Microstructure Regulation And Energy Storage Properties Of KNN-based Lead-free Ceramics Prepared By High-energy Ball Milling

Ferroelectric functional ceramics is a critical material in producing basic electronic components.Researching novel ferroelectric functional ceramics with high energy storage performance is taken to be of great strategic significance in the development of the information industry,especially under the current circumstance of the international increasing intense competition in the information field.As one of the most promising candidates to substitute Pb-based materials,Potassium sodium niobate((Ko.sNao.s)NbO3,KNN)based ceramics is always one of the frontier and spotlight fields in the last ten years.However,the problems of its low energy storage density and energy storage efficiency have severely limited its further development and application.This work focuses on increasing the energy storage performance of KNN-based lead-free ceramics.At first,by optimizing the process and dopant engineering,the study was to develop a series of KNN ceramics with sub-micrometer or nano-meter scale grain,high dielectric breakdown field strength,low remanent polarization,and high energy storage performance.Then,the effects of ion substitution on the grain size,microstructure,local micro organization,phase structure,and energy storage properties were systematically studied.The aim of the work was to establish the correlation between grain size and energy storage performance,to reveal the key factors which affect the energy storage performance of KNN ceramics and to achieve the optimal KNN-based system for energy storage performance.At last,the high-energy ball milling technology combined with a two-step sintering process that was applied to optimize the KNN-based system,to achieve KNN ceramics with ultrahigh-energy storage density and energy storage efficiency.The main conclusions obtained from experimental results and analyses are as follows:(1)The optimum process of high-energy ball milling was studied through a systematic study of the key factors that affect grain size and electrical performance of KNN ceramics,such as particle size of initial powder,milling time and milling speed.It is a fast and high-quality preparation technology for KNN ceramics and the prepared KNN ceramics have high density,fine grain,and high dielectric breakdown field strength.The results show that the initial particle size has a great influence on the grain size of pure KNN ceramics,and the smaller size of the initial particle size results in a dramatic drop in grain size;in the high-energy ball milling process,the addition of some additives decreases the problems of agglomeration and hardening of powders and improves the sintering property and densification;the milling time is more important than milling speed for obtaining pure perovskite phase structure at a same given condition.In the end,the optimal high-energy ball milling parameters are as following:PVP-K30 is 0.025g/L,the ratio of ball to powder is 8:1,the ball volume ratio is 1:10,the milling time is 100 min,the rotational speed is 600 rpm and the sintering temperature is 1140?.The prepared KNN ceramics under the optimal preparation process shows that it has a typical perovskite phase structure and fine grain(0.21 ?m),the Curie temperature(TC)is 371?,the dielectric constant(?r)is 3937,the maximum polarization intensity(Pmax)is 62 ?C/cm2,the remanent polarization intensity(Pr)is 43?C/cm2,the coercive field(EC)is 29 kV/cm,and piezoelectric constant(d33)is 78 pC/N.Compared with the traditional KNN ceramics using solid-state reaction method,the grain size of the novel KNN ceramic decreased more than 86%,and the relative density(RD)is as high as 97%,the dielectric breakdown field strength(EDBS)reaches 110 kV/cm,the total energy storage density(Wtol)is 3.79 J/cm3,and the recoverable energy storage density(Wrec)is 0.22 J/cm3.To sum up,using high-energy ball milling technology is a fast and successful method to prepare the submicron KNN ceramics and to decrease the grain size.It also achieves better dielectric,piezoelectric and ferroelectric properties.However,its energy storage density still needs to be further enhancedd for meeting the application requirements of energy storage capacitors.