Some of the most interesting and potentially useful materials are restricted in their application due to the limited manner in which they may be processed. For example it is not uncommon for useful materials to be produced in a powder form, when a single crystal or flexible wire morphology would be much more practical. Fortunately a team of researchers working at the University of Texas at Dallas have developed a method of producing yarn that contains high powder concentrations such that functional fibers can be produced for weaving, sewing, knotting, and braiding [Lima et al., Science (2011) 331, 51].
Previous methods used to produce yarns from functional powders have relied on dispersing the powder within a fiber, or by incorporating the powder onto the fiber surface. However these methods are not ideal as the concentrations of the powder are typically low, and the material is not functionally robust. Now Prof. Ray H. Baughman and colleagues have demonstrated that durable and functional yarns can be produced by biscrolling nanotube sheets. That is to say, the functional guest powder is placed on top of a host nanotube sheet which is then twisted to form a yarn. Even though the powders are not bonded to the surface, these new yarns can be washed in a conventional washing machine without losing a measurable amount of guest material. The carbon nanotube sheets are light and thin, and so the yarn is actually 95 % powder by weight. Thus, the resulting material retains the properties of the guest powder.
The sheets are drawn “at up to 2 m/s from special forests of carbon nanotubes” Baughman explained. “There are no inherent limitations on either sheet width or length, and no special difficulties in maintaining sheet quality during the draw. In fact we have already produced kilometer long yarns by twisting the narrow sheets as they are drawn from a forest”.
The sheets are just 50 nm thick, and thanks to the conductive nature of CNTs it is possible for the powder to be electrically connected. However, the authors point out that insulating hosts such as SiO2 and Si3N4 nanotubes could be also be used, depending on the situation.
To demonstrate the effectiveness of their method, the team produced several functional yarns. By biscrolling boron and magnesium powders, and then treating the yarn with magnesium vapor, they created a superconducting MgB2 yarn. The resulting material was thus far easier to produce than conventional MgB2 wires. By using LiFePO4 the researchers produced flexible Li-ion battery cathodes that were far lighter than conventional cathodes. The team is continuing their work in this area, and are working with researchers at Hanyang University, Korea to produce “flexible fuel cells, biofuel cells, and batteries using biscrolled yarns that are woven or sewn into fabrics.


Stewart Bland