Diffusion

"Diffusion - How atoms move through solids"
Diffusion means mass transport by atomic motion. The mechanisms of Gases & Liquids is known as random (Brownian) motion
and for solids is known vacancy diffusion or interstitial diffusion. Simply we can define diffusion as, the movement of particles in a solid from an area of high concentration to an area of low concentration, resulting in the uniform distribution of the substance. (Diffusion chapter 5, 2008, p.1)
Ronald D. Kriz(1999) suggests that diffusion depends on five main variables which are initial concentration, surface concentration, diffusivity, time, and temperature. The initial concentration of the material is diffusing into the base solid material is known as the "solute" or impurity, but is not always zero. The surface concentration is the amount, weight percentage, of solute near the surface of the solid base material. Diffusivity is defined as the rate at which the solute or impurity penetrates into the solid, base, and material. The time and temperature are fairly self-explanatory, though the temperature affects the rate of diffusion with increasing temperature.

There are two different types of diffusion mechanism, stems from the relationship between their relative atomic sizes, vacancy diffusion and interstitial diffusion. Vacancy diffusion occurs primarily when the diffusing atoms area of a similar size, or substitutional atoms (Physics.uwo ,2008, p5). The movement of a substitutional atom requires a vacancy in the lattice for it to move into. Interstitial diffusion occurs when the diffusing atom is small enough to move between the atoms in the lattice. This type of diffusion requires no vacancy defects in order to operate. (Physics.uwo ,2008, p6)
The rate of Diffusion may vary according to the structure and materials. For instant, an open crystal structure with lower melting temperature materials has, materials with secondary bonding, smaller diffusing atoms and cations
with lower density materials has higher diffusion rate. In contrary, close-packed structures with higher melting temperature materials with covalent bonding have larger diffusing atoms, and anions
with higher density materials have low diffusion rate.
The magnitude of the diffusion coefficient is indicative of the rate of atomic motion and depends on both host and diffusing species as well as on the temperature. By using Fick’s first diffusion law, we can find the coefficient of diffusion. Fick’s first diffusion law: for steady-state diffusion condition (no change in the system with time), the net flow of atoms is equal to the diffusivity D times the diffusion gradient dC/dx. (Diffusion chapter 5, 2008, p.2). This is when the concentration of gradient and profile are kept constant.
All in all, taking everything into consideration, by studying diffusion, now we know the pattern of how atoms move through solid and also we have looked up two different types of diffusion mechanics of solid. Moreover, the effect of structure and materials on the rate of diffusion varies with influencing factor such as temperature, volume and surface area.
References:

1)Virginia.Edu (13th Sep 2010), Introduction to Materials Science - Chapter 5 – Diffusion, MSE 2090, Available at: http://people.virginia.edu/~lz2n/mse209/Chapter5.pdf (Accessed: 4th April 2013).

2)Physics.uwo (1st Oct 2008), Chapter 5 - Diffusion, Available at: http://www.physics.uwo.ca/~lgonchar/courses/p2800/Chapter5_Diffusion_Handouts.pdf (Accessed: 4th April 2013).

3) http://www.sv.vt.edu/classes/MSE2034_NoteBook/MSE2034_kriz_NoteBook/diffusion/diffintro.html ( Accessed: 4th April 2013).
Ronald D. Kriz
 (1999) Virginia Tech
 College of Engineering: Materials Science & Engineering 2094 Class Project
(Accessed: 4th April 2013).

References: 1)Virginia.Edu (13th Sep 2010), Introduction to Materials Science - Chapter 5 – Diffusion, MSE 2090, Available at: http://people.virginia.edu/~lz2n/mse209/Chapter5.pdf (Accessed: 4th April 2013). 2)Physics.uwo (1st Oct 2008), Chapter 5 - Diffusion, Available at: http://www.physics.uwo.ca/~lgonchar/courses/p2800/Chapter5_Diffusion_Handouts.pdf (Accessed: 4th April 2013). 3) http://www.sv.vt.edu/classes/MSE2034_NoteBook/MSE2034_kriz_NoteBook/diffusion/diffintro.html ( Accessed: 4th April 2013). Ronald D. Kriz (1999) Virginia Tech College of Engineering: Materials Science & Engineering 2094 Class Project (Accessed: 4th April 2013).