Goddard technologist Vivek Dwivedi (right) and his collaborator, University of Maryland professor Raymond Adomaitis (left), are preparing to insert a sample inside a reactor that will apply a thin film using the atomic layer deposition technique. |
Space
can be a dangerous place. Micrometeorites, solar particles, and space
junk—everything from spent rocket stages to paint fragments—zip past
satellites at up to 12.4 miles (20 kilometers) per second, posing
hazards to their sensitive spacecraft optics, detectors, and solar
panels.
Although
engineers have developed different techniques to safeguard spacecraft
from these fast-moving whirling dervishes, nothing provides 100%
protection.
A
technologist at NASA’s Goddard Space Flight Center in Greenbelt, Md.,
however, is experimenting with an emerging technology that might provide
another, perhaps more effective, technique for defending sensitive
spacecraft components from the high-velocity bombardments.
Vivek
Dwivedi and his collaborator, chemical engineering professor Raymond
Adomaitis from the University of Maryland, College Park, are using
atomic layer deposition (ALD)—a rapidly evolving technology for coating
plastics, semiconductors, glass, Teflon, and a plethora of other
materials—to create a new super-strong, ultra-thin coating made of tiny
tubes of boron nitride, similar in appearance to the bristles on a
toothbrush.
“Crystalline
boron nitride is one of the hardest materials in the world,” Dwivedi
said, making it ideal as a coating to make sensitive spacecraft
component less susceptible to damage when struck by space dust, tiny
rocks, and high-energy solar particles.
Atomic layer deposition
The
ALD technique, which the semiconductor industry has adopted in its
manufacturing of computer chips, involves placing a substrate material
inside a reactor chamber and sequentially pulsing different types of
precursor gases to create an ultrathin film whose layers are literally
no thicker than a single atom.
ALD
differs from other techniques for applying thin films because the
process is split into two half reactions, is run in sequence, and is
repeated for each layer. As a result, technicians can accurately control
the thickness and composition of the deposited films, even deep inside
pores and cavities. This gives ALD a unique ability to coat in and
around 3D objects. This advantage—coupled with the fact that
technologists can create films at much lower temperatures than with the
other techniques—has led many in the optics, electronics, energy,
textile, and biomedical-device fields to replace older deposition
techniques with ALD.
According
to Dwivedi, if technicians use ALD to coat glass with aluminum oxide,
for example, they can strengthen glass by more than 80%. The resulting
thin films act like “nano putty,” filling the nanometer-scale defects
found in glass—the very same tiny cracks that cause glass to break when
struck by an object. “This ALD application has profound possibilities
for the next-generation crew modules,” Dwivedi said. “We could decrease
the thickness of the glass windows without sacrificing strength.”
“It’s
really exciting,” said Ted Swanson, Goddard’s assistant chief for
technology for mechanical systems. “This is an emerging technology that
offers a wholly new way to protect spacecraft components, perhaps more
effectively than what is possible with current techniques. Just as
important, with ALD, we can lay down material less expensively.”
Hardest materials in the world
This isn’t to say the task is easy, Dwivedi said.
Manufacturing
an ALD-based coating made of boron and other precursor gases is
exceptionally difficult to do. Currently, technologists manufacture
boron films by reacting boron powder with nitrogen and a small amount of
ammonia in a chamber that must be heated to a scorching 2,552 F—an
expensive process. With ALD, ultrathin boron-nitride film could be laid
in a chamber no hotter than 752 F.
“Our
team has studied the difficulties and think we understand why they’re
happening,” Dwivedi said. As a result, he believes the team will succeed
at depositing boron nitride on a silicon substrate by next year. If
subsequent tests at Goddard and NASA’s Langley Research Center in
Hampton, Va., prove the material’s effectiveness as a protective
coating, he believes instrument designers could one day use the
technology to coat mirrors, spacecraft buses, and other components. Such
test could occur as early as next summer.
In
addition to creating a protective coating, Dwivedi and his team are
using funding from Goddard’s Internal Research and Development program
and NASA’s Center Innovation Fund to test the technique as a possible
way to coat X-ray telescope mirrors, which must be curved to collect
high-energy X-ray photons that would otherwise pierce flat mirrors, and
radiators needed to direct heat away from sensitive instruments.
“This
technology can coat anything. It is perfect point-to-point. There are
so many applications for this technology,” Dwivedi said. “The only thing
limiting its use is your imagination.”
Source: Goddard Space Flight Center
