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Tech Front: Nanotube Soldering Process Creates Tiny Electrical Pathways

Scientists at the University of Illinois (Champaign, IL) have devised a way to heal gaps in tiny wires too small for the smallest soldering iron. A research team, led by electrical and computer engineering professor Joseph Lyding and graduate student Jae Won Do, has published its results in the journal Nano Letters.

Carbon nanotubes, which are similar to graphene but cylindrical, are like tiny hollow wires of carbon just an atom thick. Researchers have been exploring using them as transistors in place of traditional silicon, because carbon nanotubes are easier to transport onto alternate substrates, such as thin sheets of plastic, for low-cost flexible electronics or flat-panel displays.

The carbon nanotubes are high-quality conductors, but creating single tubes suitable to serve as transistors is very difficult. Arrays of nanotubes are easier to make, but the current has to hop through junctions from one nanotube to the next, slowing it down. In standard electrical wires, such junctions would be soldered, but the researchers needed to bridge these gaps on a very small scale.
Electrical and computer engineering professor Joseph Lyding led the research team that developed a way to heal gaps in wires too small for even the world’s tiniest soldering iron.
“It occurred to me that these nanotube junctions will get hot when you pass current through them, kind of like faulty wiring in a home can create hot spots,” said Lyding. “In our case, we use these hot spots to trigger a local chemical reaction that deposits metal that nanosolders the junctions.”

The nanosoldering process is simple and self-regulating. A carbon nanotube array is placed in a chamber pumped full of metal-containing gas molecules. When a current passes through the transistor, the junctions heat because of resistance as electrons flow from one nanotube to the next. The molecules react to the heat, depositing the metal at the hot spots and effectively “soldering” the junctions. Then the resistance drops, as well as the temperature, so the reaction stops. The nanosoldering process takes only seconds and improves the device performance by an order of magnitude—almost to the level of devices made from single nanotubes, but much easier to manufacture on a large scale.

Lyding’s group teamed with Eric Pop, an adjunct professor of electrical and computer engineering, and John Rogers, Swanlund professor in materials science and engineering, experts on carbon nanotube synthesis and transfer, as well as chemistry professor Greg Girolami, an expert in a process that uses gases to deposit metals on a surface, called chemical vapor deposition (CVD).

“It would be easy to insert the CVD process in existing process flows,” Lyding said. “CVD technology is commercially available off-the-shelf. The process of nanosoldering improves the performance of nanotube transistors. Metal self-deposits onto hot-spot junctions, healing gaps between nanotubes.People can fabricate these transistors with the ability to turn them on so that this process can be done. Then when it’s finished they can finish the wiring and connect them into the circuits. Ultimately it would be a low-cost procedure.”

Now, the group is working to refine the process. “We think we can make it even better,” Lyding said. “This is the prelude, we hope, but it’s actually quite significant.”

The National Science Foundation and the Office of Naval Research supported the work. Lyding and Rogers also are affiliated with the Beckman Institute for Advanced Science and Technology at the University of Illinois. The paper, “Nanosoldering Carbon Nanotube Junctions by Local Chemical Vapor Deposition for Improved Device Performance,” is available at http://tinyurl.com/Nanosoldering. ME



Liquid Crystal “Flowers” Can Be Used as Lenses

Researchers at the University of Pennsylvania (Philadelphia) have made another advance in their ongoing efforts to use liquid crystals as a medium for assembling structures.

Material scientists, chemical engineers and physicists at Penn have produced patterns of “defects,” useful disruptions in the repeating patterns found in liquid crystals, in nanoscale grids and rings in earlier studies. This new research adds a more complex pattern out of an even simpler template: a 3D array in the shape of a flower. Because the petals of this “flower” are made of transparent liquid crystal and radiate out in a circle from a central point, the ensemble resembles a compound eye and can thus be used as a lens.

The team consists of Randall Kamien, professor in the School of Arts and Sciences’ Department of Physics and Astronomy; Kathleen Stebe, School of Engineering and Applied Science’s deputy dean for research and professor in Chemical and Biomolecular Engineering and Shu Yang, professor in Engineering’s departments of Materials Science and Engineering and Chemical and Biomolecular Engineering.

A paper, “Focal Conic Flower Textures at Curved Interfaces,” describing their work was recently published in the journal Physical Review X. The researchers’ ongoing work with liquid crystals is an example of a growing field of nanotechnology known as “directed assembly,” in which scientists and engineers aim to manufacture structures on the smallest scales without having to individually manipulate each component. Instead, they set out precisely defined starting conditions and let the physics and chemistry that govern those components do the rest.

