This article says that anisotropy is an inequality in the values of a certain physical quantity along different directions of a solid. It reveals what causes anisotropy, where it occurs, how it is applied. Also briefly described is the anisotropy coefficient.
Determination of anisotropy
First, 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 to be slightly incomprehensible and clearly requires explanation. Properties are understood as 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 at a different speed across a block of stone than along. In this case, this property helps to determine the rocks lying deep in the crust. Natural distribution during an earthquake, for example, or with a specially created strong impact will give an idea of the density and angle of occurrence of various minerals.
What causes anisotropy?
When referring to this term, most often refers to the anisotropy of crystals. This section deals with solid state physics. And any scientist from 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 these atoms are connected to each other. And most importantly: they depend on the symmetry group of the resulting structure. There are thirty-two in all. 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: rotation around an axis (a certain angle), reflection from a plane or point, inversion. The symmetry group also shows what the anisotropy of the crystals will be. 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?
We gave an example above 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 term "orotropy". However, the symmetry of crystals is not only cubic or rhombic. It happens to be trigonal when a repeat of the elements of the structure occurs when turning a third of the circle, or even hexagonal, then the rotation angle is equal to one sixth of the circle. The monoclinic symmetry of the lower category allows the properties to be unequal in the crystal in three mutually non-perpendicular directions. Thus, anisotropy is the quality of crystalline bodies, which can manifest itself at any angle both in one plane and in volume.
Should all properties have anisotropy?
This question is logical. If one property in a given crystal has anisotropy, should other parameters follow this example? Not necessary. Take, for example, the crystals that are used in night vision devices. They are able to turn invisible infrared light into the visible range (most often it turns out a picture of different shades of green). In such materials, anisotropy is the main property that is suitable for use and can be useful. Moreover, in order for the effect to be the best, the crystals must be rotated at a certain angle (for this they are specially grown in a strictly defined way). In other directions, radiation conversion is less or completely absent. In this case, the thermal conductivity, the speed of sound or electrodiffusion in them spreads uniformly in all directions. It also happens that for one property the angle of difference in its characteristics is one, and for another it is another. But these are already quite exotic cases.
Where else is anisotropy?
When a person hears “crystals,” he usually imagines translucent columns of quartz or amethyst. Some girls probably think about jewelry. However, any solid can be crystalline. Products made of iron, aluminum, copper, tin also consist of crystals, only very small ones. And in every such thing at the micro level, anisotropy of metals is also observed. However, properties that are distributed in perpendicular directions in different ways are very specific and invisible in everyday life. For example, in cubic crystals of iron and aluminum , Young's elastic moduli vary depending on the chosen axis. And the linear expansion of tin in different directions differs almost twice. However, such details are usually not required to be taken into account every day. Indeed, the anisotropy of metals and its consequences, as a rule, are laid down in all their possible applications at the stage of designing things, buildings, aircraft, machines.
How to calculate anisotropy?
Everything written above, we hope, told the reader quite clearly what anisotropy is. However, another question arises: how to calculate how much the properties differ along non-coincident directions in solids? There is anisotropy coefficient for this. Immediately make a reservation, for each quantity it is calculated in its own way. Indicators experiencing anisotropy may be different from each other. The properties of a mechanical or quantum system differ radically, which is acceptable for one, for the other it will be impossible or even impossible. Therefore, it is not worth talking about a certain coefficient common to any value. In addition, most often it is not theoretically possible to calculate it purely theoretically, this value is obtained only experimentally. The anisotropy coefficient includes the ratio of the values of the studied value in different directions. Sometimes this indicator includes the angle between the selected directions. True, most often only as an indicator at the base of the value of a quantity. For example, K hu shows that this coefficient refers to the difference in the values of a physical quantity along the x-axis and the y-axis.