Study On Piezoelectric And Ultrasonic Properties Of Defect Regulated Na_0.5Bi_4.5Ti_4-x(Nb_2/3Co_1/3)_xO_y Ceramics With High Cure Temperature

Benefit from its high Curie temperature,good temperature stability,excellent piezoelectric coefficient,high dielectric constant,low dielectric loss,anisotropic electromechanical coupling coefficient,etc.,lead-free bismuth layered piezoelectric ceramics have been broadly used in the various domains,such as medical image,aeronautics and astronautics,medical instruments,defect detection,power substation and so on.In this paper,a typical bismuth layered BNT-based piezoelectric ceramic is selected.The Curie temperature is 650℃,but the cations are easily volatilized under high temperature sintering,and the"pinning effect"will hinder the domain flipping and restrain the improvement of piezoelectric performance.However,the generation of oxygen vacancy is inevitable.B-site accepter doping is designed to combine with oxygen vacancy to construct defective dipoles in this paper.By regulating defects through frequency and temperature,the turnover speed of defective dipoles is promoted and converted into polarization sources,which achieve the effect of increasing piezoelectric coefficient.The ceramic sample with the optimal piezoelectric coefficient is used as the piezoelectric array to study its application in the ultrasonic transducer,each component of the device is simulated and tested to maximize the device bandwidth.The details are as follows:Firstly,Na_0.5Bi_4.5Ti_4-x(Nb_2/3Co_1/3)_xO_y（x=0,0.075,0.09,0.105,0.12）were prepared by solid phase reaction.The microstructure of Na_0.5Bi_4.5Ti_4-x(Nb_2/3Co_1/3)_xO_y showed that it was a typical bismuth layered ceramic.Representing the Ti O₆ octahedral vibration,the v10 mode in the Raman spectral fitting peak shifted significantly,indicating that Nb⁵⁺,Co³⁺were introduced successfully into B-site to replace Ti⁴⁺.The dielectric constant was reduced with the raising of x,which was promoted again when the x is excessive,and the dielectric loss also decreased obviously.The lowest dielectric loss was 0.0014 at the room temperature of 10 KHz,which also reflected the characteristics of hard doping.The anomaly of the overall right shift of the hysteresis loop indicated a significant internal bias field.The signal near 3000 G was found by EPR spectrum test,and the calculated g factor was 2.21,which proved the existence of defects(₄^?)-(1^··)^·.The charged defect dipole formed an internal electric field.Combined with the composition of the defective dipole which was related to the oxygen vacancy,when the external conditions of low frequency and high temperature were adjusted respectively,the"pinning effect"of the static model on the piezoelectric performance was removed.By comparing the electric hysteresis loop of different frequency（0.1 Hz,1 Hz,10 Hz and 100 Hz and 1000 Hz）,found that with the decrease of the frequency,increased the flip time for defective dipoles,P_rreached its maximum at x=0.105（P_r=44?C/cm²）.With the increase of temperature（20℃to200℃）,provide defect dipole deflection with energy,increase the maximum polarization of a ferroelectric,when x was 0.105,P_r was 39?C/cm².The increasing temperature while applying the same electric field promotes the deflection of the defective dipole,increasing the polarization source,and finally achieving excellent piezoelectric properties of 27 p C/N at150℃.Secondly,the component with the largest piezoelectric coefficient（x=0.105）was used as the core element of the ultrasonic transducer.Piezo CAD simulation software was used to simulate the pulse echo curve,and the influence of the backing and matching layer on the impedance was compared.It was found that the existence of the two acts as elastic damping,which can filter the excess resonant frequency and obtain the pure impedance peak.Based on the formula of ceramic thickness and frequency as well as the difficulty of preparation,a 10 MHz ultrasonic transducer was finally prepared to test the pulse echo in the water environment.Thanks to the good focusing effect of the transducer,the measured pulse echo achieved 46.69%bandwidth,which was higher than the simulation bandwidth 44.88%.The ultrasonic transducer with two components（x=0.105）and（x=0）as piezoelectric array elements was compared.The pulse echo curves of both were acquired through FFT to obtain the frequency domain curve and were calculated the bandwidth.The results showed that the bandwidth of pure BNT with low piezoelectric coefficient was only 32.56%,which was lower than that with high piezoelectric coefficient 46.69%.The increase of bandwidth expands the working frequency of ultrasonic transducers and makes them more widely used.The high Curie point bismuth layered piezoelectric ceramic BNT has a bright application prospect in ultrasonic transducers.