Shape-Memory Alloys alter in response to changes in temperature.
In 1932, shape memory was discovered. At Naval
Ordinance Labs in 1962,
Nickel-Titanium alloys were found to exhibit shape memory
significantly, and
revolutionized the field of
research.
How it works:
At a low
temperature, the material is in its martensitic state. When
heated, it will regain its
original or memory shape. Most other materials undergo drastic
material
property changes upon
heating; materials may become brittle or stiff and can thus be prone
to breaking.
Shape-Memory alloys can
tolerate strain 3 to 25 times higher than piezoelectrics can.
Some Shape-Memory alloys:
nickel-titanium (Nitinol)
gold-cadmium
gold-cadmium
brass
ferromagnetic (a thin film, low bandwidth alloy)
Examples of applications and links:
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Six-legged robot (similar to a spider) with leg
components as shape-memory alloys. As an increase in heat
occurs, the legs contract; as the material
cools, the legs expand. The leg components function like a muscle to
yield a walking
motion. Other examples of Shape-Memory
Alloys used for joint control include a winged robot whose wings
expand and contract to produce a flight
motion, a fish robot that swims, and a snake robot that slithers.
Examples
of some impressive micro-robots made with
Shape-Memory Alloys are at: http://www.toki.co.jp/MicroRobot/_MicroRobotGallery.html
Micro tools used in minimally invasive surgeries like
miniature forceps or manipulators that can close/open,
twist, push/pull, or similar actions with a
small amount of heat applied to the tool.
Bone Plates/Stents need to fuse together or expand.
With increased heat, stents contract so they can be held
in place. Stents are extremely popular for
heart surgeries when arteries need to be expanded. A stent of
a small diameter is easily placed within a
partially clogged artery, then heat causes it to expand, pushing on
deposits, to open the artery to allow to
increased blood flow. See a picture at: http://www.designinsite.dk/htmsider/k0052.htm
Wires that expand or contract may come to replace
traditional wires used by orthodontists and dentists over several
years when straightening teeth. Shape Memory
Alloy wires could be expanding or contracting at a very gradual
but constant rate, providing desired results
but very little discomfort, rather than dramatic
expansion/contraction at irregular intervals by dentists or
orthodontists.
Microbubble Actuator. A thin film is heated, thus
creating bubbles. Bubbles act as very small actuators. These
work extremely well because of the amount of
energy created when the material returns to its remembered shape
(the largest energy density of any smart material available).
Research AML's work on micron-sized actuators
used to increase cycling speed from 1 to 100
hertz for flow control on aircraft systems.
Cell Phone Antennas with materials that are
super-elastic at room temperature, but much more brittle at extreme
temperatures, at which phone is constructed.
Featherweights/twistable glasses also rely on these materials
for the same reason. Click on the picture
at: http://www.
designinsite.dk/htmsider/k0053.htm to see
the range of flexibility.
Active Braking Systems or other open/close systems.
Smart Helicopter Rotor. Easily tunable helicopter
blades. Andreas Paul Friedrich wrote his dissertation on
this topic in 2000, and it has since been
championed by Memry Corporation. His dissertation is at:
www.glue.umd.edu/~bernhard/DISS/diss_chap1.ps.gz
Release Mechanisms including those requiring dampening,
like a low-shock release device on spacecraft systems.
Smart Skin/Deicing system for rotorcraft. For an
abstract and links to information about similar systems using
other smart materials, see: http://www.idiny.com/abstracts/
sma.html
Flexible, active endoscopes that can be used for stomach
and colon assessments. Previous endoscopes are often not
flexible enough for use in these regions, but
this group is designing an endoscope with a robot on the end that can
slither like a snake. For a picture and brief
article: http://mozu.mes.titech.ac.jp/research/snake/endscope/endscope.html
