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Fractal nanostructures used to build new supermaterials

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June 6, 2014

Caltech researchers are building nanoscale supermaterials from a CAD design (Photo: Meza/M...

Caltech researchers are building nanoscale supermaterials from a CAD design (Photo: Meza/Montemayor/Clarke/Greer at Caltech)

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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 fractal nanostructure allows for even more flexibility in choosing the material's prop...

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 features are only a few nanometers in size (Photo: Meza/Montemayor/Clarke/Greer at Cal...

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.

Source: Caltech

About the Author
Dario Borghino Dario studied software engineering at the Polytechnic University of Turin. When he isn't writing for Gizmag he is usually traveling the world on a whim, working on an AI-guided automated trading system, or chasing his dream to become the next European thumbwrestling champion.   All articles by Dario Borghino
12 Comments

I am imagining that 3d printing will have to come a loooong way before it is fast enough to print a nanostructured car. Far more likely is using materials created by foaming metals or using solvents like aerogel, both of which are very light for their strength.

Michaelc
7th June, 2014 @ 11:13 am PDT

That is very disruptive technology indeed. This trumpets every known lightweight material that serves as structures in car bodies for example right now. Truly, these nano-structured materials, if scaled up means mind-boggling weight reductions in vehicles! That would drastically reduce the energy spent on moving the vehicles from A to B and less pedestrian/cyclist fatalities

Fretting Freddy the Ferret pressing the Fret
7th June, 2014 @ 01:09 pm PDT

" 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."

i give you credit for at least giving your reader a heads up about the limitations as to what , in real world commercial terms, kind of applications these innovations might yield.

these innovations in NO direct way will be used for macroscale. maybe some of the lessons learned will be used in helping develop entirely new future processes. but as for these processes, they yield nothing in the way of progress with materials at the macro scale.

in a way, the article leads the reader to believe this, but at least includes a comment from the actual researcher telling you the exact opposite of this implication. kudos.

zevulon
7th June, 2014 @ 08:00 pm PDT

"But what if you could build an electric car so light that you could (if you were so inclined) lift it with your hands, ..."

In that case it would be a death trap - blowing over in the slightest cross-wind. Moral. Don't assume that because a little of something (weight shedding in this case) is good, then more is automatically better.

nicho
7th June, 2014 @ 09:19 pm PDT

Is the tech that made Clarke "Fountains of Paradise" real? A vertical highway from the earth to geosynchronous orbit?

Vanamonde
8th June, 2014 @ 03:13 pm PDT

This is exciting technology. The speed limitation is actually not a show stopper. The science works so now it is an engineering challenge and one that can be solved. Off the top of my head parallel processing is a solution for speed. The system used here uses only 2 lasers. If they used 512 lasers it would be very much faster. The machine would be expensive but could generate a good roi due to printing bulk material.

That's assuming the engineers don't come up with a radical new method that is fast and cheap.

VirtualGathis
9th June, 2014 @ 09:53 am PDT

It would appear that Greer is reluctant to reference one of the earliest proponents of small architecture CalTech's very own Richard Feynman.

She chooses to promote herself and her students by petty omission. Fractal nanostructuring is not new nor is the idea of design based nanostructures.

The problem is and remains creating bulk materials at reasonable cost in a reasonable time. Sadly I find nothing in this article that suggests Greer or her students have made progress in the latter realm of endeavor.

attoman
9th June, 2014 @ 11:49 am PDT

The point is made that surface area is proportionately greatly increased - however, the rate of corrosion is proportional to the surface area.

This could be a terrific innovation but I think only under specific circumstances, for instance, if the operating environment is strictly controlled.

Grunchy
9th June, 2014 @ 03:56 pm PDT

Imagine this in aviation, the reduction in energy required in flight might even make long distance electric airplanes a reality.

antiguajohn
9th June, 2014 @ 04:10 pm PDT

"But what if you could build an electric car so light that you could (if you were so inclined) lift it with your hands?" I'll tell you what would happen, it would fly off the road. The slightest breeze would make it un-driveable, and highway speeds? Forget it! Something the size of a car weighing so little you could pick it up? Totally worthless as a (ground) transportation device.

This would be excellent for making soundproofed walls and buildings, and many other things, including vehicles, but a vehicle would have to maintain a minimum weight to be safe on the road, especially at higher speeds.

As far as speeding up this nano-scale stereo lithography? (or any stereo lithography), more lasers. Rather than two lasers, use two hundred and do the job 100 times faster.

Dave Andrews
10th June, 2014 @ 07:51 am PDT

What if you built a 'solid' airship that is mostly fresh air ... and then evacuated it!

Pete_Morrison
11th June, 2014 @ 10:14 am PDT

As far as safety and impact forces it is speed and not weight that tends to kill. The military is very aware of this as a ballistic weapon that uses very lite weight warheads can penetrate tank armor without the use of an explosive if it travels fast enough. In vehicles less weight means better handling. If you take a Harley Davidson cruiser at 850 lbs. and make a bike with identical power at 250 lbs. the bike will handle better and stop far quicker than a conventional design. Cars do require some consideration as the side areas can be rather large and be attacked by wind energy. But you can take cars down below 1000 lbs. that handle OK at turnpike speeds.

Jim Sadler
16th June, 2014 @ 01:37 pm PDT
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