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Structure of materials: concept and examples

Depending on the type of chemical bond present in the structures of the materials, different mechanical, chemical, optical, thermal, electrical, or quantum properties will take place. If the bond is ionic, the material will be ionic. Meanwhile, if the bond is metallic, the material will be metallic.

Wood, for example, is a fibrous, polymeric material, made of cellulose polysaccharides. The efficient interactions between its cellulose chains define a hard body, capable of being molded, cut, dyed, polished, chiselled.

Structure of metallic materials

Metallic materials include all metals and their alloys. Their structures are composed of atoms tightly packed one next to or on top of the other, following a periodic order. Therefore, it is said that they consist of metallic crystals, which remain fixed and cohesive thanks to the metallic bond that exists between all their atoms.

Among the most common crystal structures for metals are body-centered cubic (bcc), face-centered cubic (fcc), and compact hexagonal (hcp), the latter being the most dense. Many metals, such as iron, silver, chromium or beryllium, are characterized by assigning each of them one of these three structures.

However, such a description is not enough to describe them as materials.

Metallic crystals can take on more than one shape or size. Thus, more than one crystal will be observed in the same metal. In fact, there will be many of them, which are better known by the term crystalline grain.

The distance that separates the grains from each other is known as the edge or grain boundary and is, together with crystalline defects, one of the most determining factors in the mechanical properties of metals.

Structure of ceramic materials

Zirconium dioxide spheres, a new ceramic material. Source: Lucasbosch, wikimedia commons

Most of the materials can be described as in the previous section, that is, depending on the crystals, their numbers, sizes or shapes. What varies, however, in the case of ceramic materials, is that their components do not consist only of atoms, but of ions, frequently located in an amorphous silicate base.

Ceramics are materials of highly variable compositions. For example, carbides, nitrides and phosphides are considered ceramics, and their structures made up of three-dimensional networks are governed by covalent bonding. This gives them the property of being very hard materials with high thermal resistance.

Vitreous ceramics, having a silicon dioxide base, are considered amorphous. Therefore, its structures are messy. Meanwhile, there are crystalline ceramics, such as aluminum, magnesium and zirconium oxides, whose structures are made up of ions joined by the ionic bond .

Structure of crystalline materials

The crystal structure of sodium chloride, NaCl, a typical ionic compound. The purple spheres represent sodium cations, Na +, and the green spheres represent chloride anions, Cl−.

Crystalline materials make up a large family of materials. For example, metals and ceramics are classified as crystalline materials. Strictly speaking, crystalline materials are all those whose structures are ordered, regardless of whether they are composed of ions, atoms, molecules or macromolecules.

All salts and the vast majority of minerals fall into this classification. For example, limestone, composed mainly of calcium carbonate, could be said to be a crystalline material, even though it is not always transparent and shiny, according to its natural formation process.

Sugar crystals, on the other hand, are made of sucrose molecules. As such sugar is not a material, unless castles, carcasses, furniture or chairs are built from sugar. Then, the sugar itself would go on to become a crystalline material. The same reasoning applies to all other molecular solids, including ice.

Structure of ferrous materials

Austenite, arrangement of carbon and iron atoms. Source: wikimedia commons

Ferrous materials are all those that consist of iron and its alloys with carbon. Therefore, steels count as ferrous materials. Their structures, like that of metals, are based on metallic crystals.

However, the interactions are somewhat different, since the iron and carbon atoms are part of the crystals, so it is not possible to speak of a metallic bond between the two elements.

Other examples

Nanomaterials

Carbon nanotubes. Source: wikimedia commons

Many nanomaterials, like the materials already discussed, are also described in terms of their nanocrystals. However, these comprise other more unique structural units, made up of fewer atoms.

For example, the structures of nanomaterials can be described by atoms or molecules arranged in the form of spheres, miscelas, tubes, planes, rings, plates, cubes, etc., which may or may not generate nanocrystals.

Although ionic bonding may be present in all these nanostructures, as is the case with nanoparticles of innumerable oxides, the covalent bond is more common, responsible for providing the necessary separation angles between the atoms.

Polymeric materials

Chemical structure of polyethylene

The structures of polymeric materials are predominantly amorphous. This is due to the fact that its conforming polymers are macromolecules that hardly manage to order themselves periodically or repetitively.

However, in polymers there can be relatively ordered regions, which is why some are considered semi-crystalline. For example, high- density polyethylene , polyurethane, and polypropylene are considered semi-crystalline polymers.

Hierarchical materials

Hierarchical materials are vital in nature and support living bodies. Materials science is tirelessly dedicated to mimicking these materials, but using other components. Its structures are “dismountable”, starting with the smallest parts to the largest, which would become the support.

For example, a solid that is composed of several layers of different thickness, or that has tubular and concentric cavities occupied by atoms, will be considered hierarchical in structure.

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