Synergistic Control Of Transmittance And Electrical Properties Of Potassium Sodium Niobate-based Ferroelectric Ceramics

Transparent ferroelectric ceramics are a type of multifunctional ceramic materials which can couple many physical properties such as electricity,light and mechanical deformation,etc.It not only exhibit high transparency and outstanding electro-optical coefficient,but also own good piezoelectric and ferroelectric properties,thus playing many wonderful functions in the development of modern science and technology.The most representative transparent ferroelectric ceramics are mainly lead-based materials,because lead-containing transparent ferroelectric ceramics can achieve both excellent electrical and optical performance.However,the promotion of green and low-carbon new material technologies and industrialization requirements has made research on lead-free transparent ferroelectric materials imminent.KNN-based lead-free ceramics can be combined with optical and electrical properties through doping modification to become the best substitute for lead-based transparent ferroelectric ceramics,which has focused on scholars' research hot-spots for many years.However,existing studies have shown that KNN-based ferroelectric ceramics with excellent linear transmittance are difficult to balance outstanding electrical properties.Therefore,from the perspective of control grain boundaries,takes the strategy of regulating grain boundary quality as the strategy,this paper adopts the method of introducing glass materials to dilute grain boundaries and growing grains to reduce grain boundaries,so as to reduce the light loss caused by grain boundaries as much as possible.It is expected to improve the optical transmittance of the sample while obtaining relatively excellent electrical properties.The transparency,micro-structure,phase structure,electricity performance and energy storage characteristics of the obtained ceramics were systematically studied,and the excellent transparent ceramics with both light transmittance and electrical properties were coordinated to regulate,providing alternative materials for the application of lead-free transparent ferroelectric ceramics.The main conclusions of this paper are as follows:1.A kind of SrZrO₃,which can refine the grain growth and improve the density so as to improve the transmittance of the sample,was selected as the second component to conduct doping modification of KNN ceramics.(1-x)K_0.5Na_0.5NbO₃-xSrZrO₃ transparent ferroelectric ceramics(x=0.03,0.05,0.07,0.09,0.11;KNN-SZ)with relaxor characteristics was successfully fabricated by a conventional solid-stated method,through exploration of ball milling process,pre-firing conditions,granulation process and sintering conditions.The results show that the grain size and domain size of ceramics gradually decrease with the increase of SZ content,and the crystal structure tends to change from orthogonal to pseudo-cubic phase.Both the optical transmittance and the bulk density of ceramics show a trend of increasing first and then decreasing,and the maximum values were obtained at x=0.09,which were 4.66 g/cm³ and 68%(in the nearinfrared region),respectively.Surprisingly,0.91KNN-0.09SZ ceramics also achieved outstanding energy storage performance.Under the applied electric field of 370 kV/cm,the recoverable energy storage density reached 2.81 J/cm³,and the energy storage efficiency was as high as 80%,which was better than most KNN-based lead-free transparent ceramics.In addition,the ferroelectric properties of 0.91KNN-0.09SZ ceramics have good temperature stability,fast charge and discharge speed and good electrical properties:t0.9=126 ns,d₃₃=95 pC/N,?m=1162.2.In the x=0.07 component with excellent electrical performance and good optical transmittance,the glass material(La2O₃:B2O₃:SiO₂)that can desalinate grain boundary was introduced as the third component.0.93K0.5Na0.5NbO₃-0.07SrZrO₃x%(La2O₃:B2O₃:SiO₂)(x=0,0.05,0.10,0.30,0.50;0.93KNN-0.07SZ-x%LBS)ceramics was designed and prepared.To study the effects of LBS content on transparency,microstructure,phase structure,electrical properties and energy storage characteristics of ceramics.The results show that with the increase of the La2O₃:B2O₃:SiO₂ content,the crystal structure of ceramics changed from orthogonal phase to cubic phase,which greatly improved the dielectric performance of the sample.The dielectric constant of all ceramics at room temperature was around 6000,especially when x=0.05,the maximum dielectric constant was up to nearly 14000.In addition,the introduction of La2O₃:B2O₃:SiO₂ glass material effectively inhibited the growth of grain,and the highest linear transmittance(T?72%,2000 nm)was obtained in 0.93KNN-0.07SZ-0.05%LBS samples with the smallest grain size(?180 nm).3.Through the doping of Li₂O,the grain size is increased,the grain boundary is reduced,and the light scattering from the grain boundary is reduced,so as to further improve the transmittance and electrical performance of 0.93KNN-0.07SZ ceramics.0.93K0.5Na0.5NbO₃-0.07SrZrO₃-xLi₂O(x=0,0.03,0.07,0.10,0.13;0.93KNN-0.07SZxLi₂O)ceramics were designed and prepared through a conventional solid-stated method,and the effects of different Li₂O contents on the phase structure,micro-structure,grain size,transparency,electrical properties and energy storage density of ceramics were studied.The results show that with the increase of Li₂O content,the phase structure of ceramics changes from orthogonal phase to orthogonal tetragonal mixed phase and finally to the tetragonal phase.A second phase occurs when x?0.09,and the grain size gradually increases.When the doping content of Li₂O is 0.03,the ceramics have the highest linear transmittance(T=75%,2000 nm)and good piezoelectric performance(d₃₃=150 pC/N).In addition,the strain value increases first and then decreases with the increase of Li₂O content.The maximum value obtained at x=0.03 is about 0.15%,and it shows excellent temperature stability.