Anisotropy is what? Definition and application

This article tells us that anisotropy is the inequality of values of a certain physical quantity along different directions of a solid body. Reveals what causes anisotropy, where it occurs, how it is applied. Also a brief description of the anisotropy coefficient is given.

Determination of anisotropy

To begin with, we give a definition of this concept. Anisotropy is the difference in the properties and parameters of an object in different directions. It turns out slightly incomprehensible and clearly requires explanation. Properties mean any characteristics of substances - elasticity, speed of sound, refractive index, thermal conductivity, electrical conductivity. Thus, for example, for the speed of sound, anisotropy is a phenomenon when sound waves propagate across the rock lump with a different speed than along. In this case, this property helps determine the rocks lying deep in the earth's crust. Natural propagation during an earthquake, for example, or with a specially created strong impact, will give an idea of the density and angle of occurrence of different minerals.

What causes anisotropy?

When mentioning this term is most often meant the anisotropy of crystals. This section deals with solid state physics. And any scientist in this field first of all knows: the properties of a substance depend not only on what atoms it consists of, but also in what order and by what parts of each other these atoms are connected. And most importantly: they depend on the symmetry group of the resulting structure. There are thirty-two of them. The symmetry group shows how many and what movements need to be performed so that the same elements overlap and coincide completely. These actions include: turning around an axis (at a certain angle), reflection from a plane or point, inversion. The symmetry group and shows what will be the anisotropy of the crystals. Substances with a cubic structure, for example, do not possess this property. The parameters of such solids are the same in all directions.

What angle is needed for anisotropy?

Above we gave an example when the sound propagation is not uniform in mutually transverse directions. This is a special case of how anisotropy of properties is manifested, which is called the orotropy term. However, the symmetry of crystals is not only cubic or rhombic. It is trigonal, when the repetition of the elements of the structure occurs when you turn to a third of the circle, or even hexagonal, then the angle of rotation is equal to one sixth of the circle. The symmetry of the lower category, monoclinic, makes it possible for the properties to be unequal in the crystal in three mutually perpendicular directions. Thus, anisotropy is the quality of crystalline bodies, which can be manifested at any angles both in one plane and in the volume.

Do all properties have anisotropy?

This question is natural. If one property in this crystal has anisotropy, should other parameters follow this example? Not necessary. Take, for example, crystals that are used in night vision devices. They are able to convert invisible infrared light into the visible range (most often a picture of different shades of green). In such materials anisotropy is the main property that is suitable for use and can be useful. And, in order for the effect to be best, the crystals should be turned at a certain angle (for this purpose, they are specially grown strictly in a certain way). In other directions, the conversion of radiation is less or completely absent. In this case, the thermal conductivity, the velocity of sound or the electrodiffusion in them propagate uniformly in all directions. It also happens that for one property the angle of difference of its characteristics is one, and for another - the other. But these are already quite exotic cases.

Where else is anisotropy?

When a person hears "crystals", usually imagine a semitransparent column of quartz or amethyst. Some girls probably think of ornaments. However, any solid can be crystalline. Products made of iron, aluminum, copper, tin also consist of crystals, only very small. And in each such thing at the micro level, anisotropy of metals is also observed. However, the properties that propagate in perpendicular directions in different ways, very specific and in everyday life are invisible. For example, in cubic crystals of iron and aluminum, the Young's elastic moduli vary with the chosen axis. A linear expansion of tin in different directions differs almost twofold. However, such details, as a rule, do not need to be taken into account every day. After all, the anisotropy of metals and its consequences, as a rule, are laid in all their possible applications at the stage of designing things, buildings, planes, cars.

How to calculate the anisotropy?

All written above, we hope, quite clearly told the reader what anisotropy is. However, another question arises: how do we calculate how different the properties along non-coincident directions in solids are? For this, there is an anisotropy coefficient. At once we will make a reservation, for each size it is calculated in own way. Indicators experiencing anisotropy may be different from each other. The properties of a mechanical or quantum system differ crucially, which is acceptable for one, for another it will be impossible or completely impossible. Therefore, there is no need to talk about some coefficient common for any value. In addition, it is often not possible to calculate it theoretically theoretically, this value is obtained only by an experimental method. The anisotropy coefficient includes the ratio of the values of the investigated quantity in different directions. Sometimes this figure includes the angle between the selected directions. True, most often only as an indicator at the bottom of the value of the value. For example, K _xy shows that this coefficient refers to the difference in the values of the physical quantity along the x and y axes.