Maximizing the efficiency of the combustion process requires the ability to sense oxygen levels over a broad range of concentrations with fast response times under rapidly varying conditions of pressure and temperature to maintain the correct fuel/oxygen ratio in real-time. Quenching of the luminescence from organometallic compounds by oxygen has been used to develop a number of fiber-based sensors. A major drawback of these organometallic indicators for combustion applications is that the chromophores degrade with time, have a limited operational temperature range, typically room temperature ±25°C, and lack long-term reliability. This work investigates luminescent molybdenum clusters based on Mo6Cl12 were as replacements for organometallic indicators. A study of the high temperature stability of Mo6Cl12 in air revealed irreversible changes in the optical absorption spectrum at T >250°C and a loss of the red luminescence characteristic of the pristine clusters. Thermal aging experiments run in air and under nitrogen point to oxidation of the clusters as the cause of the change in optical properties. X-ray powder diffraction measurements on samples annealed at 300°C under controlled conditions are consistent with oxidation of Mo6Cl12 to form MoO3. Optical and thermal aging experiments show that K2Mo6Cl14*1H2O, the alkali metal salt of Mo6Cl12, has higher thermal stability and remains luminescent after long-term aging in air at 280°C. Methods were developed for depositing K2Mo6Cl14*1H2O-incorporated sol-gel films on planar and optical fiber substrates by dip coating and spray coating. The mechanical properties of the films depended on the film thickness; thin films were stable, but cracks often formed in the thicker films needed for sensors. This problem was addressed using two strategies: altering the components of the sol-gel solutions used to embed the clusters and by devising a composite approach to sensing layers where a slurry of fully cured sol-gel particles containing K2Mo6Cl14*1H2O in a sol-gel "binder" were deposited on substrates. The optical properties of a large number of fiber sensors were tested up to 102°C, with the best results obtained using the K2Mo6Cl14*1H2O/sol-gel composite sensing film. Fiber M demonstrated quenching of 4-6× between <0.001% and 21.1% (v/v) oxygen at 23, 42, 60, 81 and 102°C respectively. The sensor switches abruptly between two well defined levels with a response time of less than 10 s. Quenching of the cluster luminescence by oxygen obeys a two-site Stern-Volmer relationship based on measurements of fiber 121 at 42, 73, and 102°C, with sensitivity decreasing as temperature increases. The cycle-cycle variations for six cycles between nitrogen and oxygen at 58°C for fiber 45 corresponds to an uncertainty of ±1% to ±15% in oxygen concentration over the entire measurement range from 21.1% (v/v) to 2.1% (v/v) oxygen respectively. The long-term performance data from cycling fiber 70 between <0.001% (v/v) and 21.1% (v/v) oxygen for 14 hours was stable over the entire period and variations in sensor signal were found to be synchronous with the temperature fluctuations in the flow through cell. The magnitude of the sensor signal up to 102°C is ~3-nW for ~300 μW of incident excitation power. For the current 15-cm long fiber sensor, the autofluorescence (0.011 nW) is 40× smaller than the signal (~ 0.4 nW) in 20% (v/v) oxygen.
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