Fabrication of Aligned and Random CNT films for flexible electronics, energy harvesting and artificial muscles

M.D. Lima
LINTEC of America,
United States

Keywords: carbon nanotubes, free standing films, CNT fibers, actuators, energy harvesthing

Summary:

Carbon nanotube (CNT) films can be used as starting material for a variety of products such as electrically conductive layers, filtering membranes and for manufacturing of fiber and yarns1. LINTEC have developed dry and liquid based methods to produce large size, free standing films of CNTs which can have an aligned or random orientation, reaching areal densities as low as 0.2 g/cm2 (only 2x of single layer of graphene). Their volumetric density can be as low as 1mg/cm3 (close to air) still with a weight normalize tensile strength greater than high strength steel2. Using only adhesion based on van de Waals forces, these films readily attached to virtually any solid substrates. After liquid densification, their thickness is less than 50 nm what makes than very flexible and resistant to mechanical fatigue. By modifying its topology, stretchable conductive films capable to reach more than 1000% can be manufactured from these CNT films3. Perhaps the most interesting application for CNT films is the production of CNT yarns through spinning. For single ply yarns, diameters between 3.5 µm and 150 µm can be easily manufactured. Electrical conductivity up to 160 kS/cm can be achieved by doping. When using thin yarns (< 4 µm ) arranged as grids, films with transparencies higher than 90% at 550nm and surface resistance less than 0.4 Ohm/sq can be manufactured in sizes up to 0.5 x 0.5 m. Addition of guest materials during the spinning process produces unique yarns with the guest material content as high as 99%wt 4. Such hybrid yarns have potential use as electrodes for batteries, supercapacitors and fuel cells, catalytic membranes, magnets, highly porous absorbers, and strong structures containing biomedical agents. Furthermore, it has been demonstrated5 that highly twisted carbon nanotube yarns can perform as mechanical actuators, capable to generate impressive tensile actuation, providing large strokes and vastly exceeding the work and power capabilities of natural skeletal muscle. By applying electrical pulses, contractions up to 50% and a mechanical work capacity of 1.36 kJ/kg were achieved, which exceed by two orders the performance of biological muscle6. More than a million cycles of actuation were performed without significant loss of performance. These actuators also can operate as torsional motors: a single fiber can rotate heavy rotors at speeds higher than 70 000 RPM7. Finally, besides of producing mechanical work from electric energy CNT yarns can be use for the opposite purpose. Mechanical energy harvesting using CNT yarns which are thin and flexible enough to be incorporated into conventional textiles have been shown to directly convert tensile elastic energy into electrical energy 8. That makes a very attractive energy source for wearable electronics by directly harvesting mechanical energy from the natural movements of the human body. 1. Baughman R.H., et al. Science 297 (2002) 787 2. Lima, M.D., et al. Science 338 (2012) 928 3. Liu, Z.F., et al. Science. 349, (2015) 400 4. Lima, M.D., et al. Science 331 (2011) 51 5. Lima, M.D., et al. Science 338 (2012) 929 6. Lima, M.D., et al. Small 11 (2015) 3113 7. Kim S.H., et al. E.& E. Science 8 (2015) 3336 8. Kim S.H., et al. Science 357 (2017) 773