Simulation,Design And Performance Study Of 5MHz Medical Piezoelectric Ultrasonic Transducer With Magnesium Alloy Matching Layer

In medical ultrasound imaging systems,piezoelectric ultrasonic transducers are the core components and the main structure includes piezoelectric material,matching layer and backing.The matching layer can reduce the energy loss caused by the acoustic impedance mismatch between the piezoelectric material and the human tissue,which is critical to the performance of the transducer.The traditional matching layer uses0-3 composite material,but its unstable performance,limited acoustic impedance adjustment range and large sound attenuation limit the bandwidth and sensitivity.The magnesium alloy possesses suitable acoustic impedance which is close to the theoretical acoustic impedance value of the first matching layer in the KLM model and its acoustic attenuation coefficient is small.It is an ideal matching layer material for piezoelectric ultrasonic transducers.In this paper,the magnesium alloy was selected as the first matching layer.Based on simulation study,5 MHz PZT planar and focused medical piezoelectric ultrasonic transducers with excellent performance were designed and prepared.Firstly,Piezo CAD software based on the KLM equivalent circuit model was used to simulate the 5 MHz planar transducer with double matching layers.The magnesium alloy AZ31 B and Epo-tek 301 were used as matching layers,PZT-5h was used as the piezoelectric material and a composite material consisting of tungsten powder,resin and glass ball was used as the backing.Simulation results showed that the thickness of the matching layer had a greater impact on the center frequency of the transducer and the-6 d B bandwidth: When the thickness of the Epo-tek301 decreased from 145.8 μm to 119.3 μm,the bandwidth of the transducer increased first and then decreased.The bandwidth reached the maximum at 124.6 μm and the bandwidth was 76.31%.As the thickness of the magnesium alloy AZ31 B increased from 261 μm to 464 μm,the value of the center frequency gradually decreased and the bandwidth increased first and then decreased.The bandwidth reached the maximum at 406 μm.However,the center frequency was only 4.637 MHz at 406 μm.Considering factors such as center frequency and bandwidth,the optimal thickness of the matching layer was determined,that is,the thickness of the magnesium alloy AZ31 B was 320 μm,and the thickness of the Epo-tek301 was 124.6 μm.A 5 MHz planar ultrasonic transducer with the matching layer of magnesium alloy AZ31 B was prepared.The effective electromechanical coupling coefficient was 0.51,the center frequency was4.5 MHz,the-6 d B bandwidth and bidirectional insertion loss at center frequency were 70% and-14.9 d B,respectively.Then,in order to further improve the overall performance of the piezoelectric transducer,the 5 MHz focused transducer with dual matching layers was designed.The acoustic lens was used to focus the sound waves.Comsol Multiphysics finite element software was used to simulate the sound field of the transducer.The results showed that compared with a planar transducer,the focused transducer possessed a higher sound pressure at the same position,and the-6 d B sound beam width is 0.90 mm which is only 39% of the beam width of the planar transducer,indicating lateral resolution of the focused transducer is higher.Finally,a 5 MHz focused transducer was prepared.The effective electromechanical coupling coefficient was measured at 0.52,the center frequency was 4.4 MHz,and the-6 d B bandwidth and the bidirectional insertion loss at the center frequency were 69% and-12.2 d B,respectively.Compared with the planar transducer,the bandwidth of the focused transducer did not change much and the sensitivity was higher.