The Elements of Innovation Discovered

MIT prints particle-proof metamaterial

Metal Tech News - February 28, 2024

Supersonic experiments help identify atomic resilience in metamaterials for spacecraft, vehicles, helmets, or other objects.

In a "sound-breaking" study conducted by engineers at Massachusetts Institute of Technology, a novel method has been unveiled for rapidly assessing the resilience of metamaterials and their architectures by exposure to supersonic impacts, positioning research toward possible nanoscale particle-proof protective hulls and segments that can withstand omnipresent cosmic rays.

Metamaterials are manmade materials that possess unique properties not observed in nature.

Coming to life through the infinite permutability of 3D printing, the secret of their strength is the engineering of the metamaterials' very atomic structure.

Much like massive skyscrapers can distribute weight evenly through a vast array of angles and corners, capable of withstanding high winds at their peaks, and not come crumbling down in the event of an earthquake (within reason), scientists are taking the manufacturing of exoskeletons to the microscopic level.

High speed experiments

In a study published recently in the Proceedings of the National Academy of Sciences, MIT engineers report a new way to quickly test an array of metamaterial architectures and their resilience to supersonic impacts.

In their experiments, the team suspended tiny, printed metamaterial lattices between microscopic support structures, then fired even tinier particles at the materials, at supersonic speeds. With high-speed cameras, the team then captured images of each impact and its aftermath, with nanosecond precision.

"What we're learning is, the microstructure of your material matters, even with high-rate deformation," said study co-author Carlos Portela, the Brit and Alex d'Arbeloff Career Development Professor in Mechanical Engineering at MIT. "We want to identify impact-resistant structures that can be made into coatings or panels for spacecraft, vehicles, helmets, and anything that needs to be lightweight and protected."

Brace for impact

The team's new high-velocity experiments build off their previous work, in which the engineers tested the resilience of an ultralight, carbon-based material. That material, which was thinner than the width of a human hair, was made from tiny struts and beams of carbon, which the team printed and placed on a glass slide.

They then fired microparticles toward the material, at velocities exceeding the speed of sound.

Massachusetts Institute of Technology

By firing microparticles at supersonic speeds, MIT engineers can test the resilience of various metamaterials made from structures as small as a red blood cell. Pictured are four video stills of a microparticle hitting a structure made of metamaterials.

Those supersonic experiments revealed that the microstructured material withstood the high-velocity impacts, sometimes even deflecting the microparticles and other times capturing them.

"But there were many questions we couldn't answer because we were testing the materials on a substrate, which may have affected their behavior," Portela said.

In this latest study, the engineers developed a way to test freestanding metamaterials, to better observe how the material could withstand impacts purely on its own, without a backing or supporting surface.

In their current setup, they suspended a metamaterial between two microscopic pillars made from the same base material. Depending on the dimensions of the metamaterial being tested, the researchers could calculate how far apart the pillars needed to be in order to support the material at either end while allowing it to respond to impacts, without influence from the pillars themselves.

"This way, we ensure that we're measuring the material property and not the structural property," added Portela.

Once the support design was locked in, they moved to test a variety of metamaterial architectures. For each variant, the researchers first printed the supports on a small silicon chip, then continued printing the metamaterial as a suspended layer between them.

"We can print and test hundreds of these structures on a single chip," Portela said.

Solid vs. lattice

The team printed suspended metamaterials that resembled intricate honeycomb-like cross-sections.

Each material was printed with a specific three-dimensional microscopic architecture, such as a precise scaffold of repeating octets, or more faceted polygons. Each repeated unit measuring as small as a red blood cell, with the resulting metamaterials being thinner than the width of a human hair.

Testing each metamaterial's impact resilience, the researchers fired small glass microparticles toward the suspended structures atspeeds upwards of 900 meters per second or 2,000 miles per hour – well within the supersonic range.

Massachusetts Institute of Technology

MIT engineers have captured video of a microparticle being fired through a precisely architected metamaterial, measuring thinner than the width of a human hair. Click image to watch.

Catching each impact on camera, the engineers then studied the resulting images, frame by frame, to see how the projectiles penetrated each material. Afterward, they examined the materials under a microscope and compared each impact's physical aftermath.

"In the architected materials, we saw this morphology of small cylindrical craters after impact," Portela said. "But in solid materials, we saw a lot of radial cracks and bigger chunks of material that were gouged out."

Overall, the team observed that the fired particles created small punctures in the latticed metamaterials, but the material nevertheless stayed intact.

In contrast, when the same particles were fired at the same speeds into solid, non-latticed materials of equal mass, they created large cracks that quickly spread, causing the material to crumble.

The microstructured materials, therefore, were more efficient in resisting supersonic impacts as well as protecting against multiple impact events. And in particular, materials that were printed with the repeating octets appeared to be the hardiest.

What is this for?

"At the same velocity, we see the octet architecture is harder to fracture, meaning that the metamaterial, per unit mass, can withstand impacts up to twice as much as the bulk material," Portela added. "This tells us that there are some architectures that can make a material tougher which can offer better impact protection."

Going forward, the team plans to use the new rapid testing and analysis method to identify new metamaterial designs, in hopes of establishing a library of architectures that can be scaled up to stronger and lighter protective gear, garments, coatings, and paneling.

"What I'm most excited about is showing we can do a lot of these extreme experiments on a benchtop," Portela said. "This will significantly accelerate the rate at which we can validate new, high-performing, resilient materials."

If the day comes when 3D printing can quickly turn out nanoscopically-built structures that could potentially be used in aviation or aerospace, the durability and longevity of such constructs may be capable of even resisting the dangers of cosmic rays, thereby protecting vulnerable human bodies in longer periods of space travel, but also, perhaps, long habitation of extraterrestrial planets.


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