In safety-critical applications opto-chemical sensors are preferred over conventional electrochemical ones due to several key advantages: built in media separation, harsh environment capability and electromagnetic immunity. In this thesis opto-chemical sensors whose functionality is based on the infrared absorption of fluids and/or the photoluminescence of nanostructure immersed in gaseous or fluid media are investigated. Application domains assessed relate to the fields of aeronautic safety and the smart maintenance of commercial aircrafts.Chapter 1 focuses on chemical contamination monitoring in phosphate ester based hydraulic fluids. By implementing such sensors in an aircraft, key contamination parameters can be assessed without taking fluid samples from the pressurized hydraulic system. By performing repeated measurements on the hydraulic fluid, degradation trends can be established and maintenance actions can be scheduled before any irreversible damage on the fluid or on the hydraulic system occurred. By performing the necessary maintenance actions at a pre-determined time in a pre-determined place, expensive ground time of commercial aircrafts can be minimized and significant costs due to unscheduled maintenance can be avoided. The chemical properties of aviation hydraulic fluids and their degradation mechanisms are described in the first part of this chapter. This is followed by a presentation of results gained from Fourier Transform Infra-Red (FTIR) measurements on hydraulic fluids in different states of contamination and degradation. These results show that the free water content and the Total Acid Number (TAN) can be inferred from changes in the O-H stretching vibration bands in 3200 cm-1 - 3800 cm-1 range. With this knowledge in mind, a miniaturized mid-infrared (MIR) sensor system has been realized, which allows these contamination parameters to be determined online inside a functional hydraulic system. In order to avoid flow restrictions in the hydraulic system, an extension of this MIR sensor principle into the near infrared (NIR) range of overtone vibrations is presented. The functionality of this chemical sensor system is complemented by a particle contamination sensor whose function is based on a light barrier principle. Both sensors in combination allow all practically relevant contamination parameters to be monitored online, inside an aircraft or in a ground support car directly on the airfield.Chapter 2 presents an all-optical sensor system that employs InGaN/GaN nanowire heterostructures (NWH) as optochemical transducers for gas detection. This transducer principle is best suited for safety-critical applications where reliable media separation is required, e.g. leaks in fuel and hydraulic lines. The photoluminescence intensity of the nanostructure has been found to be enhanced upon exposure to hydrogen and hydrocarbon gases and to be quenched upon exposure to oxidizing gases, such as O2, NO2 and O3. The chapter begins with the description of potential gas sensing applications in an aircraft. Afterwards the theory relating to the gas sensing mechanism using nanowires is discussed. Thereafter, the growth of group III-nitride based nanowires is presented, followed by a description of an optical fiber based sensor system that uses these nanowires as opto-chemical transducers. This experimental section also describes the gas test rig that had been used to assess the sensor performance. The presentation of the experimental results is divided in two parts: The first part focuses on the results obtained with hydrogen and hydrocarbon gases, while the second part describes the sensor response to different oxidizing gases. Minimum detectable H2 concentrations as low as 200 ppb at 50°C, and O3 concentrations as low as 10 ppb at room temperature, are demonstrated. The effects of temperature and humidity variations on the gas sensitivity are also reported in detail.Chapter 3 begins with a brief presentation of pH-monitoring needs inside an aircraft. A particularly interesting application is the monitoring of the pH value of onboard-generated drinking water, coming from a fuel cell. Thereafter GaN/InGaN quantum dots (QDs) are described with their possible use as opto-chemical transducers for measuring pH changes in fluids. The related sensing mechanism is briefly described, followed by a description of the test setup. The results section proves the variation of photoluminescence intensity with applied electrical bias and with pH changes inside a fluid medium. The accessible pH range extends from pH = 7 to pH = 2. Resolutions of less than 0.1 pH units and response times in the order of seconds have been achieved.
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