| With the rapid development of social economy and electronic information technology,the problems of energy crisis and environmental pollution have become more prominent in recent years.Thus,developing and exploiting the high-performance,green and sustainable energy and materials have received considerable attention by countries all over the world.Energy storage ceramic capacitors have a great potential to be applied in the field of electronic circuits,aerospace,medical and others due to their fast charge-discharge speed,high power density,long cycling lift as well as suitability for working in extreme environments,such as high temperature,high operating voltage and so on.However,it is difficult to optimize the maximum polarization(Pmax),remanent polarization（Pr）and breakdown strength（Eb）of environmentally friendly lead-free ceramic capacitors simultaneously,which limits the improvement of energy storage performance and the development of electronic components towards miniaturization,light weight and integration.Therefore,there are still some critical challenges to maximize the Pmax while reduce the Pr and improve the Eb simultaneously in lead-free ceramic capacitors with excellent energy storage performance.In order to be confronted with the challenge,the bismuth-based lead-free ceramics with the main component of Bi0.5Na0.5Ti O3 and Bi Fe O3 were selected as the research object and focused on developing new dielectric materials of ceramic capacitors with high recoverable energy storage density(Wrec),high energy storage efficiency（η）and outstanding stability in this dissertation.The research was started from the 0.75Bi0.5Na0.5Ti O3-0.25Sr Ti O3 systems with high polarization and a series of 0.75Bi0.5+xNa0.5-xTi O3-0.25Sr Ti O3（BNST-x）ceramics with different contents of Bi and Na were prepared by the conventional solid-state method and tape-casting technique to investigate the effect of the non-stoichiometric ratio of Bi and Na at the A-site for the structures and properties.The results indicated that by optimizing the non-stoichiometric of Bi and Na at the A site for BNST-x ceramics can inhibit the grain growth,improve the temperature stability of the dielectric constant and enhance the breakdown strength to a certain extent.At the electric field of 200 k V cm-1,the maximum value of 2.93 J cm-3 and2.63 J cm-3 can be achieved for total energy storage density(Wtot)and Wrec,respectively,when x=0.02.The optimal energy storage performance(Wrec=5.63 J cm-3,η=94%)can be abtained at the maximum applied electric field(Emax)when x=0.08.In addition,the variation of Wrec is less than±5%and the value ofηis always higher than 90%within 1～100 Hz and 30～130°C.The energy storage performance did not deteriorate significantly and theηcan be remained above 85%after 104 cycles.Meanwhile,the power density maintains 147～180 MW cm-3 while the t0.9 is less than125 ns within 30～130°C,exhibiting high power density and fast discharge speed.And then,the Sr2+ions and(Nb0.5Al0.5)4+composite-ions were introduced into the Bi0.5Na0.5Ti O3 of the A-site and B-site,respectively,to form the Bi0.5Na0.5Ti O3-Sr(Nb0.5Al0.5)O3 ceramics accompany with relaxor characteristics.Increasing the content of Sr(Nb0.5Al0.5)O3 is beneficial to refining the grain size and inducing the polar nanoregions（PNRs）with fast polarization reponse under the external electric field,resulting in the optimization of both the polarization and breakdown strength and the improvement of energy storage performance.The excellent energy storage performance(Wrec=6.64 J cm-3,η=97%)can be obtained at the electric field of 520k V cm-1 with the Sr(Nb0.5Al0.5)O3 content of 0.20 mol.Meanwhile,theηcan be maintained around 94%at 1～100 Hz and the variation of Wrec is less than±6%in the temperature range of 30～150°C.This phenomenon indicated that the energy storage performance of the Bi0.5Na0.5Ti O3-Sr(Nb0.5Al0.5)O3 ceramics possesses excellent frequency and temperature stability.In addition,a current density of 878.34 A cm-2and a power density of 131.75 MW cm-3 can be also obtained at the electric field of300 k V cm-1.Furthermore,to better optimize the polarization and breakdown strength as well as achieve high energy storage performance,the Bi0.5Na0.5Ti O3-based lead-free ceramic with the“sandwich