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04/11/2002
Memories
After a thankfully uneventful drive home, it''s back to reality in
cold New Jersey. It''s actually snowing a bit as I start this
column. I''m ready to go back to Marco Island! Before leaving
there, I took one last early morning walk on the beach. As usual,
there were things I hadn''t seen before. Sitting on the sand was
this big black bird, equal to about three or four big seagulls in
size. I''ve never heard a loon speak but this bird issued a call that
I imagined sounded like a loon. I tried finding the bird in a bird
book afterward and did find a loon that resembled my bird but
wasn''t all black in color. At any rate, it''s a loon to me. I also
noted a bird sitting on a "No Trespassing" sign posted at the
boundary of the bird sanctuary. On closer inspection, I found it
to be an eagle, another bird I had never seen on Marco. I had no
problem with our national symbol violating a No Trespassing
warning.
After the walk, it was off to New Jersey. On the day before we
arrived home, we stopped at Emporia, Virginia to find motel
guests sitting outside in shorts and shirtsleeves. Only 15 minutes
after getting our luggage in our room, there was a severe
thunderstorm and the temperature must have dropped 20 degrees.
By the next morning it was in the 30s. However, the beautiful
blossoming dogwood, forsythia and cherry trees indicated that
spring is here.
After two days spent going through the huge piles of two months
worth of mail, 90 percent of it solicitations for money, all I have
from Florida are the memories, some new walking shoes and a
truly memorable shell that I found on the beach. We all have
memories of people, places and events that have left their
impressions on us. But did you know that even some inanimate
objects have memories? Indeed, this was the subject of a
number of papers in the February issue of the Materials Research
Society (MRS) Bulletin.
What kind of inanimates has memories? We''re talking here
about objects made using the so-called "shape-memory alloy",
SMA for short. A related phenomenon is "superelasticity",
which utilizes the same type of structural transformation
responsible for the shape-memory effect. You may be saying,
"This sounds pretty esoteric to me - who cares?" If so, you may
be surprised to know that you would be unusual if you didn''t
have nearby some product employing SMAs. Indeed, you may
be wearing such a product or even have one in your body! I''m
thinking you probably have a cell phone, a coffeemaker, an
upscale pair of eyeglasses or, less likely, you may have had an
angioplasty recently. Perhaps you''re of the female sex and wear
a bra? If so, and it has an underwire, the underwire may well
depend on a "martensitic" transformation to maintain its shape.
What in the world is a martensitic transformation? To tell the
truth, I''ve never been really comfortable with it myself, but let''s
give it a quick try. Suppose this SMA, our shape-memory alloy,
has a neat crystal structure with all the atoms stacked neatly on
top of each other when the SMA is at a high temperature. I could
liken it to a perfectly stacked deck of cards. Now suppose I take
that deck of cards and push a layer of the top cards to the right,
poke the rest of the cards to the left, then poke a layer to the
right, etc., etc. We now have alternating layers of cards slanting
in opposite directions. This is sort of what happens to our SMA
on cooling to a lower temperature. The atoms are sheared into
layers (domains) of atoms slanted in opposite directions. Now if
you put a stress on the SMA the layers of atoms line up again,
only this time on a slant. It would be like shoving the whole
deck of cards to get a deck that''s smoothly slanting rather than
straight up and down vertically.
The slanted structures are the martensite structures. The striking
feature of this transformation from the original, neatly stacked
structure to the martensite structure is that the atoms don''t move
very far, yet they move in the same direction in their domains.
As a result, the shape of an object made from this alloy can
change quite noticeably, thanks to the collective motion of the
atoms in the same direction.
If you''re still not sure that this is true, let''s cut to the chase and
cite a simple example of the shape memory effect. Let''s say we
have a straight piece of wire made from an alloy of titanium (Ti)
and nickel (Ni), NiTi, at a high temperature. Now let''s cool the
wire down. It stays straight but the crystal structure changes to
the slanted layer martensite form. Now let''s take that straight
wire and bend it a bit so it has waves in it. The wire is still in
this martensite structure. Next, let''s heat that wavy wire up to a
higher temperature. The atoms move around until the original
neatly stacked structure is formed and the wire becomes straight
again with no waves! The SMA wire has "remembered" its
original shape!
