This looks to work
well and we can expect many neat applications from the produced powder. The size is uniform and around a mere two
nanometers. This is a real achievement.
It will not lead
to large crystals but no matter. These
sizes are not easy to get from mining processes and thus this tackles a
different market.
All good
Nanodiamonds
Made Under Ambient Conditions
Oct. 21, 2013 — Instead of having to use tons
of crushing force and volcanic heat to forge diamonds, researchers at Case
Western Reserve University have developed a way to cheaply make nanodiamonds on
a lab bench at atmospheric pressure and near room temperature.
The nanodiamonds are formed directly from a gas and
require no surface to grow on.
The discovery holds promise for many uses in
technology and industry, such as coating plastics with ultrafine diamond powder
and making flexible electronics, implants, drug-delivery devices and more
products that take advantage of diamond's exceptional properties.
Their investigation is published today in the
scientific journalNature Communications. The findings build on a tradition of
diamond research at Case Western Reserve.
Beyond its applications, the discovery may offer
some insight into our universe: an explanation of how nanodiamonds seen in
space and found in meteorites may be formed.
"This is not a complex process: ethanol vapor
at room temperature and pressure is converted to diamond," said Mohan
Sankaran, associate professor of chemical engineering at Case Western Reserve
and leader of the project. "We flow the gas through a plasma, add
hydrogen and out come diamond nanoparticles. We can put this together and make
them in almost any lab."
The process for making these small "forever
stones" won't melt plastic so it is well suited for certain high-tech
applications. Diamond, renowned for being hard, has excellent optical
properties and the highest velocity of sound and thermal conductivity of any
material.
Unlike the other form of carbon, graphite, diamond
is a semiconductor, similar to silicon, which is the dominant material in the
electronics industry, and gallium arsenide, which is used in lasers and other
optical devices.
While the process is simple, finding the right
concentrations and flows -- what the researchers call the "sweet
spot" -- took time.
The other researchers involved were postdoctoral
researcher Ajay Kumar, PhD student Pin Ann Lin, and undergraduate student
Albert Xue, of Case Western Reserve; and physics professor Yoke Khin Yap and
graduate student Boyi Hao, of Michigan Technical University.
Sankaran and John Angus, professor emeritus of
chemical engineering, came up with the idea of growing nanodiamonds with no
heat or pressure about eight years ago. Angus' research in the 1960s and 1970s
led him and others to devise a way to grow diamond films at low pressure and
high temperature, a process known as chemical vapor deposition that is now used
to make coatings on computer disks and razor blades. Sankaran's specialty,
meanwhile, is making nanoparticles using cool microplasmas.
It usually requires high pressures and high
temperatures to convert graphite to diamond or a combination of hydrogen gas
and a heated substrate to grow diamond rather than graphite.
"But at the nanoscale, surface energy makes
diamond more stable than graphite," Sankaran explained. "We thought
if we could nucleate carbon clusters in the gas phase that were less than 5
nanometers, they would be diamond instead of graphite even at normal pressure
and temperature."
After several ups and downs with the effort, the
process came together when Kumar joined Sankaran's lab. The engineers produced
diamond much like they'd produce carbon soot.
They first create a plasma, which is a state of matter
similar to a gas but a portion is becoming charged, or ionized. A spark is an
example of a plasma, but it's hot and uncontrollable.
To get to cooler and safer temperatures, they
ionized argon gas as it was pumped out of a tube a hair-width in diameter,
creating a microplasma. They pumped ethanol -- the source of carbon -- through
the microplasma, where, similar to burning a fuel, carbon breaks free from
other molecules in the gas, and yields particles of 2 to 3 nanometers, small
enough that they turn into diamond.
In less than a microsecond, they add hydrogen. The
element removes carbon that hasn't turned to diamond while simultaneously
stabilizing the diamond particle surface.
The diamond formed is not the large perfect crystals
used to make jewelry, but is a powder of diamond particles. Sankaran and Kumar are now consistently
making high-quality diamonds averaging 2 nanometers in diameter.
The researchers spent about a year of testing to
verify they were producing diamonds and that the process could be replicated,
Kumar said. The team did different tests themselves and brought in Yap's lab to
analyze the nanoparticles by Raman spectroscopy.
Currently, nanodiamonds are made by detonating an
explosive in a reactor vessel to provide heat and pressure. The diamond
particles must then be removed and purified from contaminating elements massed
around them. The process is quick and cheap but the nanodiamonds aggregate and
are of varying size and purity.
The new research offers promising implications. Nanodiamonds,
for instance, are being tested to carry drugs to tumors. Because diamond is not
recognized as an invader by the immune system, it does not evoke resistance,
the main reason why chemotherapy fails.
Sankaran said his nanodiamonds may offer an
alternative to diamonds made by detonation methods because they are purer and
smaller.
The group's process produces three kinds of
diamonds: about half are cubic, the same structure as gem diamonds, a small
percentage are a form suspected of having hydrogen trapped inside and about
half are lonsdaleite, a hexagonal form found in interstellar dust but rarely
found on Earth.
A recent paper in the journal Physical Review
Letters suggests that when interstellar dust collides, such high pressure
is involved that the graphitic carbon turns into londsdaleite nanodiamonds.
Sankaran and Kumar contend that an alternative with
no high pressure requirement, such as their method, should be considered, too.
"Maybe we're making diamond in the way diamond
is sometimes made in outer space," Sankaran proposed. "Ethanol and
plasmas exist in outer space, and our nanodiamonds are similar in size and
structure to those found in space."
The group is now investigating whether it can
fine-tune the process to control which form of diamond is made, analyzing the
structures and determining if each has different properties. Lonsdaleite, for
instance, is harder than cubic diamond.
The researchers have made a kind of nanodiamond
spray paint. "We can do this in a single step, by spraying the
nanodiamonds as they are produced out of the plasma and purified with hydrogen,
to coat a surface," Kumar said.
And they are working on scaling up the process for
industrial use.
"Will they be able to scale up? That's always a
crap shoot," Angus said. "But I think it can be done, and at very
high rates and cheaply. Ultimately, it may take some years to get there, but
there is no theoretical reason it can't be done."
If the scaled-up process is as simple and cheap as
the lab process, industry will find many applications for the product, Sankaran
said.
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