一种采用IDT技术的新型高频声体波换能器

压电晶体一液体界面的叉指换能器在一定频率的电信号激励下将仅向液体介质中辐射声体波，可将这一效应用于高频声体波超声换能器的制作。通过分析与计算压电晶体一液体的界面有效介电常数虚部，可确定出叉指换能器仅向液体中辐射声体波的表面慢度。根据待激发声体波的频率和慢度，可得到叉指换能器仅向液体中辐射声体波时的周期。考虑到叉指换能器直接接触液体并向液体中辐射声体波，可在叉指换能器表面敷设一层SiO3薄膜以保护电极。该类换能器的最大优点是不需匹配层而直接向液体中辐射声波，其工作频率上限原则上只受声表面波工艺限制。设计并制作出频率为31．5MHz的声体波超声换能器，实测了频响，与理论相符。

A New Type of High Frequency Acoustic Bulk Wave Transducer Using IDT Technics

The interdigital transducers (IDTs) deposited at an interface of piezoelectric crystal-liquid and powered by electrical signals of certain frequency can generate acoustic bulk waves (ABWs) only into the interior of liquid, and this phenomenon can be applied in fabricating high-frequency BAW transducers. By analyzing the imaginary parts of effective permittivities of the interface between pizeoelectric crystal and liquid, for a given piezoelectric crystal orientation, the appropriate interface slowness can be obtained, which corresponds to the case where IDTs generate BAWs only into liquid. According to the frequency of ABWs to be generated and the appropriate interface slowness, the space period of IDTs can be determined. Whereas IDTs contact liquid and generate ABWs into liquid directly, a layer of SiO_2 is deposited on the surface of IDTs as a protective film to protect the electrodes of IDTs. The main advantage of this kind of transducer is that IDTs generate BAWs into liquid directly without the need of impedance matching. The transducer working frequency is only limited by the technics of surface acoustic wave devices. A ABW transducer using IDTs with a central frequency of 31.5 MHz is designed and fabricated, and the experimental measurement results of frequency response are consistent with theoretical model.