(563d) Organic Vapor Sensing and Discrimination Using High Frequency Thickness Shear Mode Devices

Thickness shear mode (TSM) resonators, also known as quartz crystal micro-balances (QCM) are a class of acoustic wave sensors that have been used for gas/vapor sensing and for determining liquid properties. Fast and sensitive chemical vapor sensing, specifically of hydrocarbon vapors is the goal of this work. The TSM sensors typically used have a lower sensitivity compared with other acoustic wave sensors. This paper describes the development of high sensitivity organic vapor sensors using polymer thin film coatings of poly-isobutylene (PIB) on high frequency TSM devices. Commercially available AT-quartz TSM devices were milled to 17 ìm, leaving a thin quartz membrane surrounded by a 50 ìm thick outer ring. This resulted in an increased frequency and a consequent increase in sensitivity, as described by device models. TSM devices with fundamental mode resonant frequencies of 10, 20 MHz were compared to the milled 96 MHz devices. The organic vapors studied were benzene, toluene, hexane, cyclohexane, heptane, dichloroethane, dichloromethane, and chloroform at levels ranging from less than 1 to over 10 volume percentage in nitrogen gas. The Butterworth-VanDyke (BVD) equivalent circuit model was used to model both the perturbed and unperturbed TSM resonators. Monitoring the sensor response through the equivalent circuit model allowed for discriminating between the organic vapors. In particular, changes in the resistance parameter due to softening and relaxation of the PIB film allowed for this vapor discrimination. We present new results of tests conducted to demonstrate increase in sensitivity for higher fundamental frequency TSM devices. We evaluate and compare the performance of each sensor in terms of detection limit and noise level. We then suggest schemes for utilization of these sensors as continuous monitors for process vapor streams containing organic vapors of arbitrary concentration.