High Piezoelectric Response Of Relaxor Ferroelectric Single Crystals And Its Applications In Medical Ultrasonic Transducers

Piezoelectric materials transform electrical energy into mechanical energy and vice versa,acting as the core material for plenty of piezoelectric applications such as transducers and sensors.The design and preparation of piezoelectric materials with high piezoelectric and electromechanical response has always been the goal of the material science community.As an outstanding representative of Relaxor ferroelectric single crystal,PMNT single crystal has been recognized for its high piezoelectric response,high electromechanical response(d₃₃>2000 p C/N,k₃₃>90%)and excellent performance in sensors and transducers.Recently,a new domain-engineering method,AC polarization(ACP),has changed the traditional DC electric field(DCP)polarization method for ferroelectric materials.This convenient and fast way to improve the piezoelectric response of Pb based Relaxor ferroelectric single crystals has attracted extensive attention from scholars in China and abroad.However,the origin of the high piezoelectric response of Relaxor ferroelectric single crystals under ACP deserves further investigation.Therefore,based on this hot spot,this paper focuses on the research of high piezoelectric response of Relaxor ferroelectric single crystal and its application in medical ultrasonic transducer,which are mainly as follows:1)From the perspective of crystal growth process and crystal utilization,AC electric field was used to poling[001]_C oriented PMN-0.25PT single crystals.Compared with DCP sample,the piezoelectric coefficient is increased by 40%from 1220 p C/N to 1730p C/N,and the effective volume ratio of PMN-x PT single crystals for ultrasonic transducers increased to?80%.The optimal ACP conditions for the PMN-0.25PT single crystal in the thickness mode were determined:field strength of 12?18 k V/cm;polarization frequency of 20?40 Hz;and period of 20?40 cycles(the sample size is5×5×0.5 mm³).Combined with the polarization inversion model,‘the domain of frequency multiplication effect'is revealed,and it is proposed that the periodic stripe domain is the main reason for the high piezoelectric response.At the same time,it shows that the enhanced piezoelectric properties of Relaxor ferroelectric single crystals by ACP is not at the expense of temperature stability.2)According to the phase diagram of binary solid solution PMN-PT single crystal,PMN-x PT with different components was selected as the research object of high piezoelectric response of AC-polarized Relaxor ferroelectric single crystal.The piezoelectric coefficient of PMN-0.35PT under AC field can reach 3900 p C/N,which is an increased by?60%relative to the DC counterpart.High piezoelectric constant(d₃₃=2700 p C/N)was achieved in PMN-0.29PT single crystal under 30 Hz AC field.The high optical transmittance of visible light in PMN-0.29PT single crystal is more than 67%under 1 Hz AC field,which is expected to become the preferred material for the new generation of optoelectronic intelligent devices.Combined phase field simulation with domain structure characterization,it is confirmed that the excellent piezoelectric response of Relaxor ferroelectric single crystal under ACP is attributed not only to the elimination of 71°domain wall,but also to the appearance of high density 109°domain structure,resulting in a shallow free energy distribution.3)For their application in piezoelectric transducer for medical imaging,the beam mode vibration of PMN-0.25PT single crystal resonators after ACP were investigated by combining experimental impedance measurements and finite element simulation.The complete material coefficients of PMN-0.25PT single crystal were obtained by using the pulse-echo ultrasound and inverse impedance spectroscopy.Compared with traditional direct current poling(DCP),the beam mode electromechanical coupling coefficient(k₃₃,)of PMN-0.25 PT for ACP reaches up to 89.1%from 84.5%of DCP.As the attribution of the special periodic stripe domain structure(109°domain)the lateral vibration will be suppressed effectively in the array transducer.It indicates that AC-poled PMN-0.25PT single crystals can be an efficient strategy to obtain higher sensitivity and larger bandwidth in ultrasonic transducer.4)DCP and ACP single element transducers with center frequency of 3.2 MHz are designed and fabricated by finite element simulation.The results show that compared with DCP single element transducer,ACP single element transducer has higher sensitivity,which is increased from-52.46 d B to-49.81 d B,an increase of 2.65 d B.However,the-6 d B bandwidth of the single element transducer is not affected by the two different poling method,and it always keeps between 72%and 75%.Therefore,ACP is a reliable method to improve the sensitivity of single element transducer.5)The DCP and ACP PMN-0.25PT single crystals with high temperature stability were selected and combined with the KLM equivalent circuit model to design and fabricate a phased array single crystal transducer with a center frequency of 2.8 MHz.Our experiments show that the ACP transducer has less trailing and exhibits a larger bandwidth in the frequency domain,with a-6 d B bandwidth of 87.8%,an improvement of 7.7%with respect to the DCP transducer.But from the point of view of sensitivity,it only increased by 0.82 d B.Therefore,it can be said that the AC-polarized PMNT single crystal has a significant effect on increasing the bandwidth of the array transducer.At the same time,based on the multi-stage filter model,an ACP single crystal phased array ultrasonic transducer with multilayer acoustic matching layer was designed and prepared.Compared with the ACP single crystal array transducer with double-layer acoustic matching layer,its-6 d B relative bandwidth has reached 91.41%,which is an increase of 3.61%,and the sensitivity is-45.52 d B,which is an increase of 2.31 d B.The addition of multiple matching layers alleviates the impedance mismatch between the single crystal and water,improving the energy transfer coefficient of the acoustic wave to the front-end medium...