Smart materials:
Smart materials have properties that react to
changes in their environment. This means that one
of their properties can be changed by an external
condition , such as temperature, light, pressure or
electricity. This change is reversible and can be
repeated many times.
There are a wide range of different smart materials.
Each offer different properties that can be changed. some of the name and properties of smart materials are given below.
Shape-memory alloys:
For most materials, if they are bent out of shape,
they stay that way. However, if a part made from a
shape-memory alloy ( SMA) is bent out of shape,
when it is heated above a certain temperature it will
return to its original shape.
This property makes it useful for making spectacle
frames - they return to their original shape if they
are put in hot water after bending them.
SMAs are used as triggers to start the sprinklers in
fire alarm systems, controllers for hot water valves
in showers or coffee machines and for spectacle
frames.
Piezoelectric materials:
When a piezoelectric material is squeezed rapidly, it
produces a small electrical voltage for a moment. If
a voltage is put across the material it makes a tiny
change in shape.
Piezoelectric materials are being used for contact
sensors for alarm systems and in microphones and
headphones.
Quantum-tunnelling composite:
Quantum-tunnelling composite (QTC) is a flexible
polymer which contains tiny metal particles . It is
normally an insulator but if it is squeezed it
becomes a conductor.
QTC can be used to make membrane switches like
those used on mobile phones, pressure sensors
and speed controllers.
Electroluminescent materials:
Electroluminescent materials give out light when an
electric current is applied to them. Among many
possible applications are safety signs and clothing
for use at night.
Colour-change materials:
Thermochromic materials change colour as the
temperature changes. These are used on contact
thermometers made from plastic strips and test
strips on the side of batteries (where the heat
comes from a resistor under the thermochromic
film). They are also used as food packaging
materials that show you when the product they
contain is cooked to the right temperature.
Photochromic materials change colour according to
different lighting conditions. They are used for
security markers that can only be seen in ultraviolet
light.
Muscle Wire:
is a shape memory alloy that
contracts between 3 and 7% when an electrical
current runs through it. While this material is
not strong enough for heavier applications —
like rolling up heavy blinds or pulling any
significant weight
Thermochromic Pigments:
change color at a
given temperature. The two most common
types of thermochromic materials are based on
either leuco dies or liquid crystals. At specific
temperatures the liquid crystals re-orientate to
produce an apparent change of color.
Thermochromic materials can be triggered by
body heat or used in conjunction with heating
elements such as nichrome, steel thread or
even simply conductive thread.
Electro-rheostatic and Magneto-rheostatic:
Electro-rheosutatic (ER) and magneto-rheostatic
(MR) materials are fluids, which can
experience a dramatic change in their
viscosity. These fluids can change from a thick
fluid (similar to motor oil) to nearly a solid
substance within the span of a millisecond
when exposed to a magnetic or electric field;
the effect can be completely reversed just as
quickly when the field is removed. MR fluids
experience a viscosity change when exposed to
a magnetic field, while ER fluids experience
similar changes in an electric field. The
composition of each type of smart fluid varies
widely. MR fluids are being developed for use in car
shocks, damping washing machine vibration,
prosthetic limbs, exercise equipment, and
surface polishing of machine parts. ER fluids
have mainly been developed for use in
clutches and valves, as well as engine mounts
designed to reduce noise and vibration in
vehicles.
Ph-SENSITIVE POLYMERS :
pH-sensitive polymers are materials which can
swell (by absorbing water) when the pH of the
surrounding aqueous (water based) media changes.
The effect is reversible with change in pH in the
opposite direction to contract or collapse the
material (i.e. loses water).
SELF-HEALING MATERIALS:
Self-healing materials have the natural ability to
repair damage due to normal usage and so increase
the material's useful lifetime it follows same phenomena as human body has characteristic to heal wounds.
GORETEX:
Nylon is a synthetic polymer used to make fabrics
for the clothing industry, but on its own it is not very
waterproof.
If the nylon is coated with polyurethane it becomes
a much tougher durable waterproofed material for
outdoor clothing.
Unfortunately the polyurethane coating doesn't
allow water vapour through so you get too hot,
sweaty and uncomfortable, because the sweat
can't evaporate to keep you cool.
Gore-Tex is the registered trade name often
applied to a particular design of (usually outdoor)
clothing fabric that is designed to keep you dry in
the rain without getting sweaty. GoreTex fabric is made up of a layer of a plastic
based on expanded PTFE and this is laminated onto
a layer of another fabric.
The layer contains very many tiny holes called
pores.
There are apparently around 14 million pores per
square millimetre!
It is the tiny holes in the PTFE layer that let the
water vapour molecules through, i.e. it is
breathable.
BUT layer is waterproof because liquid water
droplets cannot pass through in the opposite
direction, in fact the fabric surface repels water -
hydrophobic.
CARBON FIBRES:
Carbon in the form of its allotrope graphite is soft,
slippery and easily broken solid.
However, very thin filaments or strands of graphite-
carbon ( carbon fibres) are very stiff and strong.
Precursor fibres of polyacrylonitrile are carbonised
under extreme heat is one method of making
carbon fibres.
Carbon fibres are useful for reinforcing other
materials to make them tougher i.e. strengthening
composite materials.
Carbon fibres can be incorporated into strong
plastics to make an even stronger composite
material.
Carbon fibre materials are not only strong but very
lightweight e.g. compared to strong metal alloys like
steel.
So carbon fibre composites are used for
skateboards, boat hulls and high performance
sports equipment like tennis racket frames.
