In the solid state, the intermolecular attraction energy is less than the thermal energy, therefore the molecules can hardly vibrate around more or less fixed positions. Some solids are amorphous at the molecular level, while others have a more ordered structure, such as crystals.
Some examples of solid materials are silica sand, glass, graphite (mineral coal), common salt, refined sugar, iron, copper, magnetite, quartz, and many more.
Solid state characteristics
Solid materials have the main characteristic that, under normal conditions, that is, if they are not subjected to great external forces, they maintain their volume and shape.
This is in contrast to liquids that, although they can maintain their volume, change their shape by adapting to the container that contains them. The contrast is even greater with gases, since they can be compressed or expanded by changing their volume and shape.
However, solids can vary their volume when they are subjected to changes in temperature wide enough to have significant effects, but without a phase transition to another state of matter.
Solids can be amorphous in their internal molecular structure. For example, glass is an amorphous material, even considered by many to be an overcooled liquid. However, quartz and diamond have a crystalline structure, that is, their atoms follow regular and spatially periodic arrangements.
Macroscopic and microscopic properties
Solid state physics studies the relationship between properties at the macroscopic scale (thousands or millions of times greater than the atomic scale) and properties at the molecular or atomic scale.
In a solid, the atoms are very close to each other and the interaction between them determines their properties on a macro scale, such as their mechanical characteristics: stiffness and ductility, as well as their thermal, magnetic, optical and electrical properties.
For example, conductivity, heat capacity, and magnetization are macroscopic properties of solids that depend directly on what happens at the molecular or atomic scale.
A clear example of the importance of solid physics are semiconductors. Understanding their properties at the microscopic level enables the development of devices such as transistors, diodes, integrated circuits, and LEDs, just to name a few applications.
Structure of solids
Depending on the pressure and temperature conditions, as well as the processes followed during their formation, solid materials acquire a certain microscopic structure.
For example, materials as dissimilar as graphite and diamond are composed solely of carbon atoms. But their properties are completely different, because despite being composed of the same type of atoms, their microscopic structures differ enormously.
Metallurgy specialists know that, starting from the same material, with different heat treatments, very different results are obtained in the manufacture of pieces, such as knives and swords. Different treatments lead to different microscopic structures.
Depending on their formation, solids can basically present three types of microscopic structures:
- Amorphous , if there is no spatial regularity in the arrangement of atoms and molecules.
- Monocrystalline , if the atoms are arranged in a spatial order, forming arrangements or cells that are repeated indefinitely in the three dimensions.
- Polycrystalline , composed of several regions, not symmetrical to each other, where each region has its own monocrystalline structure.
Models of solid physics and their properties
Solid physics starts from fundamental principles to explain the properties of solid materials, such as thermal conductivity and electrical conductivity.
For example, by applying kinetic theory to the free electrons in a metal, they are treated as if they were a gas.
And under the assumption that ions form an immobile substrate, it is possible to explain both the electrical conductivity and the thermal conductivity of metals. Although, in the classic version of this model, the thermal conductivity of free electrons is greater than that obtained from measurements in conductive materials.
The drawback is solved by introducing quantum corrections to the free electron model of a conducting solid. Furthermore, if they are assumed to follow the Fermi-Dirac statistic, then the theoretical predictions agree more precisely with the experimental measurements.
However, the free electron model cannot explain the thermal conductivity of solids other than metals.
In this case, the interaction of the electrons with the crystal lattice must be taken into account, which is modeled by a periodic potential in the Schrodinger equation. This model predicts energy-dependent conduction bands of electrons and explains electrical conductivity in semiconductor solids, a type of solid intermediate between insulator and conductive metal.
Solid state examples
Solid state physics has evolved to the point that it has allowed the discovery of new materials such as solid nanomaterials with unique and extraordinary properties.
Another example in the advancement of solid physics is the development of two-dimensional or monolayer materials, followed by various applications such as photovoltaic cells and the development of semiconductor integrated circuits.
The classic example of a two-dimensional material is graphene , which is nothing more than single-layer graphite and which was obtained for the first time in 2004.
Other examples of two-dimensional solids are: phosphorene, plumben, silicene, and germacene.
High temperature superconductors
Superconductivity was discovered in 1911 by the Dutchman Kamerlingh Onnes (1853-1926) when he subjected conductive materials such as mercury, tin and lead to very low temperatures (on the order of 4 K).
Superconductivity has important technological applications, such as magnetic levitation trains, as long as it can be obtained at elevated temperatures (ideally at room temperature).
Solid physics is in this search for superconductors, meaning high temperature above the temperature of liquid nitrogen (77 K), a relatively easy and cheap temperature to obtain. To date, the highest temperature superconductor is a ceramic solid that reaches this state at a temperature of 138 K or -135ºC.
Strongly correlated solids
Strongly correlated solids are heavy fermionic compounds that have unusual properties and great technological potential. For example, they can be manipulated to go from insulators to conductors through magnetic fields.
The development of this type of solids has also allowed magnetic information storage devices to exponentially increase their capacity in recent decades.
Themes of interest
Examples of solids.