A recent addition to the growing class of microelectromechanical systems (MEMS) is a single stage of a Knudsen compressor. This device was fabricated and tested to demonstrate the feasibility of Knudsen compressors as miniature vacuum pumps for future portable scientific instruments. The attributes of Knudsen compressors that make them attractive as miniature vacuum pumps are that they contain no moving parts and operate without need for lubricants or working fluids.
A Knudsen compressor exploits thermal transpiration of a rarefied gas. The principle of thermal transpiration can be described in terms of an example of two volumes of gas at different temperatures T1 and T2 connected by a tube with a radius smaller than the mean free path (λ) of gas molecules. The behavior of this system depends on the Knudsen number (Kn º λ/L, where L is a characteristic linear dimension of the tube): For Kn less than about 0.01 λ/L, the gas flows as a continuum; for Kn between about 0.01 and 10, the flow behavior of the gas is transitional between the continuum and free-molecular regimes; for Kn of about 10 or more, the flow regime is free-molecular. In the free-molecular regime, simple balancing of the equilibrium molecular fluxes leads to the following equation for the equilibrium pressures in the two volumes:
p1/p2 = (T1/T2)1/2.
The pressure differential can be exploited for pumping.
The advent of MEMS fabrication techniques and of nanopore materials with low thermal conductivities has made it possible to exploit thermal transpiration as more than a laboratory curiosity. This is because passages in pumping devices can now be made so narrow that transitional or free-molecular flow conditions can be obtained in these devices, even at pressures as high as atmospheric.
A Knudsen compressor is a cascade of multiple, individually heated compressor stages that exploit thermal transpiration. Figure 1 is a simplified schematic diagram of single stage, which includes a capillary and a connector section. By virtue of thermal transpiration, an increase in temperature along the capillaries results in an increase in pressure along the capillaries. The capillary section is followed by the connector section, where the pressure is approximately constant while the temperature falls to its lower value prior to entry to the next stage. The difference in pressure between the hotter and colder sides depends on the Kn values and other parameters; in general, it increases with the transition from the continuum to the free-molecular regime.
The prototype microscale single Knudsen compressor stage (see Figure 2) includes two silicon chips that serve as hot-side and cold-side thermal guards (the hot-side thermal guard corresponding approximately to the connector section described above), an SiO2-aerogel membrane (corresponding approximately to the capillary section described above), two low-thermal-expansion glass plenums, and aluminum vacuum connectors. The role of the thermal guards is to adjust the temperatures of molecules to the desired different values on the opposite sides of the aerogel transpiration membrane: Each silicon chip contains a dense array of 20-μm-diameter through holes, made by deep reactive-ion etching, that serve as tubes for heating or cooling the gas in them. Thin gold film heaters are patterned on both silicon chips; hence, either silicon chip can be the hot-side thermal guard. The aerogel has an average pore size of 20 nm and a very low thermal conductivity (17 W/K at atmospheric pressure), and thus satisfies the essential requirements for thermal transpiration to occur when a voltage is applied to one of the heaters.
This work was done by Stephen Vargo, E. Phillip Muntz, and Geoff Shiflett of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Physical Sciences category. NPO-21110.
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Microscale Thermal-Transpiration Gas Pump
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