The results show that the BNT powders with the particle size of about 80 nm were synthesized under the conditions of Na OH concentration of 6 mol/L,hydrothermal temperature of 200℃and holding time of 4 h.The BNT ceramics sintered at 1100℃has a recoverable energy storage density(W_rec)of 0.38 J/cm³and energy efficiency（η）of 18.24%.（2）Sr or/and Ba doped BNT ceramics were obtained by using nano powders synthesized by the hydrothermal method.It is found that Sr-doped(Bi_0.5Na_0.5)_0.75Sr_0.25Ti O₃ceramics sintered at 1100℃have a recoverable energy storage density(W_rec)of 1.27 J/cm³,which is234%higher than that of pure BNT.The Sr and Ba co-doped(Bi_0.5Na_0.5)_0.72Sr_0.24Ba_0.04Ti O₃ceramics exibit a recoverable energy storage density(W_rec)of 1.41 J/cm³,which is 271%higher than that of pure BNT.The results indicates that the energy storage properties of BNT ceramics could be effectively improved by Sr or/and Ba doping.（3）Sr,Ba and Ca co-doped BNT-based high entropy ceramics（HECs）were obtained by using nano powders（about 40 nm）synthesized by the hydrothermal method.(Bi_0.2Na_0.2Ba_0.2Sr_0.2Ca_0.2)Ti O₃（BNBSCT）HECs sintered at 1200℃displays a single tetragonal phase,and have a recoverable energy storage density(W_rec)of 1.37 J/cm³and dielectric constant（ε_r）of 3788,respectively.The lattice distortion in the BNBSCT HECs leads to the characteristics of dielectric relaxation,which significantly improves the dielectric and energy storage properties of the BNBSCT HECs.