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An industrial scale production process makes pure CNTs, new process gives shape to carbon nanotubes

An industrial scale production process makes pure CNTs, new process gives shape to carbon nanotubes

30-May-11
An industrial scale production process for manufacture of pure carbon-nanotube fibers that could lead to revolutionary advances in materials science, power distribution and nanoelectronics, has been developed over the last decade by combined research efforts of three universities-Rice University, University of Pennsylvania and Technion-Israel Institute of Technology. Houston based Rice University scientists have found the "ultimate" solvent for all kinds of carbon nanotubes (CNTs), a breakthrough that brings the creation of a highly conductive quantum nanowire ever closer, as per ACS Nano.
Nanotubes have the frustrating habit of bundling, which makes them less useful than when they're separated in a solution. Rice scientists have been trying to untangle them for years as they look for scalable methods to make exceptionally strong, ultralight, highly conductive materials that could revolutionize power distribution, such as the armchair quantum wire. The armchair quantum wire is a macroscopic cable of well-aligned metallic nanotubes envisioned by the late Richard Smalley. The Rice University team has reported that chlorosulfonic acid can dissolve half-millimeter-long nanotubes in solution, a critical step in spinning fibers from ultralong nanotubes. Current methods to dissolve carbon nanotubes, which include surrounding the tubes with soap-like surfactants, doping them with alkali metals or attaching small chemical groups to the sidewalls, disperse nanotubes at relatively low concentrations. These techniques are not ideal for fiber spinning because they damage the properties of the nanotubes, either by attaching small molecules to their surfaces or by shortening them. A few years ago, the Rice researchers discovered that chlorosulfonic acid, a "superacid" adds positive charges to the surface of the nanotubes without damaging them. This causes the nanotubes to spontaneously separate from each other in their natural bundled form. This method is ideal for making nanotube solutions for fiber spinning because it produces fluid dopes that closely resemble those used in industrial spinning of high-performance fibers. Until recently, the researchers thought this dissolution method would be effective only for short single-walled nanotubes. The acid dissolution method is also reported to work with any type of carbon nanotube, irrespective of length and type, as long as the nanotubes are relatively free of defects. The process is very easy and involves adding the nanotubes to chlorosulfonic acid that results in dissolution, without even mixing. The team discovered chlorosulfonic acid is also adept at dissolving multiwalled nanotubes (MWNTs). For the first time it was observed that long SWNTs dispersed by superacid form liquid crystals.
Study co-author opines that working with long nanotubes is key to attaining exceptional properties in fibers because both the mechanical and electrical properties depend on the length of the constituent nanotubes. Using long nanotubes in the fibers should improve their properties on the order of one to two magnitudes, and similar enhanced properties are also expected in thin films of carbon nanotubes being investigated for flexible electronics applications. Materials Today reports - "Plastics is a $300 billion U.S. industry because of the massive throughput that's possible with fluid processing," said Rice's Matteo Pasquali, a paper co-author and professor in chemical and biomolecular engineering and in chemistry. The reason grocery stores use plastic bags instead of paper and the reason polyester shirts are cheaper than cotton is that polymers can be melted or dissolved and processed as fluids by the train-car load. Processing nanotubes as fluids opens up all of the fluid-processing technology that has been developed for polymers." "That research established an industrially relevant process for nanotubes that was analogous to the methods used to create Kevlar from rodlike polymers, except for the acid not being a true solvent,� said Wade Adams, director of the Smalley Institute and co-author of the new paper. �The current research shows that we have a true solvent for nanotubes� chlorosulfonic acid � which is what we set out to find when we started this project nine years ago." Kevlar, the polymer fiber used in bulletproof vests, is about 5-10 times stronger than the strongest nanotube fibers today, but in principle the Rice scientists should be able to make their fibers about 100 times stronger.
Twisting spires, concentric rings, and gracefully bending petals are a few of the new three-dimensional shapes that University of Michigan engineers can make from carbon nanotubes using a new manufacturing process. The process is called "capillary forming," and it takes advantage of capillary action, the phenomenon at work when liquids seem to defy gravity and spontaneously travel up a drinking straw. The new miniature shapes have the potential to harness the exceptional mechanical, thermal, electrical, and chemical properties of carbon nanotubes in a scalable fashion. The 3D nanotube structures could enable countless new materials and microdevices, including probes that can interface with individual cells, novel microfluidic devices, and lightweight materials for aircraft and spacecraft. Assembling nanostructures into three-dimensional shapes is one of the major goals of nanotechnology and nanomanufacturing. The method of capillary forming could be applied to many types of nanotubes and nanowires, and its scalability is very attractive for manufacturing. Hart's method starts by patterning a thin metal film on a silicon wafer. This film is the iron catalyst that facilitates the growth of vertical carbon nanotube "forests" in patterned shapes. It's a sort of template. Rather than pattern the catalyst into uniform shapes such as circles and squares, Hart's team patterns a variety of unique shapes such as hollow circles, half circles, and circles with smaller ones cut from their centers. The shapes are arranged in different orientations and groupings, creating different templates for later forming the 3D structures using capillary action. A chemical vapor deposition process is used to grow the nanotubes in the prescribed patterns. Chemical vapor deposition involves heating the substrate with the catalyst pattern in a high temperature furnace containing a hydrocarbon gas mixture. The gas reacts over the catalyst, and carbon from the gas is converted into nanotubes, which grow upward like grass. The silicon wafer is suspended with its nanotubes over a beaker of boiling acetone. The acetone is allowed to condense on the nanotubes, and then evaporate. As the liquid condenses, it travels upward into the spaces among the vertical nanotubes. Capillary action kicks in and transforms the vertical nanotubes into the intricate three-dimensional structures. For example - tall half-cylinders of nanotubes bend backwards to form a shape resembling a three-dimensional flower.
"We program the formation of 3D shapes with these 2D patterns," Hart said. "We've discovered that the starting shape influences how the capillary forces manipulate the nanotubes in a very specific way. Some bend, others twist, and we can combine them any way we want." The capillary forming process allows the researchers to create large batches of 3D microstructures-all much smaller than a cubic millimeter. In addition, the researchers show that their 3D structures are up to 10 times stiffer than typical polymers used in microfabrication. Thus, they can be used as molds for manufacturing of the same 3D shapes in other materials. This is thought to open up the possibility to create custom nanostructured surfaces and materials with locally varying geometries and properties. Before, we thought of materials as having the same properties everywhere, but with this new technique we can dream of designing the structure and properties of a material together. Large batches of the 3D nanotube structures can be produced at once, and due to the fact that they are very stiff, they could be used as molds for creating duplicate structures using other materials. Hart believes that they could make many new microdevices and materials possible, such as probes that can interface with individual cells, novel microfluidic devices, and lightweight materials for aircraft and spacecraft.
 
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