Manufacturing Bits: July 23

Space tubes; dynamic self-assembly; desktop lithography.

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Space Tubes
In 2011, NASA produced a material that absorbs on average more than 99% of the ultraviolet, visible, infrared, and far-infrared light that hits it. NASA’s so-called “super-black” material is based on a thin layer of multi-walled carbon nanotubes.

Tiny gaps between the nanotubes collect and trap light. The carbon absorbs the photons, preventing them from reflecting off surfaces. NASA demonstrated the ability to grow nanotubes on silicon, silicon nitride, titanium and stainless steel.

The technology is superior to the current technique. At present, instrument developers apply black paint to baffles and other components to reduce stray light. Instead of black paint, NASA is interested in using nanotubes to help suppress errant light that has a way of ricocheting off instrument components and contaminating measurements.

This close-up view (only about 0.03 inches wide) shows the internal structure of a carbon-nanotube coating that absorbs about 99 percent of the ultraviolet, visible, infrared, and far-infrared light that strikes it. A section of the coating, which was grown on smooth silicon, was purposely removed to show the tubes' vertical alignment. (Credit: Stephanie Getty, NASA Goddard)

In the latest development, NASA has demonstrated that it can grow a uniform layer of carbon nanotubes using atomic layer deposition (ALD). In the production process, NASA first deposits a uniform foundation or catalyst layer of iron oxide on a component using ALD. Then, the component is heated in another oven at about 750 degrees Celsius. To enable nanotube growth, the component is bathed in carbon-containing feedstock gas.

“The significance of this is that we have new tools that can make NASA instruments more sensitive without making our telescopes bigger and bigger,” said John Hagopian, an optics engineer at NASA’s Goddard Space Flight Center, on the agency’s Web site. “This demonstrates the power of nanoscale technology, which is particularly applicable to a new class of less-expensive tiny satellites called CubeSats that NASA is developing to reduce the cost of space missions.”

NASA’s CubeSat Launch initiative (CSLI) involves small satellites, which are a new class of research spacecraft called nanosatellites. Used for smaller payloads, the cube-shaped satellites are about four inches long, have a volume of about one quart and weigh about three pounds. NASA recently selected CubeSat projects for flight opportunities as part of its CSLI program in the Human Exploration and Operations Mission Directorate.

Dynamic Self-Assembly
Self-assembly is a promising technology in which interacting bodies are autonomously aligned into an ordered structure. Static structures form through energy minimization, while dynamic ones require continuous energy.

Dynamic structures are challenging and difficult to engineer. Aalto University and Paris Tech have bridged the gap from static to dynamic self-assembly by introducing a model system based on ferrofluid droplets on superhydrophobic surfaces.

The droplets self-assemble under a static external magnetic field into simple patterns, which can be switched to dynamic dissipative structures by applying a time-varying magnetic field. The transition between the static and dynamic patterns involves kinetic trapping.

Researchers have placed water droplets containing magnetic nanoparticles on water repellent surfaces. Researchers increased the field strength and the vertical gradient acting on the droplet. They also decreased the gap between the magnet and the surface.

In one experiment, researchers used a 20-μl ferrofluid droplet. The field was increased from 80 Oe (dH/dz 3.5 Oe/mm) to 680 Oe (dH/dz 66 Oe/mm). This, in turn, “led to a deformation of the droplet into a spiked cone and cleavage into two smaller droplets at the critical field strength,” according to a research paper from Aalto University and Paris Tech. “The division takes a few tens of milliseconds, after which the daughter droplets briefly oscillate before settling at their equilibrium separation.”

This is the first time researchers have demonstrated reversible switching between static and dynamic self-assembly. “We are conducting this line of research because it opens up a way to create new responsive and intelligent systems and materials,” said Robin Ras of Aalto University, on the entity’s Web Site.

Desktop Litho
Northwestern University has put a new twist on desktop publishing and maskless lithography. Researchers have developed a desktop nanofabrication system using a massively multiplexed beam-pen lithography technology.

The technology resembles a tiny multi-beam lithography system. It makes use of beam-pen lithography (BPL) pen arrays. The arrays are based on polymeric pyramids. Each pyramid is coated with an opaque layer with a 100nm aperture at the tip. Then, a digital micromirror device projects light into the tip. A single beam of light is broken up into thousands of individual beams.

The instrument uses inexpensive components. The tool is capable of writing arbitrary patterns composed of diffraction-unlimited features over areas that are in registry with existing patterns and nanostructures.

This tool can be used to prototype functional electronic devices in a mask-free fashion. It can also be used to devise gene chips and protein arrays. “With this breakthrough, we can construct very high-quality materials and devices, such as processing semiconductors over large areas, and we can do it with an instrument slightly larger than a printer,” said Chad Mirkin, the George B. Rathmann Professor of Chemistry in the Weinberg College of Arts and Sciences, on Northwestern’s Web site.

“There is no need to create a mask or master plate every time you want to create a new structure,” Mirkin said. “You just assign the beams of light to go in different places and tell the pens what pattern you want generated.”

—Mark LaPedus