SUBJECT: Ph.D. Dissertation Defense
BY: Alireza Mahdavifar
TIME: Tuesday, November 24, 2015, 12:45 p.m.
PLACE: Love Building, 210
TITLE: Computational and Experimental Development of Ultra-Low Power and Sensitive Micro-Electro-Thermal Gas Sensor
COMMITTEE: Dr. Peter Hesketh, Chair (ME)
Dr. Hamid Garmestani (MSE)
Dr. Mostafa Ghiaasiaan (ME)
Dr. Todd Sulcheck (ME)
Dr. Shannon Yee (ME)


The growing market of gas sensing applications along with elevated need for environmental and health monitoring has provided significant motivation for new improvements in the field of gas sensing. Although several technologies are currently available, those have limitations in aspects such as: high cost, limited life time, high power consumption, need for repeated calibrations, and outdated methods. The electro-thermal sensing mechanism, on the other hand, can potentially addresses many of these problems because it doesn’t depend on chemical reactions. However implementation of this technology for practical purposes, requires more efficient designs, scientific understanding and new methods of measurements, that are aims of the present thesis. In present work a state-of-the-art micro-thermal conductivity detector is developed based on MEMS technology. Its design include a miniaturized 100 µm bridge from doped polysilicon, suspended on Si substrate and passivated by silicon nitride for improved life time. Analytical models were developed that describe relationship between sensor response and material properties. To obtain local temperature distribution, a computational three dimensional simulation based on real geometry and minimal simplifications was developed that can describe both steady-state and transient state phenomena and includes multiple physics such as flow, heat transfer, electrical current and thermal stresses. Two novel measurement methods are developed in this research, that provide lower power consumption, faster measurement, better sensitivity and less calibration. A time resolved method based on transient response of the detector to a step current pulse was introduced that correlates time constant of the response to the concentration of gas mixture. The other method is based on AC excitation of the micro detector; analysis showed that the amplitude and phase of the third harmonic of the resulting output signal is related to gas composition. The developed micro-sensor was tested in a GC system and was compared against conventional and complex FID for the detection of a mixture of VOCs. Moreover compact electronics and telemetry modules were developed that for highly portable applications.