Thermal Transpiration As a Co-Located Micro-Scale Source of High Pressure Gas for MEMS Devices

Abstract Applications of the Knudsen Compressor, a thermal transpiration pump, as a co-located source of high-pressure gas (greater than 1 atm.) for MEMS valves, actuators, and fluid flow devices has been investigated. Relying only on existing materials and technologies, it is shown that it should be possible to construct MEMS Knudsen Compressors to provide compressed gas up to pressures of about 10 atm while making use of ambient gases as the working medium. The theoretical performance of Knudsen Compressors operating from 1 atm up to 10 atm is evaluated with a previously developed rarefied gas dynamic, transitional flow model for Knudsen Compressor cascades. In one design a 27 stage Knudsen Compressor provides a pressure increase from 1 atm to 10 atm at a flow rate of 2 × 10−3 atm-cm3/s. The compressor occupies a volume of 40 mm3 and has a power requirement of 0.25W. An apparent high-pressure limit for Knudsen Compressors, roughly 100 atm, is discussed. The pressure range between 10 atm. and 100 atm. requires nanometer and smaller size capillaries in the transpiration membrane. Several physical effects that become important at these small dimensions are identified and discussed along with their qualitative influence on the performance of the Knudsen Compressor. It is concluded that existing materials with sub-nanometer capillaries have thermal conductivities that are too high, leading to inappropriate power consumption for a MEMS device; for now, the practical upper limit on the pressure to which a MEMS Knudsen Compressor can operate is around 10 atm.