What are smart materials?

Some materials are capable of changing one or more of their properties in a controlled manner in response to an external stimulus, which can be light, heat, moisture, pH changes, certain chemical compounds, electric or magnetic fields, or a mechanical force. These reactive materials, in which the changes are made automatically or are actively induced for a relevant purpose, are known as smart materials; there are many types, some of which are even used in our daily lives, for example, piezoelectric materials.

The use of piezoelectric materials in everyday life

Let's think of a quartz watch in which the battery supplies a voltage to the stone, which in response will vibrate a certain number of times per second; it is this frequency of vibration that allows the passage of time to be measured accurately. Conversely, a microphone uses a piezoelectric that transforms variations in air pressure (sound) into an electrical signal that can then be processed and reproduced on a speaker.

Piezoelectric materials have a crystalline structure that allows them to convert an applied electrical potential into mechanical voltage and vice versa. This property gives these materials the ability to absorb mechanical energy from their environment, usually ambient vibration, and transform it into electrical energy that can be used to power other devices. This phenomenon called the piezoelectric effect can be explained as follows: quartz crystals, such as the one used in watches, are made of silicon and oxygen atoms in a repeating pattern; silicon atoms are positively charged and oxygen atoms are negatively charged.

When the crystal is not under any external mechanical stress, the positive and negative charges are dispersed evenly in the molecules throughout the crystal, but when the quartz is stretched and compressed, the order of the atoms changes slightly. This change causes the negative charges to accumulate on one side and the positive charges on the opposite side, forming a positive and a negative pole, i.e., a dipole. If the ends of the crystal are connected, this potential difference (voltage) can be harnessed to produce current; similarly, sending an electric current through the crystal changes its shape.

These types of materials are being evaluated as a means of generating sustainable electrical energy, since placing these piezoelectric crystals in a street will convert the vibration and pressure generated by automobiles, and even pedestrians, into electrical energy. Similarly, a thermoelectric material can -through a temperature difference between the sides of a material- produce electricity, or vice versa, applying an electric current through a material creates a temperature difference between its two sides.

This is because charge carriers - either negatively charged electrons or places where an electron is missing (known as positively charged holes) - diffuse within the structure of the material from the hotter side to the cooler side, similar to the way gas expands when heated. These materials are used to build temperature measuring instruments, called thermocouples, and since the beginning of the last decade, automotive companies have been experimenting and evaluating the incorporation of thermoelectric generators in vehicles to harness the heat generated by cars to convert thermal energy into electricity.

Electroluminescent materials in our lives

Another type of intelligent material that we use daily is electroluminescent materials, which are used to make the light-emitting diodes (LEDs) used in television screens and lighting. Electroluminescence is an optical and electrical phenomenon in which a material emits light in response to the passage of an electric current or a strong electric field. This phenomenon is generally explained as follows: when an electric current passes through the material, the electrons of the atoms or molecules that compose it are excited by that energy and rise to a higher level. When this electron passes back to its lower energy state, the system will emit a photon, which is the particle of which light is composed, to return the excess energy it had to the outside; this light is what we perceive.

The reverse phenomenon can also occur: a material can provide an electric current when illuminated because photons have characteristic energy determined by the wavelength of light. If an atom absorbs energy from a photon and has more energy than is necessary to eject an electron from the material, and also has a trajectory directed towards the surface, then it can be ejected; this phenomenon is known as the photoelectric effect and Albert Einstein received the Nobel Prize in Physics in 1921 for its theoretical explanation.

If it changes color, it is a smart material

Since childhood we have been in contact with smart materials such as those that change color; these are known as chromogenic systems and can be thermochromic, photochromic, or electrochromic. The former change color as temperature increases or decreases, reversibly; they are usually pigments or liquid crystals and are used in contact thermometers made of plastic strips, in toys that change color when cooled or heated, and in indicator strips on the side of batteries that indicate how much they have been used (the heat comes from an electrical resistance underneath the thermochromic film).

