Research On Bi_0.5Na_0.5TiO₃-based Lead-free Ceramics And Piezoelectric Transducers

With the development of technology and social economy,Acoustic links to natural science and engineering technology more tightly.Transducer,as a common and important electronic part in Acoustic field,has received extensive attention,which involves various fields at present,like ultrasonic cleaners,atomizers in our daily life,ultrasonic testing in biomedicine and sonar for underwater communication,and so on.With the increase of production demand,the single vibration mode cannot meet the needs of special occasions,and more and more attention has been paid to the composite vibrations.The longitudinal-torsional and longitudinal-bending composite vibrations are the more common vibration modes.Longitudinal-torsional vibration is often used in ultrasonic motors,for which the inverse piezoelectric effect of ceramics was used to produce vibration,being suitable for large torque and small space occasions.Piezoelectric materials,the core part of the piezoelectric transducer,have been widely studied and rapidly developed.The lead zirconate titanate-based ceramics with excellent piezoelectric properties and good stability have occupied a high position in transducers for a long time.With the limitation of legislation on lead-containing electronic materials in various countries and the increasing awareness of environmental protection,it is a general trend to find lead-free piezoelectric ceramics which can replace lead zirconate titanate-based ceramics with excellent performance.Among the common lead-free piezoelectric ceramics,the sodium bismuth titanate(Na0.5Bi0.5TiO3)piezoelectric ceramics are suitable for the transducers using inverse piezoelectric effect because of its good strain performance and high Curie temperature,which is expected to be one of the lead-free piezoelectric ceramics instead of lead zirconate titanate-based ceramics.In order to highlight the environmental protection and increase the torsional amplitude of transducer,the preparation and performance of lead-free Na0.5Bi0.5TiO3based piezoelectric ceramics were systematically studied in this paper.The influence of doping and sintering temperatures on the structure and electrical properties of ceramics were analyzed in detail,obtaining the optimal doping content and sintering temperature.The longitudinal-torsional piezoelectric transducers that can be used to realize automobile automatic steering were designed using the prepared piezoelectric materials.The longitudinal vibration is changed to a longitudinal-torsional composite vibration by carving helix slots on the hollow tube based on the sandwich transducer.The vibration modes and amplitude of the transducer were analyzed via the finite element method,and then the performance of lead-free transducer and lead-based transducer were also compared.The main research contents and innovations in this work are as follows:1.The lead-free ceramics(1-x)(Bi0.5Na0.42K0.08)0.96Sr0.04Ti0.975Nb0.025O3-xBiAlO3(x=0?0.01?0.02?0.03?0.05)were prepared by traditional solid-state method.All the ceramics are pseudocubic phase,and the relative density are higher than 96.5%.The measurement results show two anomalies can be observed in the dielectric spectra whether the samples are poled or unpoled.There is no anomaly related to depolarization behavior in the studied temperature range(room temperature?350 ?).The dielectric spectra for unpoled and poled ceramics are similar.The doping of BiAlO3 induced the enhancement of the relaxor character of the ceramics,resulting in pinched-shape P-E and symmetrical sprout bipolar S-E hysteresis loops.The four current peaks on the I-E curves gradually weakened with the increase in the BiAlO3 amount.The ceramic with x=0.02 has relatively good ferroelectric and strain properties,which has the strain value of 0.35%.2.The influence of different sintering temperatures(T=1100?,1115?,1130 ?,1145?,1160 ?,1175?)on the ceramics with x=0.02 were investigated.The results show that the sintering temperatures have a significant effect on the density,average grain size,high temperature impedance spectra and conductivity of the samples.The grain sizes increase from 0.73 ?m to 1.33 ?m with the increase of sintering temperatures.The dielectric spectra for all the unpoled and poled samples are similar,with two anomalies in the dielectric spectra of the ceramics.There is no anomaly related to depolarization behavior in the measurement temperature range(room temperature?400?)for poled ceramics,indicating that depolarization behavior occurs at room temperature.All of the samples exhibit pinched-shape P-E and symmetrical sprout bipolar S-E hysteresis loops.The four current peaks were observed on the I-E curves.These characters indicate strong relaxor properties of the samples.The maximum strain Smax(0.33%)and inverse piezoelectric constant d33*(331 pm/V)were obtained from the unipolar S-E loops of the samples sintered at T=1115?.3.The longitudinal-torsional piezoelectric transducer is designed using PZT-8 ceramics.The longitudinal vibration mode was changed to the longitudinal-torsional composite vibration mode by means of the helix slots,which realizes the coupling vibration of longitudinal vibration and torsional vibration at the same frequency.Besides,the thrust bearing structure of the transducer is used to replace the spring of the traditional piezoelectric motor,which provides a reliable theoretical basis for the design and manufacture of high power longitudinal-torsional piezoelectric motor.The resonant frequency,vibration mode and amplitude in X,Y and Z directions of the transducer are obtained by means of the finite element method.In order to further improve the amplitude of the transducer,a certain number of square grooves are set on the front face of the hollow tube.The comparison results show that the amplitudes of the transducer with square grooves in all directions are significantly increased via the finite element method.The longitudinal-torsional piezoelectric transducers with square grooves composed of PZT-8 ceramics and BNT-based ceramics with x=0.02 sintered at 1115? were studied comparatively.Compared with the transducer of PZT-8 ceramics,the amplitude in the Z direction decreases slightly,while in the Y direction greatly increases for the transducer of BNT-based ceramics.The torsional vibration of the transducer of BNT-based ceramics is dramatically improved.