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Simulasi  rancangan  material near-frictionless

Date:

July 21, 2015

Source:

DOE/Argonne National Laboratory

Summary:

Scientists used the Mira supercomputer to identify and improve a new mechanism for eliminating friction, which fed into the development of a hybrid material that exhibited superlubricity at the macroscale for the first time. Researchers helped enable the groundbreaking simulations by overcoming a performance bottleneck that doubled the speed of the team's code.

Para ilmuwan menggunakan superkomputer Mira untuk mengidentifikasi dan memperbaiki mekanisme baru untuk menghilangkan gesekan , yang dimasukkan ke dalam pengembangan bahan hibrida yang pamerkan superlubricity di macroscale untuk pertama kalinya . Para peneliti membantu mengaktifkan simulasi terobosan dengan mengatasi hambatan kinerja yang dua kali lipat kecepatan kode tim .

............. While reviewing the simulation results of a promising new lubricant material, Argonne researcher Sanket Deshmukh stumbled upon a phenomenon that had never been observed before.

............. Sementara meninjau hasil simulasi dari bahan pelumas baru yang menjanjikan , peneliti Argonne Sanket Deshmukh menemukan sebuah fenomena yang belum pernah diamati sebelumnya .....more

Simulations lead
to design of near-frictionless material

Date:

July 21, 2015

Source:

DOE/Argonne National Laboratory

Summary:

Scientists used the Mira supercomputer to identify and improve a new
mechanism for eliminating friction, which fed into the development of a hybrid
material that exhibited superlubricity at the macroscale for the first time.
Researchers helped enable the groundbreaking simulations by overcoming a
performance bottleneck that doubled the speed of the team's code.

...........................

Argonne scientists used Mira to identify and improve a new mechanism for
eliminating friction, which fed into the development of a hybrid material that
exhibited superlubricity at the macroscale for the first time. Argonne
Leadership Computing Facility (ALCF) researchers helped enable the
groundbreaking simulations by overcoming a performance bottleneck that doubled
the speed of the team's code.

While reviewing the simulation results of a promising new lubricant
material, Argonne researcher Sanket Deshmukh stumbled upon a phenomenon that
had never been observed before.

"I remember Sanket calling me and saying 'you have got to come over
here and see this. I want to show you something really cool,'" said
Subramanian Sankaranarayanan, Argonne computational nanoscientist, who led the
simulation work at the Argonne Leadership Computing Facility (ALCF), a DOE
Office of Science User Facility.

They were amazed by what the computer simulations revealed. When the
lubricant materials--graphene and diamond-like carbon (DLC)--slid against each
other, the graphene began rolling up to form hollow cylindrical
"scrolls" that helped to practically eliminate friction. These
so-called nanoscrolls represented a completely new mechanism for
superlubricity, a state in which friction essentially disappears.

"The nanoscrolls combat friction in very much the same way that ball
bearings do by creating separation between surfaces," said Deshmukh, who
finished his postdoctoral appointment at Argonne in January.

Superlubricity is a highly desirable property. Considering that nearly
one-third of every fuel tank is spent overcoming friction in automobiles, a
material that can achieve superlubricity would greatly benefit industry and
consumers alike. Such materials could also help increase the lifetime of
countless mechanical components that wear down due to incessant friction.

Experimental origins

Prior to the computational work, Argonne scientists Ali Erdemir, Anirudha
Sumant, and Diana Berman were studying the hybrid material in laboratory
experiments at Argonne's Tribology Laboratory and the Center for Nanoscale
Materials, a DOE Office of Science User Facility. The experimental setup
consisted of small patches of graphene (a two-dimensional single-sheet form of
pure carbon) sliding against a DLC-coated steel ball.

The graphene-DLC combination was registering a very low friction
coefficient (a ratio that measures the force of friction between two surfaces),
but the friction levels were fluctuating up and down for no apparent reason.
The experimentalists were also puzzled to find that humid environments were
causing the friction coefficient to shoot up to levels that were nearly 100
times greater than measured in dry environments.

To shed light on these mysterious behaviors, they turned to
Sankaranarayanan and Deshmukh for computational help. Using Mira, the ALCF's
10-petaflops IBM Blue Gene/Q supercomputer, the researchers replicated the
experimental conditions with large-scale molecular dynamics simulations aimed
at understanding the underlying mechanisms of superlubricity at an atomistic
level.

This led to their discovery of the graphene nanoscrolls, which helped to
fill in the blanks. The material's fluctuating friction levels were explained
by the fact that the nanoscrolls themselves were not stable. The researchers
observed a repeating pattern in which the hollow nanoscrolls would form, and
then cave in and collapse under the pressure of the load.