They are also used in frying pans that change color when cooked at the right temperature and in cups that show funny images and patterns when filled with some hot liquid: this is because after absorbing a certain amount of heat, the crystalline or molecular structure of the pigment changes in such a way that it absorbs and emits light at a different wavelength than at lower temperatures. Photochromic materials change color according to different lighting conditions; they are used as security markers on tickets that can only be seen when illuminated with ultraviolet light. Electrochromic materials change color or opacity when a voltage is applied to them, as in liquid crystal displays (LCDs).

Does it return to its original form? It is also an intelligent material

Another type of amazing smart material has shape memory, which can be alloys, ceramics, or polymers; they can convert heat to mechanical stress and vice versa. They exhibit two unique properties: memory effect, which is the ability to transform when heat is applied, and superelasticity, which is the ability to deform greatly and reversibly, dissipating energy in the form of heat in the process.

The principle by which shape memory alloys work can be explained as follows: the atoms that compose it can switch between two different atomic configurations, depending on whether or not they are being subjected to electricity, heat, or even magnetism; for example, imagine we have a shape memory alloy at room temperature, with atoms that come together like stacked cubes, once we add heat they rearrange their structure and connect in a new hexagonal pattern, like a honeycomb that changes the shape of the whole piece of metal. If we stop heating the alloy, the atoms return to the cubes, and the metal returns to its original configuration.

One of the major applications of this type of alloy is in the manufacture of orthodontic appliances, popularly known as brackets, whose purpose is to exert a constant force on the teeth. Another application is smart concrete, reinforced by wires of these alloys that can detect cracks and contract to repair them. Shape memory polymers have two phases in their structure, one of which is responsible for the permanent or hard shape of the material, to which it can always return under certain conditions, such as when heated.

The other phase is responsible for the temporary or soft shape of the material, which can be reached by cooling or deformation of the permanent shape. They have advantages over shape memory alloys in that they are more elastic and cheaper. These polymers have been used in the construction industry as a foam for sealing window frames that expand with heat, in automobile fenders that return to their original shape after being hit, and in helmets.

Self-repairing? It must be mutant!

Perhaps the smart materials whose properties are most surprising are the self-healing ones. These are polymers, ceramics, and metals that when damaged by shocks or heat can "heal" themselves, as if they were living organisms, and return to their original state. For many years mankind has developed some materials that exhibit this ability, such as stainless steel, which is an alloy of iron, chromium, and nickel or molybdenum. Chromium oxidizes on contact with oxygen because it is very reactive and forms a protective layer on the surface of the metal that can self-repair if scratched. Another example is Roman concrete, which contained a particular type of volcanic ash that allowed cracks to crystallize, which is why it has lasted for almost 2000 years.

At present there are different methods to make a self-repairing material; some incorporate in their structure microcapsules containing small amounts of glue-like substances, when the material is cracked they break and fill this crack, thus joining the material again. Others have a system of small channels, analogous to the vascular systems of living organisms, which can transport adhesives that allow the material to repair itself, just as when the vascular system of people transports blood and platelets so that a wound can coagulate and heal later.

Another method is present in certain newly manufactured polymers, when broken, the ends or fragments are highly reactive and can rejoin; when dispersed and upon receiving heat or light, i.e. energy, they naturally attempt to rejoin other nearby molecules of the same material, effectively reversing the damage and repairing themselves. Some break to expose electrically charged ends, so the broken fragments will have an electrostatic attraction, like when you rub a balloon against your hair and then attract each other. When damage occurs, electrostatic forces pull the fragments together, allowing the material to self-heal.

Other smart materials

In addition to the smart materials mentioned above, there are many others, such as electroactive polymers that change volume when a voltage is applied to them. Magnetocaloric materials, which change temperature when exposed to a magnetic field, and halochromic materials, which change color when there is a change in the acidity of the surrounding environment.

In conclusion, this article states that, by definition, smart materials are those that have properties that react to changes in their environment in a reversible way and this change can be repeated many times. This means that one of their properties can be modified by an external condition such as temperature, light, pressure, electricity, voltage, pH, or chemical compounds and that the system will return to its original condition when the primary conditions are reached again. There is a wide range of smart materials that can be used for the generation of new technologies.

Source: ¿Qué son los materiales inteligentes?, p.4 Luis Carlos Ortiz Dosal, graduate from the Institutional Doctorate in Engineering and Materials Science at UASLP.