Researchers
have built a sheet of nickel with nanoscale pores that make it as strong as
titanium but four to five times lighter.
High-performance
golf clubs and airplane wings are made out of titanium, which is as strong as
steel but about twice as light. These properties depend on the way a metal’s
atoms are stacked, but random defects that arise in the manufacturing process
mean that these materials are only a fraction as strong as they could
theoretically be.
Design
and build new materials
An
architect, working on the scale of individual atoms, could design and build new
materials that have even better strength-to-weight ratios.
In
a study published in ‘Nature Scientific Reports’, researchers at the University
of Pennsylvania’s School of Engineering and Applied Science, the University of
Illinois at Urbana-Champaign, and the University of Cambridge have done just
that. They have built a sheet of nickel with nanoscale pores that make it as
strong as titanium but four to five times lighter.
The
empty space of the pores, and the self-assembly process in which they’re made,
make the porous metal akin to a natural material, such as wood.
And
just as the porosity of wood grain serves the biological function of
transporting energy, the empty space in the researchers’ ‘metallic wood’ could
be infused with other materials.
Infusing
scaffolding with anode and cathode materials
Infusing
the scaffolding with anode and cathode materials would enable this metallic
wood to serve double duty: a plane wing or prosthetic leg that’s also a
battery.
The
study was led by James Pikul, assistant professor in the Department of
Mechanical Engineering and Applied Mechanics at Penn Engineering. Bill King and
Paul Braun at the University of Illinois at Urbana-Champaign, along with Vikram
Deshpande at the University of Cambridge, contributed to the study.
Even
the best natural metals have defects in their atomic arrangement that limit
their strength. A block of titanium where every atom was perfectly aligned with
its neighbors would be ten times stronger than what can currently be produced.
Materials
researchers have been trying to exploit this phenomenon by taking an
architectural approach, designing structures with the geometric control
necessary to unlock the mechanical properties that arise at the nanoscale,
where defects have reduced impact.
Pikul
and his colleagues owe their success to taking a cue from the natural world.
“The
reason we call it metallic wood is not just its density, which is about that of
wood, but its cellular nature,” says Pikul.
‘Cellular
materials are porous’
“Cellular
materials are porous; if you look at wood grain, that’s what you’re seeing –
parts that are thick and dense and made to hold the structure, and parts that
are porous and made to support biological functions, like transport to and from
cells.
“Our
structure is similar. We have areas that are thick and dense with strong metal
struts, and areas that are porous with air gaps. We’re just operating at the
length scales where the strength of struts approaches the theoretical maximum.”
The
struts in the researchers’ metallic wood are around 10 nanometres wide, or
about 100 nickel atoms across. Other approaches involve using 3D-printing-like
techniques to make nanoscale scaffoldings with hundred-nanometre precision, but
the slow and painstaking process is hard to scale to useful sizes.
“We’ve
known that going smaller gets you stronger for some time,” says Pikul, “but
people haven’t been able to make these structures with strong materials that
are big enough that you’d be able to do something useful.
“Most
examples made from strong materials have been about the size of a small flea,
but with our approach, we can make metallic wood samples that are 400 times
larger.”
Pikul’s
method starts with tiny plastic spheres, a few hundred nanometres in diameter, suspended
in water. When the water is slowly evaporated, the spheres settle and stack
like cannonballs, providing an orderly, crystalline framework.
Using
electroplating, the same technique that adds a thin layer of chrome to a
hubcap, the researchers then infiltrate the plastic spheres with nickel. Once
the nickel is in place, the plastic spheres are dissolved with a solvent,
leaving an open network of metallic struts.
“We’ve
made foils of this metallic wood that are on the order of a square centimeter,
or about the size of a playing die side,” says Pikul. “To give you a sense of
scale, there are about one billion nickel struts in a piece that size.”
Because
roughly 70 per cent of the resulting material is empty space, this nickel-based
metallic wood’s density is extremely low in relation to its strength. With a
density on par with water’s, a brick of the material would float.
Replicating
this production process at commercially relevant sizes is the team’s next
challenge. Unlike titanium, none of the materials involved are particularly
rare or expensive on their own, but the infrastructure necessary for working
with them on the nanoscale is currently limited.
Once
that infrastructure is developed, economies of scale should make producing
meaningful quantities of metallic wood faster and less expensive.
A
better understanding of its tensile properties
Once
the researchers can produce samples of their metallic wood in larger sizes,
they can begin subjecting it to more macroscale tests. A better understanding
of its tensile properties, for example, is critical.
“We
don’t know, for example, whether our metallic wood would dent like metal or
shatter like glass,” says Pikul. “Just like the random defects in titanium
limit its overall strength, we need to get a better understand of how the
defects in the struts of metallic wood influence its overall properties.”
In
the meantime, Pikul and his colleagues are exploring the ways other materials
can be integrated into the pores in their metallic wood’s scaffolding.
“The
long-term interesting thing about this work is that we enable a material that
has the same strength properties of other super high-strength materials but now
it’s 70 per cent empty space,” says Pikul.
“And
you could one day fill that space with other things, like living organisms or
materials that store energy.”
