Study On Structure-performance Regulation Of Lead-free Potassium Sodium Niobate Piezoelectric Nanowires And Micro-nano Sensors

Sensing technique is one of the three pillars in information technology,which constitutes a vital guarantee of national industrial development together with communication and computer technology.With the development of 5G technology and advent of intelligent era,artificial intelligence and internet of everything have been developing trend and will be main application field of sensor technology in the future.Micro-nano sensors are gradually developing towards miniaturization and high flexibility.Piezoelectric material can realize the mutual conversion of mechanical energy and electric energy,which is a kind of very important functional materials.Microelectromechanical systems（MEMS）based on piezoelectric film have been widely applied in the field of mechanical detection such as pressure sensing,vibration sensing and acceleration sensing.However,the integrated density of the film material is relatively low,and the piezoelectric constant of films is generally limited by the clamping effect of the substrate.The two-dimensional planar structure also restricts the accurate perception of the micro-area pressure distribution and multi-dimensional pressure information,which is difficult to meet the technical requirements of pressure sensors in the fields of artificial touch,micro-nano robot and intelligent wearable devices.One-dimensional nanostructures such as nanowires,nanorods and nanobelts exhibit excellent piezoelectric properties beyond thin film systems.Besides,the bottom-up growth strategy enables the integration of high-density array in three-dimensional structures,so as to realize multidimensional mechanical properties and pressure distribution sensing accurately and sensitively.Lead-based one-dimensional materials such as lead zirconate titanate（PZT）exhibit excellent piezoelectric properties but do harm to the human health and ecological development.Potassium sodium niobate（KNN）is considered to be one of the most promising materials in lead-free piezoelectric systems because of its high Curie temperature,good stability,good biocompatibility.However,the piezoelectric properties of lead-free one-dimensional piezoelectric materials are still far from that of lead-based materials.Therefore,improving the piezoelectric properties of lead-free one-dimensional piezoelectric materials is an important and urgent issue to be solved in this field.In this paper,the large-scale and controllable preparation of KNN-based nanorods（nanowires）was achieved by hydrothermal synthesis technology,and the piezoelectric properties were regulated and optimized by modulating orthorhombic-tetragonal phase boundary.Besides,the pressure sensors with various structures were also assembled.The main research contents are as follows:（1）Large-area and controllable preparation of KNN nanorod arrays and study on mechanical-electrical coupling characteristics.The large-area preparation of orthorhombic KNN nanorod arrays was realized by the inducement of single crystal Sr Ti O₃ substrate and hydrothermal synthesis technology.The preferred and oriented growth of KNN 1D nanostructure origins from the helical dislocation line of the nucleation center under the internal stress of the substrate.The growth behavior not only leads to the production of oriented growth of nanorods,but also result in the formation of other KNN micro-nano structures such as chimney shaped hollow nanotubes and step shaped films.The mechanical properties of KNN nanorods were studied by in-situ SEM,and the flexibility and super elasticity different from bulk materials was confirmed.KNN nanorods could withstand tensile strain of 4.15%and recover immediately to original state after the releasement of applied force,which is an excellent carrier to develop highly sensitive and flexible micro-nano pressure sensor.Besides,the piezoelectric force microscopy（PFM）results show that there are four possible polarization direction in the axial plane of KNN nanorods,and the radial piezoelectric constant d₃₃^* of KNN nanorods is about 40 pm/V.（2）Controllable preparation,phase regulation and performance optimization of Li-doped KNN nanorod arrays.To enhance the piezoelectric performance of KNN nanorods,the controllable doping of Li in lattice of KNN nanorod arrays was realized by hydrothermal synthesis technology.It turns out that with the increasement of Li content,KNN nanorods gradually transform from orthorhombic phase to tetragonal phase,and the orthorhombic-tetragonal phase transition temperature(T_O-T)decreases gradually.When the Li doping content is 0.06,the phase transition temperature is reduced to room temperature,that is,the phase boundary is established at room temperature.The PFM results show that with the increasement of Li content,the axial and radial piezoelectric properties both improved obviously.When the doping amount of Li is 0.06,the average axial piezoelectric properties of the nanorods could reach 554 pm/V,and the maximum piezoelectric properties could be as high as 814 pm/V,which greatly improves the piezoelectric properties of lead-free piezoelectric nanomaterials.（3）Controllable preparation,phase regulation and performance optimization of Ta doped KNN nanorods.KNNT nanorods were prepared at large scale on the etched STO（111）substrate by hydrothermal synthesis technology.With the increasement of Ta content,the lattice structure of KNNT nanorod varied,the radial plane spacing increases,and the axial plane spacing decreases.The KNNT nanorods gradually transferred from orthorhombic phase to tetragonal phase,and the spontaneous polarization and ferroelectric domain distribution also changed.The PFM results show that when the doping amount of Ta is 20%,the radial piezoelectric constant d₃₃^* of KNNT nanorods reaches a peak value of about 120 pm/V,which is comparable to KNLN nanorod and is twice as high as that of undoped samples.Besides,the dielectric constant of KNNT nanorods was also studied by SPM technique,which is measured and calculated to be about 180.（4）Fabrications and practical applications of micro-nano sensors.Sandwich structure,single-electrode structure and two-dimensional pressure imaging device based on KNN 1D nanostructures were assembled,and the sensing properties of these devices were studied systematically.Among them,the sensitivity of the pressure sensor based on KNLN nanorod arrays is about 0.094 V/N cm^-2.The sandwich based on 3D interlaced structure of KNNT nanorods exhibits significantly improved energy conversion efficiency,which could light up 15blue LEDs while pressed by a finger.Besides,the output electrical signal of flexible single-electrode micro-nano sensor exhibit linear relationship with the applied external force,which could realize self-powered detection under various circumstance.The assembled two-dimensional pressure imaging device could help to analyze and determine the force position at any moment with excellent resolution and stability.In addition,the sensor array based on the dot-matrix KNN nanorod array can realize the highly sensitive detection of pressure beyond0.2 N at nano-micro scale.Based on the above sensors,not only the self-powered monitoring of human finger,elbow,running,jumping position,but also two-dimensional pressure distribution sensing of micro-scale pressure signals with high sensitivity and high resolution are realized.