FLEXURAL PLATE WAVE DEVICES FOR CHEMICAL-ANALYSIS

The mass sensitivities, vapor sensitivities, and vapor detection limits of flexural plate wave (FPW) and surface acoustic wave (SAW) vapor sensors are compared both theoretically and experimentally. FPW devices offer high mass sensitivity at much lower operating frequency. Mass sensitivity increases as membrane thickness is decreased; frequency decreases at the same time. This scaling law is in contrast to SAW devices where mass sensitivity increases as frequency increases. FPW devices with sorbent polymer films respond to vapors in a manner similar to that of SAW devices coated with the same polymer. The FPW vapor sensor, however, offers lower absolute noise levels and hence lower vapor detection limits. It is also demonstrated experimentally that FPW devices can monitor changes in polymer films as the polymer undergoes the glass transition. Plots of frequency vs temperature show a discontinuity in slope at the static glass transition.