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Micro Machining Enhances Precision Fabrication
Industrial Productivity
Originating Technology/NASA Contribution
In President Ronald Reagan’s 1984 State of the Union address,
he announced plans for a U.S. space station, the equivalent
of the Russian space station, Mir. This announcement set
off a flurry of congressional funding debates, and it was
not until 1988 that the President announced that a consensus
had been reached and the project would go forward. The project
was named “Space Station Freedom.”
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Artist’s
concept of the Space Station Freedom, a design that
led to many of the technologies currently aboard
the International Space Station. |
It was to be a permanently manned, Earth-orbiting outpost
that could serve as a repair shop for the shuttle fleet,
a microgravity laboratory, an observation point for astronomers,
and an assembly station for spacecraft heading even farther
into space. The project came to an end, unrealized, in 1990,
when it was revealed that the design was over budget, overweight,
and had been so complicated and compromised by the political
debates and budget restrictions that it could no longer
be realized.
Although Freedom never made it past the design stages, the
science, and many aspects of the original designs, made their
way onto the International Space Station (ISS), whose first
component was launched in 1998. For instance, several advanced
thermal systems developed for cooling everything from battery
components to crew cabins originally designed for Space Station
Freedom are still in use on the ISS.
These thermal systems, while more advanced and specialized
than most used on Earth, like many terrestrial refrigerators,
employ evaporative ammonia as their coolant. To create this
simple refrigerant, heat is applied to a mixture of water
and ammonia until the solution reaches the boiling point
of the ammonia. The boiling solution then flows to another
chamber, where the ammonia gas separates from the water.
The gas floats upwards to a condenser, where a series of
fins and coils cool it, allowing it to condense back into
a liquid. The liquid ammonia then flows to an evaporator,
where it is mixed with hydrogen gas, and, when it evaporates,
produces cold temperatures. Having fulfilled its refrigerating
purposes, the ammonia, along with the hydrogen, mixes again
with the water. This solution releases the hydrogen gas,
where it returns to await the ammonia gas, as the cycle continues.
Even though this same series of chemical reactions is used
in space-bound refrigerators as in terrestrial versions,
the space-bound coolers must have one major difference: they
must be significantly smaller. Refrigerators are notoriously
heavy, and weight is always a payload concern for space-bound
equipment, so the Space Agency needed to engineer smarter,
more efficient thermal systems.
Partnership
In the 1980s, through two Small Business
Innovation Research (SBIR) contracts with Johnson Space Center, Dr. Javier Valenzuela
worked on a project to develop an ammonia evaporator for
thermal management systems aboard Freedom. At the time, he
was serving as the principal investigator for these contracts
while working with Creare Inc., of Hanover, New Hampshire.
In 1991, Valenzuela formed Mikros
Technologies Inc., based
in Claremont, New Hampshire, to commercialize the work he
had done under the NASA contracts. In 2001, the company was
awarded two SBIR research contracts from Goddard Space Flight
Center and is, to this day, actively engaged in advancing
micro-fabrication and high-performance thermal management
technologies.
Product Outcome
|
Breadboard
and test station of Mikros’ High-Heat Flux Evaporator
for the Phase II NASA SBIR project. |
The technique Valenzuela developed was an advanced form of
micro-electrical discharge machining (micro-EDM) to make
tiny holes in the ammonia evaporator. The evaporator relied
on droplet impingement cooling to achieve high heat flux
and low thermal resistance. Many thousands of small nozzles
were required to “print” a thin layer of ammonia over the
surface of the evaporator. However, no technology existed
at that time for the fabrication of suitably shaped micro-nozzles
in metals. Mikros’ micro-EDM technology was developed to
meet this need. Micro-EDM is an erosion process, in which
electrical discharges between two electrodes (one on the
tool, and the other on the conductive work surface) machine
very small holes, channels, and cavities.
Micro-EDM can cut complex shapes into a variety of conductive
materials, regardless of the material’s hardness, even hard
materials such as steels and carbides. It can also be used
on materials like ferrites and silicon, which have the tendency
to crack or become brittle when exposed to traditional (macro)
EDM.
All EDM has the additional advantage of noncontact machining.
The method, which relies on a gap between the tool and work
surface for the discharging of the spark, allows the process
to take place with no pressure being placed on the material.
This means that micro-EDM can be used on very thin and very
fragile surfaces, and even on curved surfaces, without damaging
them.
Mikros has had great success applying this method to the
fabrication of micro-nozzle array systems for industrial
ink jet printing systems. The company is currently the world
leader in fabrication of stainless steel micro-nozzles for
this market. It routinely fabricates nozzle arrays with hundreds
or even thousands of shaped nozzles with diameters ranging
from 30 to 80 microns. The micro-nozzle arrays are machined
in thin, free-standing foils, or on foils that are diffusion
bonded to thicker substrates with complex internal flow passages.
The company has continued its relationship with the Space
Agency, too. Its other division, in addition to micro-EDM,
is thermal systems, which is currently still involved with
Goddard through the SBIR contracts to develop a high-heat
flux capillary evaporator for use in loop heat pipes or capillary
pumped loops.
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