Design, Fabrication and Testing of Micronozzles for Gas Sensing Applications
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Real-time identification and quantitative analysis of volatile and semi-volatile chemical vapors are critical for environmental monitoring. Currently available portable instruments lack the sensitivity for routine air quality monitoring, so preconcentrators are employed as front-ends for miniaturized chemical sensors. However, commonly used techniques for sensitivity enhancement have a time constant associated with adsorption/desorption or permeation of gas molecules being concentrated. Little work has been reported on fast-response concentrating techniques for gas sensing applications. This research is devoted to the development of a fast-response microfluidic gas concentrating device with appropriate flow dynamic shapes and pressure gradients based on the separation nozzle method. It is capable of concentrating heavy gas molecules diluted in light ones when they are flowing at high speeds, thus maintaining the measurement system response time. This is promising for developing real-time preconcentrators to improve the sensitivity of miniature chemical sensors. In the initial phase of this work, linear test structures were used to characterize viscous effects in microfluidic devices. Unit processes were developed to fabricate encapsulated micronozzles with through-hole inlets and outlets. The mass flow efficiency of the test structures was measured to be in the range of 0.36-0.81, increasing with rising Reynolds number as a result of the decreasing influence of boundary layers. Single-stage gas concentration devices were designed and fabricated on the basis of the test structures. A gas separation experimental setup and a mass spectrometric analysis apparatus were developed to evaluate the performance of the devices. Analytical and finite element analyses were conducted to better understand and verify the experimental results. As a proof-of-concept, gas separation experiments with two different inert gas mixtures were carried out in conjunction with mass spectrometric analysis. More than two-fold enrichment of SF6 molecules with a response time on the order of 0.01 ms was demonstrated through the device. The effects of design parameters and operating conditions on the separation factor were determined experimentally and compared to the numerical simulation results. This study forms the basis for developing a cascade of the single-stage elements envisioned as a preconcentrator for miniature chemical sensors to realize real-time environmental monitoring.