Study On Controllable Synthesis,Piezoelectric Properties And Energy Harvesting Of Lead-free （K,Na）NbO₃ Nanorod Arrays

Low dimensional ferro-piezoelectric nano-materials are widely used in non-volatile ferroelectric storage,micro electro mechanical system（MEMS）,energy harvesting device,advanced sensors and photocatalysis areas.With the development of modern society,sustainable and environmental friendly have become the key development themes,the potential applications for low dimensional ferro-piezoelectric nano-materials in energy harvesting attract extensive attention.And one dimensional ferro-piezoelectric nano-materials（1D）（nanorod,nanotube et al.）is the minimum dimension structure which can realize energy conversion,1D has important application value in energy harvesting area for ultra high specific surface area.Howerver,most common used piezoelectric materials are lead oxide based,such as Pb（Zr,Ti）O3（PZT）,in which the content of Pb O（or Pb₃O₄）is about 60%.The highly toxic lead composition is harmful to human bodies and ecological environment during the manufacturing,using and discarding process,which limits their application in multiple fields such as biological and medical treatment fields.Thus,researches and developments for lead-free piezoelectric materials have significant social influences and economic values.（K,Na）NbO₃（KNN）has attracted great attention for its high piezoelectric constant,high Curie temperature and adjustable phase structure.Nevertheless,researches about 1D KNN nano-structure are still faced with certain challenges,it is difficult to realize oriented growth and its piezoelectric properties remain to be enhanced.Focusing on these problems,in this paper,Sr TiO₃（STO）assistant hydrothermal process was applied to realize oriented growth of high quality KNN nanorod arrays（KNN NRAs）,then the piezoelectric properties was enhaced and finally assembled high output nanogenerators.The main research results can be summarized as follows:1.KNN NRAs were synthesized on 100-STO through hydrothermal process.Influences of hydrothermal temperature,reaction time and alkalinity on morphology and phase structure of synthesized KNN were carefully investigated.The results indicated that a buffer layer of KNN film would be preferential grown before oriented growth of KNN NRAs,and lattice parameters of the film were between KNN nanorod（KNN NR）and STO substrate.As the increase of hydrothermal temperature,reaction time and alkalinity,anisotropy growth of KNN was gradually transformed to isotropy growth,leading to length to diameter ratio of KNN NR gone up firstly and then decreased.Well crystalized[110]oriented KNN NRAs were obtained with high length to diameter ratio were obtained when hydrothermal temperature,reaction time and alkalinity were 190 ^oC,24 h and 8 M,respectively.2.Piezo-response researches along radial and axial direction were performed and both results present representative butter-fly piezo-response,the piezo-electric constant along axial direction（[110]crystal orientation）is about 140 pm/V,which was 2.5 times of that along radial direction（[001]crystal orientation）（⁴0 pm/V）.First-principles calculation results showed that Nb atom in KNN lattice would move along[110]direction,so the spontaneous polarization of orthorhombic KNN was along[110]crystal orientation,the intensity of polarization was about38.857?C/cm² along[110]direction,which was much higher than that along other direction,resulting in high piezoelectric constant along axial direction of KNN NR.3.High temperature annealing process in oxygen was applied to modify the phase structure,defects and piezoelectric properties of KNN NR,the results showed that piezoelectric properties could be greatly enhanced through high temperature annealing process in oxygen,the piezoelectric constant was increased from 140 pm/V to 360 pm/V after annealed at 800 ^oC for12 h.Firstly,K element would be volatilized in annealing process,and K/Na ratio of KNN NR was decreased from 1.7:1 to 1:1 as the temperature increased,leading to phase structure of KNN transformed from orthorhombic to tetragonal.KNN was located at orthorhombic–tetragonal（O–T）phase boundary region after annealed at 800 ^oC.Thus,O–T phase boundary was built through annealing process.Secondly,surface oxygen vacancies could be passivated during high temperature annealing process in oxygen,which improved the electron transportation and dielectric properties.Hence,great improvement of piezoelectric properties was the combined contribution of O–T phase boundary and oxygen vacancies passivation after annealed in oxygen at high temperature.4.Vertically integrated nano-genrators（VING）and laterally integtated nano-generators（LING）were respectively packaged based on KNN NRAa and parallel-arranged nanorods to investigate the piezoelectric power generation performance.According to these results,output voltage of VINGs was more than 10 V and maximum output power was 2.25 nW with load resistance.VINGs,with simple structure,high output,strong ability and low flexibility,were suitable in acceleration sensors areas.Output voltage of LINGs were also more than 10 V and maximum output power was 0.36 nW with load resistance.LINGs,with high output and excellent flexibility,were appropriate for flexible sensors fields.