Why don't solids fuse spontaneously?

[Smacal1072]

Suppose I have single crystal of Quartz, and I break it in half. It took energy to break the crystalline bonds of the crystal.

Now, I take my 2 crystals, and fit them together along the break line, so that they are flush with each other.

Why should the 2 crystals not spontaneously conjoin? If the crystalline bond is energetically favorable, what prevents the crystalline bond from re-forming?

 

[nbo10]

Normally an oxide layer is formed on a free surface.

 

[Smacal1072]

Thats true for most metals, but quartz is already silicon dioxide.

 

[Oudeis Eimi]

I think there's an energy barrier, so you need an activation energy before reaching the actual energy minimum (the 'fused' solid). You can get them to fuse by applying pressure and/or temperature (while remaining in the solid state).

 

[f95toli]

You can indeed fuse solids together using pressure and high temperatures. A good example are bicrystal substrates which made by breaking a single crystal subtrate into two pieces, rotating one piece and then fusing them toghether again. This creates an artifical grain boundary which if made correctly will be very "thin", ideally a few angstrom (there are always a few defects/dislocations so the GB is a bit wider than the "ideal" case).

 

[PhaseShifter]

Thats true for most metals, but quartz is already silicon dioxide.

Even when an oxide layer isn't formed, the surface structure is often (always?) different than the internal structure.

Solid materials have a surface energy just like liquids do, only in liquids it's called "surface tension." This is one of the main deciding fators in why crystals tend to preferentially grow in some directions rather than others. In order to reduce this surface energy, the surface layers of a solid tend to reorganize.

Also, you might be interested in the related topic of contact welding.

Also, oxygen isn't the only reactive substance in the atmosphere...I wouldn't be too surprised if the surface of quartz were covered with hydroxyl groups (from reaction with water vapor) after exposure to air.

 

[Smacal1072]

So theoretically, when we cut a surface perfectly, all of the energy that went into breaking the lattice is transformed into the surface energy of the new surfaces. That makes sense. Thanks for the contact welding link PhaseShifter.

Off topic just a bit, but suppose we take a solid, and pulverize it into fine particles. Since we've created a lot of surfaces, we've transformed a lot of lattice energy into surface energy. When we melt this powder, would it take less heat of fusion than melting the initial single crystal, since we've already broken the lattice bonds?

 

[Studiot]

Unfortunately the increased surface energy is lost to the environment on phase change so no, the latent heat remains the same.

Incidentally no one has mentioned Free energy and in particular Gibbs Free Energy, in relation to your original question.

You started from the premise that the crystalline phase is energetically favourable. You need to be careful about this 'minimum energy' approach because it is not the obvious energy that tends to a minimum but something more subtle called the free energy.

The free energy is a subtle blend of the entropy gain that occurs when you disorganise a lattice and the energy that you have to put into effect the disruption. these factors often work in opposite directions so the end result is a compromise.

http://en.wikipedia.org/wiki/Gibbs_free_energy

 

[Smacal1072]

Unfortunately the increased surface energy is lost to the environment on phase change so no, the latent heat remains the same.

Incidentally no one has mentioned Free energy and in particular Gibbs Free Energy, in relation to your original question.

You started from the premise that the crystalline phase is energetically favourable. You need to be careful about this 'minimum energy' approach because it is not the obvious energy that tends to a minimum but something more subtle called the free energy.

The free energy is a subtle blend of the entropy gain that occurs when you disorganise a lattice and the energy that you have to put into effect the disruption. these factors often work in opposite directions so the end result is a compromise.

http://en.wikipedia.org/wiki/Gibbs_free_energy

Ah, thanks. I completely disregarded entropy. So the transition from a pulverized powder of a crystal to a single crystal is not spontaneous because it would require a decrease in entropy, and an increase in Gibbs energy. Thanks