Thermal transpiration in mixed cellulose ester membranes: Enabling miniature, motionless gas pumps

Abstract

In this paper, we introduce the use of thin mesoporous mixed cellulose ester (polymer) membranes as the enabling element of miniature, motionless gas pumps. The pores within these membranes serve as channels that constrain gas molecules to the free molecular or transitional gas flow regimes. A temperature gradient across the membranes causes a transpiration based gas flow from the cold side to the hot side; this type of flow is the basis of Knudsen pumps. Gas flow characteristics and vacuum generation capabilities of polymer membranes with three different pore-sizes are reported. In this group, membranes with 25 nm pore-size provide superior functionality. For an input power of 1.4 W/cm 2, Knudsen pump test structures based on this membrane material provide a typical gas flow rate of ≈0.93 sccm/cm 2 in the absence of pressure load. The transient pressure response is used to quantify various non-idealities. Experiments suggest that these polymer membranes are relatively defect-free as compared to bulk microporous ceramics that were previously evaluated for similar applications. In longevity tests performed to date, a polymer pump has operated continuously for ≈6000 h without significant deterioration in its performance.