Responses And Applications Of Piezoelectric Sensors Under Several Non-ideal States

Piezoelectric sensors,which became popular after the original work of Sauerbrey in 1959,are very sensitive to mass change at the sensing surface. According the Sauerbery's Equation, An increase in mass loading onto quartz surface causes a linear decrease in the oscillation frequency of the resonator. A piezoelectric sensor is also called as quartz crystal microbalance (QCM) because it is very sensitive to the change in surface mass loading. QCM has been applied extensively in chemical and biological sensors. In this dissertation, the recent developments in piezoelectric sensors are reviewed briefly. Usually,Sauerbery's Equation is used to estimate the mass change in the application of QCM sensors. However, Sauerbery's Equation is only valid under the ideal conditions of a rigid, homogeneous and thin film. If these conditions were not holded, there would be a large systemical error in the mass change estimation from Sauerbery's Equation. Up to now, the responses and applications of piezoelectric sensors under non-ideal situations have few been reported. In this thesis, the response of QCM in three non-ideal cases, including longtidinal effect, cleavage phenomenon in resonant peak and the formation of ice film, were investigated. And the influence of excitation electric level on the response of a QCM was discussed. The details were given below:1) We report a new phenomenon of a quartz crystal microbalance (QCM) that was mounted horizontally at the bottom of the detection cell. The small shoulder resonant peak appears and disappears periodically in the conductance-frequency curve with increasing liquid height. The frequency shifts of the small shoulder resonant peak were in linear correlations with the liquid height in a given range. The overlap between the primary resonant peak and the small shoulder resonant peak caused a decrease in the intensity of the primary resonant peak and an increase in the intensity of the small shoulder resonant peak. At some liquid heights, the primary and small shoulder resonant peak have similar intensities in the conductance-frequency curves. But the areas of the conductance-frequency curve were nearly constant in the case with or without peak cleavage. Finally, the response pattern of QCM was employed to monitor the liquid density. In a combined impedance analysis method, the changes in surface mass loading, liquid viscosity and density were measured simultaneously. In a model application, the viscosity,density of water at different temperatures were measured in this method. The results measured were consistent with the data in literature.2) An ice film was prepared on the surface of the QCM and the sublimation and adsorption processes of the film were monitored in real-time. The sublimation of ice flim was observed even at the temperature of -18°C. Non-homogeneous film was obtained due to the sublimation of ice. A group of cleavage resonant peaks occur in the conductance-frequency curv and move from low frequency region to high one. The changes in the shape of the ile film and the resonant frequency of the peaks were dicussed. This phenomenon has not been reported in the literature. And then the adsorption behavior of ethanol on the rigid ice film was investigated by QCM. It will provide some theoretical guidance for the novel piezoelectric ice sensors.3) Impedance analysis method is an important method to measure the response of a quartz crystal microbalance (QCM), which can provide the values of the resonant frequency and equivalent circuit parameters. Among the measuring conditions in the impedance analysis method, the influence of the excitation electric volt supplied to the quartz crystal disc on the response of the QCM was rarely discussed in references. A recently published article claimed that the excitation electric lever is a key parameter for QCM measurement in impedance analysis method. The response of a QCM was reported to be dependent on the excitation volt supplied. In this work, the influence of excitation volt on the equivalent circuit parameters of the QCM and its responses to surface mass loading, viscosity and density of liquid was investigated. Experimental results show that,in the range of 5-1000mV, the excitation volt supplied to QCM has little influence on its equivalent circuit parameters. This conclusion gives another suggestion for the experimental condition choice for impedance analysis method in QCM measurements.