超声换能器带宽对光声成像的影响

研究了不同尺寸吸收体产生的光声压的频谱特性：对于厘米量级、毫米量级和几百个微米量级的吸收体,产生光声压频谱的主要范围分别约为20～300kHz、70kHz～2．5MHz和400kHz～20MHz;讨论了不同频率范围的光声信号对重建图像的影响,低频段的光声信号能反映物体的非边界区域,而高频段的光声信号能突出物体的细微结构,尤其是物体的边界特征。提出了不同尺寸的吸收体要选用或设计不同带宽范围的探测器进行检测的方法．当探测器的带宽范围与光声压频谱范围基本吻合时,损失的频率成份较少,重建的光声图像效果较好,这一结论在仿真和实验结果中都得到了证明。实验用的光源为YAG激光器,波长为532nm,重复频率为30Hz,脉宽为7ns,探测器为针状的PVDF膜水听器,接收面积的直径为1mm。     

2019年中国声学学会青年人才托举工程项目交流会在南昌召开

为改善电磁超声换能器(electromagnetic acoustic transducer,EMAT)激发横波的特性,提出一种基于正交试验的优化方法,并利用有限元仿真软件COMSOL Multiphysics建立了横波EMAT的有限元模型,应用正交试验和极差分析法,分析了线圈各参数对电磁超声横波的近场长度和半扩散角的影响,比较了线圈各参数影响程度的大小。结果表明,激励电流的频率和线圈的尺寸对横波的传播特性有着明显影响,提高频率会使得横波的近场长度增大及半扩散角减小;其次,减小导线的宽度及间距,以缩小线圈尺寸能有效改善横波的传播特性。     

Capacitive micromachined ultrasonic transducers using commercial multi-user MUMPs process: Capability and limitations

The objective of this work is to construct capacitive micromachined ultrasound transducers (cMUTs) using multi-user microelectromechanical systems (MEMS) processess (MUMPs) and to analyze the capability of this process relative to the customized processes commonly in use. The MUMPs process has the advantages of low cost and accessibility to general users since it is not necessary to have access to customized fabrication capability such as wafer-bonding and sacrificial release processes. While other researchers have reported fabricating cMUTs using the MUMPs process none has reported the limitations in the process that arise due to the use of standard design rules that place limitations on the material thicknesses, gap thicknesses, and materials that may be used. In this paper we explain these limitations, and analyze the capabilities using 1D modeling, Finite Element Analysis, and experimental devices. We show that one of the limitations is that collapse voltage and center frequency can not be controlled independently. However, center frequencies up to 9 MHz can be achieved with collapse voltages of less than 200 V making such devices suitable for medical and non-destructive evaluation imaging applications. Since the membrane and base electrodes are made of polysilicon, there is a larger series resistance than that resulting from processes that use metal electrodes. We show that the series resistance is not a significant problem. The conductive polysilicon can also destroy the cMUT if the top membrane is pulled in the bottom. As a solution we propose the application of an additional dielectric layer. Finally we demonstrate a device built with a novel beam construction that produces transmitted pressure pulse into air with 6% bandwidth and agrees reasonably well with the 1D model. We conclude that cMUTs made with MUMPs process have some limitations that are not present in customized processes. However, these limitations may be overcome with the proper design considerations that we have presented putting a low cost, highly accessible means of making cMUT devices into the hands of academic and industrial researchers.     

Fabrication and evaluation of fully-sampled, two-dimensional transducer array for "Sonic Window" imaging system

A low-cost, fully-sampled, 3600 element 2D transducer array operating at 5 MHz and designed for use in a hand-held ultrasound system is described here. Four array configurations are presented - (1) array with both matching and pedestal backing layers, (2) array with a matching layer but no backing pedestal, (3) array with a backing pedestal but no matching layer, and (4) array with neither matching layer nor backing pedestal. Each array was characterized in terms of impedance measurements, pulse-echo response, and experimental beamprofile. Comparative finite element analysis simulations are also presented. Average estimated active element yield for the four arrays was 94%. The array with pedestal layer proved the most promising, providing a 26% bandwidth and a 1.7 dB improvement in sensitivity with respect to the array with neither pedestal nor matching layer. Although this bandwidth is acceptable for our specific application (C-scan imaging), reverberations within the substrate material remain a potential challenge. We are currently working to fabricate a custom PCB material to address this concern, and may also consider using a pre-compensated transmit waveform or matched digital filter approach to further reduce the effects of such reverberations.     

