Heat Transfer Implications in the First MEMS Fabricated Thermal Transpiration-Driven Vacuum Pump for Gases

Abstract

The success of NASA’s future space missions and the development of portable, commercial instrument packages will depend greatly on miniaturized components enabled by the use of microelectromechanical systems (MEMS). Both of these application markets for miniaturized instruments are governed by the use of MEMS components that satisfy stringent power, mass, volume, contamination and integration requirements. An attractive MEMS vacuum pump for instruments requiring vacuum conditions is the Knudsen Compressor, which operates based on the rarefied gas dynamics phenomenon of thermal transpiration. Thermal transpiration describes the regime where gas flows can be induced in a system by maintaining temperature differences across porous materials under rarefied conditions. This pumping mechanism provides two overwhelmingly attractive features as a miniature vacuum pump — no moving parts and no working fluids or lubricants. Due to favorable power, volume and mass estimates a Knudsen Compressor fabricated using MEMS fabrication techniques (lithography, deep reactive ion etching) and new materials (silicon, aerogel) has been completed. The experimental testing of this MEMS Knudsen Compressor device’s thermal and pumping performance are outlined in this manuscript. Good agreement between experiments and numerical predictions using a transitional flow analysis have also been obtained although simple simulations based on the aerogel’s structure are difficult to perform.