Research On The Preparation Of KNN Based Lead-free Piezoelectric Ceramics And Micro Control Technology Of Micro Displacement Characteristics

The trend in the development of manufacturing technology are high efficiency,precision,flexibility and automation.Micro displacement actuator technology is the key technology of precision machining,which has been the research hotspot in the field of mechanical manufacturing in various countries.At present,lead zirconate titanate piezoelectric(PZT)ceramics,which have lead compositions up to 60%,are still the raw materials for all actuators for precision machining and positioning.Therefore the development of lead-free piezoelectric ceramic micro displacement actuators have become the research hotspot of micro-displacement driving technology.However,the inherent hysteresis nonlinearity of lead-free piezoelectric ceramics is large,and the major method currently used to compensate for it is the mechanical control.In order to improve the accuracy of micro displacement actuator fundamentally,theoretical analysis,numerical calculation,finite element simulation and experimental verification were conducted to study the relationship between phase composition and hysteresis nonlinearity,the effects of grinding force on the control of microstructure of the KNN based ceramics,as well as the influence of grinding residual stress on piezoelectric and mechanical properties of materials.The main contents,results and innovations are listed as follows:(1)The effects of material composition and sintering temperature on phase structure were studied,and the piezoelectric properties of the ceramics were improved by the creation of R-T phase boundary.(1-x)(K_0.48Na_0.52)(Nb_1-ysb_y)O₃-x Bi_0.5(Na_0.82K_0.18)_0.5Zr O₃(KNNS-BNKZ)series piezoelectric ceramics were prepared by conventional solid-state reaction method.The rhombohedral-tetragonal(R-T)polycrystalline phase boundary(PPB)was constructed by the replacement of K⁺and Na⁺ions at A site by Bi³⁺with larger electronegativity and smaller radius,and the replacement of Nb⁵⁺and Sb⁵⁺ions at B site by Zr⁴⁺ions with larger radius.At y=0.04,x=0.0375,with sintering temperature of 1140?,the piezoelectric constant reached the maximum value(d₃₃=198p C/N),which is 32%higher than that of R phase ceramics sintered at 1160?(d₃₃=150p C/N).(2)The effect of phase composition on hysteresis nonlinearity of ceramics was studied.A method to improve the hysteresis nonlinearity of materials by constructing the PPB was proposed.The phase compositions of the samples were analyzed.The P-E loops,S-E curves and displacement data under DC-voltage were tested.The hysteresis models were established with regression analysis.It was found that the ceramics with R-T polycrystalline phase structure had the advantages of easy domain switching,high residual polarization strength(Pr)and low coercive field strength(Ec)due to the lowered anisotropy.Under DC voltage,the ceramic showed large strain,small hysteresis and excellent linearity,and the hysteresis was 31.4%lower than that of the ceramic in R phase.In conclusion,through the creation of R-T PPB the hysteresis nonlinearity is reduced,which will fundamentally improve the accuracy of the actuator.(3)The effect of grinding force on the phase composition of KNN based piezoelectric ceramics was studied,and it was found that the grinding force could control the microstructure and reduce the hysteresis nonlinearity.By controlling the grinding parameters,the plastic grinding of ceramics was achieved,and the changes of phase composition and hysteresis nonlinearity for the ceramics before and after grinding were studied.The results showed that the grinding force could induce the transformation of tetragonal phase(T)into orthorhombic phase(O).And at the same time coexistence of R-T phase changed into R-O-T phase polycrystalline structure.As a result the hysteresis nonlinearity of ceramics was further reduced due to the increase of spontaneous polarization direction.(4)The effect of grinding residual stress on the properties of KNN based ceramics was studied,and a method to improve the piezoelectric and mechanical properties of ceramics by using the residual compressive stress was proposed.The changes of mechanical and piezoelectric properties of ceramics before and after grinding were studied.It was found that the density raised,E_C decreased while Pr increased.The piezoelectric constant d₃₃ increased from 198p C/N to 275p C/N,the maximum strain reached 0.13%,and the linearity could even be improved.The relationship between piezoelectric property and residual compressive stress was deduced based on thermodynamic principle,proving that the residual compressive stress could improve the piezoelectric properties of ceramics theoretically.The effects of grinding compressive stress on fracture toughness and strength of ceramics were analyzed.In conclusion,grinding residual compressive stress can improve the piezoelectric properties,fracture toughness and mechanical strength of KNN based ceramics,which will ultimately improve the reliability and service life of the ceramic actuators(5)The KNN based ceramic laminated actuator were manufactured,and its output displacement and mechanical characteristics were studied.13 pieces of ceramic sheets after grinding to the thickness of 0.5mm were used to make a laminated actuator.The simulations and experimental studies were conducted on the input and output characteristics of the actuators.The simulation results showed that the displacement reached 0.76?m under the maximum DC voltage of 200V.And the maximum displacement was 0.73?m in the experiment.The maximum output force reached720nm when the displacement was 0?m.Finally,the data measured in the experiment was analyzed with regression analysis,and the regression equations were established.The results showed that the actuator had excellent linearity,with the correlation coefficient R² as high as 0.9903.