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Friday, October 3rd, 2014
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| 12:00a |
Fast, cheap nanomanufacturing Luis Fernando Velásquez-García’s group at MIT’s Microsystems Technology Laboratories (MTL) develops dense arrays of microscopic cones that harness electrostatic forces to eject streams of ions.
The technology has a range of promising applications: depositing or etching features onto nanoscale mechanical devices; spinning out nanofibers for use in water filters, body armor, and “smart” textiles; or propulsion systems for fist-sized “nanosatellites.”
In the latest issue of the IEEE Journal of Microelectromechanical Systems, Velásquez-García, his graduate students Eric Heubel and Philip Ponce de Leon, and Frances Hill, a postdoc in his group, describe a new prototype array that generates 10 times the ion current per emitter that previous arrays did.
Ion current is a measure of the charge carried by moving ions, which translates directly to the rate at which particles can be ejected. Higher currents thus promise more-efficient manufacturing and more-nimble satellites.
The same prototype also crams 1,900 emitters onto a chip that’s only a centimeter square, quadrupling the array size and emitter density of even the best of its predecessors.
“This is a field that benefits from miniaturizing the components, because scaling down emitters implies less power consumption, less bias voltage to operate them, and higher throughput,” says Velásquez-García, a principal research scientist at MTL. “The topic we have been tackling is how we can make these devices operate as close as we can to the theoretical limit and how we can greatly increase the throughput by virtue of multiplexing, with massively parallel devices that operate uniformly.”
When Velásquez-García speaks of a “theoretical limit,” he’s talking about the point at which droplets — clumps of molecules — rather than ions — individual molecules — begin streaming off of the emitters. Among other problems, droplets are heavier, so their ejection velocity is lower, which makes them less useful for etching or satellite propulsion.
The ions ejected by Velásquez-García’s prototype are produced from an ionic salt that’s liquid at room temperature. Surface tension wicks the fluid up the side of the emitters to the tip of the cone, whose narrowness concentrates the electrostatic field. At the tip, the liquid is ionized and, ideally, ejected one molecule at a time.
Slow the flow
When the ion current in an emitter gets high enough, droplet formation is inevitable. But earlier emitter arrays — those built both by Velásquez-García’s group and by others — fell well short of that threshold.
Increasing an array’s ion current is a matter of regulating the flow of the ionic salt up the emitters’ sides. To do that, the MIT researchers had previously used black silicon, a form of silicon grown as closely packed bristles. But in the new work, they instead used carbon nanotubes — atom-thick sheets of carbon rolled into cylinders — grown on the slopes of the emitters like trees on a mountainside.
By carefully tailoring the density and height of the nanotubes, the researchers were able to achieve a fluid flow that enabled an operating ion current at very near the theoretical limit.
“We also show that they work uniformly — that each emitter is doing exactly the same thing,” Velásquez-García says. That’s crucial for nanofabrication applications, in which the depth of an etch, or the height of deposits, must be consistent across an entire chip.
To control the nanotubes’ growth, the researchers first cover the emitter array with an ultrathin catalyst film, which is broken into particles by chemical reactions with both the substrate and the environment. Then they expose the array to a plasma rich in carbon. The nanotubes grow up under the catalyst particles, which sit atop them, until the catalyst degrades.
Increasing the emitter density — the other improvement reported in the new paper — was a matter of optimizing existing manufacturing “recipe,” Velásquez-García says. The emitters, like most nanoscale silicon devices, were produced through photolithography, a process in which patterns are optically transferred to layers of materials deposited on silicon wafers; a plasma then etches the material away according to the pattern. “The recipe is the gases, power, pressure level, time, and the sequence of the etching,” Velásquez-García says. “We started doing electrospray arrays 15 years ago, and making different generations of devices gave us the know-how to make them better.”
Nanoprinting
Velásquez-García believes that using arrays of emitters to produce nanodevices could have several advantages over photolithography — the technique that produces the arrays themselves. Because they can operate at room temperature and don’t require a vacuum chamber, the arrays could deposit materials that can’t withstand the extreme conditions of many micro- and nanomanufacturing processes. And they could eliminate the time-consuming process of depositing new layers of material, exposing them to optical patterns, etching them, and then starting all over again.
“In my opinion, the best nanosystems are going to be done by 3-D printing because it would bypass the problems of standard microfabrication,” Velásquez-García says. “It uses prohibitively expensive equipment, which requires a high level of training to operate, and everything is defined in planes. In many applications you want the three-dimensionality: 3-D printing is going to make a big difference in the kinds of systems we can put together and the optimization that we can do.”
