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Rensselaer researchers
demonstrated that a small carbon nanotube block such as this can
be used to create an effective, highly sensitive pressure sensor.
When the block is compressed,
individual carbon nanotubes start to buckle, which in turn
decreases the block's electrical resistance. Researchers can
measure this resistance in order to determine precisely how much
pressure is being placed on the block.
Photos ďż˝ by
Rensselaer/V. Pushparaj |
A sensor incorporating the carbon nanotube
block would be able to detect very slight weight changes and would be
beneficial in any number of practical and industrial applications,
Sreekala said. Two potential applications are a pressure gauge to
check the air pressure of automobile tires, and a
microelectromechanical pressure sensor that could be used in
semiconductor manufacturing equipment.
Despite extensive research over the past decade into the mechanical
properties of carbon nanotube structures, this study is the first to
explore and document the materials strain-resistance relationship.
The paper, titled "Effects of compressive strains on electrical
conductivities of a macroscale carbon nanotube block" was published
in a recent issue of Applied Physics Letters.
Over the course of the experiment, the researchers placed the carbon
nanotube block in a vice-like machine and applied different levels of
stress. They took note of the stress applied and measured the
corresponding strain put on the nanotube block. As it was being
squeezed, the researchers also sent an electrical charge through the
block and measured its resistance, or how easily the charge moved from
one end of the block to the other.
The research team discovered that the strain they applied to the block
had a linear relationship with the blocks electrical resistance. The
more they squeezed the block, the more its resistance decreased. On a
graph, the relationship is represented by a neat, straight line. This
means every time one exposes the block to a load of X, they can
reliably expect the blocks resistance to decrease by Y.
This reliability and predictability of this relationship makes the
carbon nanotube block an ideal material for creating a highly
sensitive pressure sensor, Sreekala said.
The pressure sensor would function similarly to a typical weight scale.
By placing an object with an unknown weight onto the carbon nanotube
block, the block would be squeezed down and its electrical resistance
would decrease. The sensor would then send an electrical charge
through the nanotube block, and register the resistance. The exact
weight of the object could then be easily calculated, thanks to the
linear, unchanging relationship between the block�s strain and
resistance.
A study published earlier this year (http://news.rpi.edu/update.do"artcenterkey=2217),
written by Rensselaer senior research specialist Victor Pushparaj, who
is also an author of the pressure sensor paper, showed that carbon
nanotubes are able to withstand repeated stress yet retain their
structural and mechanical integrity. Electrical resistance decreases
as the block is squeezed, as the charged electrons have more pathways
to move from one end of the block to the other.
In the new study, Sreekala and the research team found that the
nanotube blocks linear strain-resistance relationship holds true
until the block is squeezed to 65 percent of its original height.
Beyond that, the blocks mechanical properties begin to fail and the
linear relationship breaks down.
The team is currently thinking of ways to boost the nanotubes
strength by mixing them with polymer composites, to make a new
material with a longer-lived strain-resistance relationship.
"The challenge will be to choose the correct polymer so we don't lose
efficiency, but retain the same response in all directions", Sreekala
said.
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