Research On High Performance Piezoelectric Composite Ultrasonic Device

Ultrasound imaging is one of the important means to diagnose diseases.It has the advantages of safety,noninvasive and good real-time.It is widely used in clinical fields such as obstetrics,gynecology,cardiovascular medicine and so on.Clinical diagnosis requires ultrasonic imaging to have higher definition and larger imaging range,which requires ultrasonic transducer to have higher spatial resolution and sensitivity.By optimizing the piezoelectric array,the piezoelectric composite ultrasonic transducer can improve the resolution and sensitivity,and has great potential in imaging applications.However,there are many deficiencies in the research of piezoelectric composite transducer: the design is highly dependent on experience and lacks a complete design methods;The lateral mode limits the performance improvement of piezoelectric composites;The research on the optimization of composite structure is not deep enough.Based on the research method combining theory,simulation and calculation,this dissertation systematically studies the material optimization,composite structure optimization and artificial intelligence optimization of piezoelectric composite transducer from material to structure and then to optimization method.The main research contents are as follows:(1)Material optimization.The highly empirical dependence of piezoelectric composite design affects its application and popularization in new piezoelectric materials.In this dissertation,a systematic research method is established,and the bismuth scandate lead titanate(BSPT)1-3 composite transducer is developed.Theoretical calculation combines 1-3 composite thickness mode model and transverse mode model to design the structural parameters of piezoelectric composite.Then the performance of 10 MHz 1-3 composite transducer is simulated by PZFlex simulation software based on finite element analysis.Finally,the composite material was prepared by dice and fill method and the transducer was encapsulated.The prepared BSPT 1-3 composite transducer has a center frequency of 8.93 MHz,a bandwidth of 41.59% and an electromechanical coupling coefficient of 0.608,and its performances are better than those of BSPT transducer(10.57 MHz,24.31% and 0.579).(2)Lateral structure optimization.In conventional design,piezoelectric composites are usually regarded as a whole,and the periodic arrangement of composite structures is convenient for preparation and research.However,the periodic arrangement of the structure introduces the lateral mode,which becomes the main interference mode and limits the improvement of the performance of the piezoelectric composite transducer.Based on the above established research methods,we deeply study the lateral mode of 1-3 composite.Taking the piezoelectric phase as an independent individual,through the wave equation,we establish the theoretical model of piezoelectric composite transverse mode.Based on this model,the composite structure is optimized in the transverse direction,and the non-periodic1-3 composite structure is designed to eliminate the lateral mode.Then,the performances of4 MHz non-periodic 1-3 composite transducer are simulated by finite element simulation.Finally,the non-periodic 1-3 composite transducer is experimentally prepared and characterized.Its center frequency is 3.98 MHz,bandwidth is 38.9%,electromechanical coupling coefficient is 0.68,peak-to-peak voltage is 2.44 V,insertion loss is-17.40 d B,and its performances are better than those of the 1-3 composite transducer used for comparison(4.22 MHz,28.4%,0.55,1.19 V and-22.86 d B).The experimental and simulation results are consistent,which proves that the lateral optimization of the composite structure can eliminate the lateral vibration and improve the performance of the composite transducer.(3)Structural optimization in thickness direction.The optimization of composite structure is not only limited to the lateral direction,but also has great potential in the thickness direction.We propose a new functional gradient composite structure,which realizes the parameter gradient through the structural gradient to improve the sensitivity of the transducer.In this work,a 5 MHz functionally graded composite transducer is developed by using the method of finite element simulation and experiment.Its center frequency is 5.37 MHz,the bandwidth is 22.5%,and the insertion loss is-13.57 d B.Each performance is better than that of the conventional transducer(5.26 MHz,18.3% and-17.72 d B),greatly improving the sensitivity.Then,this design is extended to 1-3 composite,and the multi-layer 1-3 composite theoretical model and transmit receive sensitivity model are established.The performance of multi-layer1-3 composite transducer and conventional composite transducer is simulated and compared by finite element method,and finally verified by experiment.Multilayer 1-3 composite transducer has a center frequency of 5.03 MHz,a bandwidth of 40.6% and a peak-to-peak voltage of 166.18 m V,which proves that it improves the sensitivity while inheriting the advantages of conventional 1-3 composite transducer(large pitch: 4.6 MHz,38.9% and98.74 m V;small pitch: 4.7 MHz,46.6% and 88.2 m V).(4)Artificial intelligence optimization.The structural parameters of the composite transducer in the above work have not been optimized.In addition,the optimal design of the matching layers of the composite transducer are also a difficulty.The traditional optimization design of piezoelectric composite transducer uses finite element simulation software,which has high accuracy,but it takes a long time.In this dissertation,artificial intelligence optimization method is used to improve the optimization efficiency of composite structure parameters and matching layer parameters of composite transducer.The data are obtained through finite element simulation and trained by BP neural network.Optimization criteria are formulated,and particle swarm optimization algorithm is used to obtain the optimization parameters,which are verified by finite element simulation and experiment.The optimized4 MHz 2-2 composite transducer without matching layer has a center frequency of 4.57 MHz,a bandwidth of 41.6%,an peak-to-peak voltage of 2.89 V and an electromechanical coupling coefficient of 0.71.The optimized 9.5 MHz 2-2 composite transducer with two matching layers has a center frequency of 9.8 MHz,a bandwidth of 79.6%,and an echo peak to peak value of 2.11 V.It is used for imaging experiments after spherical pressure focusing.The lateral resolution and axial resolution of tungsten wire imaging characterization are 125 μm and 132 μm,respectively.In the B-mode imaging experiment,the pig eye structure is clearly visible,which proves that the intelligently optimized 2-2 composite transducer has good imaging quality.