“Typically the interest of this type of emitter is to be able to emit a beam of ions and not a beam of droplets,” says Herbert Shea, an associate professor in the Microsystems for Space Technologies Laboratory at the École Polytechnique Fédérale de Lausanne. “Using their nanotube forest, they’re able to get the devices to operate in pure ion mode but have a high current typically associated with the droplet mode.”
Shea believes that, at least in the near term, the technology’s most promising application is in spacecraft propulsion. “It would take a lot of effort to make it into a practical micromachining tool, whereas it would take very little effort to use it as propulsion for small spacecraft,” he says. “The reason you’d like to be in ion mode is to have the most efficient conversion of the mass of the propellant into the momentum of the spacecraft.” | | 12:00a |
Untangling how cables coil The world’s fiber-optic network spans more than 550,000 miles of undersea cable that transmits e-mail, websites, and other packets of data between continents, all at the speed of light. A rip or tangle in any part of this network can significantly slow telecommunications around the world.
Now engineers at MIT, along with computer scientists at Columbia University, have developed a method that predicts the pattern of coils and tangles that a cable may form when deployed onto a rigid surface. The research combined laboratory experiments with custom-designed cables, computer-graphics technology used to animate hair in movies, and theoretical analyses.
In the lab, MIT engineers set up a desktop system to spool spaghetti-like cables onto a conveyor belt. They adjusted parameters such as speed of deployment and the speed of the belt, and observed how the cable coiled as it hit the surface.
At Columbia, computer scientists adapted a source code used for simulating animated hair and, incorporating the parameters of the MIT experiment, found that the simulation accurately predicted the coiling patterns seen in the lab.
The researchers say the coil-predicting method may help design better deployment strategies for fiber-optic cables to avoid the twisting and tangling that can lead to transmission glitches and data loss.
“We now have a set of design guidelines that allow you to tune certain parameters to achieve a particular pattern,” says Pedro Reis, an associate professor of mechanical engineering and civil and environmental engineering at MIT. “We have a description that applies to many systems.”
Reis and his colleagues publish their results this week in the Proceedings of the National Academy of Sciences. His co-authors are Khalid Jawed of MIT and Fang Da, Jungseock Joo, and Eitan Grinspun of Columbia University.
Shipping up to Boston
Fiber-optic cables are typically deployed from a sailing vessel, which unfurls lengths of cable from a large spool. Depending on how the sailing speed of the boat relates to the speed of the spool, cable can be deposited on the seafloor in straight lines, or in meandering, coiling patterns.
“If the boat is sailing slower than the rate of the cable, then you’re putting more cable down, which generates loops, coils, and tangles,” Reis says. “That can lead to signal attenuation. But if the boat is traveling faster, then the cable can get taut and fracture, which is really bad news. So we wanted to understand what was underlying those patterns.”
To do this, Reis set up a small-scale version of a cable-deploying system in his lab. He and his students fabricated filaments from silicone-based rubber, and rigged a spool to automatically reel out the wire onto a conveyor belt. They altered various parameters of the setup, including the speed of the belt and the spool.
The team used a digital video camera to record the filaments’ motion as they hit the belt, and observed three main patterns: meandering waves, alternating loops, and repeated coils.
A Hollywood makeover
To see if these patterns could be predicted in simulations, Reis teamed up with Grinspun, an expert in discrete differential geometry. Grinspun has applied sophisticated mathematical methods to simulating the movement of thin filaments such as hair and cloth — notoriously difficult features to animate realistically — for films including “The Hobbit” and Disney’s “Tangled.”
“The eye is very good at picking up what’s physical and what’s not,” Grinspun says. “We want to capture the motion of hair and clothing in a realistic way, so a lot of algorithms we develop, we need to think about geometry.”
Grinspun had previously upgraded a code he developed to simulate hair to model the flow of viscous fluids like honey. As honey is poured from a jar, it can resemble rope or thread, drizzling onto a surface in wavelike patterns. Reis wondered if the same code could be adopted to simulate the coiling of cables.
“We realized that I’m using geometry to scale up and down problems, and he’s using geometry to speed up his codes, so we thought that we should port some of his algorithms into engineering, and test if these patterns can be predicted,” Reis says.
At first, the collaborative effort produced mixed results: Patterns seen in experiments could not be replicated in simulations. The researchers eventually identified a key feature they were not originally factoring into the simulations: the natural curvature of the filament, which, when wound on a spool, retains a certain amount of curve as it’s unwound. This initial mismatch between experiments and simulations motivated Reis to devise an experimental protocol to fabricate rods with customizable natural curvature.
