Thermally driven Knudsen gas pump enhanced with a thermoelectric material.

Abstract

iii ACKNOWLEDGMENTS I am indebted to my advisor, Dr. Shamus McNamara, for this thesis. His abundant knowledge guided me right from the start to finish making sure the project headed in the right direction and the goals were achieved. He continued to educate and support me right from my Master's degree and it was under his able guidance that I was introduced to the exciting world of MEMS and microfabrication. Whether it was theory or the practical nuances of laboratory experiments he instilled the confidence within me and made me structure my independent thought process to carry out experiments with positive results. I have learned how to be a good researcher by following his principles. I developed the insight to observe minute details which could make a difference in the outcome of an experiment. He supported me financially as a research assistant in his laboratory and gave me the opportunity to work on multiple projects which broadened my horizon, which also included the cleanroom experience. This thesis was developed over a period of five years. The basic proof was already established by Dr. McNamara, while he was at the University of Michigan and by other researchers. I was very fortunate to have the opportunity to continue this project and to improve the performance of the project. The thesis focuses on improving the flowrate of the Knudsen gas pump. The Knudsen pump uses thermal transpiration as the driving mechanism to pump gas. It is a motionless gas pump as the pump does not require any moving actuators for pumping.The thermally driven gas flow is accomplished in the molecular or transitional gas flow regime. The advantage of this pump is that without any moving parts it avoids friction losses and stiction problems which devices in micro scale are prone to suffering due to scaling issues. Thus, this pump is highly robust and reliable. Knudsen pumps in the past have suffered from the drawback of low flowrates and inability to operate at atmospheric pressure. In the early days lack of micromachining technologies limited minimum channel size which had to be operated at lower than atmospheric pressure to achieve free molecular flow. Various designs have been implemented with an impetus on increasing the flowrate of the pump.The key to this pump is establishing a temperature difference along the length of the channel. A higher temperature difference over a shorter channel length makes the vi pump more efficient. Pump channels have been made out of various materials like silicon, glass and polymer. The silicon microfabricated single channel conventional design pump suffered from the high thermal conductivity of silicon, which limited the thermal gradient that could be achieved. Silicon was replaced by glass, which has a lower thermal conductivity. The glass micro fluidic pump could pump water in reservoirs but at a slow rate. Renewable forms of Knudsen pump were also made by using nanoporous silica colloidal crystals which are robust and could use solar energy and body heat to create a temperature difference ...