One team devised a pick-and-place system that uses electro-adhesion to handle tiny electronics. Another team developed a new method of making aerospace-grade composites, opening the door to potentially faster ways to manufacture airplane parts. In both cases, carbon nanotubes and electricity were at the center of the innovation.
In the two reports that follow, Jennifer Chu, a writer for MIT News, describes how MIT research engineers came to these discoveries, and why they hold promise for manufacturers of electronics and composite parts.
‘Electro-adhesive’ Stamp Picks Up and Puts Down Microscopic Structures
New technique could enable assembly of circuit boards and displays with more minute components.
Jennifer Chu | MIT News
CAMBRIDGE, Mass.—If you were to pry open your smartphone, you would see an array of electronic chips and components laid out across a circuit board, like a miniature city. Each component might contain even smaller “chiplets,” some no wider than a human hair. These elements are often assembled with robotic grippers designed to pick up the components and place them down in precise configurations.
As circuit boards are packed with ever smaller components, however, robotic grippers’ ability to manipulate these objects is approaching a limit.
“Electronics manufacturing requires handling and assembling small components in a size similar to or smaller than grains of flour,” said Sanha Kim, a former MIT postdoc and research scientist who worked in the lab of mechanical engineering associate professor John Hart. “So a special pick-and-place solution is needed, rather than simply miniaturizing [existing] robotic grippers and vacuum systems.”
Now Kim, Hart, and others have developed a miniature “electro-adhesive” stamp that can pick up and place down objects as small as 20 nanometers wide — about 1,000 times finer than a human hair. The stamp is made from a sparse forest of ceramic-coated carbon nanotubes arranged like bristles on a tiny brush.
When a small voltage is applied to the stamp, the carbon nanotubes become temporarily charged, forming prickles of electrical attraction that can attract a minute particle. By turning the voltage off, the stamp’s “stickiness” goes away, enabling it to release the object onto a desired location.
Hart said the stamping technique can be scaled up to a manufacturing setting to print micro- and nanoscale features—for instance, to pack more elements onto ever smaller computer chips. The technique may also be used to pattern other small, intricate features, such as cells for artificial tissues. And, the team envisions macroscale, bioinspired electro-adhesive surfaces, such as voltage-activated pads for grasping everyday objects and for gecko-like climbing robots.
“Simply by controlling voltage, you can switch the surface from basically having zero adhesion to pulling on something so strongly, on a per unit area basis, that it can act somewhat like a gecko’s foot,” Hart said.
The team published its results on October 11, 2019, in the journal Science Advances.
Like Dry Scotch Tape
Existing mechanical grippers are unable to pick up objects smaller than about 50 to 100 microns, mainly because at smaller scales, surface forces tend to win over gravity. You may see this when pouring flour from a spoon — inevitably, some tiny particles stick to the spoon’s surface, rather than letting gravity drag them off.
“The dominance of surface forces over gravity forces becomes a problem when trying to precisely place smaller things — which is the foundational process by which electronics are assembled into integrated systems,” Hart says.
He and his colleagues noted that electro-adhesion, the process of adhering materials via an applied voltage, has been used in some industrial settings to pick and place large objects, such as fabrics, textiles, and whole silicon wafers. But this same electro-adhesion had never been applied to objects at the microscopic level, because a new material design for controlling electro-adhesion at smaller scales was needed.
Hart’s group has previously worked with carbon nanotubes (CNTs) — atoms of carbon linked in a lattice pattern and rolled into microscopic tubes. Carbon nanotubes are known for their exceptional mechanical, electrical, and chemical properties, and they have been widely studied as dry adhesives.
“Previous work on CNT-based dry adhesives focused on maximizing the contact area of the nanotubes to essentially create a dry Scotch tape,” Hart said. “We took the opposite approach, and said, ‘let’s design a nanotube surface to minimize the contact area, but use electrostatics to turn on adhesion when we need it.’”
