International Team Shows Stronger CNT Yarns for Nano Motors
Yarns made from carbon nanotubes are the power behind a new generation of artificial muscles developed by an international team of researchers from Australia, Canada, Korea and the U.S. The work could lead to low-cost, simple-to-produce novel nanomotors for a wide range of applications, according to the researchers.
An international team is working with high-strength carbon nanotube yarns to power a new generation of nanomotors.
In the research, the team constructed CNT-based muscles which can accelerate a 2000-times heavier paddle up to 590 revolutions per minute in 1.2 seconds, and then reverse this rotation. The team also demonstrated a rotation of 250 per millimeter of muscle length (1,000 x that of previous artificial muscles).
The higher output power (per-yarn-weight) is comparable to that for large electric motors, and the weight-normalized performance of these conventional electric motors severely degrades when they are downsized to millimeter scale.
"The discovery, characterization, and understanding of these high performance torsional motors shows the power of international collaborations", said Prof. Ray H. Baughman, Robert A. Welch Professor of Chemistry and director of The University of Texas at Dallas Alan G. MacDiarmid NanoTech Institute
Torsional CNT muscles can be used as nanoscale motors for a number of applications, Prof. Baughman added, because they can be simpler and more inexpensive to construct than it is to miniaturize conventional motors,
The CNT-based torsional motors consist of a yarn electrode and a counter-electrode, which are immersed in an ionically conducting liquid. A low voltage battery can serve as the power source, which enables electrochemical charge and discharge of the yarn to provide torsional rotation in opposite directions.
In the simplest case, researchers attach a paddle to the CNT-based yarn, which enables torsional rotation to do useful work – like mixing liquids on "micro-fluidic chips" used for chemical analysis and sensing.
Charging CNT-based yarns is like charging a supercapacitor - ions migrate into the yarns to electrostatically balance the electronic charge electrically injected onto the nanotubes. Although the CNT yarns are porous, this influx of ions causes the yarn to increase volume, shrink in length by up to a percent, and torsionally rotate.
This shrinkage in yarn length as volume increases is due to the yarn's helical structure, which acts like a child’s finger cuff toy – which traps a finger when elongated, but frees it when shortened.
The team’s use to torsional rotation follows many examples from nature, Prof. Baughman added. Nature has used torsional rotation based on helically wound muscles for hundreds of millions of years, and can be seen in elephant trunks and octopus limbs where helically wound muscle fibers cause rotation by contracting against an essentially incompressible, bone-less core.
Helically-wound CNTs in CNT-based yarns, in contrast, are undergoing little change in length, but are instead causing the volume of liquid electrolyte within the porous yarn to increase during electrochemical charging, so that torsional rotation occurs, according to Prof. Baughman.
The work demonstrates the strength of flexible of CNT yarns, which are created by spinning or rotating CNTs into helical yarns. Because rotation is torsional, a twist occurs in one direction until a limiting rotation results. Upon hitting that limit, rotation can be reversed by changing the applied voltage. The result is “left hand” and “right hand” yarns, which rotate in opposite directions when electrically charged.
Funding for this research was provided by grants from the Air Force Office of Scientific Research, the Air Force AOARD program, the Office of Naval Research MURI program, and the Robert A. Welch Foundation in the United States; the Creative Research Initiative Center for Bio-Artificial Muscle in Korea; the Natural Sciences and Engineering Research Council of Canada; and the Australian Research Council.
The work appears in the Oct 13 issue of Science magazine.