Responsive smart materials

Material

Responsive (smart) materials

"Smart" materials respond to environmental stimuli with particular changes in some variables. For that reason they are often also called responsive materials.

Depending on changes in some external conditions, "smart" materials change either their properties (mechanical, electrical, appearance), their structure or composition, or their functions.

Mostly, "smart" materials are embedded in systems whose inherent properties can be favorably changed to meet performance needs.

Material

Photochromic materials

Photochromic materials change reversibly colour with changes in light intensity.

Usually, they are colourless in a dark place, and when sunlight or ultraviolet radiation is applied molecular structure of the material changes and it exhibits colour. When the relevant light source is removed the colour disappears.

Changes from one colour to another colour are possible mixing photochromic colours with base colours. They are used in paints, inks, and mixed to mould or casting materials for different applications.

Material

Electroluminescent materials

Electroluminescent materials produce a brillant light of different colours when stimulated electronically (e.g. by AC current). While emitting light no heat is produced.

Like a capacitor the materials is made from an insulating substance with electrodes on each side. One of the electrodes is transparent and allows the light to pass. The insulating substance that emits the light can be made of zinc sulphide or a combinatio

They can be used for making light stripes for decorating buildings, or for industrial and public vehicles safety precautions.

Material

Conducting polymers

Conducting polymers are conjugated polymers, namely organic compounds that have an extended p-orbital system, through which electrons can move from one end of the polymer to the other. The most common are polyaniline (PAni) and polypyrrole (PPY).

Polipyrrole has been used for the development of micro muscles. Polyaniline films sandwiched around a ion-conducting film are considered as material for artificial muscles for robots.

A current flow reduces one side and oxidises the other. Ions are transferred. One side expands and the other contracts, resulting in a bending of the sandwich. Electrical and chemical energies are so transformed in mechanical energy.

Material

Shape memory alloys (SMA)

Shape-Memory Alloys are metals that, after being strained, at a certain temperature revert back to their original shape. A change in their crystal structure above their transformation temperature causes them to return to their original shape.

SMAs enable large forces (generated when encountering any resistance during their transformation) and large movements actuation, as they can recover large strains.

Material

Polymer gels

Polymer gels consist of a cross-linked polymer network inflated with a solvent such as water. They have the ability to reversibly swell or shrink (up to 1000 times in volume) due to small changes in their environment (pH, temperature, electric field).

Micro sized gel fibres contract in milliseconds, while thick polymers layers require minutes to react (up to 2 hours or even days). They have high strength and can deliver sizeable stress (approximately equal to that of human muscles).

The most common are polyvinylalcohol (PVA), polyacrylicacid (PAA) and polyacrylonitrile (PAN). Many potential applications (e.g. artificial muscles, robot actuators, adsorbers of toxic chamicals), but presently, few of them have a commercial diffusion.

Material

Piezoelectric materials

They produce an electric field when exposed to a change in dimension caused by an imposed mechanical force (piezoelectric or generator effect). Conversely, an applied electric field will produce a mechanical stress (electrostrictive or motor effect).

They transform energy from mechanical to electrical and vice-versa. The stress is very small, 0.1-0.3%. They are used for sensing purposes (e.g. microphone, transducer), and for actuating applications.

Similar to piezoelectric materials are electrostrictive and magnetostrictive materials used in high prescision actuation. They are ferromagnetic materials which experience an elastic strain when subjected to an electric or magnetic field respectively.

Material

Thermoelectric materials

Thermoelectric materials are special types of semiconductors that, when coupled, function as a "heat pump". By applying a low voltage DC power source, heat is moved in the direction of the current (+ to -). (See glossary, Peltier effect).

Usually, they are used for thermoelectric modules where a single couple or many couples (to obtain larger cooling capacity) are combined. One face of the module cools down while the other heats up, and the effect is reversible.

Thermoelectric cooling allows for small size and light devices, high reliability and precise temperature control, and quiet operation. Disadvantages include high prices and high operating costs, due to low energy efficiency.