James Hone once said,
so strong that "it would take an elephant, balanced on a pencil, to
break through a sheet of graphene the thickness of Saran Wrap." Credit:
Andrew Shea for Columbia
Read more at: http://phys.org/news/2013-05-defects-graphene-strongest-material-world.html#jCp
the strongest material
ever measured and, as Columbia Engineering Professor James Hone once
said, so strong that "it would take an elephant, balanced on a pencil,
to break through a sheet of graphene the thickness of Saran Wrap."
Credit: Andrew Shea for Columbia Engineering
Read more at: http://phys.org/news/2013-05-defects-graphene-strongest-material-world.html#jCp
In a new study, published in Science, Columbia Engineering
researchers demonstrate that graphene, even if stitched together from
many small crystalline grains, is almost as strong as graphene in its
perfect crystalline form. This work resolves a contradiction between
theoretical simulations, which predicted that grain boundaries can be
strong, and earlier experiments, which indicated that they were much
weaker than the perfect lattice. Graphene consists of a single atomic
layer of carbon, arranged in a honeycomb lattice. "Our first Science
paper, in 2008, studied the strength graphene can achieve if it has no
defects -- its intrinsic strength," says James Hone, professor of
mechanical engineering, who led the study with Jeffrey Kysar, professor
of mechanical engineering. "But defect-free, pristine graphene exists
only in very small areas. Large-area sheets required for applications
must contain many small grains connected at grain boundaries, and it was
unclear how strong those grain boundaries were. This, our second
Science paper, reports on the strength of large-area graphene films
grown using chemical vapor deposition (CVD), and we're excited to say
that graphene is back and stronger than ever."
The study verifies that commonly used methods for post-processing
CVD-grown graphene weaken grain boundaries, resulting in the extremely
low strength seen in previous studies. The Columbia Engineering team
developed a new process that prevents any damage of graphene during
transfer. "We substituted a different etchant and were able to create
test samples without harming the graphene," notes the paper's lead
author, Gwan-Hyoung Lee, a postdoctoral fellow in the Hone lab. "Our
findings clearly correct the mistaken consensus that grain boundaries of
graphene are weak. This is great news because graphene offers such a
plethora of opportunities both for fundamental scientific research and
industrial applications."
In its perfect crystalline form, graphene (a one-atom-thick carbon
layer) is the strongest material ever measured, as the Columbia
Engineering team reported in Science in 2008 -- so strong that, as Hone
observed, "it would take an elephant, balanced on a pencil, to break
through a sheet of graphene the thickness of Saran Wrap." For the first
study, the team obtained small, structurally perfect flakes of graphene
by mechanical exfoliation, or mechanical peeling, from a crystal of
graphite. But exfoliation is a time-consuming process that will never be
practical for any of the many potential applications of graphene that
require industrial mass production.
Currently, scientists can grow sheets of graphene as large as a
television screen by using chemical vapor deposition (CVD), in which
single layers of graphene are grown on copper substrates in a
high-temperature furnace. One of the first applications of graphene may
be as a conducting layer in flexible displays.
"But CVD graphene is 'stitched' together from many small crystalline
grains -- like a quilt -- at grain boundaries that contain defects in
the atomic structure," Kysar explains. "These grain boundaries can
severely limit the strength of large-area graphene if they break much
more easily than the perfect crystal lattice, and so there has been
intense interest in understanding how strong they can be."
The Columbia Engineering team wanted to discover what was making CVD
graphene so weak. In studying the processing techniques used to create
their samples for testing, they found that the chemical most commonly
used to remove the copper substrate also causes damage to the graphene,
severely degrading its strength.
Their experiments demonstrated that CVD graphene with large grains is
exactly as strong as exfoliated graphene, showing that its crystal
lattice is just as perfect. And, more surprisingly, their experiments
also showed that CVD graphene with small grains, even when tested right
at a grain boundary, is about 90% as strong as the ideal crystal.
"This is an exciting result for the future of graphene, because it
provides experimental evidence that the exceptional strength it
possesses at the atomic scale can persist all the way up to samples
inches or more in size," says Hone. "This strength will be invaluable as
scientists continue to develop new flexible electronics and ultrastrong
composite materials."
Strong, large-area graphene can be used for a wide variety of
applications such as flexible electronics and strengthening components
-- potentially, a television screen that rolls up like a poster or
ultrastrong composites that could replace carbon fiber. Or, the
researchers speculate, a science fiction idea of a space elevator that
could connect an orbiting satellite to Earth by a long cord that might
consist of sheets of CVD graphene, since graphene (and its cousin
material, carbon nanotubes) is the only material with the high
strength-to-weight ratio required for this kind of hypothetical
application.
The team is also excited about studying 2D materials like graphene.
"Very little is known about the effects of grain boundaries in 2D
materials," Kysar adds. "Our work shows that grain boundaries in 2D
materials can be much more sensitive to processing than in 3D materials.
This is due to all the atoms in graphene being surface atoms, so
surface damage that would normally not degrade the strength of 3D
materials can completely destroy the strength of 2D materials. However
with appropriate processing that avoids surface damage, grain boundaries
in 2D materials, especially graphene, can be nearly as strong as the
perfect, defect-free structure."
The study was supported by grants from the Air Force Office of Scientific Research and the National Science Foundation.
Source: Columbia University
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