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.

Figure 1. One Stage of a Knudsen Compressor exploits thermal transpiration to sustain a small net increase in pressure. Multiple stages like this one are cascaded in order to sustain a usefully large overall increase in pressure.
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.

Figure 2. Advanced Materials and Fabrication Techniques are essential to the realization of this prototype of a one stage of a microscale Knudsen compressor. The transpiration medium is a 520-¼m-thick SiO2-aerogel membrane. The hot- and cold-side thermal guards are 400-¼m-thick micromachined silicon chips.
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.

Topics:
Compressors Conductivity Connectors and terminals Electrical systems Fabrication Gases Glass Manufacturing processes Materials properties Parts Physical Sciences Pressure Pumps Vacuum
This Brief includes a Technical Support Package (TSP).
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Microscale Thermal-Transpiration Gas Pump

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NASA Tech Briefs Magazine

This article first appeared in the February, 2003 issue of NASA Tech Briefs Magazine (Vol. 27 No. 2).

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Overview

The document discusses the development of a microscale thermal-transpiration gas pump, specifically a Knudsen compressor, created by researchers at NASA's Jet Propulsion Laboratory (JPL). This innovative device operates on the principle of thermal transpiration, which involves the movement of gas molecules through a porous medium in response to temperature differences. The Knudsen compressor consists of multiple stages, each designed to achieve a small net increase in pressure, ultimately resulting in a significant overall pressure increase.

The design features a capillary section and a connector section. The capillary section, made from a silica aerogel membrane, allows gas to flow through tiny pores, while the connector section maintains a constant pressure as the gas temperature decreases before entering the next stage. The temperature gradient is crucial for the operation of the compressor, as it drives the thermal transpiration process. The document highlights that the efficiency of the compressor increases as the flow transitions from the continuum to the free-molecular regime.

The prototype described includes two silicon chips functioning as thermal guards, which can be heated or cooled to create the necessary temperature differential across the aerogel membrane. The aerogel's low thermal conductivity and small pore size are essential for effective thermal transpiration. The design also incorporates low-thermal-expansion glass plenums and aluminum vacuum connectors, ensuring stability and performance.

The document emphasizes the advantages of this solid-state compressor, particularly its lack of moving parts, which eliminates the need for lubricants and allows for more compact and reliable operation. This technology is particularly suited for applications in environments where traditional pumps may be impractical, such as in space exploration or portable scientific instruments.

In summary, the microscale Knudsen compressor represents a significant advancement in vacuum technology, leveraging the principles of thermal transpiration and micromachining to create an efficient, compact, and reliable gas pump. The work is a collaboration among researchers Stephen Vargo, E. Phillip Muntz, and Geoff Shiflett, and it showcases the potential for future applications in various fields, including aerospace and microelectronics.