Sciencemadness Discussion Board

Pressing powders into a pellet: how it works

Metallus - 10-11-2015 at 08:42

Hi there

This is more of a doubt I have concerning "how" the particles stick together in the pellet/platelet you make when you press a powdered solid.

From what I understood, when you grind a solid (eg: KBr, Al2O3, ZrB2) into a fine powder of an average particle size of 1-5 μm, and then you press said powder into a small platelet, what happens is that you stack all those small particles in a small volume, while still keeping their microscopic size (so the particles don't actually fuse together to give e.g 500μm particle size, but just stick together because reasons)

What is the force that keeps the pellet/platelet together? Is it just weak interactions? Because afterall the pellet is still very fragile and can be more or less easily broken and pulverized.

Also, in the case of KBr, you can actually make a perfectly transparent platelet starting from a white powder. A professor of mine told me that you actually melt the material together when pressing, but that explanation seemed a bit "odd" to me. Did he mean "you locally melt the particles because with an high surface area the melting point is lower"? Still, I never noticed a change in temperature whatsoever, so when he told me this I was a bit like "is he BSing me?".

I also know that in order to actually "fuse" the particles together, there is a process called sintering (like HP) during which you heat at high temperatures (even >2k°C) and press the material together, so I'm somewhat sure you don't actually fuse the particles together when you press them for 5min at RT, but more like "stack them together for a while".

I hope you can solve this little doubt of mine. If I got it right, it's just a difference between microscopic and macroscopic changes.

Thanks for your attention

aga - 10-11-2015 at 09:10

Dunno about powders, but squashing stuff at high mechanically-induced pressures certainly causes localised heating.

The Krone system for telephone wires is basically ramming a copper wire between two blades.

The heating at the blade edge causes a 'microweld' between the copper wire and the blades.

Detonationology - 10-11-2015 at 09:21

Quote: Originally posted by aga  
Dunno about powders, but squashing stuff at high mechanically-induced pressures certainly causes localised heating.

I too don't have any practical experience with metal powders, but this reminds me of explosion welding (without the explosion). I assume that when powdered, particles can make better contact, but high temperature and pressure seems to be necessary across the board.

Bert - 10-11-2015 at 09:35
Quote:

(so the particles don't actually fuse together to give e.g 500μm particle size, but just stick together because reasons)


Um, what? "because reasons"?!

Lots of different physical and chemical mechanisms may be in play, depending on the materials being pressed and conditions.

Got a specific question?

annaandherdad - 10-11-2015 at 10:38

If two perfectly clean surfaces are pressed together, I find myself wondering why they would *not* weld.

One reason would be that neither surface is a perfect plane, or even if they are (because they are crystals cleaved along a plane of the lattice), the two planes cannot be made adjacent because of bumps elsewhere on the surface, or even if they can be made adjacent, the crystal lattices would not be aligned. But pressure would overcome misalignment, and the pressure is magnified at small points of contact. And a misalignment of crystals is no worse than what happens at domain boundaries.

I'm just speculating, I have no official knowledge. Of course, it could be that the surfaces are not perfectly clean.

phlogiston - 10-11-2015 at 12:19

I suspect van der Waals forces are a big part of the answer.

Lots of substances undergo phase transitions when pressure changes. Possibly, KBr melts, but not because heat is generated when you apply pressure, but rather because at the high pressure and room temperature, KBr is a liquid. (However, googling for a few minutes I can't find a reliable references quickly that supports that KBr actually melts. Some sites say that it becomes plastic) so whether that is really true in the case of KBr I am not sure (but it is certainly possible for other solid substances).

Similarly, you can boil water at room temperature if you lower the pressure. You can melt ice at well below 0 deg C if you apply pressure. You can compress gasses to store them as liquids at room temperature. Look up pressure-temperature phase diagrams to find out more.

careysub - 10-11-2015 at 15:12

Quote: Originally posted by phlogiston  
...
Lots of substances undergo phase transitions when pressure changes. Possibly, KBr melts, but not because heat is generated when you apply pressure, but rather because at the high pressure and room temperature, KBr is a liquid. (However, googling for a few minutes I can't find a reliable references quickly that supports that KBr actually melts. Some sites say that it becomes plastic) so whether that is really true in the case of KBr I am not sure (but it is certainly possible for other solid substances).
...


I believe the phenomenon you are looking for is plastic deformation - the pressure exceeds the yield point for the substance and it flows, even though it is still fully solid.