Development and application of mass sensors based on flexural resonances in alumina beams

New mass sensors are described, which are based on the change in the flexural resonant frequencies of ceramic cantilever beams subject to piezoelectric excitation. The devices have been fabricated by screen-printing and firing a PZT-based paste on 96% alumina substrates with the methods of thick film technology and are, therefore, low cost and easy to manufacture. Inserted in an electronic feedback loop sustaining and tracking oscillations at one of the resonant frequencies, the sensors work as resonant microbalances with frequency output and can be employed for gravimetric chemical measurements. The beams have been implemented in two different sizes and, as a consequence, operated at different frequencies (about 82 and 149 kHz), in order to vary the mass sensitivity. The sensors´ manufacturing and theory of operation are illustrated, and experimental results on their characterization are reported. For the two sensor sizes, mass sensitivities of about -280 and -1200 Hz/mg have been measured, in agreement with theoretical predictions. The influence of temperature has been investigated showing that, for slow thermal drifts, a satisfying degree of compensation can be achieved with a differential configuration. The devices have been successfully applied as sorption sensors for the measurement of air relative humidity (RH) by sensitizing the beams´ surface with hydrophilic polymeric coatings. On the basis of past investigations, poly(N-vinylpyrrolidinone) and poly(ethyleneglycol) have been adopted as coating materials, obtaining respective frequency shifts of about 500 and 1400 Hz for RH ranging from 12 to 85%.