Characterizing the failure processes that limit the storage of energy in carbon nanotube springs under tension

We report measurements of the mechanical properties and energy storage capabilities of carbon nanotube (CNT) springs under tensile loading, including correlated measurements of their cyclic loading and electrical resistance behavior. Tests are conducted on fibers of multi-walled CNTs fabricated from 6 mm tall forests. The highest measured strength and stiffness of the fibers are 2 N tex−1 and 70 N tex−1 respectively. The highest recorded energy density is approximately 7 kJ kg−1 or 500 kJ m−3, more than an order of magnitude higher than the gravimetric energy density of steel springs, and half the volumetric energy density of steel springs. The resistance and stress responses of the fibers during loading to failure and cyclic loading demonstrate that disorder at the nanoscale affects the bulk response. CNT springs show limited effects of fatigue under 75 tensile cyclic loading cycles. Improving the structural quality of the CNTs and the organization of the fibers offers potential to significantly increase the energy storage capacity of the springs