Nano-Particulate Platinum / Ni-Ti Shape Memory Alloy
Continuously Shorted Fuel Cell for Artificial Muscle
Chemo-mechanical muscles (continuously shorted fuel cells) utilizing Ni-Ti shape
memory alloy coated with nano-particulate Pt have demonstrated substantial activation
stresses and strains, comparable to natural muscle (higher actuation stress, but lower
elongation. Upon contact with a hydrogen / oxygen containing fuel mix, the shape
memory element heats up above its austenitic transition temperature, and contracts,
producing ~ 5% strain and ~ 150 MPa stress. When the fuel and/or oxidant is removed
the shape memory element cools below the martensitic transition temperature, and
contracts to its original length. Heat transfer and transformation hysteresis (~ 30C)
controls cycle time and thus output power: (recall that Power may be defined as Work
per unit Time = (Force x Distance) / Time).
Project goals would include characterizing and/or optimizing the Shape
Memory Alloy’s phase transformation behavior (note that the phase transformation is
not reversible for an infinite number of cycles), optimizing efficiency and chemomechanical power transduction, and improving response time through optimized
material selection, cell design and associated heat transfer. The deliverables for this
project would be a working prototype of a chemo-mechanical actuator, suitable for use
in a prosthetic device (or other application, subject to staff approval), accompanied by a
detailed technical analysis of the capabilities and limitations of the prototype (such as
actuator force, stroke, thermodynamic efficiency, cycle time, suitability for various along
with suggested avenues for improvement and directions for further research).
The photos above show a ‘proof-of-concept’ demonstration of the effect.
Extended (left) and contracted (right) states during the cyclic operation of a fuel-powered
shape-memory muscle to lift and lower a 50 g weight. Methanol evaporates from the
labeled container and reacts with air on the platinum-coated surface of the Ni-Ti shape-
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memory wire spring. The resulting heating causes the spring to contract, thereby lifting
both the underlying weight and a Teflon plug, which eventually blocks the flow of
methanol vapor to the shape-memory spring attached above. In the absence of this fuel,
the shape-memory wire cools, extends to lower the weight and unblock the flow of
methanol vapor to the shape-memory spring, enabling the process to repeat automatically
(note that in the example shown, the cycle time is controlled by heat transfer from the
wire to a quiescent atmosphere and is not optimized).
Image courtesy of J Oh, M Kozlov, V H Ebron, and R H Baughman.
SOURCE: http://nanotechweb.org/articles/news/5/3/18/1/ebron
Continuously shorted fuel cell muscle based on a Ni-Ti shape-memory alloy. (A)
Schematic illustration, with cut-away to reveal details, of the fuel-powered artificial
muscle mounted in the dynamic mechanical analyzer used for measurements. (B)
Actuator strain versus time during exposure of the chemically powered actuator to a
mixture of N2, 2.5% by volume hydrogen and 50 % oxygen (red curves) and during
exposure to pure oxygen (blue curves). (C) Actuator strain versus time for different
volume percents of hydrogen for the experiment in B. The insert shows the dependence
of actuator strain on the H2 volume % in the fuel.
SOURCE: http://www.utdallas.edu/chemistry/frg/research.html
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