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Microsystems in Spacecraft Thermal Control 197

high due to the short length of the supports. In addition, coating the gold membrane
has not yet achieved the high emissivity required.

9.4.4 MICROHEAT PIPES
To dissipate thermal energy, thermal engineers may use thermal doublers and heat
pipes to spread the heat within the structure. With tight footprint restrictions, the
conductive heat transfer enhancement by thermal doublers may be limited. For high
heat flux dissipation, heat pipes are proven to be more effective than thermal doublers
in achieving a uniform temperature distribution. Heat pipes achieve this through a
capillary driven, fluid phase change process. They are sealed tubes partially filled
with a working fluid, with a capillary wick acting as the pump. Large heat transfer
rates can be achieved at an almost constant temperature in the system.
Conventional heat pipes were used for thermal control on spacecraft as early
as 1964. A conventional heat pipe may have a capacity range from as high as a
few kilowatts to as low as a few watts. Heat pipes may be classified into two main
categories, constant-conductance and variable-conductance. Constant-conductance
heat pipes are used for stable heat loads while variable-conductance heat pipes
(VCHP) are used when the environmental sink or heat source varies, or when tighter
temperature control is desired. Common heat pipes are extruded from square or
finned aluminum or copper tubes ranging from about 0.5 to 2.0 cm in diameter, with
lengths usually less than 2.0 m. They have the capillary grooves or a mesh wick on
the inside. Common operating fluids include ammonia, water, and propylene.
MEMS-based heat exchange techniques have been investigated for cooling a
central processing unit (CPU) on ground applications. The technique targeted high-
performance CPUs that are used in very restricted spaces in workstations. Other
versions have been planned for other types of ICs, including graphics processors
and other dense ICs. In dealing with miniature or microscale heat removal within
high-power density electronics, SNL has been one of the leading institutions in
developing micro-machined vapor chamber heat spreaders. SNL has patented a
passively ‘‘smart’’ heat transfer mechanism to remove heat dissipated by computer
2
chips in the 50 W/cm range. The ‘‘chip heat pipes’’ mechanism uses small amounts
of vaporized liquid sealed in tiny flat pipes to move heat to the side edge of the
computer. 14 Air fins are used to dissipate the heat into its environments.
SNL has expanded chip heat pipes into the MEMS heat pipes arena. 15 Similar
to conventional heat pipes, SNL’s microheat pipes contain basic components of a
working fluid, a wick structure, and an envelope. As the fluid heats up and evaporates,
it moves to the cooler area where it condenses. This cyclic evaporation and conden-
sation distribute, or evaporation, and condensation distributes heat throughout the
substrate. Through a capillary action, the microheat pipes are capable of removing
heat from its source to a nearby heat sink passively and efficiently.
The structure of SNL’s microheat pipes comprises a copper ring separating two
copper plates. The advantage of SNL’s microheat pipes is that they can be etched to
follow curved or bent paths from a heat source to a heat sink, and go around
mounting holes, screws, or standoffs. These microheat pipes are made of two pieces

© 2006 by Taylor & Francis Group, LLC