Identification of the properties of a complex layer deposited on the surface of a quartz crystal microbalance (QCM) by the impedance measurement

Abstract

The quartz crystal microbalance (QCM) is a quartz crystal plate resonator for measuring a minute mass according to the resonant frequency change. In some applications, an adsorbing layer must be formed on the resonator surface, which adsorbs the material whose mass is measured. This layer affects not only the resonant frequency but also its damping as the layer is viscoelastic. Its presence cannot simply be ignored but should be included in the modelling. In our previous work, the algorithm to characterize the viscoelastic layer's properties was developed, in which the multiple resonant frequencies and the corresponding resonant resistances were considered including the overtone operation of the quartz crystal plate (Wakatsuki N, Wada S, Kagawa Y and Haba M 2003 Inverse Problems in Engineering Mechanics vol 4 (Oxford: Elsevier) pp 121–6). It is unrealistic however to assume that the layer's properties are unchanged for such a wide frequency range. In the present paper, no overtone resonance is considered. Mass of the adsorbed material and the thickness of the viscoelastic layer are identified by means of Newton's method.