Manufacturing Bits: Nov. 29

Supersonic kinetic spraying; tiny electron gun.


Supersonic kinetic spraying
Low-cost flexible electronics could enable a new class of products, such as roll-up displays, wearable electronics, flexible solar cells and electronic skin.

There is a major barrier to enable these technologies, however. The problem is to make flexible transparent conducting films that are scalable and economical.

The University of Illinois at Chicago and Korea University have taken a step to solve the problem. Researchers have developed a supersonic kinetic spraying technique that enables new films for use in flexible and wearable electronics.

The films are made of fused silver nanowires. The ultra-thin film is transparent and conductive to electric current.

In a system, researchers first suspend the silver nanowires in water. Then, the nanowires are propelled through a nozzle at supersonic speeds. The ideal speed is 400 meters per second, according to researchers. Then, the liquid evaporates in flight and strikes a surface, namely a flexible substrate. Then, silver nanowires fuse together. The nanowires are about 20 microns long, but the diameters are a thousand times smaller.

Left, photograph of a large-scale silver nanowire-coated flexible film. Right, silver nanowire particles. (Credit: S.K. Yoon, Korea University)

Left, photograph of a large-scale silver nanowire-coated flexible film. Right, silver nanowire particles. (Credit: S.K. Yoon, Korea University)

The resulting film has the electrical conductivity of a silver-plate. These films show low sheet resistance of <10 Ω sq−1, combined with high transmittance at >90%, according to researchers. The films can be applied to flexible plastic films and three-dimensional objects. “The surface shape doesn’t matter,” said Alexander Yarin, a professor of mechanical engineering at the University of Illinois at Chicago. “It should be easier and cheaper to fabricate, as it’s a one-step versus a two-step process. You can do it roll-to-roll on an industrial line, continuously.”

Tiny electron gun
The Deutsches Elektronen-Synchrotron (DESY) organization and the Massachusetts Institute of Technology (MIT) have built a small electron gun for use in scientific research.

Typically, electron guns are large systems that are used to generate beams of electrons. They are the electron source for giant linear particle accelerators, which drive X-ray free-electron lasers (FELs). Generally, FELs are used in X-ray metrology applications to explore various materials.

Electron guns are sometimes the size of a car. In comparison, the new electron gun from DESY and MIT is the size of a matchbox. It measures 34- x 24.5- x 16.8-mm.

A miniature electron gun driven by Terahertz radiation. (Source: DESY)

A miniature electron gun driven by Terahertz radiation. (Source: DESY)

The new device uses laser-generated terahertz radiation. This compares to traditional systems, which use radio-frequency fields to accelerate electrons. “Electron guns are used ubiquitously for making atomic-resolution movies of chemical reactions via ultrafast electron diffraction,” said DESY scientist Franz Kärtner, on the organization’s Web site. “With smaller and better electron guns, biologists can gain better insight to the intricate workings of macromolecular machines, including those responsible for photosynthesis. And physicists can better understand the fundamental interaction processes in complex materials.”

W. Ronny Huang from MIT added: “Our device has a nanometer thin film of copper which, when illuminated with ultraviolet light from the back, produces short bursts of electrons. Laser radiation with Terahertz frequency is fed into the device which has a microstructure specifically tailored to channel the radiation to maximize its impact on the electrons.”

As a result, the device reaches an accelerating power of 350 megavolts per meter. “The accelerating field was almost twice that of current state-of-the-art guns,” Huang said. “We achieved an acceleration of a dense packet of 250,000 electrons from rest to 0.5 kilo-electronvolts (keV) with minimal energy spread. Because of this, the electron beams coming out of the device could already be used for low-energy electron diffraction experiments.”

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