Pseudo-LFE study in AT-cut quartz for sensing applications

Lateral field excitation (LFE) AT-cut quartz devices have recently been investigated for liquid sensing applications. However, previous studies concentrate on LFE device with Psi = plusmn90deg due to the maximum theoretical LFE coupling coefficient. Here Psi represents the angle between the lateral electric field and the crystallographic X-axes in the plate surface. In the present study, AT-cut LFE devices with different electric field directions were studied and a TSM (Thickness shear mode) was observed in the LFE device with Psi = 0deg, where the calculated LFE coupling coefficient is zero. This indicates that the observed TSM may not be excited by the lateral field but the thickness field with the liquid acting as an analog electrode, and the device is in fact a pseudo-LFE device. Moreover, experiments have also shown that LFE devices with different electric field directions are of similar sensitivities to liquid viscosity, relative permittivity and conductivity.