(2)We introduced Sr2+/2n2+ ion into A/B sites to form submicron relaxor ferroelectric(1-x)KNN-xSZN ceramics with decreased Pr and improved EDBS.This KNN had high energy storage density and excellent power density.Meanwhile,it reveals the physical mechanism of SZN doping KNN ceramics,which is of increased EDBS and decreased grain size decreasing.The results show that the average grain size of ceramics decreased from 1.39 ?m to 0.18 ?m by regulating the content of SZN(x=0.02-0.15).It achieves the goal of reducing the grain size of ceramics via tune various doping components to obtain the submicron grain size,which has greatly contributed to the EDBS.Meanwhile,the dielectric temperature spectra and TEM showed the frequency dispersion and dispersion phase transition in(1-x)KNN-xSZN ceramics.The PNRs also can be observed via TEM,which confirms dielectric relaxation response in x?0.06 samples.The relaxor behavior and submicron grain size are beneficial to improve energy storage density.In 0.92KNN-0.08SZN ceramic,it showed a submicron grain size(0.19 ?m)and a dense microstructure(relative density about 97.6%),contributed to improving breakdown strength.A high Wtol of 6.8 J/cm3 and Wrec of 2.75 J/cm3 is also achieved under 220 kV/cm.Furthermore,the pulse discharge properties of 0.92KNN-0.08SZN were studied,both a huge power density of 208.7 MW/cm3 and the current density of 2197 A/cm2 was obtained.Also,the discharge rate is up to 38 ns,and the temperature is steady over a wide range of 25-140?.Therefore,introducing Sr2+/Zn2+ion into A/B sites to form submicron relaxor ferroelectric(1-x)KNN-xSZN ceramics are able to improve the submicron grain size and to receive a higher energy storage capacity density.(3)We introduced Ba2+/Zn2+ion into A/B sites to form nonomicron relaxor ferroelectric(1-x)KNN-xBZN ceramics with ultrahigh Wrec and excellent PD.Meanwhile,it reveals the physical mechanism and the key factors of BZN doping KNN ceramics.The results show the average grain size of ceramics decreased from 1620 nm to 69 nm by regulating the content of BZN(x=0-0.15),meaning nanocrystalline ceramics were successfully obtained,which have a great influence on the phase structure,domain structure and electrical properties of ceramics,exhibits a Pseudo-cubic phase and good dielectric temperature stability due to relaxation behavior,resulting a great polarization difference,so as to obtain a huge energy storage density and power density.In 0.95 KNN-0.05 BZN ceramic,it showed nano scale grain size about 69.3 nm,EDBS up to 220 kV/cm,t0.9 fast to 36 ns and Wrec up to 4.87 J/cm3.It also shows a good temperature stability,excellent current density(CD=2208.0 A/cm2)and power density(PD=331.2 MW/cm3).In addition,the ceramic is an excellent fatigue material which can withstand 105 loading electric field cycles.However,it also needs further to enhance its energy efficiency.(4)SZN/BZN doping KNN ceramics were investigated in terms of their mechanism of obtaining huge energy storage density.The results shows that the mainly depends on three aspects:? Contribution of sub micron or nanocrystalline grains The doping engineering causing the change of grain size and domain,which leads to the decrease of Pr and the increase of EDBS,thus increasing the energy storage density.?Contribution of ferroelectric's relaxation response The crystal structure of KNN ceramics change from prthogonal symmetry(O)to pseudo-pubic symmetry(Pc)by introducing SZN and BZN.The heterovalent ions occupied the A/B site,which leads to the distribution of charge is not uniform,and disturb the order of long-range ferroelectricity,causing the formation of ferroelectric island or PNRs in the cubic(pseudo-pubic)lattice matrix.The dielectric temperature spectrum,diffusion factor and empirical Vogel-Fulcher(VF)formula were analyzed,which comfired the relaxation of x=0.07 and 0.09 ceramics.The lower activation energy(Ea=0.06-0.12 eV)indicates that is polaron relaxation due to the jumping movement of captured electrons and/or holes.? Contribution of the resistance A higher resistance with the decreasing electrical conductivity shows that the resistance can block the electron transfer,which is conducive to the EDBS and W.Moreover,the DC/AC loading electric field shows that the material can get higher breakdown field strength under DC loading.(5)The 0.95KNN-0.05BZN ceramics prepared by a new process of high-energy ball milling technology combined with two-step sintering were investigated in terms of their energy storage density,energy efficiency and power density,which solved the problems of low energy efficiency and narrow temperature zone of sintering,and achieved a comprehensive energy storage ceramic.The results show that,compared with pure KNN ceramics,the main effect for grain size of the 0.95KNN-0.05BZN ceramic is attributed to the composition and sintering process,rather than its initial particle size.In addition,the P-E loop become "thinner" with an ultrahigh EDBS(415 kV/cm),Wtol up to 7.48 J/cm3,Wrec up to 6.05 J/cm3,as well as the ? up to 81%.Thus,the 0.95KNN-0.05BZN ceramics achieves the design goal.Moreover,it also shows an excellent charging-discharge performance(t0.9=36.8 ns,CD=1821.7 A/cm2,PD=227.7 MW/cm3),fatigue performance(105 cycles)as well as frequency stability.Furthermore,the ceramics with excellent relative density is obtained by two-step sintering over a wide sintering temperature range.Therefore,the 0.95KNN-0.05BZN ceramic is a comprehensive energy storage ceramic prepared via the new process,which not only provides an alternative material for APPCs,but also provides an efficient method for the preparation of other energy storage ceramics.