Question

It is believed that interstitial diffusion is faster than vacancy diffusion. Can you show this by comparing the values of diffusion coefficients of a pair of cases (one values for each type of diffusion) from Table 5.2...... (5 points) Describe a real example of Fick's first law of diffusion....(2 points)

Answer 1

Hi.

1) Interstitial diffusion is faster than vacancy diffusion.It is because of following reasons.

• Interstitial diffusion occurs only for the atoms having small size ,so that it can be easily fit in the interstitial space.This small atoms has more interstitial space than that of space in vacancy diffusion.
• Interstitial space bonds with its surrounding atoms are weaker than that of the bonds between atom to atom.

Small atoms such as Hydrogen ,Carbon are the ones which can show interstitial diffusion.Speed of diffusion is mainly dependent on the diffusion coefficient.If diffusion coefficient is high, then the speed will be high.Let us consider For interstitial diffusion to occur diffusing atom should be significantly smaller than the host atom(in terms of atomic radius).Diffusion coefficient depends mainly on temperature.If temperature is high,diffusion coefficient will also be high.

By considering a standard table of diffusion data,Copper in Aluminium will be interstitial diffusion(because copper has significantly less atomic radius than Aluminium),and the diffusion coefficient will be in terms of 10^-14 m²/s.If we consider the the other case of Zinc in Copper which will be vacancy diffusion(Zinc has higher atomic radius than Copper) ,here the diffusion coefficient will be of the range of 10^-18m²/s.

2)Ficks first law of diffusion states that diffusion flux will be proportional to the the concentration gradient.And also diffusion will occur from higher concentration region to lower concentration region.

We can take a simple example of mixing of salt with water.In a beaker Lets say there is a quantity of salt is present and then some quantity of water is added.Since there is a concentration difference,diffusion occurs from salt side to water side and after some time it will be equalised by the formation of salt water.