Characteristics of Shear-Horizontal-Mode Circumferential Waves Propagated in ZnO Films/Silica Glass Pipe Structure under Liquid Load

Sensors using shear horizontal (SH) acoustic waves are good candidates for liquid measurements such as viscosity. We measured the propagation characteristics of shear-horizontal-mode acoustic waves transmitted circumferentially around an IDT/c-axis parallel-oriented ZnO film/silica glass pipe structure for a highly sensitive liquid sensor. Two laps of acoustic waves were observed in the time response. The insertion loss of the first lap consisted of two frequency components at 129 MHz and 160-330 MHz. The arrival time and amplitude of the 129 MHz wave decreased as the liquid was loaded outside the pipe surface. Therefore, the 129 MHz circumferential wave propagated mainly at the outside boundary of the silica glass pipe. On the other hand, the 252 MHz circumferential wave propagates mainly in the interior of the silica glass pipe because the arrival time and amplitude of the wave changed slightly with the liquid loading. The pipe structure can measure the liquid loaded on the outside surface using a 129 MHz circumferential wave.