Kazys Rymantas J, Sliteris Reimondas, Sestoke Justina
Ultrasound Institute of Kaunas University of Technology, LT-51423 Kaunas, Lithuania.
Sensors (Basel). 2017 Oct 16;17(10):2365. doi: 10.3390/s17102365.
For improvement of the efficiency of air-coupled ultrasonic transducers PMN-32%PT piezoelectric crystals which possess very high piezoelectric properties may be used. The electromechanical coupling factor of such crystals for all main vibration modes such as the thickness extension and transverse extension modes is more than 0.9. Operation of ultrasonic transducers with such piezoelectric elements in transmitting and receiving modes is rather different. Therefore, for transmission and reception of ultrasonic signals, separate piezoelectric elements with different dimensions must be used. The objective of this research was development of novel air-coupled ultrasonic receivers with PMN-32%PT strip-like piezoelectric elements vibrating in a transverse-extension mode with electromechanically controlled operation and suitable for applications in ultrasonic arrays. Performance of piezoelectric receivers made of the PMN-32%PT strip-like elements vibrating in this mode may be efficiently controlled by selecting geometry of the electrodes covering side surfaces of the piezoelectric element. It is equivalent to introduction of electromechanical damping which does not require any additional backing element. For this purpose; we have proposed the continuous electrodes to divide into two pairs of electrodes. The one pair is used to pick up the electric signal; another one is exploited for electromechanical damping. Two types of electrodes may be used-rectangular or non-rectangular-with a gap between them directed at some angle, usually 45°. The frequency bandwidth is wider (up to 9 kHz) in the case of non-rectangular electrodes. The strip-like acoustic matching element bonded to the tip of the PMN-32%PT crystal may significantly enhance the performance of the ultrasonic receiver. It was proposed to use for this purpose AIREX T10.110 rigid polymer foam, the acoustic impedance of which is close to the optimal value necessary for matching with air. It was found that in order to get a wide bandwidth the length of the matching strip should be selected not a quarter wavelength λ/4 at the antiresonance frequency but at lower frequency. It allowed achieving the frequency bandwidth (14-18)% with respect to the central frequency at -3 dB level.
为提高空气耦合超声换能器的效率，可使用具有非常高压电性能的PMN-32%PT压电晶体。此类晶体在所有主要振动模式（如厚度伸缩和横向伸缩模式）下的机电耦合系数均大于0.9。使用此类压电元件的超声换能器在发射和接收模式下的操作有很大不同。因此，为了发射和接收超声信号，必须使用不同尺寸的单独压电元件。本研究的目的是开发新型空气耦合超声接收器，其具有以横向伸缩模式振动且运行受机电控制、适用于超声阵列应用的PMN-32%PT带状压电元件。由以这种模式振动的PMN-32%PT带状元件制成的压电接收器的性能可通过选择覆盖压电元件侧面的电极几何形状来有效控制。这等同于引入机电阻尼，而无需任何额外的背衬元件。为此，我们提出将连续电极分成两对电极。一对用于拾取电信号，另一对用于机电阻尼。可使用两种类型的电极——矩形或非矩形，它们之间有一定角度（通常为45°）的间隙。在使用非矩形电极的情况下，频率带宽更宽（高达9kHz）。粘结到PMN-32%PT晶体尖端的带状声匹配元件可显著提高超声接收器的性能。为此建议使用AIREX T10.110刚性聚合物泡沫，其声阻抗接近与空气匹配所需的最佳值。研究发现，为了获得宽带宽，匹配条的长度不应选择在反谐振频率下的四分之一波长λ/4，而应选择在较低频率下。这使得在-3dB水平下相对于中心频率可实现(14 - 18)%的频率带宽。
