Thermal Transpiration in Microsphere Membranes

Abstract. Self-assembled glass microsphere membranes as an alternative transpiration membrane for application in a Knudsen Compressor are discussed. A performance model is constructed and used to compare the performance of glass microsphere membranes to silicon aerogel membranes for this application. An initial experimental Knudsen Compressor stage based on glass microsphere membranes has been designed and experimentally tested. Preliminary performance results show a discrepancy between the predicted and observed pressure differences produced by the single stage. Several possible explanations for the discrepancy are discussed. Two variations of a proposed design for a Knudsen Compressor employing a microsphere transpiration membrane are discussed. It is concluded that beds of glass microspheres may be attractive candidates for transpiration membrane materials over the entire pressure range of operation for a micro-scale vacuum pump, 10mTorr to 760 Torr