Mechanical energy storage in carbon nanotube springs

Abstract

Energy storage in mechanical springs made of carbon nanotubes is a promising new technology. Springs made of dense, ordered arrays of carbon nanotubes have the potential to surpass both the energy density of electrochemical batteries and the power density of capacitors due to the effective Young's modulus of carbon nanotubes of 1 TPa and their high elastic strain limit of up to 20%. The energy density of springs made of carbon nanotubes is predicted to be more than three orders of magnitudes higher than the energy density of conventional springs made of steel. The present work studies the mechanical properties and energy storage capabilities of two types of carbon nanotube arrays: fibers made of continuous, millimeter-long carbon nanotubes prepared from forests, and spun yarn made of carbon nanotubes. The highest recorded strength and stiffness of the fibers are 2 N/tex and 68 N/tex, respectively, and the fibers have demonstrated reversible energy storage of 670 kJ/m³ or 6.9 kJ/kg. The spun yarn has a specific strength of 0.8 N/tex, a specific stiffness of 48 N/tex, a maximum energy density of 7720 kJ/m³ or 6.7 kJ/kg, and a maximum demonstrated power density of 190 MW/m³ or 170 kW/kg. The energy density of the current springs is three orders of magnitude lower than the theoretical limit. Mechanical testing, Raman spectroscopy during loading, scanning electron microscope imaging during loading, and simultaneous stress and electrical resistance measurements have provided valuable insights into the mechanisms governing the mechanical behavior of carbon nanotube fibers and yam. While elastic loading is optimal for reversible energy storage, the results indicate that disorder in the structure of both materials causes loading to deviate from purely elastic behavior. Densification of carbon nanotubes in fibers using capillary effects is shown to be an effective way to consolidate CNTs and create high performance fibers since the CNTs selfassemble into dense, interacting networks that promote load transfer. The first demonstrations of carbon nanotube springs that store energy and power small-scale systems are presented. These systems include mechanical watches, escapements, slingshots, and a mechanical energy harvester. Power regulation mechanisms were implemented to control the rate at which energy was released from the springs. Electric batteries were developed that store energy mechanically in carbon nanotube springs and release the energy in the electrical domain. The energy density, power density and efficiency of the springs in each of the systems are characterized to evaluate the performance of the springs as an energy storage medium.