应用于医用超声换能器的PMN-PT基铁电单晶高频强场性能研究

The low frequency and weak field properties of PMN-PT based relaxor ferroelectric single crystals with excellent piezoelectric properties have been widely studied. However, the related work concerning on the characterization of high frequency and strong field properties is not too much. In this project, with the application background of high performance medical ultrasonic transducer, we shall investigate the important high frequency and strong field performance and its mechanism of PMN-PT based single crystal. The main contents are as follows. Firstly, by exploring the use of depolarizing critical value method and ultrasonic pulse echo method, we shall characterize high-frequency coercive field of ferroelectric single crystal. Then, we will characterize the frequency and temperature dependence of the coercive field of PMN-PT based on single crystal with different cutting type and composition. Especially, we shall study the anomalous frequency-dependent crossover behavior between two power-law scaling regimes of frequency-dependent coercive field. Secondly, by establishing different measurement systems, the performance degradation, self-heating, and piezoelectric and dielectric nonlinear responses of PMN-PT based single crystal excited by high frequency and strong electric field will be further studied. We aim to know the influence of different experimental parameters on the performance of single crystal. Thirdly, by applying the phase field simulation and the thermodynamic constitutive model, we are going to declare the different response mechanisms of the relaxor ferroelectric single crystals excited by high frequency and strong electric field. After discussing all above crucial and attractive problems, it will enrich the evaluation methods of ferroelectric single crystals as well as providing theoretical and technological basis for material selection and device design of medical ultrasonic transducers.

目前，具有优异压电性能的PMN-PT基弛豫铁电单晶的低频弱场性能已被广泛研究，但高频强场性能表征并不多见。本项目以高性能医用超声换能器为应用背景，研究PMN-PT基单晶重要的高频强场性能及其机理。主要内容包括：探索利用退极化临界值法和超声脉冲回波法等来表征铁电单晶高频矫顽场，对不同切型和组分的PMN-PT基单晶的矫顽场频率和温度依赖特性进行表征，并着重研究矫顽场频率特性存在不同幂律标度区域的反常现象。在此基础上，构建测量系统来研究高频强电场激励下PMN-PT基单晶的性能退化、自发热现象以及压电和介电非线性响应等，明确不同实验参数对单晶性能的影响。再有，通过相场模拟和运用铁电热力学本构模型等方法阐明高频强电场激励下弛豫铁电单晶不同的响应机制。对这些关键问题的研究，将丰富铁电单晶材料评价方法和体系从而为医用超声换能器的材料选择和器件设计提供理论和技术基础。

本项目以高性能医用超声换能器为应用背景，对具有优异压电性能的PMN-PT基二元系及三元系弛豫铁电单晶的高频强场性能及其内在机理进行了系统研究。通过该项目的研究，我们完善了铁电单晶高频矫顽场测量方法和系统，获得医用超声换能器常用PMN-PT及PIN-PMN-PT单晶在不同频率和温度时的高频矫顽场数据，并揭示了PMN-PT及PIN-PMN-PT单晶高频矫顽场反常频率特性的内在机理。同时，利用电滞回线方法获得了了不同电场和不同温度下各种组分PMN-PT单晶的矫顽场、回线面积和极化强度的动力学标度关系，基于铁电热力学本构模型等分析了其对应的微观机理。发现了[001]c极化的PIN-PMN-PT单晶存在强电致疲劳现象，阐明了其在强电场下的性能退化机理，揭示了温度对于PIN-PMN-PT单晶疲劳恢复的重要影响。在另一方面，建立了Tone-burst电场激励下压电材料的自发热模型，通过数值仿真以及实验研究分析了脉冲重复频率和占空比等电场激励参数对PMN-PT等压电材料温升特性的影响。建立了压电材料高频强场介电测量方法和装置，克服了当前PMN-PT等压电材料强场介电性能测量局限在低频的不足。发展了提出了基于温度和振动测量的强场机械品质因数方法，在此基础上提出了基于Tone-Burst瞬态共振激励方法的强场压电系数测量方法，大大提高了强场压电系数测量的灵敏度和频率范围。通过本项目的研究，初步建立了高频强场弛豫铁电压电材料的评价方法，为医用超声换能器的材料选择和器件设计提供了理论和实验基础。