Coarsening: Exploring the Mechanism Behind Solid Grain Growth

[RPI_Quantum]

When solid grains are immersed in a liquid, I know that there is a tendency for larger particles to grow at the expense of smaller particles (and this phenomenon I have heard referred to as coarsening).

Can anybody explain the mechanism behind this to me? I've tried to think of it in terms of solubility, but that doesn't seem to explain the larger grain growth. I have a feeling that surface energy considerations are the underlying cause, but I can't seem to come up with a coherent explanation on my own.

 

[Astronuc]

Coaresening simply means 'getting larger'. In metals, the smaller grains genereally have a higher surface energy, and so if the metal is heated, the atoms from the smaller grains migrate to the larger grains, and so the smaller grains disappear.

In the context of a solution, and let's assume that it is saturated or nearly saturated, there is a contants equilibrium of dissolution and precipitation. Smaller grains are more likely to lose atoms, and I believe higher surface energy is part of it, but also, in a solution, the average energy within the smallest particles is likely to be somewhat higher.

 

[SiO2]

I don't know if this is going to be correct. What I am thinking is that the solubility thermodynamics is mechanism of what you are seeing. The reason that the solid phase is growing is simply because the solid phase is more stable in this part in the phase diagram. And the reason that big grain is getting larger at the expense of smaller grains might be due to the property that, for this material, the bulk energy (function of volume) is lower than the surface energy (function of surface). And the reason is you are seeing smaller grains disappearing might be due to the fact they are swallowed by larger grain (big + small = bigger, I don't want to call this growth an "expense" of the smaller grain).

 

[Modey3]

The answer comes from the Gibbs-Thompson Effect. In a two component system, the equilibrium concentration of solute in a precipitate depends on its curvature. Large curvature (r-> infinity) precipitates have equilibrium concentration corresponding to the equilibrium diagrams commonly seen. Small curvature particles have higher equilibrium concentrations. What ends up happening is that the solute from the small particles diffuse to the larger particles due to concentration gradient.

Modey3

 

[naeem421]

i agree with Modey3, Cheer up!