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198 MEMS and Microstructures in Aerospace Applications

of silicon with microscopic wicking surfaces etched into the surfaces. The fluid path
in the pipe contains finely etched lines about as deep as fingerprints and 60 mm
wide. SNL’s microheat pipes use methanol as cooling fluid. Methanol or other types
of fluids will circulate inside the pipes and remove heat from, for example, a heat
source such as a distributed battery. As is true for all heat pipes, the operation of the
microheat pipes involves the phase change of the working fluid to maximize its heat
transfer capability. This provides an added advantage of maintaining the hardware
in a constant temperature range. Once heated, vaporized methanol flows by con-
vection to the heat sink where it gives up its heat, condenses back to liquid phase,
and returns to the hot end. The heat and cooling of the working fluid completes the
evaporation and condensation cycle of the microheat pipes.
One basic issue in spacecraft thermal control is heat dissipation from densely
packaged electronic parts assembled in electronic boxes mounted on a spacecraft
platform. 16 In solving thermal related problems, SNL has actively engaged in the
development of MEMS-based heat pipes for future space application. To further
ensure MEMS advances for defense, aerospace, and commercial applications, SNL
has established several cooperative research and development agreements with
aerospace companies. Preliminary results demonstrated that the development of
microheat pipes is steady and relatively successful. As shown in the literature,
microheat pipes range in size from 1 mm in diameter and 60 mm in length to 30
mm in diameter and 10 mm in length. The methods currently used to fabricate
microheat pipes with hydraulic diameters on the order of 20 to 150 mm into silicon
or gallium arsenide wafers are also available. 17

9.4.5 MEMS PUMPED LIQUID COOLING SYSTEM
Pumped fluid loops are active thermal control systems. A simplified loop consists of
a pumping device, a heat exchanger, and a space radiator. Pumped fluid loops are
devices operated under the principle of forced liquid convective cooling. Cooling
is accomplished when the working fluid absorbs excess heat and transports it to a
heat sink. The loops can effectively maintain temperatures even when the spacecraft
dissipates high power or operates under environmental extremes. For example,
NASA’s Mars Pathfinder used a mechanically pumped single-phase cooling loop
with Freon-11 as a working fluid to achieve a cooling power capacity of 90 to
180 W. Chip level temperature control by micropumped loop device was reported
by Pettigrew et al. 18 In this work, an evaporator, condenser, reservoir, and liquid
and vapor lines were etched into the silicon wafer, while the glass wafer serves as a
cover plate into which grooves were etched for capillary pumping. The device had a
1 2 mm evaporator and was capable of operating at a constant 1008C.
As spacecraft get ever smaller, many thermal control devices will be required to
miniaturize. Aiming at future deep space science exploration, the NASA Jet Pro-
pulsion Laboratory (JPL) has investigated a MEMS cooling system for micro- or
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nanospacecraft. Although the current pumped liquid cooling system is designed to
transfer large amounts of thermal energy between two locations on a spacecraft, it is
not capable of handling heat transfer in high power density applications. Power

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