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A Motionless Gas Micropump Using Thermal Transpiration in Bulk Nanoporous Materials.

By Naveen Kumar Gupta


Thermal transpiration-driven Knudsen pumps have the ability to pump gas molecules without the use of any moving parts. This promises high structural reliability and low frictional losses. However, the dearth of suitable transpiration materials with appropriate properties has limited their performance, especially for atmospheric pressure operation. This thesis describes the use of bulk nanoporous materials for thermal transpiration-driven gas pumping at atmospheric pressure. A naturally-occurring zeolite, clinoptilolite, is used to demonstrate the feasibility of thermal transpiration-driven Knudsen pumps using bulk nanoporous ceramics. For an input power of 5.35W, the initial prototype has a temperature bias of 38K across the thickness of the zeolite disc. This results in a gas flow of ≈0.12sccm with a nominal pressure load of ≈50Pa at the output, or a maximum pressure head of ≈1kPa. Transient pressure response at the sealed outlet of a Knudsen pump is analyzed using a fitted model, which allows us to quantify various non-idealities. Several other synthetic nanoporous ceramics are also evaluated for their thermal transpiration-driven gas flow characteristics. A clay-based ceramic 15PC is identified as suitable for multistage Knudsen pumps that may accommodate higher pressure heads. While operating at 55K above room temperature, a 9-stage Knudsen pump is demonstrated to generate a maximum pressure head ≈12kPa, or a gas flow of ≈3.8µL/min. against a pressure head of 160Pa. The pump has a footprint of ≈8x8mm2/stage. To date, a multistage Knudsen pump has operated continuously for more than 7000 hours without any deterioration in its performance. Higher gas flow generation capabilities are demonstrated using thermal transpiration through nanoporous cellulose ester polymer membranes. For an input power of 1.4W, a single stage Knudsen pump with 11.5mm diameter and 105µm thick polymer membrane has a temperature bias of 30K across the membrane, which provides 0.4sccm flow against a 330Pa pressure head. Experiments suggest that the polymer Knudsen pump results in a thermal transpiration-driven gas flow of ≈1 sccm in absence of any external load. It has a final packaged volume of 14x14x4.5mm3. To date, a polymer pump has operated continuously for more than 600 hours without deterioration

Topics: Knudsen Pump, Thermal Transpiration, Bulk Nanoporous Materials, Polymer Membranes
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