A Sticky On/Off Switch
The team found that if they coated CNTs with a thin dielectric material such as aluminum oxide, when they applied a voltage to the nanotubes, the ceramic layer became polarized, meaning its positive and negative charges became temporarily separated. For instance, the positive charges of the tips of the nanotubes induced an opposite polarization in any nearby conducting material, such as a microscopic electronic element.
As a result, the nanotube-based stamp adhered to the element, picking it up like tiny, electrostatic fingers. When the researchers turned the voltage off, the nanotubes and the element depolarized, and the “stickiness” went away, allowing the stamp to detach and place the object onto a given surface.
The team explored various formulations of stamp designs, altering the density of carbon nanotubes grown on the stamp, as well as the thickness of the ceramic layer that they used to coat each nanotube. They found that the thinner the ceramic layer and the more sparsely spaced the carbon nanotubes were, the greater the stamp’s on/off ratio, meaning the greater the stamp’s stickiness was when the voltage was on, versus when it was off.
In their experiments, the team used the stamp to pick up and place down films of nanowires, each about 1,000 times thinner than a human hair. They also used the technique to pick and place intricate patterns of polymer and metal microparticles, as well as micro-LEDs.
Hart said the electro-adhesive printing technology could be scaled up to manufacture circuit boards and systems of miniature electronic chips, as well as displays with microscale LED pixels.
“With ever-advancing capabilities of semiconductor devices, an important need and opportunity is to integrate smaller and more diverse components, such as microprocessors, sensors, and optical devices,” Hart said. “Often, these are necessarily made separately but must be integrated together to create next-generation electronic systems. Our technology possibly bridges the gap necessary for scalable, cost-effective assembly of these systems.”
This research was supported in part by the Toyota Research Institute, the National Science Foundation, and the MIT-Skoltech Next Generation Program.
Jennifer Chu is a writer for MIT News.
Reprinted with permission of MIT News.
A New Approach to Making Airplane Parts, Minus the Massive Infrastructure
Carbon nanotube film produces aerospace-grade composites with no need for huge ovens or autoclaves.
Jennifer Chu | MIT News
A modern airplane’s fuselage is made from multiple sheets of different composite materials, like so many layers in a phyllo-dough pastry. Once these layers are stacked and molded into the shape of a fuselage, the structures are wheeled into warehouse-sized ovens and autoclaves, where the layers fuse together to form a resilient, aerodynamic shell.
Now MIT engineers have developed a method to produce aerospace-grade composites without the enormous ovens and pressure vessels. The technique may help to speed up the manufacturing of airplanes and other large, high-performance composite structures, such as blades for wind turbines.
The researchers detail their new method in a paper published today in the journal Advanced Materials Interfaces.
“If you’re making a primary structure like a fuselage or wing, you need to build a pressure vessel, or autoclave, the size of a two- or three-story building, which itself requires time and money to pressurize,” said Brian Wardle, professor of aeronautics and astronautics at MIT. “These things are massive pieces of infrastructure. Now we can make primary structure materials without autoclave pressure, so we can get rid of all that infrastructure.”
Wardle’s co-authors on the paper are lead author and MIT postdoc Jeonyoon Lee, and Seth Kessler of Metis Design Corporation, an aerospace structural health monitoring company based in Boston.
Out of the Oven, into a Blanket
In 2015, Lee led the team, along with another member of Wardle’s lab, in creating a method to make aerospace-grade composites without requiring an oven to fuse the materials together. Instead of placing layers of material inside an oven to cure, the researchers essentially wrapped them in an ultrathin film of carbon nanotubes (CNTs). When they applied an electric current to the film, the CNTs, like a nanoscale electric blanket, quickly generated heat, causing the materials within to cure and fuse together.
With this out-of-oven, or OoO, technique, the team was able to produce composites as strong as the materials made in conventional airplane manufacturing ovens, using only 1 percent of the energy.
