The diffusion coefficient is where and how much

Summer! The sun, the heat, the smell of freshly cut grass in the air. The grass is as far away from here, and the smell is felt. This is physics, a similar phenomenon is called diffusion. And it is defined as the penetration of particles of contiguous substances into each other. For example, as now, the grass touches the air, and the smell from the mown grass spreads far away from the mowing place. A characterized by a similar process is usually as large as the diffusion coefficient.

We can say that everything that is happening around us is largely related to the processes of interpenetration. Thanks to this phenomenon, we even live. And what's so surprising? Penetration of oxygen or nutrients into the blood - this is the most real diffusion, one substance penetrates into another. Diffusion in nature is much more widespread than it seems to us. This phenomenon is not something exotic, but, on the contrary, it is widely represented in the surrounding world. Even the fact that the exhaled air does not accumulate around, but dissipates in space, is also caused by diffusion.

There can be interpenetration in a variety of bodies - liquid, gaseous, solid. Its cause is the chaotic motion of molecules of matter. By the way, such a phenomenon as diffusion is considered to be one of the factors supporting it in the molecular-kinetic theory. So, returning to the aggregate state of the body: the rate of penetration of a substance depends on its aggregate state and on such a value as the diffusion coefficient.

What is this mysterious concept? This is the name of the quantitative characteristic of the rate of transport of molecules of one substance to another. The diffusion coefficient, the formula of which is quite specific, makes it possible to estimate the amount of matter that has passed through a unit of area (square meter) per unit of time (second).

In practice, it has been established that in mutual gases, mutual penetration occurs at the highest rate, while in solids the rate of penetration is minimal. The diffusion coefficient is influenced by the temperature of the body and the mutual concentration of the substances nearby. With increasing temperature, the interpenetration rate increases, with increasing concentration of matter, too.

Thus, the phenomenon of interpenetration is due to the so-called concentration gradient or the temperature gradient. By type, diffusion is divided into free and forced. Forced arises under the influence of external forces. Depending on their type, the forced diffusion is determined as thermo-, electro-, baro- and ascending.

Diffusion is widely used in engineering. One of the most typical examples is diffusion welding. The essence of this technology is simple: connect two different bodies (let it be two metals), and then subject them to pressure and temperature. The latter is smaller in magnitude than the melting point of the substances.

The result is a mix of two dissimilar materials. Such technology is widely used in instrument making and electronic industry, in the manufacture of large-sized parts of complex shape, in experimental and small-scale production. Such welding can be carried out under different conditions, up to the conditions of vacuum, all determined by the specific requirements imposed on the finished product.

Another, no less common, use of the phenomenon of diffusion is its use for the production of semiconductor structures. One of the technologies for creating pn junctions is based on the phenomenon of interpenetration. Under the influence of high temperatures approaching the melting temperature, regions with the required impurity concentration are obtained in the crystal structure.

Consideration of the concept of "diffusion coefficient" made it possible to verify the extremely wide spread of the phenomenon of interpenetration of substances in nature, as well as in various variants of its use in engineering.