Diffusion in metal Diffusion in metal is a phenomenon where the net movement of atoms occur from higher concentration region to low concentration region [ ]

Diffusion in metal
Diffusion in metal is a phenomenon where the net movement of atoms occur from higher concentration region to low concentration region . It also defines that the tendency of atoms to move in a predictable space to eliminate the concentration differences between two metals, create a temperature and composition equilibrium between two different metals. The magnitude of net movement of atoms depends on the initial concentration gradient and temperature.

Factors that affect diffusion rate
A diffusion rate increases with rise in temperature, large surface area, and smaller particle size. Rise in temperature increases the kinetic energy of atoms which allow atoms to move quickly and the host atoms can easily pass through the internal interstitial space between atoms. Diffusion rate also increase with large surface area due to more contact with outside ambient and higher concentration. The smaller host particle size of atoms diffuses very quickly as compare to the large size atoms in main metal structure.

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Industrial application of Diffusion.

Carburization for surface hardening of steels:- The application of diffusion in hardening the teeth of the gear. In carburization process, a source of carbon such as graphite powder or gaseous phase containing carbon, is diffused into steel components such as gears.
Dopant Diffusion for semiconductor devices:- The entire microelectronics industry use the diffusion of different atoms into silicon or other semiconductors.

Conductive Ceramics:- In polycrystalline ceramics tend to be good insulators of electricity. The diffusion of ions also play an important role in the electricity conductivity of many conductive ceramics such as lithium cobalt oxide (LiCoO2) use in the lithium batteries.

Oxidation of Aluminium through diffusion:- The use of diffusion on the surface of aluminium convert into aluminium oxide which is a protecting layer of aluminium from further corrosion.

Silica coating on optical fibers:- Optical fibers made from silica (SiO2) are coated with polymeric materials to prevent diffusion of water molecules. As a result, it improves the optical and mechanical properties of the fibers.

Stability of atoms
Stability of atoms are not constant still in the room temperature because room temperature has enough activation energy or thermal energy to the atoms from one lattice position to others. For example, an atom can move from one lattice position to their nearby vacancy or to the nearby interstitial sites, or jump across a grain boundary and causing the grain boundary to move. The probability of atoms and ions increases with increase in temperature, or thermal energy, carried by the atoms.
The rate of atoms movement depends on the Arrhenius equation :-
Rate=C0 exp {-QRT}
Where C0 is a constant, R is the gas constant (1.987 calmol.k), T is the absolute temperature (K), and Q is the activation energy (cal/mol) required to cause an Avogadro’s number of atoms to move. This equation is derived from a statistical analysis of the probability that the atoms will have the extra energy Q needed to cause movement.
Mechanism for diffusion.

There are various types of diffusion mechanism which occurs into the atomic structure of materials. Some of the important mechanism for diffusion is discussed below: –
Self-diffusion: – This is a diffusion mechanism in which atoms move from one lattice position to another in the structure’s vacancies. The self-diffusion can be traced by using the radioactive tracers. For example, the self-diffusion of a radioactive atoms isotopes of gold (Au198) onto the surface of normal gold. Eventually, the radioactive atoms would be uniformly distributed throughout the entire regular gold sample. Although self-diffusion occurs continually in all materials, its effects on the material’s behaviour is generally not significant.

Inter-diffusion: – Inter-diffusion mechanism occurs between two sheets such as between nickel sheet bonded to a copper sheet. At elevated temperatures, nickel atoms gradually diffuse into the copper and copper atoms migrate into the nickel. Again, the nickel and copper atoms eventually are uniformly distributed. In the case of inter-diffusion atoms can move to different directions.

Vacancy Diffusion: – In Self-diffusion and diffusion involving substitutional atoms, an atom leaves its lattice site to fill a nearby vacancy (thus creating a new vacancy at the original lattice site). In this case the diffusion occurs in the form of counterflows of atoms and vacancies called vacancy diffusion. The number of vacancies, which increases as the temperature surges, helps determine the extent of both self-diffusion of substitutional atoms.

Interstitial Diffusion: – When a small interstitial atom in the crystal structure try to move from one interstitial site to another. No vacancies are required for this mechanism. Partly because there are many more interstitial sites than vacancies, interstitial diffusion occurs more easily than vacancy diffusion. Interstitial atoms that are relatively smaller can diffuse faster.


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