Study On Design,Preparation And Related Mechanisms Of Lead-free Dielectric Ceramics For High Energy Storage Performances

In the future,the world energy consumption will continue to increase,CO₂and other greenhouse gas emissions are required to be reduced at the same time,which inspired an enthusiasm effort towards the development of efficient,clean and renewable energy sources.Efficient electrical energy storage devices are the key to achieving this goal.Dielectric energy storage ceramics are favored due to their extremely high power density.Unfortunately,most of dielectric ceramic materials with high energy storage characteristics contain lead element.This feature brings great harm to the environment and human health.In this paper,we designed and prepared sodium bismuth titanate(Na_0.5Bi_0.5Ti O₃)and silver niobate（Ag Nb O₃）based lead-free ceramics with high energy storage properties by introducing other components or optimizing sintering methods.Furthermore,we also carefully studied their related mechanisms.The results of this paper may help to expand the future application prospects of lead-free dielectric energy-storage ceramic materials.The important contents and conclusions are listed below:（1）Ag_0.91Sm_0.03Nb O₃modifiedNa_0.5Bi_0.5Ti O₃-Ba Ti O₃ceramics（0.94-x）Na_0.5Bi_0.5Ti O₃-0.06Ba Ti O₃-x(Ag_0.91Sm_0.03)Nb O₃（x=0,0.03,0.06,and 0.09）were prepared by solid-state reaction method.The structural and electrical properties of all the samples were systematically studied.Our results indicate that all the samples exhibit pure perovskite structure with dense microstructure.All the elements in the sample are homogeneously distributed.The sample with the doping level x=0.09,possesses good temperature stability of the dielectric constant(Δε’/ε’_150°C≤±15%)and low dielectric loss（<0.02）over a wide temperature of 84-318°C.A large energy-storage density value of W_rec=2.12 J/cm³,high efficiency of?=83%under a low electric field of 18 k V/mm,as well as excellent temperature/frequency stabilities were simultaneously achieved in the sample.This work provides a viable way to design high energy storage performance lead-free ceramics operating at low fields.（2）Exploring eco-friendly energy-storage ceramics simultaneously featuring large recoverable energy storage density(W_rec),high energy storage efficiency（?）,and excellent temperature/frequency stabilities is highly desirable for the applications of pulsed power systems.Herein,we used Ag Nb_0.85Ta_0.15O₃to modify Na_0.5Bi_0.5Ti O₃based lead-free relaxor ferroelectric ceramics in an effort to enhance the breakdown electric field strength.The ceramics（1-x）(0.75Na_0.5Bi_0.5Ti O₃-0.25Sr Ti O₃)-x Ag(Nb_0.85Ta_0.15)O₃（x=0,0.05,0.1,and 0.15）were prepared by the conventional solid-state reaction method.The microstructure,dielectric,and energy storage properties of all the ceramics have been systematically studied.Our results show that the introduction of Ag Nb_0.85Ta_0.15O₃leads to a homogeneous microstructure and small grain size,thereby increasing the strength of the dielectric breakdown field（29 k V/mm）.In addition,it can also decompose the macroscopic long-range ferroelectric order into randomly-oriented polar nano-regions（PNRS）.A large W_rec（3.6 J/cm³）and high?（80%）as well as excellent temperature and frequency stabilities were simultaneously achieved in the sample with x=0.1.This work provides a feasible method for designing lead-free ceramics with high energy storage performances.（3）In this work,we systematically investigated the effects of different sintering methods on the structural,dielectric and energy storage properties of pure Ag Nb O₃lead-free antiferroelectric ceramics.Both ceramics exhibit pure perovskite structure and no secondary phase is found.Moreover,the ceramic prepared by two-step sintering method has small grain size,dense and homogeneous microstructure.In addition,the results of dielectric temperature spectrum shown that the two-step sintering method hardly changes the phase transition temperature of pure Ag Nb O₃ceramics.Most importantly,the ceramic prepared by two-step sintering method displayed high breakdown electric field strength（22 k V/mm）,large W_rec（2.59 J/cm³）and high?（45%）as well as excellent temperature stability,than that of the ceramic by single-step sintering method.Furthermore,it also exhibited high power density（P_D=25.7 MW/cm³）and extremely fast charge-discharge speed（25 ns）.Our results provide a simple and novel way to design high-performance Ag Nb O₃based energy-storage lead-free ceramics.（4）The Ag Nb_0.85Ta_0.15O₃and Ag_0.85Bi_0.05Nb O₃ceramics were successfully prepared by two-step sintering method.We have systematically studied the microstructure,dielectric and energy storage characteristics of the above these ceramic samples.The dielectric temperature spectrum results show that these samples are between the M1-M2 and M2-M3antiferroelectric phases at room temperature,respectively.The most important thing is that Ag Nb_0.85Ta_0.15O₃and Ag_0.85Bi_0.05Nb O₃ceramic samples have large recoverable energy storage density(W_rec).Among them,A large energy-storage density value of W_rec（3.53 J/cm³）,high efficiencyη（86%）,as well as excellent temperature/frequency stabilities were simultaneously achieved in the Ag_0.85Bi_0.05Nb O₃ceramic sample.On the other hand,the charge-discharge tested results show that it also has an ultrahigh power density（73.57 MW/cm³）and extremely fast charge-discharge speed（51 ns）.The above results indicate that the Ag_0.85Bi_0.05Nb O₃ceramic prepared by the two-step sintering method has great potential for the applications of pulsed power systems.This work shows that the two-step sintering method combined to tuning phase boundary opens up a new way for the design of Ag Nb O₃ceramic materials with excellent energy storage characteristics.（5）All the（1-x）(0.94Na_0.5Bi_0.5Ti O₃-0.06Ba Ti O₃)-x(Sr_0.85Bi_0.1)(Mg_1/3Nb_2/3)O₃（x=0,0.18,0.24,0.3）ceramics samples were successfully prepared by traditional solid-state reaction method.The SEM results show that the introduction of(Sr_0.85Bi_0.1)(Mg_1/3Nb_2/3)O₃can achieve the effect of small grain size,and dense microstructure.Moreover,when x=0.24,the dielectric temperature spectrum results also show that the T_Scan move to around room temperature and it has good dielectric temperature stability.In addition,a large energy-storage density value of W_rec（3.05 J/cm³）and high efficiencyη（89%）,as well as excellent temperature/frequency stabilities were simultaneously achieved in above sample.And variation of recoverable energy storage density is less than 7%in the temperature range of 25～140°C.On the other hand,it also has an high power density（67.86 MW/cm³）regardless of variation of temperature.