structure”was designed and fabricated by tape-casting technique and two-step sintering method via the superposition effect of composite materials.First of all,the component of(Bi0.5(Na0.8K0.2)0.5)0.96Sr0.04Ti0.99Ta0.01O3 with high polarization and the component of 0.70Bi0.5Na0.5Ti O3-0.30Sr(Nb0.5Al0.5)O3 with high breakdown strength were selected as the outer layer and the middle layer dielectric materials of the“sandwich structure”,respectively.These two different dielectric materials were combined by stacking,and the thickness ratio between the middle layer and the outer layer for the performance was investigated.Furthermore,the optimized 0.80Bi0.5Na0.5Ti O3-0.20Sr(Nb0.5Al0.5)O3 and 0.75Bi0.52Na0.48Ti O3-0.25Sr Ti O3 were used as the dielectric materials of high breakdown strength layer and high polarization layer,respectively.Studying the effect of different spatial distributions of high breakdown strength layer and high polarization layer in a“sandwich structure”for the performance.The results indicated that these different dielectric layers were combinated very well with co-firing.The temperature stability of the dielectric constant as well as the polarization and breakdown strength can be improved via the“sandwich structure”design and regulation.The Wrec andηreached up to 6.78 J cm-3 and 90%,respectively,for prepared“sandwich structure”ceramics at the electric field of 572 k V cm-1.Meanwhile,the energy storage performance also possesses excellent stability.On the basis of aforesaid studies,the Bi0.5Na0.5Ti O3-based lead-free ceramics with multilayer structure were further designed and prepared.Herein,the dielectrics with the component of 0.80Bi0.5Na0.5Ti O3-0.20Sr(Nb0.5Al0.5)O3 and 0.75Bi0.52Na0.48Ti O3-0.25Sr Ti O3 were selected to provide high breakdown strength and high polarization,respectively.The electric field distribution of the materials can be redistributed according to the multilayer structure design,resulting in slowing down the development speed of the electrical tree and improving the breakdown strength while keeping in high polarization.At the electric field of 300 k V cm–1,the energy storage performance of the multilayered ceramics is superior to the0.75Bi0.52Na0.48Ti O3-0.25Sr Ti O3 and 0.80Bi0.5Na0.5Ti O3-0.20Sr(Nb0.5Al0.5)O3ceramics.Combined with the results of finite element simulation and experimental can be found that the alternating distribution of 0.80Bi0.5Na0.5Ti O3-0.20Sr(Nb0.5Al0.5)O3 and 0.75Bi0.52Na0.48Ti O3-0.25Sr Ti O3 dielectric layers is beneficial to the improvement of breakdown strength.Meanwhile,a recoverable energy storage density of about 7.00 J cm-3 and an energy storage efficiency of 95%can be obtained at an electric field of 565 k V cm-1.At the same time,the energy storage performance exhibited excellent frequency stability,temperature stability and cycling stability.Moreover,a power density of 405.50 MW cm-3 and a current density of 1763.06 A cm-2 can be also achieved.Finally,considering the fact that the low breakdown strength,large leakage current density and poor stability of the Bi Fe O3-based dielectrics,the binary solid solution ceramics with the simple component of Bi Fe O3-Sr Ti O3 were selected as the research object to better improve the energy storage performance of the bismuth-based lead-free ceramics.The ceramic samples were fabricated by tape-casting method as well as sintered by one-step and two-step sintering methods.Composition and structural optimization strategy were adopted and the related properties were studied and analyzed in detail.With increasing the content of Sr Ti O3for Bi Fe O3-Sr Ti O3 binary solid solution ceramics,the domain size,leakage current density and remanent polarization tend to decrease.Combining the experimental and finite element simulation results can be found that the two-step sintering method helps to refine the grain size and improve the breakdown strenugth.A high recoverable energy storage density of 8.43 J cm-3 together with high energy storage efficiency around 91%can be achieved at an electric field of about 750 k V cm-1 with the Sr Ti O3content of 0.65 mol.Meanwhile,the energy storage performance exhibited excellent stability within 1～100 Hz and 20～120°C and a power density of 282.19 MW cm-3can be also achieved. |
