Structural Design And Energy Storage Performance Of Na_0.5Bi_0.5TiO₃-SrTiO₃-Based Lead-free Thin Films

Dielectric capacitors have been receiving increasing attention because of their capability of delivering high power density with an ultrafast charging-discharging process,which is of importance for modern electrical and electronic applications such as advanced pulsed power systems,hybrid electric vehicles,high frequency inverters,energy systems,defibrillator and so on.So far,among various dielectric capacitors,film capacitors for dielectric energy storage have attracted increasing attention due to their high BDS.Despite the progress that has been made in the research on film capacitors for energy storage,their energy storage performance is still found to be unsatisfactory.In this paper,Sr_x(Na_0.5Bi_0.5)_1-xTiO₃?0.3?x?1?thin film was selected as the research object to improve the energy-storage performance by optimizing polarization behavior,reducing defect concentration,and constructing multilayer thin films based on interface charge coupling effect and electric field amplifying effect.The effect of defect dipole on polarization mechanism is analyzed in detail.At the same time,a new method of embedding polar clusters into amorphous thin films is proposed to optimize the energy storage efficiency.Firstly,the relaxor ferroelectric Sr_0.3(Na_0.5Bi_0.5)_0.7TiO₃ thin film was taken as the research object,and the energy storage performance was improved by introducing Mn²⁺ into Ti⁴⁺site.On the other hand,doping Mn²⁺ can suppress the valence of Ti⁴⁺ and reduce the leakage current path formed by free electrons jumping between Ti⁴⁺ and Ti³⁺.By changing the leakage mechanism,the carrier potential well becomes deeper,thereby reducing leakage current density and increasing the breakdown strength.1% Mn²⁺-doped thin film obtained a large energy storage density at low electric field strength.In order to further improve the breakdown strength,by adjusting the composition of the film,the Na+ and Bi³⁺ volatilization is reduced,and the oxygen vacancy concentration is reduced.By adjusting the Sr content,energy storage density and energy storage efficiency have been further improved.In order to obtain high breakdown field strength,non-ferroelectric SrTiO₃ thin film was selected as the main research subject.First,introduce Mn²⁺ at the Ti⁴⁺ site to suppress the valence of Ti⁴⁺ and ensure a larger breakdown strength,and then introduce Na⁺_-Bi³⁺ at the Sr²⁺ site to optimize the polarization behavior by constructing locally polarized regions.Finally,the Na⁺_-Bi³⁺-doped ST thin film obtained ferroelectricity at room temperature,the polarization was significantly improved,and an excellent energy storage density was obtained.In order to improve the energy storage efficiency,Ca²⁺ is doped at the Sr²⁺ site.By weakening the interaction between the domains,the domains are easier to rotate with the electric field and the polarization hysteresis is reduced.The Sr_0.9Ca_0.1Ti_0.99Mn_0.01O₃ thin film exhibits a slim hysteresis loop,and the remanent polarization is greatly reduced.At 4910 k V/cm,an energy storage density of 63.9 J/cm³ is obtained under ultra-high electric field?5015 k V/cm?,and the energy storage efficiency is 65.8 %.This paper also proposes a new idea of embedding polar clusters rich in Na and Bi into amorphous SrTiO₃ thin films to further optimize energy storage performance.Amorphous thin films have ultra-high breakdown strength due to low heat treatment temperature,few defects,no grain boundaries and domain boundaries.The anisotropic polar nano-regions caused by polar clusters can freely change direction with the external electric field,and are not constrained by the periodic structure of the crystal,so that the amorphous film exhibits a fine hysteresis loop line shape,accompanied by larger maximum polarization and smaller polarization hysteresis.Finally,an energy storage density of 65.3 J/cm³ was obtained in the prepared small amount of Na-Bi doped ST amorphous film,and the energy storage efficiency was 70.8 %.The Na_0.5Bi_0.5TiO₃-SrTiO₃ relaxor ferroelectric thin films with similar composition and large dielectric constant difference is selected to form multilayer thin films to further improve the energy storage performance of the relaxor ferroelectric thin film.On the one hand,based on the amplifying effect of electric field,the electric field actually applied to the high dielectric constant film layer will be amplified to achieve the effect of increasing the breakdown field strength.On the other hand,by increasing the number of stacking cycles to introduce multilayer interfaces,the use of interface charge coupling effect and stress relaxation increases the polarization.Based on this design,the breakdown strength of the Na_0.5Bi_0.5TiO₃-SrTiO₃ multilayer thin film is 44 % higher than that of the singlelayer film,the polarization is greatly improved,and the energy storage density of 60 J/cm³ is finally obtained.Compared with current research status,high energy storage density is achieved at lower electric field strength?<2700 k V/cm?.