So much to thank for, there are many concepts in chemical equipment's functional mechanism I just took for granted as common sense. Now it is not only making sense, the solid logic and math behind it just making the use of these equipment more powerful and appreciated! Thanks again!!!
If the enthalpy of motion of a vacancy is 100 kJ mol-1, the time that a vacancy takes to jump to an adjacent site at 25°C is about ....? Looking for the solution to this problem sir.
Sir, I have a question, referring to 31:14 in the video. Since the size of window available to come out is in the ratio of 0.155, is the size of window to go in will also be 0.155? So does interstitial diffusion also cause distortion when accepted into an OHV? Also when a C atom is present in an OHV of a FCC Fe lattice, will the size of the octahedron increase due to present of another atom? Just curious to know.
The window need to open, means its size will have to increase which is more reasonable than to salute atom contraction. So the energy barrier is to increase the size of window and let solute atom jump and try it's luck to diffuse. Size of atom present in the void can have larger or small size in comparison to void size, so there can be compression or tension strain fields.
Sir, does alloying of an element reduce equillibirium vacancies, ie in substitutional solid soln addition of other elements in small quantity fills up the voids right?
It is called substitutional as it is the mechanism of diffusion in substitutional solid solution. Also, the net effect of the diffusion on a given lattice site is substitution one element by another. But the individual steps of the process involves exchange of place between an atom and a vacancy.
Dear Sir, I was asked in exam that how does an interstitial atom knows that which are vacant sites? How do they know the next vacant position to occupy? I was also asked to discuss diffusion from statistics point of view. What do we mean by statistics of diffusion?
This appears to be a difficult exam question. Was it a written exam or a viva? As you have not shared the actual question let me make a guess. Substitutional diffusion takes place down the concentration gradient by jumping of an atom in a nearby vacancy. But vacancies are randomly distributed. Thus there are vacancies in the forward direction (decreasing concentration) as well as backward direction (increasing concentration). And jump happens to be due to thermal vibration which is also random and equally likely to be in the forward direction as well as in the backward direction. Then why the diffusion happens in the forward direction? The answer is that due to decreasing concentration there are more atoms jumping in the forward direction than jumping in the backward direction. This is where statistics is coming. Individual atoms jump with equal probability in both forward and backward direction. But there are more atoms moving in the forward rather than backward direction giving an overall forward diffusion. So actually an individual atom does know the vacant sites around it but has no preference for the one in the right (forward) direction. I do not know if I have answered the question which was asked you. But I now have an interesting question to ask my students. Thanks :-)
