Besides the energy storage the researchers are making 10 to 20 centimeter long carbon nanotubes that have no defects and have a strength of 200 GPa.
Wei Fei and colleagues from Tsinghua University in Beijing and the Chinese Academy of Sciences in Shenyang have now succeeded in producing ultralong defect-free CNTs and demonstrating their potential for use in nanoscale mechanical energy storage.
“We set out to produce ‘perfect’ CNTs without defects,” says Wei. CNTs without defects are much more durable than lower-quality CNTs, and so can withstand repeated mechanical stress — a property that grants them an extraordinary capacity for storing mechanical energy. The researchers grew their ultralong CNTs using a recently developed technique that involved exposing iron catalyst particles on a specially designed silicon substrate to a stream of methane gas. The CNTs grew to lengths of 10–20 cm by a kite mechanism in the gas flow.
The researchers then investigated the mechanical properties of the CNTS by fixing them across a 750 μm-wide gap and exposing them to a high-speed flow of nitrogen gas (see image) and then sound from a loudspeaker. The behavior of the CNTs under these physical stresses was monitored by spraying the nanotubes with nanoparticles of titanium dioxide, which could be visualized easily by high-speed camera.
The tests revealed the CNTs to have remarkable mechanical durability with the ability to store mechanical energy at energy densities 5–8 times higher than that of lithium batteries, and 25,000 times that of steel springs. The stored energy could be used to power nano- and microelectromechanical systems.
The challenge now, Wei explains, is harnessing the storage power of CNTs. “We are now attempting to make a nano-electricity generator driven entirely by the mechanical energy stored in CNTs,” he says. The researchers also intend to refine their fabrication techniques to allow pure, long CNTs to be produced on a larger scales.
Microscopy image showing the TiO2-bearing carbon nanotube suspended across a 750 μm-wide gap and subjected to nitrogen gas flow at various speeds (yellow arrows)
© 2011 Wiley-VCH
Advanced Materials - Superstrong, ultralong, individual carbon nanotubes (CNTs) are deposited with TiO2 particles and visualized under an optical microscope with excellent strain-relaxation reversibility and high fatigue resistance capability. The CNTs with perfect structures have tensile strengths of up to 200 GPa, densities to 1.34 TPa, energy density as high as 1125 Wh per kg and the power density can be up to 144 MW per kg for mechanical energy storage. The superb mechanical properties confirm the potential of an individual CNT as an effective storage medium with mechanical energy for nano-electromechanical systems, flexible devices, sensors, actuators, antennas, etc.
Acoustic-wave-driving-vibration system used for testing the mechanical strength of CNT/TiO2 strings. The yellow layer on the silicon substrate was the TiO2 layer in which the unsuspended segments of a CNT were embedded. The suspended CNT/TiO2 strings can be excited to vibrate by the acoustic wave from the loudspeaker linked to a digital signal generator. Changing the frequency, the amplitude of the vibrating suspended CNTs will change accordingly. When the amplitude reaches the highest value, a resonance occurs. The resonance frequency can be read directly from the window of the signal generator. No higher order modes on the suspended nanotubes were detected in our experimental research. The measurement of amplitude is under optical microscope for the reason that the optical visualization of suspended CNTs/TiO2. The whole process of CNT vibration can be recorded by a digital camera. The resonant amplitude can be measured from the photo directly.
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