Date of Award

2015

Document Type

Open Access Thesis

Department

Electrical Engineering

Sub-Department

College of Engineering and Computing

First Advisor

Goutam Koley

Abstract

Detection of volatile organic compounds (VOCs), which are widely used in industrial processes and household products, is very important due to significant health hazards associated with them. VOCs are commonly detected using photo-ionization detectors (PIDs), suspended hot bead pellistors, or heated metal oxide semiconductor functionalization layers. However, these techniques used for detecting VOCs often suffer from one or more of the following issues - high power consumption, limited selectivity, complicated functionalization technique and expensive characterization tools. On the other hand, microcantilevers offer excellent avenues for molecular sensing that arises out of their high sensitivity to various physical parameter changes induced by the analyte molecules. Microcantilever heaters, which are extremely sensitive to changes in thermal parameters, have been widely utilized for calorimetry, thermal nanotopography and thermal conductivity measurements. Due to the small area of the microcantilever that needs to be heated (i.e. the tip of a triangular microcantilever), they also offer the possibility of reduced power consumption for high temperature operation. The present study reports the multimodal VOC detection capability of unfunctionalized microcantilever heaters made of AlGaN/GaN heterostructure, which can address many of the limitations observed in other techniques.

The microcantilevers, fabricated on an AlGaN/GaN on Si wafer, were found to be excellent heating elements with high degree of localization and low power consumption (K). While most of the microcantilevers had a

single conducting channel along the arms, some were specially designed to have two parallel channels, isolated by semi-insulating GaN or air. The single channel microcantilevers exhibited a dc response to different VOCs above particular threshold voltages, which were found out to be strongly correlated to the latent heat of evaporation for those analytes. At a constant dc bias which is above that threshold voltage, the magnitude of the response for any VOC is a function of concentration and molecular dipole moment of the VOC, which is another metric that can be easily determined and calibrated. While threshold voltage is a reliable indicator for uniquely identifying a VOC, the response magnitude can be used to estimate the concentration of the analyte also, down to low ppm range with a response time less than 40 s. The microcantilevers with two parallel channels are suitable for thermal conductivity based detection of any vapor or gas, therefore it helps pinpointing the VOCs even better in an event where two different VOCs have very close threshold voltages but significantly different thermal conductivities. A numerical model, based on three dimensional heat transfer and Joule heating equations, has also been developed for these microcantilevers. This model has been employed to explain the physical phenomena associated with the sensor under different bias conditions, and also to predict the response time of the heater alone, which is much smaller than the response time of the overall system. The noise limited resolution from the theoretical model is in the range of parts per billion and shows excellent promise for the future application of this kind of sensor in detecting VOCs with very low power consumption.

Rights

© 2015, Ifat Jahangir

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