Study On Structural Design And Energy Storage Performance Of SrTiO₃/BiFeO₃ Thin Films

In recent years,with the rapid development of new energy,mobile electronic equipment,power systems,hybrid vehicles,pulse energy instruments,etc.,the demand for dielectric energy storage capacitors with small volume,high power density and large energy storage density has become more urgent.However,compared with batteries and supercapacitors,the energy storage density of dielectric capacitors is low,which limits their applications.Therefore,the research of solid dielectric materials with high energy storage density has become a key technical issue to be solved.Compared with bulk materials,dielectric thin films show higher withstanding electric field and larger energy storage density,and gradually become a research hotspot in the field of dielectric energy storage materials and devices.In this paper,after optimizing the electrical properties of Sr Ti O₃（STO）thin films and Bi Fe O₃（BFO）thin films,STO/BFO heterostructure thin films and STO-BFO composite thin films were prepared by a sol-gel method,and the structure and energy storage properties were studied.The main research work is as follows:First,STO thin films and BFO thin films were optimized.Different amounts of Mn-dopd STO thin films were prepared by layer-by-layer annealing to optimize the energy storage properties.When Mn doping amount is≤1%,the thin films are pure perovskite structure;the dielectric constantε_r and loss tanδshow excellent frequency stability.When Mn doping amount is≥3%,the main crystalline phase of the thin films is perovskite structure,and a small amount of second phase appears;the thin films appear relaxation:the dielectric constant and loss decrease with increasing frequency.The dielectric tunability of all samples at 200 k V/cm is less than 10%.Optimized energy storage properties were obtained in 1%Mn doped STO thin film:discharge energy density U_rec=24 J/cm³ with efficiencyηof 70%at 2286 k V/cm.BFO thin films with different Mn substitution at B site were studied to optimize the electrical properties.All thin films were pure perovskite structure without any second phase.Compared with pure BFO thin film,the leakage current of Mn-substituted thin films is decreased.Bi Fe_0.97Mn_0.03O₃ thin film shows the largest difference between maximum polarization and remnant polarization,which is favorable for energy storage.Therefore,in the later experiments,the BFO layers in the heterostructure films were prepared with 3%Mn substituted BFO sol.The effects of BFO layer positions on the structure and energy storage properties of STO/BFO heterostructure thin films were investigated.Compared with bilayered thin film,sandwich-structured thin film has higher electric breakdown strength of2431 k V/cm and larger energy storage density,and its energy storage performance under 2333 k V/cm is:U_rec=30 J/cm³,η=55%.Its dielectric properties at 10 k Hz are:ε_r=111,tanδ=0.036,and the capacitance-temperature variation is less than 15%from room temperature to 145°C.The effects of BFO layer thickness on the structure and energy storage properties of sandwich-structured STO/n BFO thin films（n is the number of spin coating of BFO;n=1-3）were investigated.The breakdown strength and energy storage density decrease with the increase of n.The energy storage properties of STO/1BFO thin film is:U_rec=41 J/cm³ andη=45%at 2059 k V/cm.The dielectric properties at 10 k Hz are:ε_r=274,tanδ=0.030,dielectric tunabilityτ=4.2%at 200 k V/cm,and the capacitance-temperature variation（25-200°C）is-5.5%,which shows excellent frequency stability,bias stability and temperature stability of dielectric constant.The effects of interface number on the structure and energy storage properties of STO/BFO heterostructure thin films（Marked as F-n;the larger n with more STO/BFO interfaces）were investigated.With the increase of interface number,the recoverable energy storage density increases first and then decreases,and the energy storage efficiency increases gradually.F-3 thin film has the optimized energy storage performance:U_rec=48 J/cm³ andη=56%at 2333 k V/cm.The dielectric properties are:ε_r=195,tanδ=0.031 at 10 k Hz.La Ni O₃（LNO）thin films were prepared on Si（100）substrates and the resistivity was～3×10^-4Ω·cm,which can be used as electrode materials.Then,STO/BFO heterostructure thin films were prepared on Pt/Ti/Si O₂/Si substrate and LNO/Si substrate,respectively.The effects of bottom electrodes（LNO,Pt）on the structure and energy storage properties of STO/BFO heterostructure thin films were investigated.For STO/BFO/Pt thin film,U_rec=14 J/cm³ andη=64%at 1000 k V/cm.For STO/BFO/LNO thin film,the discharge energy density is 47 J/cm³ at 1533k V/cm,which is 250%higher than the former,but the efficiency is decreased to 49%;the dielectric properties at 10 k Hz are:ε_r=215,tanδ=0.15,and the capacitance-temperature variation is<15%from room temperature to 200°C.The effects of annealing temperature on the structure and energy storage properties of STO thin films were investigated.When the annealing temperature is lower than 600°C,the thin films are amorphous.When the annealing temperature is higher than 650°C,the thin films begin to crystallize.As the annealing temperature increases,the critical electric field of the film increases from 3682 k V/cm annealed at500oC to 4451 k V/cm annealed at 550oC,and then gradually decreases.For the STO thin film annealed at 500oC,the U_rec is 28 J/cm³ with anηof 60%.The optimized energy storage properties are obtained in the thin film annealed at 550°C:U_rec=54J/cm³ andη=78%at 4541 k V/cm.And the dielectric properties at 10 k Hz are:ε_r=43,tanδ=0.005.The effects of BFO content on the structure and energy storage properties of STO-BFO thin films were investigated.With the increase of BFO content,the breakdown strength and energy storage density of the films decrease.When the BFO content is 10%,the thin film has the highest breakdown strength and the largest energy storage density among all samples.Its dielectric properties at 10 k Hz are:ε_r=35,tanδ=0.003,and the capacitance-temperature variation is 2.4%from room temperature to 200°C;the energy storage performance at 5876 k V/cm is:U_rec=101J/cm³ andη=93%.The change rate of U_rec is 1.3%andηkeeps more than 83%from room temperature to 150oC.The results show that STO-BFO thin films have excellent dielectric temperature stability,good bias voltage stability and outstanding energy storage performance,indicating potential applications in high temperature energy storage fields.