Diffusion is a process of moving atoms in a material from a high concentrated area to low concentrated area. The material can be solid, liquid or gas. diffusion is widely used in everyday life, various industries and for various experiments. In this tutorial, we are going to lean about basics about diffusion, examples and applications of diffusion. Diffusion is a important concept in chemical engineering.
Written by: Madusha Ruwansara (undergraduate), Department of Civil Engineering, Faculty of Engineering, University of Peradeniya
In this tutorial, we will learn followings.
In addition to the above examples, diffusion is widely used in everyday life, various industries and for various experiments. At present, the phenomenon of diffusion has become a very important and very useful phenomenon, so it is very important to know what it is and to study its use.
Diffusion of an individual atom is random. If there is a gradient of concentration (high concentration to low concentration) the overall atomic movement is not random and it is directional. WE can study the diffusion of solid in two main parts as the type of diffusion and the mechanism of diffusion.
When it comes to the type of diffusion, it is further divided into two parts; inter-diffusion and self-diffusion. Depending on the mechanism of diffusion, it can also be divided into two parts; substitutional diffusion and interstitial diffusion.
Self-diffusion is the mechanism of migration in pure materials. When considering a pure material, a particular atom of the material is not always at a one equilibrium site. It moves from place to place in the material. This phenomena is called self-diffusion.
Inter diffusion is a process of diffusional exchange of atoms across two materials that are in contact. For example, consider an alloy with Cu and Ni atoms. Atoms tend to migrate from regions of huge concentration to low concentration.
When considering a binary alloy, usually the component which have the lower melting point diffuses faster than the others. This effect is known as Kirkendall effect.
Consider a binary alloy of A and B. The melting point of A is lower than B. Therefore, the diffusion flux of A is larger than B.
Basically diffusion of solid material is happen in two major ways.
Substitutional diffusion is aided by a vacancy in a lattice. Explain further it happens by the movement of atoms from one atomic site to another. Substitutional diffusion applies to Substitutional impurities. It means that the atoms exchange with vacancies in lattice. The rate of substitutional diffusion depends on the number of vacancies in the lattice and the activation energy needed to exchange.
When it comes to interstitial diffusion, the diffusing atom is not on a lattice site/lattice point. It is on an intersite located between lattice points.
Energy is needed for atoms to break bonds with adjacent atoms to jump from a lattice site or an interstitial site to another lattice site or an interstitial site. Therefore, there is an energy barrier to overcome. The energy needed to diffusion comes from the thermal energy of atomic vibration. Atoms need enough thermal energy to break bonds and squeeze through their neighbor atoms. This is called activation energy.
Substitutional diffusion is happened by the atoms which migrate between the lattice points. Therefore, the bonds which arise between the host atoms in lattice are very strong and hard to break. Further, it is relatively not easy to squeeze through the host atoms for diffusing atoms. To do this high activation energy is needed for Substitutional diffusion.
When it comes to interstitial diffusion, diffusing atoms are not in the lattice points. There are in the interstitial sites between the lattice points. Therefore the bond strength between host atoms and interstitial atoms is not high and it is relatively easy to squeeze through the host atoms during the diffusing process. Therefore relatively less activation energy is needed for the interstitial diffusion. Generally, less energy is needed for interstitial diffusion than Substitutional diffusion. The bonding of interstitial atoms to adjacent atoms is normally weaker and there are more Interstitial sites than vacancy sites to migrate. Therefore, interstitial diffusion is generally faster than Substitutional diffusion.