The researchers next looked for ways to make high-performance composites without the use of large, high-pressure autoclaves — building-sized vessels that generate high enough pressures to press materials together, squeezing out any voids, or air pockets, at their interface.
“There’s microscopic surface roughness on each ply of a material, and when you put two plys together, air gets trapped between the rough areas, which is the primary source of voids and weakness in a composite,” Wardle said. “An autoclave can push those voids to the edges and get rid of them.”
Researchers including Wardle’s group have explored “out-of-autoclave,” or OoA, techniques to manufacture composites without using the huge machines. But most of these techniques have produced composites where nearly 1 percent of the material contains voids, which can compromise a material’s strength and lifetime. In comparison, aerospace-grade composites made in autoclaves are of such high quality that any voids they contain are negligible and not easily measured.
“The problem with these OoA approaches is also that the materials have been specially formulated, and none are qualified for primary structures such as wings and fuselages,” Wardle said. “They’re making some inroads in secondary structures, such as flaps and doors, but they still get voids.”
Part of Wardle’s work focuses on developing nano porous networks — ultrathin films made from aligned, microscopic material such as carbon nanotubes, that can be engineered with exceptional properties, including color, strength, and electrical capacity. The researchers wondered whether these nano porous films could be used in place of giant autoclaves to squeeze out voids between two material layers, as unlikely as that may seem.
A thin film of carbon nanotubes is somewhat like a dense forest of trees, and the spaces between the trees can function like thin nanoscale tubes, or capillaries. A capillary such as a straw can generate pressure based on its geometry and its surface energy, or the material’s ability to attract liquids or other materials.
The researchers proposed that if a thin film of carbon nanotubes were sandwiched between two materials, then, as the materials were heated and softened, the capillaries between the carbon nanotubes should have a surface energy and geometry such that they would draw the materials in toward each other, rather than leaving a void between them. Lee calculated that the capillary pressure should be larger than the pressure applied by the autoclaves.
The researchers tested their idea in the lab by growing films of vertically aligned carbon nanotubes using a technique they previously developed, then laying the films between layers of materials that are typically used in the autoclave-based manufacturing of primary aircraft structures. They wrapped the layers in a second film of carbon nanotubes, to which they applied an electric current to heat it up. They observed that as the materials heated and softened in response, they were pulled into the capillaries of the intermediate CNT film.
The resulting composite lacked voids, similar to aerospace-grade composites that are produced in an autoclave. The researchers subjected the composites to strength tests, attempting to push the layers apart, the idea being that voids, if present, would allow the layers to separate more easily.
“In these tests, we found that our out-of-autoclave composite was just as strong as the gold-standard autoclave process composite used for primary aerospace structures,” Wardle said.
The team will next look for ways to scale up the pressure-generating CNT film. In their experiments, they worked with samples measuring several centimeters wide — large enough to demonstrate that nano porous networks can pressurize materials and prevent voids from forming. To make this process viable for manufacturing entire wings and fuselages, researchers will have to find ways to manufacture CNT and other nano porous films at a much larger scale.
“There are ways to make really large blankets of this stuff, and there’s continuous production of sheets, yarns, and rolls of material that can be incorporated in the process,” Wardle says.
He plans also to explore different formulations of nano porous films, engineering capillaries of varying surface energies and geometries, to be able to pressurize and bond other high-performance materials.
“Now we have this new material solution that can provide on-demand pressure where you need it,” Wardle said. “Beyond airplanes, most of the composite production in the world is composite pipes, for water, gas, oil, all the things that go in and out of our lives. This could make all those things, without the oven and autoclave infrastructure.”
This research was supported, in part, by Airbus, ANSYS, Embraer, Lockheed Martin, Saab AB, Saertex, and Teijin Carbon America through MIT’s Nano-Engineered Composite aerospace Structures (NECST) Consortium.
Jennifer Chu is a writer for MIT News.
Reprinted with permission of MIT News.