Study On The Preparation Of K_1-xNa_xNbO₃ Nanostructure And Its Piezoelectric Properties

The study on phase composition,preparation and properties of K_1-x-x Na_xNbO₃?KNN?,lead-free piezoelectric material,is far less those of Pb(Zr_0.52Ti_0.48)O₃?PZT?,in consideration of the difficulty in synthesizing KNN nano material and the lower piezoelectric properties,KNN nanowires and nanowire arraies with the morphotropic phase boundary?MPB?structure were synthesized in this paper.The growth behavior of nanowires and nanowire arraies was investigated by changing the preparation conditions,and the piezoelectric properties of KNN nanowires were improved.The effects of different surfactants,hydrothermal temperature and alkali concentration on synthesis of the KNN powders via hydrothermal method were studied using Nb₂O₅ as the source.Under the condition of 10M alkali concentration and PEG400 addition,when the hydrothermal temperature is 200?C?180?C and160?C,tetragonal and rhombohedral KNN powders,orthogonal micron cubic with uniform dispersion,K₄Na₄Nb₆O₁₉·H₂O particles were synthesized,respectively.There are only intermediate product of K₄Na₄Nb₆O₁₉·H₂O micron particles when the alkali concentration is 8M.Under the condition of 10M alkali concentration and PAA addition,when the hydrothermal temperature is 200?C?180?C and 160?C,a large number of NaNbO₃ nanoparticles and a small amount of orthogonal KNN particles,long micron rod KNN and NaNbO₃ nanoparticle and a small quantity of K₄Na₄Nb₆O₁₉?9H₂O particles,long micron rod K₄Na₄Nb₆O₁₉·9H₂O were synthesized,respectively.Orthogonal KNN nanowires were synthesized using KNN gel as precursor prepared by sol-gel system with ethylene glycol and metal alkoxides via hydrothermal method under the condition of 190?C,12h,10M?alkali concentration?,the diameter and length of orthogonal KNN nanowires were 150nm and 5?m.The growth process and growth mechanism of orthogonal KNN nanowires were analyzed by SEM and TEM analysis.The growth direction of nanowires was[001]direction,and the growth mechanism was crystal orientation growth.The electric domain changes are observed on the surface of KNN orthogonal nanowires using the piezo response mode of AFM,and the piezoelectric constant of the KNN nanowires is 17.56pm/V.The KNN micron rods coexisted by Rhombohedral-Tetragonal phases were prepared via hydrothermal method under the condition of 220?C,12h and 10M?alkali concentration?.The micron rods were metastable at room temperature?RT?,analyzed by XRD and Fourier transform infrared spectra?FTIR?,the phase of KNN changed into orthogonal which is stability at RT,and maintain the same shape after cooling to room temperature.The ferroelectric paraelectric phase transition occurs at555?C measured by dielectric temperature spectrum.The addition of Sodium dodecyl sulfate?SDS?to the hydrothermal solvent can effectively reduce the content of NN in the product.The orthogonal KNN nanowire arrays were grown on the Nb-doped STO substrate?Nb-STO?with?100?orientation at 190?C,12h and 10M.The growth process and growth mechanism of the orthogonal KNN nano arrays w were studied by changing the hydrothermal time.The piezoelectric constant of the orthogonal KNN nanowires was measured by the PFM is 26.37pm/V,and the piezoelectric constants of the KNN nanowires with the coexistence of R-T two phases is28.93pm/V.The effect of polarization on the domain of the KNN nanowires was investigated.The changes in the domain at different polarization voltages were observed for the first time on the surface of nanowire.Sb is selected as the B bit doping element.The K_1-x-x Na_x Sb_wNb_1-w-w O₃?KNN-Sb?nano arrays with R-T coexisting were grown under the condition of 220?C,12h and10M?alkali concentration?on the Nb-STO substrate.The influence of Sb doping on R-T nano arrays is studied by using the PFM.It is confirmed that Sb doping can greatly improve the piezoelectric properties of KNN nanowires,and the piezoelectric coefficient of KNN-Sb nanowires reaches 250.4pm/V when the initial concentration of Sb is 6mol%.Ca is selected as A site doping element.The R-T K_1-xNa_x Ca_yNbO₃?KNN-Ca?nanowires were grown under hydrothermal conditions of 220?C 12h,and 10M?alkali concentration?without the guidance of Nb-STO.The symmetry of R-T coexisting KNN nanowires under different crystal axes is investigated by the convergent beam electron diffraction technique.It is further proved that the rhombohedral and tetragonal phases coexist in the same KNN nanowire.The piezoelectric constant of Ca doped KNN nanowires is 37.1pm/V by using the PFM.Compared with pure phase KNN nanowires,the piezoelectric properties are increased by about 30%,and the morphology of KNN is changed from granular to one-dimensional nanowire by Ca doping.