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Inqui ry I s sue
1
| 2016
Inqui r y I s sue
1
| 2016
15
reating materials in their solid state can be
tricky, but offers some advantages over other
methods. It typically involves subjecting the
component elements to some type of mechanical
force—such as stress, shear or strain—to drive a reaction.
“You eliminate the need for solvents, so it removes
potentially harmful substances from the waste stream,”
said Ames Laboratory scientist and Iowa State University
Distinguished Professor Vitalij Pecharsky, “and it offers
greater selectivity so you can steer it toward a specific
reaction. Most processing is done at room temperature so
energy inputs are reduced and the resulting end products
may be meta-stable as well.”
It also offers a pathway to materials that aren’t
typically possible by other methods. One example is
the work Pecharsky has done using ball milling. Using
this mechano-chemistry technique, you can create a
homogeneous mixture—a consistent blend throughout
the entire sample—even though you start with a mixture
of components that can be 99.9 percent of one component
and only 0.1 percent of the second component.
“You can get complete dispersion,” Pecharsky said,
“something that would be very difficult to achieve by
melting the two components together.”
Because it doesn’t require solvents and often can
be done without heat and with relatively low energy
inputs, solid-state processing costs less than other
methods. In many cases, it’s also scalable to industrial/
commercial applications.
MECHANO-CHEMICAL BALL MILLING
As the name implies, ball milling uses metal balls
in a closed canister to shake, rattle and roll a chemical
reaction that turns individual chemical components into
a compound. Pecharsky said the impact of the balls with
the container and each other, with the material mixture
getting smashed between them, transfers the mechanical
energy of the rattling balls into chemical energy that in
turns drives the reaction.
The shear, stress and strain fractures the normal
molecular structure of the component materials, allowing
them to combine in ways that normally require a solvent
C
to break the molecular bonds and let the reaction
take place.
Pecharsky’s group is using the technique to investigate
creation of metal hydrides to serve as a hydrogen storage
medium. The group recently added a low temperature
ball mill that allows processing of materials that are
plastic or ductile.
“These materials will deform, but don’t fracture at
room temperature,” Pecharsky said. “By lowering the
temperature to that of a liquid nitrogen bath, like most
things, they become brittle and we’re able to process them
using this technique as well.”
FRICTION CONSOLIDATION
A brand new area for Ames Laboratory, friction
consolidation uses high pressure and friction to grind,
tear and press new materials into existence.
“It’s very fundamental,” said
Ames Laboratory scientist and ISU
associate professor of materials
science Jun Cui. “We put material
in a die and apply pressure with a
rotating plunger. The friction from
the rotating plunger creates shear
stresses within the materials. They eventually heat up,
soften and flow homogeneously. It’s a violent and chaotic
process, but there’s also a certain amount of order to it.”
The process typically uses powdered metals which are
easily consolidated because of the small initial size of the
particles. Similar to ball milling, friction consolidation allows
creation of microstructures not possible by other means.
“For example, you can take copper and process it with
carbon nanotubes and wind up with a nanocomposite
material that has greater mechanical strength than normal
copper without any reduction in electrical conductivity, ”
Cui said, “or may create a magnesium-titanium alloy that
is corrosion resistant.”
Once the material has been consolidated, it can
then be extruded or processed by a number of standard
industrial methods.
GLEEBLETHERMOMECHANICAL SYSTEM
Another new technology for Ames Laboratory is a
Gleeble system that allows laboratory simulation of any
number of commercial materials processing techniques.
The new equipment recently installed in the Laboratory’s
Metals Development building lets researchers precisely
control and measure what happens to materials during an
array of industrial processes from casting and forging to
sintering and extrusion.
“It allows us to do the precise measuring and monitoring
of physical simulations of complex processes,” said
Pete Collins, Ames Laboratory associate scientist and
Iowa State University associate professor of Materials
Science and Engineering, “as opposed to computational
simulations. However, the two really go hand in hand
—our measurements can validate and inform modeling
simulations, and modeling can suggest the physical
simulations we need to run.”
The equipment uses resistive heating to bring samples
quickly to high temperatures needed to simulate melting,
casting and welding—thousands of degrees in a few
seconds. The electrical demands—enough power to
run two or more average homes—were a primary reason
for locating the equipment at Ames Laboratory. The
equipment was part of Collins’ research startup agreement
when he accepted the faculty position at Iowa State.
“It also made sense from a materials processing
standpoint to have it located near the (Laboratory’s)
other additive manufacturing tools, such as the LENS™
(laser engineered net shaping) 3D printer,” Collins said.
“The Gleeble can be an important component of a high-
throughput suite of capabilities, so we can rapidly test
the array of alloy samples that the LENS™ system can
produce. In addition, we now have the capability to assess
other powder consolidation techniques. We can also take
metals powders and simulate how those powders are
processed under pressure and temperature to optimize
the conditions for the best results.”
New paths to newmaterials
An array of ball milling canisters and stainless steel balls Vitalij Pecharsky’s group uses to process materials in their solid state.
Solid-StateProcessing:
B Y K E R R Y G I B S O N
A canister containing elements
and stainless steel balls is clamped
in place in a ball mill in
preparation for shaking.
Ames Laboratory researcher
Shalabh Gupta loads material into
a cryogenic ball mill used to process
ductile or plastic materials. A liquid
nitrogen bath cools the materials,
making them brittle enough to be
mechano-chemically milled.