Study On Acoustic Impedance Characteristics Of Relaxation Ferroelectric Single Crystal Transducer During Acoustic Wave Transmission

With the development of the times,the total investment in global medical ultrasonic diagnostic equipment has shown a linear growth trend from 2013 to 2019.With the new crown virus epidemic approaching in 2020,the development of the medical industry has attracted great attention of the world.Piezoelectric materials and acoustic materials are the core parts of piezoelectric ultrasonic transducers.Piezoelectric ultrasonic transducers based on(100-x)PbZrO3-xPbTiO3(PZT)ceramics have been widely used at present.Compared with PZT ceramics,(100-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3(PMNT)relax or ferroelectric single crystal exhibits better piezoelectric coefficient d33(～2000 pC/N)and electromechanical coupling coefficient(k33>92%).The development of PMNT relaxor ferroelectric single crystal ultrasonic transducer and its matching acoustic materials is expected to greatly improve the sensitivity and bandwidth of ultrasonic transducer.This thesis focuses on the research on the acoustic impedance characteristics during the acoustic wave transmission of the relaxor ferroelectric single crystal transducer.The specific research contents and results are as follows:For the relaxation ferroelectric single crystal with a composition of 72Pb(Mg1/3Nb2/3)O3-28PbTiO3(PMN-28PT),the 3 MHz phased array piezoelectric ultrasound transducer with the thickness vibration mode is studied.The acoustic materials for relaxor ferroelectric single crystal ultrasonic transducer were prepared by epoxy resin and inorganic material powder,such as zirconia powder,alumina powder,silica powder and tungsten powder,etc.The density and sound velocity of samples were measured by Archimedes drainage method and ultrasonic spectrum method,the variation law of acoustic impedance of which was counted and analyzed.The results show that the density and sound velocity of samples are linearly related to the mass fraction of inorganic filler.The acoustic material parameters of ultrasonic transducer with acoustic impedance range of 1.4-10.1 MRayl were obtained by changing the mass ratio of inorganic filler and epoxy resin.In addition,the acoustic materials without internal defects were successfully prepared by the combination of vacuum degassing method and centrifugal degassing method,which laid a foundation for the preparation of the actual transducer.Based on the acoustic material data,PiezoFlex finite element simulation software is used to design piezoelectric single crystal phased array ultrasonic transducer and adjust the parameters of acoustic material,so as to achieve the best performance of ultrasonic transducer.Compared with the traditional single-layer backing ultrasonic transducer,the improved ultrasonic transducer adopts a double-layer backing structure design,the sensitivity and the-6 dB relative bandwidth of which are increased by 6.4 dB and 12.4%.On this basis,through the optimized design model after parameter adjustment,the relative bandwidth can reach 102.2%when the sensitivity change is small.The sensitivity and bandwidth performance of the ultrasonic transducer can be effectively improved through the adjustment of acoustic material parameters and the structural design of ultrasonic transducer,which plays a good role in promoting the development of high-end market of relaxor ferroelectric single crystal transducer in the future.In order to explore the environment-friendly lead-free piezoelectric ultrasonic transducer,the electrical properties and crystal structure of lead-free single crystal(100-x)Na0.5Bi0.5TiO3-xBi(Zn0.5Ti0.5)O3(NBT-xBZT)were systematically studied.With the addition of BZT,the electrical performances of NBT-xBZT crystals exhibit a trend of increasing firstly and then decreasing.Similar to PMNT binary single crystals,NBT-8BZT single crystals are near the morphotropic phase boundary(MPB)and exhibit the best electrical properties.The piezoelectric constant d33 of pure NBT single crystals is only 52 pC/N,while that of NBT-8BZT single crystals is 127 pC/N.In terms of temperature stability,the Curie temperature Tc of PMNT single crystal is about 130℃,while that of NBT-8BZT single crystal is higher than 328℃.This improvement in electrical performance and excellent temperature stability provide the possibility for the development of lead-free piezoelectric ultrasonic transducers and the subsequent study of acoustic impedance characteristics to improve the sensitivity and bandwidth performance of ultrasonic transducers.