"The friction was dipping to very low values at the moment the scroll
formation took place and then it would jump back up to higher values when the
graphene patches were in an unscrolled state," Deshmukh said.

The computational scientists had an idea to overcome this issue. They tried
incorporating nanodiamond particles into their simulations to see if the hard
material could help stabilize the nanoscrolls and make them more permanent.

Sure enough, the simulations proved successful. The graphene patches
spontaneously rolled around the nanodiamonds, which held the scrolls in place
and resulted in sustained superlubricity. The simulation results fed into a new
set of experiments with nanodiamonds that confirmed the same.

"The beauty of this particular discovery is that we were able to see
sustained superlubricity at the macroscale for the first time, proving this
mechanism can be used at engineering scales for real-world applications,"
Sankaranarayanan said. "This collaborative effort is a perfect example of
how computation can help in the design and discovery of new materials."

Not slippery when wet

Unfortunately, the addition of nanodiamonds did not address the material's
aversion to water. The simulations showed that water suppresses the formation
of scrolls by increasing the adhesion of graphene to the surface.

While this greatly limits the hybrid material's potential applications, its
ability to maintain superlubricity in dry environments is a significant
breakthrough in itself.

The research team is in the process of seeking a patent for the hybrid
material, which could potentially be used for applications in dry environments,
such as computer hard drives, wind turbine gears, and mechanical rotating seals
for microelectromechanical and nanoelectromechanical systems.

Adding to the material's appeal is a relatively simple and cost-effective
deposition method called drop casting. This technique involves spraying
solutions of the materials on moving mechanical parts. When the solutions
evaporate, it would leave the graphene and nanodiamonds on one side of a moving
part, and diamond-like carbon on the other side.

However, the knowledge gained from their study is perhaps even more
valuable, said Deshmukh. He expects the nanoscroll mechanism to spur future
efforts to develop materials capable of superlubricity for a wide range of mechanical
applications.

For their part, the Argonne team will continue its computational studies to
look for ways to overcome the barrier presented by water.

"We are exploring different surface functionalizations to see if we
can incorporate something hydrophobic that would keep water out,"
Sankaranarayanan said. "As long as you can repel water, the graphene
nanoscrolls could potentially work in humid environments as well."

Simulating millions of atoms

The team's groundbreaking nanoscroll discovery would not have been possible
without a supercomputer like Mira. Replicating the experimental setup required
simulating up to 1.2 million atoms for dry environments and up to 10 million
atoms for humid environments.

The researchers used the LAMMPS (Large-scale Atomic/Molecular Massively
Parallel Simulator) code to carry out the computationally demanding reactive
molecular dynamics simulations.

With the help of ALCF catalysts, a team of computational scientists who
work directly with ALCF users, they were able to overcome a performance
bottleneck with the code's ReaxFF module, an add-on package that was needed to
model the chemical reactions occurring in the system.

The ALCF catalysts, in collaboration with researchers from IBM, Lawrence
Berkeley National Laboratory, and Sandia National Laboratories, optimized
LAMMPS and its implementation of ReaxFF by adding OpenMP threading, replacing
MPI point-to-point communication with MPI collectives in key algorithms, and
leveraging MPI I/O. Altogether, these enhancements allowed the code to perform
twice as fast as before.

"With the code optimizations in place, we were able to model the
phenomena in real experimental systems more accurately," Deshmukh said.
"The simulations on Mira showed us some amazing things that could not be seen
in laboratory tests."

And with the recent announcement of Aurora, the ALCF's next-generation
supercomputer, Sankaranarayanan is excited about where this line of research
could go in the future.

"Given the advent of computing resources like Aurora and the wide
gamut of the available two-dimensional materials and nanoparticle types, we
envision the creation of a lubricant genome at some point in the future,"
he said. "Having a materials database like this would allow us to pick and
choose lubricant materials for specific operational conditions."

Contributors to the code optimization work include Nichols A. Romero, Wei
Jiang, and Chris Knight from the ALCF; Paul Coffman from IBM; Hasan Metin
Aktulga from Lawrence Berkeley National Laboratory (now at Michigan State
University); and Tzu-Ray Shan from Sandia National Laboratories.







Story Source:

The above post is reprinted from materials provided byDOE/Argonne
National Laboratory. The original item was written by Jim Collins. Note:
Materials may be edited for content and length.







Journal Reference:

1.   
D. Berman, S. A. Deshmukh, S. K. R. S. Sankaranarayanan, A. Erdemir, A. V.
Sumant. Macroscale superlubricity enabled by graphene nanoscroll
formation. Science, 2015; 348 (6239): 1118 DOI: 10.1126/science.1262024

http://www.sciencedaily.com/releases/2015/07/150721194001.htm

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