PURPOSE: To illustrate shape-memory alloy.
DESCRIPTION: A loop of 0.012 inch NITINOL wire is wound around an upper plastic pulley and a lower brass pulley. When the lower brass pulley is immersed about half way into water at 80 degrees celcius, the wire rotates, acting as a heat engine.
EQUIPMENT: Thermobile, beaker of hot water on hot plate, classroom thermometer as photographed.
SETUP NOTES:
The place that manufactured the thing:
Innovative Technology International, Inc.
10747-3 Tucker St.
Beltsville, MD 20705
Telephone # was 301-937-3688
They will send replacement loops plus all kinds of
other memory wire stuff.
The shape memory comes from changes in crystal structure as a
function of temperature.
The high temperature phase (austenite) is much stronger than the low
temperature phase (martensite). At room temperature, the wire is in the
martensite phase; it's very weak and deforms easily. When you heat this,
the x-tal changes phase to the much stronger austenite phase to regain
it's original shape.
Go visit http://www.sma-inc.com/
They can send an engineering sample (about 2') for free!
The unannealed wire is used in this demo. Form it into any shape, then heat it to anneal
it.
Form it into the shape of a person and heat it with a propane torch (or
electrically). Crumble him up. Then throw him into hot (70 C) water and he'll
suddenly turn back into a man again.
Another source of memory wire is Educational Innovations, Inc.:
(phone 203-629-6049).
They sell it for $5.00/foot for short lengths,
$4.50/foot if ordering 6 feet or more (cat. no. #HS-6).
They also sell packages of 10 wires, 3" long each,
for $6.50 (cat. no. HS-9).
Regarding the "memory wire": These are the Shape-memory Alloys (SMAs), and
Nitinol is the most well-known
example of them. However, there are also several brass type, and steel type SMAs.
These materials are available in various forms: wires, strips, plates, etc.
Applications of SMAs include devices in aeronautics, medicine, dentistry,
electronics, agriculture, domestic appliances, clothing (shirts) and
even intimate apparel (no joke !!, commercially a big success).
SMAs drastically change shape (and in general all physical properties) when heated.
As pointed out earlier, SMAs work due to the fact that when heated
these materials undergo a reverse martensitic transformation.
The temperature range at which the most significant changes are observed
is about 5-10 degrees. Within this range we find what is called
the "transformation temperature" of the alloy (i.e., the temperature at
which a crystal transformation occurs: from the "martensitic (soft) phase"
to the "austenitic (rigid) phase"). There are ONE-WAY SMAs and TWO-WAY
SMAs :
The nitinol wire is a ONE-WAY SMA, that is it recovers its "hot" shape
when heated, but it DOES NOT return to its original shape after cooling.
Brass type SMAs are generally TWO-WAY SMAs, they return to their original
shape after cooling.
Because of these properties SMAs are very useful for designing thermal
actuators. With brass SMAs you get two-way actuation in the same part,
while with one-way SMAs you need to provide a biasing force.
Designing and making SMA parts to perform a specific function IS NOT
that easy. One needs to understand that designing the metallurgical
process for making the part is an integral element of the part-design
procedure.
The transformation temperature is determined through alloying, and to
a certain degree modified in the metallurgical/"training" procedure used
in the making of the part. Furthermore, virtually all physical properties
of these alloys vary non-linearly with temperature. This means that all
of the nice engineering design formulas, such as the ones used for
designing a coil spring, are out the window (these formulas assume linear
behaviour with temperature).
If one is interested in simply training a piece of wire to adopt a specific
shape when it is heated, all that is required is to put the wire through
a series of thermomechanical "training" cycles to impart the high
temperature shape (clamp the wire down in the desired "high-temp"shape
and heat it; if the material is a two-way SMA then one must also force
the material to adopt the "cold-temp" shape while quenching it. Repeat
the cycle several times; 10, 20,...depending on the material and part).
However, if one wishes to get a certain amount of volumetric work output
from the SMA part, one has to be more careful with the "training" and
design process. This is particularly important if one wishes to make
a part that delivers a high number of "shape-changes" or cycles
(a good design will deliver ten, hundred thousands, or even millions
of cycles); or if one wishes to reduce the thermal hysteresis of the part.
Well, enough for now. I hope that this is of some value.
Please let me know if I can help you find out more about these materials.
For those familiar with this memory Nitinol wire, I
have the the Thermobile device made up of a loop of Nitinol
around two pulleys. The wire just broke at one point, possibly due
to fatigue or inherent weakness, and I'm wondering how to best fix
it. Could anybody that has worked with the stock wire tell me the
best way to fix it? Actually I think it might have broken at the edge
of an area where an adjunct faculty member placed the wire in a
FLAME(!) thinking that was supposed to make it work.
NiTi, otherwise known as Nitinol wire is the shape memory wire
you want to know about. The shape memory comes from changes in
crystal structure as a function of temperature.
The high temperature phase (austenite) is much stronger than
the low temperature phase (martensite). At room temperature, the wire
is in the martensite phase; it's very weak and deforms easily. When
you heat this, the x-tal changes phase to the much stronger austenite phase
to regain it's original shape.
References:
L. McDonald Schetky, Shape Memory Alloys, Scientific American, (Nov. 1979).
Kevin Sanders, Miracle Metal, Science Digest, (Oct. 1981).
Kevin Sanders, Grassroots Genius, Science Digest, (Mar. 1982).
R. D. Spencer and Michael J. Harrison, Demonstration Solid State Engine, AJP 52(12) ,
(Dec. 1984).
Earl Zwicker, Ed McNeil an Norbert Zarumba, Doing Physics: A Mystery Heat Engine, TPT,
(Sept. 1984).
Frederick E. Wang, Society of Automotive Engineers Publication, SAE Technical Paper Series
851495: The Thermobile
Nitinol Engine, (Sept. 9-12, 1985).
Jane A. Slezak and Ronald W. Veresko, Undergraduate Investigation of Nitinol, TPT 30,
42-43 (1992).