this spring fascinates me I'll
completely uncoil it except for a few
kinks now watch as I put it in hot water
75 degrees or so rapidly the spring
reforms
let's watch it again but close up I
stretch the spring and then to slow down
the action use a hair dryer to heat it
this spring is made from night long wire
the name comes from the elements it
contains and where it was discovered
nickel and titanium and the naval
ordnance lab it's showing what's called
the shape-memory effect a length of this
wire was formed at very high
temperatures into a spring which shape
setting occurs at temperatures above
about 500 degrees Celsius and then
cooled almost no matter how it's bent at
room temperature it returns to its
original shape when heated at 75 degrees
Celsius or so so why does night null
have this memory the key is
understanding how the atoms move in
response to stretching and bending of a
piece of knife null typically any metal
whether or not or not is comprised of
small grains that depending on the
material are microns to nanometers
across and each grain is comprised of
atoms arranged in a regular repeating
pattern if this metal rod were held at
the top and stretched then the grains
might elongate they can do this because
the atoms inside move a bit in a metal
bonds can be form more easily than in
nonmetals and so the atoms slip although
it's a bit more complicated than applied
here which looks like spherical atoms
rolling along each other it involves
defects in the crystal that allow the
atoms to move only small amounts in
night and although generally the atoms
don't slip they accommodate the stresses
from bending stretching compression and
so on in a different way which produces
the memory effect here's a simplified
two-dimensional version of the atomic
structure of nitinol but one that
captures the essence of why it has a
temperature dependent memory the
crystallites and the small grains have a
nice tidy highly symmetric arrangement
the atoms sit on the corners of squares
at temperatures above 500 degrees
Celsius or so the nightfall can be
shaped into whatever form is useful for
example the spring I showed you earlier
when it cools the crystal structure the
arrangement of the atoms changes only
slightly notice that it's very similar
all the atoms are connected in the same
way but the nice tidy squares are now
rhombuses notice that even though the
atoms move a bit the overall shape too
really change so for example if you
fashioned a spring it still looks like a
spring this doesn't seem very exciting
but this structure has a unique property
it's composed of rhombuses that are
mirror images for example notice the
position of the atoms above them below
this line aren't identical but are
reflections of each other this is called
twinned structure you could have all of
the rhombuses oriented this way or this
way and as you can now see when the
night now cools it forms equal amounts
of these two orientations of rhombuses
ones oriented this way and ones this way
what produces the memory effect in
nitinol is that these two orientations
can easily be changed from one to the
other with very small motions of the
atoms for example if I take the cool
night ball and change its shape some of
the crystallites might experience a
shear of force across the top notice
that in response the atoms move and
change the ratio of the two types of
rhombuses in this example they all
change to one type you could imagine
that in response to different forces
then the large enough crystal there
would be hundreds of mixtures of these
two rhombuses and many many more in a
three-dimensional structure but notice
this at all times the atoms are
connected in the same way as in the high
temperature phase like in slip where
atoms can rearrange quite a bit more so
when the temperature is raised again
these Robins no matter how they are
distributed returned to tidy squares the
only structure that can result is the
original structure and so the night not
reverts to the shape at hand at high
temperature there are some clever uses
for nitinol for example this device is
fascinated me since childhood it's a
type of engine its construction is very
simple a large and small wheel each
grooved underside with a loop of nitinol
wire that runs through the grooves and
connects them to make a run I just
contact the lower edge of the small
bottom wheel with water which i've
heated again 275 degrees Celsius
then spin the top wheel a bit in the
device that runs on its own it uses the
temperature difference between the water
and the air to power the engine when I
remove the water the wheel stops
spinning this engine runs because this
loop of nitinol wire was originally
straight at a very high temperature so
as it's heated it tries to straighten to
see how that causes the engine to
operate
watch what happens is I contact the
lower wheel with the heated water I'll
mark the position of the wire on the
left of the yellow line then when I
start the wheel the wires distance from
the initial position is much greater
it's hard to see so I'll mark its
position with a purple line this happens
because when the bottom wheel comes into
contact with a hot water the section of
the wire highlighted here in orange is
heated if it were not no loop then the
wire would be strained like this but
because it's in a loop as it tries to
straighten it exerts a force on the
wheel which causes it to turn let's
break this down on the right the air
cooled wire returns it wraps on to the
wheel as the water heats the wire it
starts to straighten but is restrained
by being in a loop this creates a force
at this point on the wheel from this
fulcrum which is the last point of
contact of the wire in the wheel this
generates a force about the center of
the wheel which is transmitted as torque
and sets in motion of the engine
although a lot of nitropaste engines
were patented off the practical uses of
night nor not as a shape memory metal
but is a super elastic material let me
show you compare Knight null wire to a
piece of copper wire if I move the
copper wire a bit and let it go it
returns to its original position
that's an elastic deformation but if I
go too far it doesn't return to its
original shape that's plastic
deformation compare that to this piece
of nitinol wire it shows that elastic
response for small deviations but if I
do that same extreme deformation and
then let go the wire returns to its
original state even
our sighting for a while if I bend the
wire into multiple loops and then
release it it becomes straight this
super elasticity is closely related to
the shape memory effect let's compare
the two phenomena and look at what's
happening at the atomic level
I start with shape memory here's a piece
of night null wire that's been
conditioned at 500 degrees Celsius or so
to be straight at that very high
temperature the crystal structure in the
grains is those tidy neat squares as
it's cooled to room temperature the twin
structure forms the one with the equal
number of left and right Rob is's which
you recall doesn't change the overall
shape much then when I deform the wire
the atoms and the crystal light shift in
the grains to some unequal mixture left
and right Rob assists there are hundreds
of possibilities when I heat it with a
hair dryer but tidy neat squares return
and the wire becomes straight again and
then as it returns to room temperature
the twin structure returns the one with
an equal number of both rhombuses now
let's look at a nightmare wire that
showed super elasticity it's been
conditioned so that it's crystal
structure is this tidy neat squares at
room temperature it would form the twin
structure at minus 15 degrees Celsius so
a much lower temperature than the shape
memory effect when I apply an external
force to the wire that crystallites and
the grains deform into the rhombuses
whatever mix of those would allow it to
accommodate the reshaping when I remove
the force then because it's at room
temperature it returns to the tidy
squares
here's an example of a commercial device
that uses super elastic nitinol it's a
cage-like metal tube called a stint
that's used by a cardiologist the doctor
inserts it into a vessel or duct here
it's shown in a coronary artery and it
holds the walls in place because these
are super elastic watch this I can
severely deform it and when I release it
it returns to its original shape to use
it a cardiologist chooses a stint with a
slightly larger diameter than the vessel
wall which then crimped to a size
smaller than the vessel inserted and
allowed to expand because it's larger
than the vessels diameter it keeps a
force on the wall
and resists compression night nulls
other commercial uses are in high-end
products premium eyeglasses built from
super elastic nitinol so they can be
bent and twisted yet return to their
original shape and it's used in designs
who are reducing weight is critical for
example in the 2014 Chevrolet Corvette a
night null device replaced a heavy
motorized actuator to open and close
event in the car's trunk there are
thousands of more uses for nitinol and
perhaps for other shape-memory
materialists for example polymers that
exhibit this phenomena are now in
development I'm Gail Hammack the
engineer guy