SUBJECT: Ph.D. Dissertation Defense
BY: Ardalan Lotfi
TIME: Friday, March 5, 2021, 9:00 a.m.
PLACE:, Online
COMMITTEE: Dr. Peter J Hesketh, Chair (ME)
Dr. S Mostafa Ghiaasiaan (ME)
Dr. F Levent Degertekin (ME)
Dr. Satish Kumar (ME)
Dr. Milad Navaei (GTRI)


The recent advancement in Micro and Nano technology have made possible very small and multi-target gas sensors. This work focuses on developing a new generation of the thermal conductivity detectors. This research is aimed at enhancing the Limit of Detection (LOD), developing new measurement methods, developing new models to predict sensor behavior and bring TCD gas sensor technology together with gas chromatography so ultimately it can be used in portable applications, such as micro-GC systems. A key attribute in the design of TCD is material selection. Therefore, platinum was chosen as sensing element. In this research, the goal of materials selection for the TCD insulating layer was to reduce thermal induced stress by choosing aluminum oxide, which has the closest value of thermal expansion to that of platinum. The developed COMSOL model for the TCD sensor was used to study the influence of the cross-section variation and gap size effect on the sensor performance. As a result of studying the TCD sensor design principle and sensitivity enhancement, which required high operating temperature, the cantilever-based TCD was proposed. The combination of proposed materials and new concept for TCD geometrical design promised further miniaturization of the TCD and gave birth to a new generation of TCD sensor; the cantilever-based platinum TCD. Several generations of cantilever-based TCD were designed and fabricated. The geometrical design of the sensor and the lithography mask both were precisely engineered to overcome the lithography and lift-off process limitations. The fabrication of thin film cantilever using balanced platinum sandwich structure was a novel design and considered a breakthrough in the TCD sensor engineering. A 2D COMSOL model was developed to study the TCD sensor behavior using the 3-Omega technique. A new concept of TCD was proposed that has two active layers; a heating layer and a sensing layer. A model for the two active layers TCD was developed to study the time delay between the sensing and heating layer. The adaption of 3-Omega technique for double active layers TCD was investigated. The fabricated TCD sensor was integrated with a commercial GC to test the real-world application of such a sensor. The fabricated TCD sensor successfully detected 5 ppm ammonia gas and the limit of detection of the sensor was calculated to be 1.1 ppm. As a result, the concept of cantilever-based TCD combined with 3-Omega technique could successfully operate at a temperature of 300 ˚C and, consequently, enhance the limit of detection of such a sensor. A noise model for the cantilever-based TCD sensor was developed to study the influence of a different source of noise and their accumulated effect on the sensor limit of detection. The 3-omega technique reduced the total noise contributed to the measurements lowering the limit of detection, despite abundance of noise sources.