Preparation And Properties Of Rare Earth Doped KNN-based Multifunctional Transparent Ceramic

Multi-functional transparent materials have received widespread attention from a large number of researchers because of their novel properties.Potassium sodium niobate ceramic(K_0.5Na_0.5Nb O₃,abbreviated as KNN)is a classic lead-free ferroelectric material with good electrical properties and high Curie temperature,which shows significant potential to be a multifunctional material.First,KNN is a natural photochromic material due to its large number of vacancy defects((1^′/(1^′_（6）formed by K/Na volatilization during high temperature sintering.Secondly,KNN is a good luminescent substrate due to its low phonon energy,and it can become transparent luminescent material when doped with rare earth（RE）ions.In addition,its piezoelectric properties can be enhanced through phase boundary engineering strategies by doping transition metals or rare earth elements.The coupling between the different performances and the transparency greatly enhances its application prospects in the field of advanced optics.For example,the coupling between transparency and luminescence can be used in scintillation detectors and high-power LEDs,whilethe coupling of transparency and photochromism can be applied in the novel 3D optical information storage,and the transparent and piezoelectric coupling can be applied in medical photoacoustic imaging devices.However,the preparation of high-performance multifunctional KNN-based ceramics remains a huge challenge up to now.It is difficult to prepare KNN-based transparent ceramics with both high transparency（T>65%）and high optical modulation ratio（ΔR_T>20%,ΔR_L>70%）,which limits their applications in optical information storage and optical switches.In addition,considering the coexistence of high piezoelectric properties and high transparency,the piezoelectric constant is still smaller than 200 p C/N)in highly transparent KNN ceramics（T>65%）,which is much lower than that of lead-based transparent piezoelectric ceramics(d₃₃～1400 p C/N).In this article,KNN-x Pr transparent photochromic ceramics with high transmittance and optical modulation ratio were prepared first,which expands the application of KNN ceramics in the field of 3D transparent optical information storage.Then,KNN-x Nd transparent photochromic ceramics with strong luminescence in the near-infrared region（NIR）were prepared,which can be applied in optical information storage or optical switches in the near-infrared region.In addition,0.96KNNTS_x-0.04BNKZ transparent piezoelectric ceramics have also been prepared.The ceramic has a certain transparency and a piezoelectric constant of up to 240 p C/N.The various properties and microstructure of KNN-based multifunctional transparent ceramics were characterized,the relationship between their microstructure and macroscopic properties was analyzed.The main conclusions are as follows:1.Rare earth Pr can significantly improve the transmittance of KNN ceramics by reducing grain size and improving phase symmetry.The transmittance of KNN-3Pr ceramics can reach72.9%at 900 nm,which is very close to its theoretical transmittance of 74%.Pr doping will introduce more and deeper defect levels into KNN and enhance photochromism,causingΔR_Tto decrease andΔR_L to increase with the increase of Pr doping concentration.TheΔR_T of KNN-2Pr transparent ceramics is up to 29%,and theΔR_L of KNN-3Pr transparent ceramics is up to 81%,which both are the highest values among KNN-based transparent ceramics with transmittance over 50%so far.KNN-x Pr ceramics have stable reversibility when writing and erasing optical information（more than 10 cycles）,as well as a long information storage life（more than one week）.The KNN-x Pr ceramics with high transmittance and optical modulation ratio show potentials to be used in the fields of 3D optical information storage,optical switches,and"smart windows."2.Nd doping makes KNN ceramics have strong near-infrared photoluminescence and excellent transmittance.The transmittance of KNN-3Nd ceramics can reach 68.9%（@2500nm）,which is mainly due to the very small average grain size（～118 nm）and the high symmetry of the crystal structure（pseudocubic phase）.Nd doping can also significantly enhance the photochromism of KNN and effectively regulate its near-infrared fluorescence intensity.KNN-x Nd transparent luminescent ceramics have great application prospects in near-infrared laser,optical switch,optical information memory,and other fields.3.The 0.96KNNTS_x-0.04BNKZ ceramic is on the rhombohedral-orthogonal-tetragonal（R-O-T）phase boundary when x=0.04,containing domains with size of only～20 nm and exhibiting the highest piezoelectric constant(d₃₃=240 p C/N)as well as a certain transparency（T=31.5%@900 nm）.Although its piezoelectric constant is higher than the highest value（～185 p C/N）reported in transparent piezoelectric KNN ceramics up to now,its transmittance still needs to be improved.This ceramic provides a research basis and idea for the preparation of a new type of lead-free transparent piezoelectric ceramic that can replace lead-based transparent piezoelectric materials.