In their latest work, the researchers used a much simpler cue. “Before we were growing these liquid crystals on something like a trellis, a template with precisely ordered features,” Kamien said. “Here, we’re just planting a seed.” The seeds, in this case, were silica beads—essentially, polished grains of sand. Planted at the top of a pool of liquid crystal flower-like patterns of defects grow around each bead.

In their experiment that generated grid patterns of defects, those patterns stemmed from cues generated by the templates’ microposts. Domains of elastic energy originated on the flat tops and edges of these posts and travelled up the liquid crystal’s layers, culminating in defects. Using a bead instead of a post, as the researchers did in their latest experiment, makes it so that the interface is no longer flat.

This type of directed assembly could be useful in making optical switches and in other applications. The research was supported by the National Science Foundation, Penn’s Materials Science Research and Engineering Center, and the Simons Foundation. ME


Rapid-Review Journal Presents Innovations across Manufacturing

The first two issues of Manufacturing Letters, a new journal from SME and its publishing partner Elsevier, were recently introduced. This online publication joins the Journal of Manufacturing Processes and Journal of Manufacturing Systems on the advanced manufacturing reading list from SME.

The brief, concise letters format and a rapid review process provide an interdisciplinary forum for significant new findings, innovations and know-how from across manufacturing’s broad spectrum, explains Manufacturing Letters’ editor-in-chief Kornel F. Ehmann, PhD, FSME (Northwestern University; Evanston, IL). Submissions merit acceptance by detailing the key elements of an emerging field of research, describing the potential of an experimental or theoretical improvement or demonstrating a new, nontrivial contribution to practice or policy.

While access to SME journal papers is generally fee-based and member-restricted, several of the new articles in Manufacturing Letters are available free of charge until June 2014. For more information on submissions, subscriptions and member access to all SME journals, go to www.sme.org/journals.

In the paper (http://tinyurl.com/Zhang-Jun) by Y. Zhang and M.B.G. Jun of the University of Victoria (Victoria, BC), a new system delivers independently atomized water and oil for effective cooling and lubrication in micromilling, where rapid tool wear is a significant problem. The system requires no surfactant or emulsifiers because water and oil are mixed in the air. Better cutting performance was observed when a water and oil mixture was used, compared to water only, oil only and conventional cutting fluid. The results also show strong potential for effectively tailoring oil and water amounts for different materials and cutting conditions.

Schematic (a) and photo (b) of a system to deliver independently atomized water and oil in micromilling (http://tinyurl.com/Zhang-Jun).

AFB (abrasive fluidized bed) finishing is a promising technique in the reprocessing of hard coatings. M. Barletta and colleagues from Italy applied AFB to finish axial-symmetric workpieces coated by high-velocity oxygen fuel with a cobalt-chromium alloy (http://tinyurl.com/Barletta-et-al). Al2O3 abrasives were found suitable to finish the coatings with a satisfactory morphology, and the process did not have a significant impact on dimensional properties.

The feasibility of a haptic position approach to measure the shape of soft and compliant objects with a hand-held magnetic sensor is demonstrated by University of Michigan (Ann Arbor, MI) researchers R. Chen, Y. Wang, B. Tai and A. Shih (http://tinyurl.com/Roland-Chen). Applications of the haptic position measurement system (HPMS), when complex geometries preclude use of conventional CMMs, include measuring the deflection of thin guide wire and needle insertion into soft tissue, deformation of tissue during surgical manipulation and displacement of compliant mechanisms.

The current issue of the Journal of Manufacturing Processes (http://tinyurl.com/JMP14-4) presents 33 papers, including nine focused on advanced machining systems. Shiv G. Kapoor, PhD, FSME, of the University of Illinois (Urbana, IL) notes articles covering ionic fluid lubricants, atomized cutting fluid concentrations for titanium machining, modulation-assisted high-speed machining of compacted graphite iron and spindle dynamics identification using particle swarm optimization, among others.

The 30 articles in the most recent Journal of Manufacturing Systems (http://tinyurl.com/JMS32-4) include two review papers on cloud manufacturing and aircraft assembly. JMS now has a new editor-in-chief, Neil A. Duffie, PhD, FSME, CMfgE (University of Wisconsin-Madison), following the successful six-year tenure of S. Jack Hu, PhD, from the University of Michigan.


TechFront is edited by Senior Editors Patrick Waurzyniak, pwaurzyniak@sme.org, and Ellen Kehoe, ekehoe@sme.org.

 

This article was first published in the February 2014 edition of Manufacturing Engineering magazine.  Click here for PDF


Published Date : 2/1/2014

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