POLITECNICO DI MILANO School of Industrial and Information Engineering Master of Science in Mechanical Engineering Quartz Crystal Microbalance : Temperature Effects Analysis

This study investigates the effects of temperature on the quartz crystal microbalance for thermogravimetric analyses in space. The studied microbalance differs from literature ones because of the device’s innovative built-in heater and temperature sensor, made with deposited film over the crystal. Temperature gradient is expected on the crystal due to the presence of the built-in heater, an issue that has not yet been deeply investigated. The crystal temperature was measured by means of infrared thermography, while a finite element approach served the purpose of investigating mechanical resistance and dynamic behavior. A non-uniform temperature distribution was detected when the heater circuit was fed, and thanks to the developed FE model, deformed shape and stress values were determined. A dynamic model of the microbalance has been developed and the thickness shear frequency at ambient temperature was correctly found at 10 MHz. Uniform temperature distribution has been applied to the crystal in order to evaluate the frequency shift. The model was tested at temperatures up to 90 °C, providing differences with the experimental values smaller than 60 Hz. Moreover, a temperature gradient (90 °C at the center, 23°C at the edge of the disk) was applied in the model. The resulting resonance was 1.7 kHz higher with respect to the uniform 90 °C temperature field case, highlighting the importance of the temperature gradients on the microbalance performance. The conclusion is that the microbalance’s innovative heater causes thermal gradients that, although not critical from the point of view of mechanical resistance, ultimately cause frequency shifts. Thus, an experimentally determined correction factor for frequency will be necessary to correct the measurements during heating and retrieve correct results from thermogravimetric analyses.