Mechanism for Diffusion

 

Mechanism of Diffusion 

In materials containing vacancies, the atom jump from one lattice position to another. This process is called self-diffusion. Although self-diffusion occurs continually in all materials, but its effect on the material’s behaviour is not significant. Diffusion of different atoms in different directions is known as interdiffusion.

There are 2 important mechanisms by which atoms or ions can diffuse:    

                          Vacancy Diffusion

·       In self diffusion and diffusion involving substitutional atoms, an atom leaves its lattice site and fill its nearby vacancy, thus creating a new vacancy in its original lattice site.

·       As this kind of diffusion continues, we have countercurrent flows of atoms and vacancies, called vacancy diffusion

*Countercurrent flow means two bodies flowing in opposite direction 

Interstitial Diffusion

·       No vacancies are needed in this type of diffusion

·       The interstitial atom or ion that presents in the crystal structure will move from one interstitial site to another in this type of diffusion

 

Is interstitial diffusion occurs more easily and faster than vacancies diffusion?

Yes. Because

1. Interstitial atoms are relatively smaller

2. Usually there are more interstitial sites than vacancies in a crystal structure

3. In some materials like ceramics with ionic bonding structure, smaller anions often diffuse faster than bigger cations

References:

Donald R.Asekeland, Pradeep P.Phule.(2003). The Science and Engineering of Material (4th Edition). Bill Stenquist.