The Design And Properties Study Of Bismuth Titanatebased Relaxor Ferroelectric Ceramics For Energy Storage

The sodium bismuth titanate-based dielectric ceramics have high dielectric constant and good thermal stability,thus are the most promising dielectric materials for ceramic energy storage capacitors.The polarization strength and breakdown strength of bismuth sodium titanate-based ceramics largely determine the energy storage performance.This article focuses on the composition optimization,grain refinement,and local electric field regulation to improve the energy storage density of bismuth sodium titanate-based relaxaor ferroelectric ceramics.The specific research contents and conclusions are as follows:（1）BNT-ST ceramic with a morphotropic phase boundary is selected.At the MPB,the materials exhibit a larger P_max（about 40μC/cm² at 60 k V/cm）and a lower P_r than pure BNT ceramic,which is a prerequisite for ensuring a high energy storage density under a lower electric field.The substitution of Ag Nb O₃ in the MPB composition further disrupts the long-range ferroelectric order,causing a transition from the nonergodic relaxor to ergodic relaxor state and thereby leading to a large P_maxand small P_r in BNT-ST-100x AN.When the Ag Nb O₃ was added in excess,the long-range ordering of ferroelectrics is severely disrupted,and a very high electric field is required to re-induced the relaxor ferroelectric-ferroelectric phase transition,so P_max also decreases significantly at this time.Therefore,under the same test electric field of 100 k V/cm,BNT-ST-5AN achieved the highest recoverable energy storage density.It also has excellent temperature（25-175°C）and frequency（1-100 Hz）stability.Finally,BNT-ST-5AN achieved an energy storage density of more than 2 J/cm³ under a lower electric field of 120 k V/cm.（2）BNT-ST-5AN:Si O₂ composite ceramic particles were constructed by the hydrolysis reaction of tetraethyl orthosilicate.BNT-based composite ceramics with ultrafine grain size were successfully prepared.Si O₂ as a sintering aid drastically reduces the sintering temperature from 1150°C to 980°C.The reduced sintering temperature is not only conducive to the energy consumption of the production process,but also reduces the rate of element diffusion during the sintering process and suppresses grain growth.The nano-sized Si O₂ at the grain boundaries of the matrix particles bears a higher local electric field while hindering the growth of grain boundaries,and further increases the breakdown strength.Finally,the composite ceramics obtained a recoverable energy storage density of 3.22 J/cm³ under a breakdown field of 317 k V/cm,showing the great potential of fine-grained ceramics in increasing the breakdown strength of dielectric materials and increasing the energy storage density.（3）In view of the problem of the unsatisfactory energy storage efficiency of BNT-ST-5AN and its composites,BNT-SBT relaxor ferroelectric ceramic with ultrahigh energy storage efficiency（>90%,）was selected as the matrix,the Al N fillers with low dielectric constant,high breakdown strength and excellent chemical stability at high temperature was introduced into the matrix,and finally composite ceramics with obvious two-phase structure was successfully prepared.Due to the excellent chemical stability at high temperature,Al N grains are dispersed independently at the grain boundaries of the matrix ceramics,and there is no significant diffusion reaction between the matrix and Al N.The lower dielectric constant Al N filler phase will share a higher local electric field,resulting in the actual local electric field that the matrix ceramics bear is smaller than the apparent electric field calculated from the sample size（ie,the test electric field）.Due to the excellent insulation properties of Al N ceramics,it can withstand ultrahigh local electric fields.Therefore,compared to the matrix ceramics,the overall breakdown strength of composite ceramics has increased by 104%,the energy storage density has increased by108%to 4.24 J/cm³,and the energy storage efficiency is more than 90%.