(University
of Texas at Dallas scientists have constructed novel fibers by wrapping
sheets of tiny carbon nanotubes to form a sheath around a long rubber
core. This illustration shows complex two-dimensional buckling, shown in
yellow, of the carbon nanotube sheath/rubber-core fiber. The buckling
results in a conductive fiber with super elasticity and novel electronic
properties./Credit: UT Dallas Alan G. MacDiarmid NanoTech Institute)
An
international research team based at The University of Texas at Dallas
has made electrically conducting fibers that can be reversibly stretched
to over 14 times their initial length and whose electrical conductivity
increases 200-fold when stretched.
The research
team is using the new fibers to make artificial muscles, as well as
capacitors whose energy storage capacity increases about tenfold when
the fibers are stretched. Fibers and cables derived from the invention
might one day be used as interconnects for super-elastic electronic
circuits; robots and exoskeletons having great reach; morphing aircraft;
giant-range strain sensors; failure-free pacemaker leads; and
super-stretchy charger cords for electronic devices.
In
a study published in the July 24 issue of the journal Science, the
scientists describe how they constructed the fibers by wrapping
lighter-than-air, electrically conductive sheets of tiny carbon
nanotubes to form a jelly-roll-like sheath around a long rubber core.
The
new fibers differ from conventional materials in several ways. For
example, when conventional fibers are stretched, the resulting increase
in length and decrease in cross-sectional area restricts the flow of
electrons through the material. But even a "giant" stretch of the new
conducting sheath-core fibers causes little change in their electrical
resistance, said Dr. Ray Baughman, senior author of the paper and
director of the Alan G. MacDiarmid NanoTech Institute at UT Dallas.
One
key to the performance of the new conducting elastic fibers is the
introduction of buckling into the carbon nanotube sheets. Because the
rubber core is stretched along its length as the sheets are being
wrapped around it, when the wrapped rubber relaxes, the carbon
nanofibers form a complex buckled structure, which allows for repeated
stretching of the fiber.
"Think of the buckling
that occurs when an accordion is compressed, which makes the inelastic
material of the accordion stretchable," said Baughman, the Robert A.
Welch Distinguished Chair in Chemistry at UT Dallas.
"We
make the inelastic carbon nanotube sheaths of our sheath-core fibers
super stretchable by modulating large buckles with small buckles, so
that the elongation of both buckle types can contribute to elasticity.
These amazing fibers maintain the same electrical resistance, even when
stretched by giant amounts, because electrons can travel over such a
hierarchically buckled sheath as easily as they can traverse a straight
sheath."
Dr. Zunfeng Liu, lead author of the study
and a research associate in the NanoTech Institute, said the structure
of the sheath-core fibers "has further interesting and important
complexity." Buckles form not only along the fiber's length, but also
around its circumference.
"Shrinking the fiber's
circumference during fiber stretch causes this second type of reversible
hierarchical buckling around its circumference, even as the buckling in
the fiber direction temporarily disappears," Liu said. "This novel
combination of buckling in two dimensions avoids misalignment of
nanotube and rubber core directions, enabling the electrical resistance
of the sheath-core fiber to be insensitive to stretch."
By
adding a thin overcoat of rubber to the sheath-core fibers and then
another carbon nanotube sheath, the researchers made strain sensors and
artificial muscles in which the buckled nanotube sheaths serve as
electrodes and the thin rubber layer is a dielectric, resulting in a
fiber capacitor. These fiber capacitors exhibited a capacitance change
of 860 percent when the fiber was stretched 950 percent.
"No
presently available material-based strain sensor can operate over
nearly as large a strain range," Liu said. Adding twist to these
double-sheath fibers resulted in fast, electrically powered torsional --
or rotating -- artificial muscles that could be used to rotate mirrors
in optical circuits or pump liquids in miniature devices used for
chemical analysis, said Dr. Carter Haines BS'11, PhD'15, a research
associate in the NanoTech Institute and an author of the paper.
In
the laboratory, Nan Jiang, a research associate in the NanoTech
Institute, demonstrated that the conducting elastomers can be fabricated
in diameters ranging from the very small -- about 150 microns, or twice
the width of a human hair -- to much larger sizes, depending on the
size of the rubber core. "Individual small fibers also can be combined
into large bundles and plied together like yarn or rope," she said.
"This
technology could be well-suited for rapid commercialization," said Dr.
Raquel Ovalle-Robles MS'06 PhD'08, an author on the paper and chief
research and intellectual properties strategist at Lintec of America's
Nano-Science & Technology Center. "The rubber cores used for these
sheath-core fibers are inexpensive and readily available," she said.
"The only exotic component is the carbon nanotube aerogel sheet used for
the fiber sheath."
Last year, UT Dallas licensed
to Lintec of America a process Baughman's team developed to transform
carbon nanotubes into large-scale structures, such as sheets. Lintec
opened its Nano-Science & Technology Center in Richardson, Texas,
less than 5 miles from the UT Dallas campus, to manufacture carbon
nanotube aerogel sheets for diverse applications.
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