| Because of their excellent piezoelectric properties,lead-oxide-based piezoelectric materials have been widely used in many applications,such as sensors,actuators,and energy harvesting devices.However,with the development of the times,lead element becomes more and more unacceptable due to its irreparable damage to the human body and the environment.In some European countries,the use of lead has already been limited in electronic devices.Considerable efforts dedicated to develop high-performance lead-free piezoelectrics have been made in the past decade.On the other hand,with the rapid development of the automotive industry and aerospace industry,the high-temperature stability of piezoelectric materials has also got increasing attention.However,the piezoelectric properties of lead-free piezoelectric ceramics are still inferior to those of lead-oxide-based materials.Furthermore,it has been observed that the increase of piezoelectric properties through the composition modification of these materials normally leads to a lower temperature stability of the properties and hence a reduction of operating temperature.One approach that can get over the inverse correlation between the electromechanical properties and the operating temperature of the piezoelectric materials is to design piezoelectric metamaterials based on the flexoelectric effect.At the present stage,it has been found that the flexoelectric coefficient of ferroelectric materials are 3 to 8 orders of magnitude larger than theoretically predicted.This experimental discovery stimulates a mass of researches on flexoelectricity in the following years,which focused on searching larger flexoelectric coefficient materials,exploring the mechanism of the measured enhancement of measured flexoelectric effect,and the practical application.Furthermore,all the research studies in these fields have made significant progress.However,so far,there are still some questions in the flexoelectric effect phenomenon,and further research are demanded.The first problem is that the reasons for the large discrepancy between the experimental and theoretical flexoelectric coefficients of ferroelectric materials are not fully understood.Secondly,the maximum flexoelectric coefficient reported in the literature is about 1.0 mC/m which is still difficult to meet the application requirements,so promoted methods to boost flexoelectric effect in materials are needed.Finally,in the application of the flexoelectric effect,more flexoelectric piezoelectric composite materials with simple structure and excellent piezoelectric response are required to realize the widely application based on flexoelectric effect.In this research,by measuring the flexoelectric coefficients of(1-x)Na0.5Bi0.5TiO3-xBaTiO3(NBBT)system lead-free ferroelectric ceramics,we analyzed the mechanism of the large discrepancy between the experimental and theoretical flexoelectric coefficients,explored the methods to improve flexoelectric effect in ferroelectric ceramics,and fabricated a flexoelectric piezoelectric composite with strong apparent piezoelectric response and high temperature stability.There are five chapters in this research,the main body of each chapter as follows.In Chapter 1,the flexoelectric effect in solid materials is introduced in detail,including the definition of the flexoelectric effect,the measurement methods of the flexoelectric coefficient,the magnitude of the flexoelectric coefficient of ferroelectric ceramics,the mechanism of the enhanced flexoelectricity in ferroelectric ceramics,and the application of the flexoelectric effect.In Chapter 2,we measured the flexoelectric coefficient of the NBBT-based lead-free ferroelectric ceramics.By analyzing the relationships between the flexoelectric coefficient and the dielectric constants,piezoelectric properties,mechanical properties,phase composition,structure composition and lattice parameters of ceramics,the influence of internal contribution on the flexoelectric effect was discussed.The effect of external contributions such as built-in electric field,defects or polar microdomains and surface effects on the flexoelectric effect was analyzed.Chapter 3 mainly describes that the flexoelectric properties can be greatly improved by applying an asymmetric chemical reduction to NBBT-based ferroelectrics.We characterized the phase structure and electrical properties of the reduced NBBT-based ceramics.The experimental results show that the flexoelectric coefficient of the reduced NBBT ceramics can be increased by 14 to 190 times,and it can be further improved by optimizing the reduction conditions.A maximum flexoelectric coefficient of about 1.1 mC/m was discovered in reduced NBBT8 ceramic.Furthermore,a new type of lead-free piezoelectric metamaterial can be fabricated by using the reduced NBBT ceramics,the metamaterials exhibit an apparent piezoelectric response up to 3500 pC/N and an inverse piezoelectric response of 4×104 pm/V.Besides,the apparent piezoelectric response of the lead-free piezoelectric metamaterials can be sustained above Curie temperature.In Chapter 4 and 5,we applied the asymmetric chemical reduction method to PZT-based ferroelectric ceramics,and we showed that a lead-free RAINBOW devices can be fabricated using the reduced NBBT-based ceramics.In the 4th chapter,we applied the chemical reduction method to PZT-based ceramics and fabricated a PZT-based piezoelectric metamaterials.The PZT-based piezoelectric metamaterial exhibits a strong piezoelectric response up to 2900pC/N,which is much higher than the corresponding lead-based single crystals materials.Furthermore,the piezoelectric response remains above the Curie temperature.In the 5th chapter,we characterized the field-induced displacement of the NBBT8-based lead-free RAINBOW devices.A displacement of approximately 17 ?m under an electric field of 900 V/mm and high piezoelectric sensitivity(>4000 pC/N)under a stress,which are related to the reduction induced curvature,can be measured in the devices.We proposed that apart from the piezoelectric properties,flexoelectric effect could also be a contributing mechanism for the observed apparent piezoelectric response in RAINBOW devices. |
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