High-temperature piezoelectric wafer active sensors (HT-PWAS) have been developed for structure health monitoring at hazard environments for decades. Different candidates have previously been tested under 270 °C and a new piezoelectric material langasite (LGS) was chosen here for a pilot study up to 700 °C. A preliminary study was performed to develop a high temperature sensor that utilizes langasite material. The Electromechanical impedance (E/M) method was chosen to detect the piezoelectric property. Experiments that verify the basic piezoelectric property of LGS at high temperature environments were carried out. Further validations were conducted by testing structures with attached LGS sensors at elevated temperature. Additionally, a detection system simulating the working process of LGS monitoring system was developed with PZT material at room temperature. This thesis, for the first time, (to the best of author’s knowledge) presents that langasite is ideal for making piezoelectric wafer active sensors for high temperature structure health monitoring applications.

This study experimentally investigated the effect of hydrophobic and hydrophilic surfaces characteristics on nucleate boiling heat transfer performance for the application of direct immersion cooling of electronics. A dielectric liquid, HFE – 7100 was used as the working fluid in the saturated boiling tests. Twelve types of 1-cm2 copper heater samples, simulating high heat flux components, featured reference smooth copper surface, fully and patterned hydrophobic surface and fully and patterned hydrophilic surfaces. Hydrophobic samples were prepared by applying a thin Teflon coating following photolithography techniques, while the hydrophilic TiO2 thin films were made through a two step approach involving layer by layer self assembly and liquid phase deposition processes. Patterned surfaces had circular dots with sizes between 40 – 250 μm. Based on additional data, both hydrophobic and hydrophilic surfaces improved nucleate boiling performance that is evaluated in terms of boiling incipience, heat transfer coefficient and critical heat flux (CHF) level. The best results, considering the smooth copper surface as the reference, were achieved by the surfaces that have a mixture of hydrophobic/hydrophilic coatings, providing: (a) early transition to boiling regime and with eliminated temperature overshoot phenomena at boiling incipience, (b) up to 58.5% higher heat transfer coefficients, and (c) …