Piezoelectric Properties And Thermal Stability In AZrO₃ Modified KNN-based Piezoelectric Ceramics

High-performance piezoelectric ceramics are widely used in sensors,actuators and transducers,due to piezoelectric effect and electromechanical coupling ability.Potassium sodium niobate?KNN?lead-free piezoelectric ceramics are the most extensively investigated piezoceramic systems on the account of excellent piezoelectric properties.The piezoelectric properties of KNN ceramics can be greatly improved by the modification of Li⁺,Sb⁵⁺,Ta⁵⁺or the second component in the past decades.Limited by its inherent phase transition characteristics,however,the electrical properties fluctuate around the phase boundary,which result that the poor of temperature stability of electrical properties.Moreover,the piezoelectric properties still have a large gap with lead zirconate titanate?PZT?,which cannot far too meet the practical application.In this thesis,KNN-xA?Ca,Sr,Ba?ZrO₃ lead-free piezoelectric ceramics were prepared by solid-state reaction method.The piezoelectric properties and temperature stability of KNN ceramics were improved by adjusting the phase transition behavior.Specific works and conclusions in this thesis are as follows:In KNN-xCaZrO₃ ceramics,the addition of an appropriate amount of CaZrO₃ leads to an increase in density.The TO-T and TC of the KNN-based ceramics move toward lower temperature with the increase of x.At the compositional range of x=0.02⁰.04,the O-T coexistance polymorphic phase boundary?PPB?with O-T coexistence near room temperature of KNN-based ceramics is formed,resulting in easier domain wall motion and domain inversion under external electric field.and the piezoelectric property is improved,the best piezoelectric performance was achieved at x=0.03,showing d33=252pC/N,kp=0.55.However,when x?0.05,the KNN-based ceramics transition from the O-T coexistence to the pseudo-cubic phase,and the piezoelectric properties also decrease.In KNN-xSrZrO₃ ceramics,the introduction of SrZrO₃ moves T_O-T and T_C toward lower temperature.In the range of components of x=0.04⁰.06,there is almost no change in the O-T phase ratio near room temperature.The degree of relaxation increases,which leads to an increase in the degree of dispersion of the O-T phase transition process,considering to be the main reason for obtaining stable two-phase ratios in a wide range of compositions,and stable piezoelectric properties were achieved at x=0.04⁰.06,showing d₃₃=250²56pC/N,k_p=0.42⁰.46.In KNN-xBaZrO₃ ceramics,in the range of components x=0.04⁰.06,the T_O-T-T peaks move towards lower temperature,while the falling speed becomes slower or even unchanged,indicating that BZ has a certain range for the adjustment of O-T phase ratio,accompany with increase of the relaxation degree.The coexistence of O-T at room temperature results in easier domain wall motion and domain inversion under an external electric field,and piezoelectric performance is enhanced at the composition of x=0.05?BZ5?,showing d33=295pC/N,kp=0.46.Furthermore,the electric-field induced strain up to 0.20%are available as well as d^*₃₃ up to 605pm/V,which exceeds the current works of the mainstream KNN-based ceramics,showing great application prospects in the field of actuators and sensors.KNN-0.05AZ?A=Ca,Sr,Ba,abbreviated to CZ5,SZ5 and BZ5?ceramics were selected to research the temperature stability of phase structure,dielectric,ferroelectric and strain properties.As the ionic radius of the IIA elements increases,the temperature stability of the electrostrictive strain of CZ5,SZ5 and BZ5 ceramics increases gradually.KNN-0.05AZ with relaxor ferroelectric characteristcis will lead to O-T phase dispersion,which makes a stable state of the polar nano-regions?PNRs?in a wide temperature range.At the electric field of 2kV/mm and in a temperature range of30¹20?,compared with CZ5 ceramics posseses d^*₃₃ between 286³84pm/V,SZ5ceramics posseses d^*₃₃ between 359⁴38pm/V,BZ5 ceramics posseses with great electrical strain and excellent temperature stability of d^*₃₃ between 359⁴38pm/V,is expected to be achieved in applications in the field of actuators and sensors.