ANALYSIS OF THE RADIAL DEPENDENCE OF MASS SENSITIVITY FOR MODIFIED-ELECTRODE QUARTZ-CRYSTAL RESONATORS

The radial dependence of mass sensitivity of the sensing surface is an alytically calculated for two examples of ''modified-electrode'' quart z crystal resonators (QCR), The term ''modified-electrode'' QCR is use d here with respect to the conventional QCR which has two identical ci rcular and concentric electrodes, For these QCRs, the sensing surface is divided into a fully electroded, a partially electroded, and an une lectroded region, and the efficiency of each region is evaluated in te rms of the electrode mass loading factor, Such QCRs are typically inve stigated for sensor applications in which the electrical properties of the liquid load or the coating deposited on the sensing surface (elec troded and partially electroded regions) are being measured in additio n to mass loading, While modified-electrode QCRs can be viewed as a si mple capacitance sensor in those applications, the use of a piezoelect ric crystal resonator in the narrow range of frequencies near resonanc e and antiresonance allows for a direct measurement of the capacitance through the antiresonant frequency, provided that the device damping (motional resistance) is not too high or that the resonance quality fa ctor, Q, is high enough for a stable vibration under the load, It is s hown that, for some values of the electrode mass loading factor, the o ff-electrode efficiency (partially electroded and unelectroded region efficiency) can still have a significant contribution to the overall s urface area mass sensitivity, Knowledge of the efficiencies is needed to determine the loading area required for stable QCR sensor operation , This is because additional dissipation of energy into the load can o ccur, especially for cases where the sample load extends to the unelec troded surface, which has a nonnegligible particle displacement amplit ude, It is also shown that, for some applications involving a liquid l oad and for some values of the electrode thickness, the shear particle displacement profile is such that compressional wave generation can c ontribute significantly to device damping, thus making the device unst able, Experimental measurements of the mass sensitivity profile on the surface are also performed for those QCRs and compared to theory.