A photo-optical technique for measuring flow-induced vibrations in cantilevered tube bundles

A photo-optical technique has been developed for monitoring the dynamic displacement of cantilevered tubes in fluid flow. The technique employs an optical fiber to transmit light through the tube and a phototransistor array to measure the motion of the light beam that is projected from the end of the tube. The device is simple, inexpensive, and very sensitive to small displacements. Details are given for the development of the technique, analysis of performance, and static calibration. The device was tested by monitoring the dynamic response of a bundle of cantilevered tubes in both single-phase and two-phase Freon 11 flows. The results are compared with those of a standard strain gauge bridge.     

简易弓形位移传感器及其在工程岩体研究中的应用

本文介绍了一种用于岩体力学物理模拟试验的位移传感器及其结构，力学原理和制造方法建立了传感器位移量与基本身应变的关系，并进行了标定和验证，同时，介绿了该位移传感器在某双曲拱坝坝肩软岩带变形特征的地质力学模拟试验的应用情况，结果表明，该传感器用模型内部的变形量测，具有独特的优点。     

Extremely high-frequency piezoelectroacoustic transducer based on BN-tube/SiC-whiskers rope

Innovative idea of piezoelectric electroacoustic transducer in extremely high-frequency terahertz range on the basis of BN-tube/SiC-whiskers rope is suggested and substantiated. Unlike an acoustic spectrum of solid rectangular pins and films used so far in ultrasonic pulsers and receivers, in the acoustic spectrum of circular hollow nanotubes, the peculiar squash E_2g and the subsequent E_ng modes of starlike chain belonging to a gallery of whispering acoustic modes was shown by ab initio RHF/6-31G calculations to exist in the Raman spectra. Inherent important feature of these standing vibrations is their weak attenuation and high frequency, which, as depended on the nanotube diameter, fall in the range of about 1 GHz–1 THz. Hypersound was suggested to be excited by resonant microwaves using the piezoelectric properties of BN heteropolar nanotubes and then to transmit it into a sample by high modulus encapsulated SiC-whiskers. Such BN-tube/SiC-whiskers of 100–800 nm in diameter and with 20 aspect ratio were synthesized by carbothermal and CVD techniques. Cactus-like arrays of SiC nanowhiskers were synthesized by CVD technique. A sketch of the hypersound generator/detector, with the piezoelectroacoustical transducer on the basis of the BN-tube/SiC-whisker assembly serving as hypersonic antenna, was advanced.     

Phase-response characteristics of the transfer function of a spatially limited electrochemical cell

Investigation of the dynamic characteristics of diffusion transducers is of great practical importance because of the wide utility of these devices in instruments for measuring the movement parameters and wave fields. Previously, cathode currents were the object of investigation because their difference was used as an output signal in several useful devices. In this work we investigated the amplitude and phase relationships between currents measured on both cathodes and anodes over a wide range of frequencies in a four-electrode electrochemical cell. It was shown in experiment that the sum of the currents recorded on the four electrodes of a molecular electronic cell (MEC) is zero, as follows from the condition of conservation of an electric charge. At frequencies as low as ～0.1 Hz the frequencies of the signal currents of the anode and the nearest cathode coincided. As the frequency grew to f ～80 Hz, the phase difference monotonously increased to values close to π. Previous theoretical models suggested, explicitly or implicitly, that the phases of the signal currents coincided over the whole range of frequencies in each anode-cathode pair. The results of the experiment were explained within the framework of the theoretical model of a four-electrode electrochemical cell, which is based on the one-dimensional equation of convective diffusion and takes into account the limited space of the cell.     

一种新型光纤压力传感器的设计

设计了一种工作波长为800 nm的光纤压力传感器,提出了一种用补偿光纤对温度和双折射进行补偿的方法,并从理论上进行了解释.在不同温度和压力条件下对该传感器进行了实验,相对误差小于1.5％.实验结果表明,该补偿方法是有效的.在0℃~50℃的温度下,对传感器在0～40 MPa的压力范围内进行了测试,其标准偏差在1%以内.