Materials Design And Performance Optimization Of Bi-based Perovskite-type Dielectrics Thin Films

With the development of pulse power technology and the trend of miniaturization of integrated circuits,stricter requirements have been imposed on the performance parameters of dielectric materials for energy storage capacitors.Materials are required to process higher discharge energy storage density and energy storage efficiency.Compared with bulk ceramics,due to the size effect,significantly improved electric breakdown strength while greatly reduced polarization in ceramic film,resulting in a limited increase in the energy storage density.Expect to develop a film substrate with high polarization strength in ceramic films to further increase the energy storage density of dielectric materials.The subject of this paper is based on Bi Mg_0.5Ti_0.5O₃(abbreviated as BMT)with high polarization strength,and the energy storage performance of the dielectric film is regulated through material design and structure optimization.The effects of heat treatment process conditions on dielectric films were studied.In the temperature range of 620°C-700°C,all the films exhibited cubic perovskite structure.When the heat treatment temperature is 640°C,the film has the best electrical properties,with a polarization intensity of 106.01?C/cm² at a breakdown electric field of 912 k V/cm,a breakdown electric field,and a discharge energy storage density of 26J/cm³.1.Energy storage properties optimizationby compositon modification:(a)The non-stoichiometric ratio of Bi surplus makes up for the volatile Bi during heat treatment and reduces the oxygen vacancy concentration.However,no obvious improvement in electrical performance was seen,and the optimal energy storage performance was still obtained when the Bi content was 1.0.(b)According to the energy storage equation,breakdown strength is one of the key parameters affecting energy storage density.A non-stoichiometric Mg excess and Zr replacement Ti were used to prepare a dielectric thin film with a uniform fine-grained structure,in order to improve the breakdown electric field.Studies have shown that the excessive introduction of Mg does not change the phase structure of the base film but significantly refines the grains of the film,which greatly improves the breakdown electric field of the film.When the Mg content is increased from 0.5 to 0.56,The breakdown electric field was increased from 900 k V/cm to 1800 k V/cm,which was nearly doubled.At the same time,the optimal discharge energy storage density value was 44.14 J/cm³,and the overall energy storage efficiency changed little(50%-60%).Similarly,Zr replacement of Ti stabilizes the crystal structure of the film,reduces the dielectric constant and polarization strength,but greatly improves the breakdown electric field of the film.When the Zr content is 0.09,the discharge energy storage density is 67 J/cm³ It is three times that of the base film,which indicates that the introduction of an appropriate amount of Zr content is beneficial to improve the energy storage performance of the BMT base film.Under the breakdown electric field,the effect of the Zr content on the efficiency is not large,the value is between52-61%.2.Optimizing bismuth,magnesium and titanium films by microstructure design:In order to further optimize the breakdown electric field,the purpose of designed the heterogeneous interface is to improve energy storage performance;The Ba_0.7Sr_0.3Ti O₃(BST)/Bi Mg_0.5Ti_0.5O₃(BMT)heterogeneous laminated film is designed.The BST film layer with high breakdown electric field and low dielectric constant leads to a significant reduction in the polarization of the heterostructure film and further reduces energy storage density.Inspired by the introduction of the glass phase to improve the breakdown electric field and energy storage characteristics of ceramics,an amorphous phase with high breakdown electric field was introduced in the film by adding an excessive amount of Ti.Studies have shown that when the Ti content is 0.65,an amorphous phase appears in the film and the crystal grains of the film are significantly refined.The best energy storage performance is obtained when the Ti content is 0.75,that is,the discharge energy storage density is as high as 126 J/cm³ at a breakdown electric field intensity of 5000 k V/cm.The energy storage efficiency gradually increases with the increase of Ti content and has the same trend as the leakage current.When x?0.65,the leakage current of the film gradually decreases with the increase of Ti content,however,with the further increase of the Ti content,the leakage current does not change significantly,which may be because the appearance of the amorphous phase in the film increases the resistivity and reduces the leakage current.The slope of the straight line fitted to the data of leakage current and voltage is close to 1,indicating that the leakage mechanism of the film is ohmic conductance.In order to further reveal the factors influencing the performance of energy storage films and the relationship between various parameters,this study uses the change of saturation polarization intensity with the electric field to represent the delay effect of polarization saturation.The increase of heat treatment temperature and the increase of Bi content are not conducive to the delay of polarization saturation,while the increase of Mg content,Zr content,the number of BST insertion layers and Ti content can effectively delay polarization saturation.Moreover,the interface effect was revealed through the relationship between the breakdown electic field of the films with different conditions and the number of interfaces.The insertion of the heterostructure design thin film,namely the BST thin film layer and the excessive Ti,lead to an increase in the number of internal interfaces of the thin film,which is helpful to the improvement of the breakdown electic field.In addition,the difference between the saturation polarization intensity and the residual polarization intensity under the breakdown electric field,the breakdown electric field,and the discharge energy storage density depend on various variables(heat treatment conditions,Bi content,Mg content,Zr content,BST insertion layer number,Ti content)revealed that the size of the discharge energy storage density depends on the synergistic effect of polarization strength and breakdown electric field.