Chemistry Reference
In-Depth Information
8.2.2
Shape-memory materials
Shape-memory materials are those materials that return to a specific shape
after being exposed to specific temperatures. In other words, these materi-
als are able to 'remember' their initial shape. This process of changing the
shape of the material can be repeated several times. The shape-memory
effect has been observed in different materials, such as metallic alloys,
ceramics, glasses, polymers and gels.
Shape-memory alloys (e.g. Cu-Zn-Al, Fe-Ni-Al, Ti-Ni alloys) are
already in use in biomedical applications such as cardiovascular
stents, guidewires and orthodontic wires. The shape-memory effect of
these materials is based on a martensitic phase transformation. Shape
memory alloys, such as nickel-titanium, are used to provide increased pro-
tection against sources of (extreme) heat. A shape-memory alloy possesses
different properties below and above the temperature at which it is acti-
vated. Below this temperature, the shape of the alloy is easily deformed due
to its flexible structure. At the activation temperature, the alloy can be
changed by applying a force, but the structure resists this deformation and
returns back to its initial shape. The activation temperature is a function
of the ratio of nickel to titanium in the alloy. In contrast with Ni-Ti,
copper-zinc alloys are capable of a two-way activation, and therefore a
reversible variation of the shape is possible, which is a necessary condition
for protection purposes in textiles used to resist changeable weather
conditions.
In products, a shape-memory alloy is usually in the shape of a spring. The
spring is flat below the activation temperature but becomes extended above
it. By incorporating these alloys between the layers of a garment, the gap
between the two layers can be substantially increased above the activation
temperature. The increased amount of air between the textile layers forms
additional insulation, and as a consequence, provides considerably
improved protection against changes in external temperature. Shape-
memory polymers have the same effect as the Ni-Ti alloy but, being poly-
mers, they have the potential to be more compatible with textiles. However,
a limitation of polymers with shape-memory properties (polyurethanes) is
that they have a relatively low melting point. The shape-memory effect is
observed when a plastic deformed to a specific shape returns to its initial
shape at a particular temperature. For clothing applications, the desirable
temperatures for the shape-memory effect to occur will be close to body
temperature. Established applications for shape-memory alloys also include
domestic appliances (e.g. shower mixer valves, coffee makers and rice
cookers) and utility applications (e.g. safety shut-off valves for fuel lines in
the event of fire, and air-conditioning systems). The shape-memory alloys
can also contribute to the miniaturisation of equipment and systems, and
