I MATERIALI SMART

SMART MATERIALS

The word ‘intelligence’ comes from the Latin adverb intus, ‘within’, and the Latin verb legere, ‘to read’. It, therefore, means ‘to read within’, i.e. the ability to perceive external stimuli and act accordingly. The adjective ‘intelligent’ is also attributed to certain metals. Why?

Any material subjected to external stress (heating, electric field, radiation…) undergoes some change and therefore, in a broad sense, has a response that can be considered a kind of perception of external reality. At the end of the 1980s, materials capable of responding to different stimuli were developed, adapting their response to particular needs. The adjective ‘smart’ is reserved for this particular category of materials.

There are now numerous materials that fall into this category and, depending on the type of stimulus to which they are sensitive and the type of response they provide, they can be classified in different ways: piezoelectric, shape memory, photovoltaic, electroactive polymers, dielectric elastomers, magnetostrictive, chromogenic, ferrofluidic, photomechanical, self-repairing, magnetocaloric, thermoelectric.

Shape memory materials

Some materials have the property when deformed, to return to their original shape when brought to a certain temperature. There are two types of materials that have this property: metal alloys and polymers. Alloys are combinations of two or more elements, one of which is a metal, and in which the resulting material has different metallic properties.

The first shape of memory metal alloys was identified in the 1930s; the best-known one, made in

the 1960s, is Nitinol, from Nickel Titanium Naval Ordnance Laboratory (the name of the laboratory where it was first made). Nitinol is composed of Nickel and Titanium, and these two elements form an intermetallic compound that can exist in two crystalline forms depending on temperature. Thanks to research, it has been possible to obtain special alloys in which the shape memory is activated not by temperature, but by a magnetic field; these are ferromagnetic materials consisting of nickel, manganese, and gallium alloys. Shape memory alloys find applications in various fields. Among the most common are fasteners, damping systems, and biomedical devices.

As mentioned, there are also polymeric materials that have shape memory. Some of them even can store two or three shapes, each of which is assumed again at a certain temperature value, or by light or an electric field. Shape memory polymers are used in the same applications as alloys, but with several additional advantages, including much lower costs, ease of processing, and biodegradability.

Photovoltaic materials

The Sun radiates an enormous amount of energy, the devices capable of transforming sunlight into electrical energy are called photovoltaics. The material most commonly found in photovoltaic panels, those now seen above the roofs of many buildings, is silicon, a semiconductor (a material belonging to the category of semimetals) whose energy jump, energy gap, of the electrons on the last orbit corresponds precisely to the energy of the photons of sunlight. The explanation of this effect, called the ‘photoelectric effect’, earned Albert Einstein the Nobel Prize in 1921. It was 33 years from Einstein’s theory to practice when, in 1954, the first photovoltaic cell was realised using silicon at Bell Laboratories in Murray Hill, New Jersey. Silicon offers several advantages: it is widespread in the earth’s crust, is non-toxic, and is easy to process.

In addition to silicon, other semiconductors that can be used in photovoltaic cells are now available; these include cadmium telluride (CdTe), cadmium sulphide (CdS), and gallium arsenic (GaAs). These semiconductors are generally used in the form of a thin film on glass substrates or other transparent materials.

Ferrofluids

In 1963, Steve Papell, an engineer working at a NASA research centre near Cleveland, was trying to create a liquid rocket fuel that could be pumped through the application of a magnetic field. To achieve this, he had the idea of dispersing ferromagnetic nanoparticles within a liquid. The results were extremely promising: the liquid was called ferrofluid and patented.

The ferromagnetic particles of a ferrofluid have diameters in the order of 10 nanometres (0.000001 meters) and usually consist of magnetite, hematite, or some other iron compound; e.g. the toner in ordinary photocopiers works well for this purpose. The composition of a typical ferrofluid is about 5 percent dispersed magnetic solids, 10 percent surfactant, such as soy lecithin, and 85 percent liquid consisting mostly of water and a hydrocarbon. The surfactant ensures the stability of the ferrofluid, which can thus last for several years even when subjected to repeated magnetisation.

When a ferrofluid is subjected to the action of a magnetic field, its surface takes on a conformation full of sharp points and depressions; this phenomenon is called the ‘Rosenzweig effect’. The points that form on the surface are the sharper the more intense the magnetic field.

Ferrofluids, apart from the purpose for which they were created, have various applications. A significant example of their application in the medical field is with Targeting Drug Delivery, i.e. the targeted distribution of a drug when injected into the body. Magnetic nanoparticles are attached to the drug and, using magnetic fields, it is possible to guide the particles to the part of the body where the drug is to be delivered.