You might ask, "What happens if we cool the wire back down?
Will it become wavy again?" There are so-called "one-way"
and "two-way" shape memory alloys. In a one-way material, our
wire will stay straight on cooling back down. By fiddling with
the alloy compositions and /or other treatments, one can make a
two-way material that can be cycled back and forth repeatedly.
I also mentioned superelasticity. Without going into detail, the
martensitic transformation is also key to this type of alloy, which
could be described as a really springy metal. Its shape memory
is more of a stress effect than a temperature effect. Practical
examples are the cell phone antenna, which we don''t want to
break if bent, and the underwire bra, the function of which needs
no elaboration. The eyeglasses with the springy frames that
stand up to flexing are another market for superelastic alloys.
To me the most exciting applications of shape memory and
superelasticity are in the medical field, where the Ni-Ti shape-
memory alloy known as Nitinol is a familiar player. You may
recall recent reports in the press about studies on the success
rates of angioplasty operations, in which clogged blood vessels
are opened by inserting and inflating a balloon. A big problem
with angioplasties has been restenosis, a reclosing of the blood
vessel. It has been found that the number of cases of restenosis
is markedly reduced if the surgeon inserts a stent into the vessel
after angioplasty, the stent being like a cylindrical screen that
keeps the vessel open.
Today, Nitinol is used as a "self expanding" stent material, the
stent being initially formed into a compact shape that can be
threaded into the blood vessel easily and quickly. When its
sheath is removed the stent expands to prop open the vessel.
This same feature is used when the surgeon introduces a filter in
a blood vessel to catch errant blood clots that could work their
way to the heart or elsewhere. In fact, in an angioplasty, there
will likely be such a filter or catcher device to capture any debris
that breaks off while the balloon is being threaded through the
blood vessel. This captured material is then removed at the end
of the operation. Nitinol may also be used as a guide wire to lead
the balloon to the clogged site.
The NiTi alloy and alloys of NiTi with other elements can be
used to adjust the temperatures and stresses that are consistent
with the particular application. NiTi-based alloys are particularly
good alloys from the shape memory standpoint and also from the
biocompatibility of these alloys when inserted in the body, which
explains its popularity in the medical field. Another medical
application is in the dental field as wires in the braces used to
move teeth so as to obtain that gorgeous smile. Thanks to the
superelasticity and the shape memory properties the wires can be
adjusted to keep constant pressure on the teeth to move them
along to their new locations.
NiTi SMAs also have served in our military establishment as, for
example, in the F-14 fighter plane. The application in the F-14
was as a coupling for connecting two tubes. In this application,
the coupling is expanded while in the martensitic phase. The
tubes are inserted in to the coupling, which is then heated. The
magic transformation occurs and the coupling shrinks back to its
original size, clamping the tubes tightly together in a snug fit.
To cite another important practical application, consider a simple
device that could actuate an alarm or a response to a fire. Let''s
take a shape-memory alloy wire or rod and bend it into the shape
of a horseshoe shape at a high temperature. Next, let''s cool it
down and push the ends of the horseshoe closer together. Now
put the "closed" horseshoe between two electrical contacts in a
fire detector of some sort. If the temperature of the detector goes
up due to a fire, the horseshoe will remember its "opened-up"
shape and open up to close the electrical circuit, thus sounding an
alarm or activating a shower system.
Speaking of horseshoes, remember that I had been having an
interesting time turning over horseshoe crabs on Marco? On that
last walk, there wasn''t a single horseshoe to be seen. No doubt
they realized that I was leaving and were making sure not to get
stranded on their backs waiting for the Good Samaritan to come
along. They obviously have their own memories.
Allen F. Bortrum
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