With natural curvature now incorporated in the simulations and controlled in the lab, the researchers found that they were able to simulate the exact patterns observed in experiments. They then tuned the dimensions of various features in the simulation, and found they were able to predict the shape and amplitude of curves formed, based on several main factors: the speed of the wire deployed, the speed of the conveyor belt, the stiffness and diameter of the filament, and the size of its spool (a measure that determines a wire’s natural curvature).
They also found, surprisingly, that the height from which a filament is deployed does not influence its coiling patterns — good news for ships that navigate choppy waters to deploy fiber-optic cables.
“This is important because, as a ship sails, the height of the ocean floor relative to the surface is changing all the time,” Grinspun says. “We also know that how big you make the spools on the ship does matter. So we now have a map of how cables coil, and an understanding of what variables are important if you’re trying to achieve certain patterns.”
Going forward, Reis says that he and Grinspun may collaborate on other projects to understand and simulate the motion of thin filaments with features such as fluid drag and friction. For example, an understanding of such relationships from an engineering standpoint may improve the animation of phenomena such as hair blowing in the wind.
“I think what we now have is a bridge between these two fields, and we can start having traffic back and forth,” Reis says.
This research was funded in part by the National Science Foundation. | | 12:00a |
Crumpled graphene could provide an unconventional energy storage When someone crumples a sheet of paper, that usually means it’s about to be thrown away. But researchers have now found that crumpling a piece of graphene “paper” — a material formed by bonding together layers of the two-dimensional form of carbon — can actually yield new properties that could be useful for creating extremely stretchable supercapacitors to store energy for flexible electronic devices.
The finding is reported in the journal Scientific Reports by MIT’s Xuanhe Zhao, an assistant professor of mechanical engineering and civil and environmental engineering, and four other authors. The new, flexible supercapacitors should be easy and inexpensive to fabricate, the team says.
“Many people are exploring graphene paper: It’s a good candidate for making supercapacitors, because of its large surface area per mass,” Zhao says. Now, he says, the development of flexible electronic devices, such as wearable or implantable biomedical sensors or monitoring devices, will require flexible power-storage systems.
Like batteries, supercapacitors can store electrical energy, but they primarily do so electrostatically, rather than chemically — meaning they can deliver their energy faster than batteries can. Now Zhao and his team have demonstrated that by crumpling a sheet of graphene paper into a chaotic mass of folds, they can make a supercapacitor that can easily be bent, folded, or stretched to as much as 800 percent of its original size. The team has made a simple supercapacitor using this method as a proof of principle.
The material can be crumpled and flattened up to 1,000 times, the team has demonstrated, without a significant loss of performance. “The graphene paper is pretty robust,” Zhao says, “and we can achieve very large deformations over multiple cycles.” Graphene, a structure of pure carbon just one atom thick with its carbon atoms arranged in a hexagonal array, is one of the strongest materials known.
To make the crumpled graphene paper, a sheet of the material was placed in a mechanical device that first compressed it in one direction, creating a series of parallel folds or pleats, and then in the other direction, leading to a chaotic, rumpled surface. When stretched, the material’s folds simply smooth themselves out.
Forming a capacitor requires two conductive layers — in this case, two sheets of crumpled graphene paper — with an insulating layer in between, which in this demonstration was made from a hydrogel material. Like the crumpled graphene, the hydrogel is highly deformable and stretchable, so the three layers remain in contact even while being flexed and pulled.
Though this initial demonstration was specifically to make a supercapacitor, the same crumpling technique could be applied to other uses, Zhao says. For example, the crumpled graphene material might be used as one electrode in a flexible battery, or could be used to make a stretchable sensor for specific chemical or biological molecules.
“This work is really exciting and amazing to me,” says Dan Li, a professor of materials engineering at Monash University in Australia who was not involved in this research. He says the team “provides an extremely simple but highly effective concept to make stretchable electrodes for supercapacitors by controlled crumpling of multilayered graphene films.” While other groups have made flexible supercapacitors, he says, “Making supercapacitors stretchable has been a great challenge. This paper provides a very smart way to tackle this challenge, which I believe will bring wearable energy storage devices closer.”
The research team also included Jianfeng Zang at Huazhong University of Science and Technology and Changyang Cao, Yaying Feng, and Jie Liu at Duke University. The work was supported by the Office of Naval Research, the National Science Foundation, and the National 1000 Talents Program of China. |
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