Mikro-çubukların gaz, sıvı ve süperkritik CO2 içindeki sıklık yanıtları

Micromechanical resonators have been extensively studied in recent years because of their potential for high-sensitive, low cost, compact device applications, such as in situ viscometers and densitometers. This thesis is the first study in the literature that investigates the response of microcantilevers in a supercritical fluid. In this study, the frequency response of microcantilevers in CO2 was investigated in order to relate the thermophysical properties such as density (?), viscosity (?) and isothermal compressibility (KT) to the resonance frequency (fR), quality factor (Q) and damping parameter (ß). 200 µm, 225 µm, 250 µm long, 20 µm wide and 1 µm thick multilayered Ni microcantilevers consisting thin Cr and Au films were fabricated by conventional microfabrication techniques. The cantilevers were placed in a high pressure view cell equipped with a custom designed Teflon housing holding the microcantilever chip and the electromagnetic coil which was actuated sinusoidally. A Laser Doppler Vibrometer (LDV) was used for detection. Experiments were carried out in the pressure range of 1 atm to 200 atm and at 25 °C, 35 °C, 45 °C and 55 °C. First, it was observed that fR decreased with increasing ? and Q decreased with increasing ?. The changes in fR and Q were larger in the gas phase than those in the liquid or supercritical (sc) phase. Second, fR was observed to vary linearly with ? as well as (??)0.5 in the gas phase which enabled the determination of ? and ? through calibration with percentage errors less than 1 %. A quadratic relationship was observed between Q and ?. Apart from that, the decreases of fR observed in successive measurements (i.e. drifts) at a constant temperature and pressure were found to correspond to the shifts of the maxima of the density multiplied by compressibility, defined as ? -?K ? _T=(??/?P)_T. Moreover, microscale bubbles were observed at a pressure less than the bubble point pressure at 25 °C due to heating of cantilevers by LDV.