This is a pretty good review piece of just were our current research is. Material science is advancing steadily into nano engineered materials capable of miraculous properties and ultimately cost effective manufacturing.
What is described here is a natural one off process that
will still have serious application. Rocket engines come to mind.
Yet we are well on the way to producing my multilayered
skin for my Magnetic Field Exclusion Vessel.
Fractal nanostructures used to build new supermaterials
By Dario Borghino
June 6, 2014
Researchers at the California
Institute of Technology are developing a disruptive manufacturing
process that combines nanoscale effects and ad-hoc architectural
design to build new supermaterials from the ground up. The materials
can be designed to meet predetermined criteria such as weighing only
a tiny fraction of their macroscopic counterpart, displaying extreme
plasticity, or featuring outstanding mechanical strength.
The pickup car
A Tesla Model S has a curb weight of
2,108 kg (4,650 lb), the battery pack making up a significant portion
of that figure, but in most cases the cargo it carries is only around
one tenth of that number. From an efficiency standpoint, this is an
unmitigated disaster: everything that isn't cargo is simply overhead,
and having to carry that much of it around will shorten the car's
range considerably.
But what if you could build an
electric car so light that you could (if you were so inclined) lift
it with your hands, with a range in the thousands of miles, and that
was still as structurally strong as a normal car? Or what if you
could build a bridge using only one percent of the materials that
would normally be required?
According to Julia Greer, professor
of materials science and mechanics at Caltech, the technology that
would allow us to do that might soon be within our reach. But to get
there, we need to take our approach on manufacturing materials and
turn it completely on its head.
Normally, the manufacturing
processes that are used on a material will dictate the mechanical
properties of the final product. But according to the Caltech
researchers, the better approach is to start from the properties you
want the material to have and then go backwards, designing the
nanoscale architecture that would allow the material to reach the
desired specifications.
"I can start with a property
and say that I want something that has this strength or this thermal
conductivity, for example," says Greer. "Then
I can design the optimal architecture with the optimal material at
the relevant size and end up with the material I wanted."
A three-step process
Using a technique they originally
developed last year, Greer and her team are able to first create a
computer design of the structure they want to build, and then process
it in such a way that it can be made out of any material class,
whether they be metals, ceramics or semiconductors, with a precision
on the order of a few nanometers.
The researchers first
use a laser writing method called two-photon lithography, which uses
the constructive interference of photons from two different lasers to
sculpt a three-dimensional pattern onto a polymer. When the laser
beam is focused, it hits the polymer and cross-links it, dissolving
away the rest. The end product is a three-dimensional polymer
scaffold.
In the
second step, the researchers use an industry-standard deposition
process to coat the scaffold with a thin and uniform layer of the
desired material, obtaining a composite of the polymer and the
coating material.
In the
final step, the researchers etch out the polymer from within the
structure, leaving a hollow architecture comprised entirely of the
desired material.
As Greer tells us, the challenge
with scaling up the nanotruss production is that the two-photon
lithography process, which they use in the first step of creating the
structures, is very time-consuming.
"The process is effectively a
3D printer, but allows for much more refined features that have
dimensions three orders of magnitude smaller than those of
conventional printers," Greer tells us. "It would have to
be a completely different 3D scaffold generating process to produce
large-scale nano-trusses in a feasible way."
Changing the rules
If you were to plot the strength of
every known material against its density, you would find a very clear
trend: density and strength always go hand in hand. Strong materials
like steels and other metals are very heavy, and materials that are
light, like foam, are also quite weak.
However, tinkering with materials at
the nanoscale can change their macroscopic properties in ways that
are dramatic and often surprising.
"It turns out that almost every
class of materials has size-dependent properties, even in the
physical world," Greer told Gizmag. "Single-crystalline
metals become stronger when reduced to the nano-scale, and some
intrinsically brittle materials become ductile."
The over-arching reason for this,
Greer tells us, is that these nanostructures have a very high surface
area to volume ratio. This completely transforms the way in which the
microscopic features of the material – for instance, a small defect
or the carriers of plasticity – interact with each other and
propagate throughout the material. An intelligently designed
nanoarchitecture can further manipulate the way in which the features
interact with each other.
As the researchers have shown, some
metals can become about 50 times stronger in this way, decoupling
strength from density and making materials that are strong, tough and
extremely lightweight despite containing 99 percent air.
Fractal nanotruss
Greer and colleagues have taken this
process one step further to build fractal nanostructures. Simply put,
these are nanomaterials whose architecture displays a nested,
self-similar pattern, which allows the researchers to incorporate
hierarchical design into the material's architecture.
"When materials are arranged in
hierarchical manner, they end up possessing many lucrative
properties, beyond the so-called rule of mixtures," Greer tells
us. "Hard biological materials like nacre shells and bird beaks,
for example, draw much of their damage tolerance from the
hierarchical arrangements of platelets and other hard constituents
within their structure. Adding each level of hierarchy likely allows
for even more degrees of freedom in tuning the specific properties
because each level offers this amplification beyond the rule of
mixtures."
Applications
The range of applications is
practically limitless. Transportation certainly jumps to mind as an
area where this technology could have a major impact, leading to much
lighter vehicles with improved performance and which would
drastically cut energy consumption without sacrificing safety.
There could also be disruptive
changes in battery technology. Silicon has a much greater capacity
for storing energy than the graphitic carbon used in today's lithium
ion batteries, but it isn't currently practical for commercial use
because it suffers from cracking, leading to catastrophic failure.
Using nanotrusses, the silicon can be made to be both extremely
lightweight and capable to expand by as much as 400 percent,
preventing failure and paving the way for batteries that are both
lighter and better-performing than today's.
In photovoltaics, Greer's group has
demonstrated geometries for the antireflective coating of a solar
cell that would be able to trap photons across the entire spectrum.
And finally, a lightweight but
mechanically strong nanostructure could be very well-versed as a
scaffold – from the nanoscale, where it could be used to support
cell growth, to the macroscale, to construct tougher buildings at a
fraction of the cost.
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