超声共振法测定圆柱体材料弹性常数的影响因素分析

为明确超声共振频谱分析法(RUS)测定圆柱体试样弹性常数的影响因素, 由此针对各向同性圆柱体开展实验, 观察圆柱体试样测量结果的分散性和稳定性, 且主要研究横纵比和有效模态数量对测量结果的影响, 并与拉伸试验的结果进行比较. 以测量结果标准差、变异系数和95%置信度下均值的区间估计3个指标来衡量结果的稳定性和分散性, 且从模态简并角度对实验结果进行分析. 最终结果表明, 当超声共振频谱分析法的有效模态数量在17左右、试样横纵比接近1时, 测量结果的稳定性和分散性较小.     

Thickness shear mode quartz crystal resonators with optimized elliptical electrodes

Quartz crystal resonators (QCRs) with circular electrodes have been widely used for various liquid and gas sensing applications. In this work,quartz crystal resonators with elliptical electrodes were studied and tested for liquid property measurement. Mindlin’s theory was used to optimize the dimension and geometry of the electrodes and a 5-MHz QCR with minimum series resistance and without any spurious modes was obtained. A series of AT-cut QCRs with elliptical electrodes of different sizes were fabricated and their sensing performances were compared to devices with circular electrodes. The experimental result shows that the device with elliptical electrodes can obtain lower resonance impedance and a higher Q factor,which results in a better loading capability. Even though the sensitivities of devices with elliptical and circular electrodes are found to be similar,the sensor with elliptical electrodes has much higher resolution due to a better frequency stability. The study indicates that the performance of QCRs with elliptical electrodes is superior to that of traditional QCRs with circular electrodes.     

Brittle-Ductile Transition of Ferroelectric Ceramics Induced by Thermally Activated Dislocation Emission

Thermally activated dislocation emission in high-temperature ferroelectric ceramics is investigated through an assumption of thermal stability and a novel analytical method. The stress intensity factor (SIF) arising from domain switching is evaluated by using a Green's function method, and the critical applied electric field intensity factor (CAEFIF) for brittle fracture at room temperature is obtained. Besides, the lowest temperature for single dislocation emission before brittle fracture is also obtained by constructing an energy balance. The multi-scale analysis of facture toughness of the ferroelectric ceramics at high temperature is carried out. Through the analysis, the CAEFIF for crack extension is recalculated. The results show that the competition and interaction effects between dislocation emission and brittle fracture are very obvious. Besides, the higher critical activation temperature, the more columns of obstacles will be overcome. Additionally, the shielding effect arising from thermally activated dislocations is remarkable, thus, the brittle-ductile transition can promote the fracture toughness of high-temperature ferroelectric ceramics.     

A0模态Lamb波聚焦透镜的结构设计及实验研究

基于折射率的弯曲波透镜多在板结构上打孔或挖槽,这种设计破坏了原有的结构,降低了板的刚度和稳定性。鉴于此,通过在板表面镶嵌棱柱的方式设计了A0模态Lamb波的聚焦透镜。首先,详细讨论了棱柱的结构参数对Lamb波带隙的影响机理,获取了波速与结构尺寸的定量关系,并实现了透镜的结构设计;其次,有限元仿真了该聚焦透镜的工作性能,包括聚焦位置、焦点处能量分布、聚焦尺寸、工作带宽等;最后,通过实验验证了该透镜设计的正确性。研究结果表明,设计的透镜能够使弯曲波聚焦在预先设定位置,且在不改变结构参数的情况下具有一定的工作频率带宽。该透镜设计方法具有聚焦性能优越、刚度强、易于加工等优点,为声聚焦透镜在无损检测、能量收集等领域的实际应用提供了参考。     

Properties study of LiNbO3 lateral field excited device working on thickness extension mode

This paper investigates the properties of thickness extension mode excited by lateral electric field on LiNbO3 by using the extended Christoffel-Bechmann method. It finds that the lateral field excitation coupling factor for amode (quasi-extensional mode) reaches its maximum value of 28% on X-cut LiNbO3. The characteristics of a lateral field excitation device made of X-cut LiNbO3 have been investigated and the lateral field excitation device is used for the design of a high frequency ultrasonic transducer. The time and frequency domain pulse/echo response of the LiNbO3 lateral field excitation ultrasonic transducer is analysed with the modified Krimholtz-Leedom-Matthae model and tested using traditional pulse/echo method. A LiNbO3 lateral field excitation ultrasonic transducer with the centre frequency of 33.44 MHz and the -6 dB bandwidth of 33.8% is acquired, which is in good agreement with the results of the Krimholtz-Leedom-Matthae model. Further analysis suggests that the LiNbO3 lateral field excitation device has great potential in the design of broadband high frequency ultrasonic transducers.     

Lateral field excitation properties of langasite single crystal

In this work, bulk acoustic wave propagation properties of langasite single crystal excited by lateral electric field have been investigated. Three important crystal cuts have been identified for different operational modes of lateral field excitation (LFE) on langasite substrate, namely the (yxl)65o (pure-LFE mode), (yxl)45o (quasi-LFE mode), and (yxl)0o (pseudo-LFE mode). Devices on langasite substrate with the above cuts were fabricated and tested, and the experimental results agree well with the theoretical analysis. It is found that a pure thickness shear mode exists in the (yxl)65o langasite LFE device with the bare side facing liquid, and no spurious mode is found due to its moderately large piezoelectric coupling factor. In addition, (yxl)0o langasite LFE device is also found suitable for liquid phase sensing applications.