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How much air could really be in the copper powder if the copper powder crystals are only 1.8 µm (microns) maximum, in size?
If you assume perfect spherical particles all of equal size, the maximum packing factor is about .75, which means you'll always have at least 25% empty space.
Having a range of sizes improves packing, but being in the real world means you'll never, ever reach the full theoretical maximum either.
You'll always have empty space when using a powder, always.
Now if you sintered a powder, as they do to make solid parts in powder metallurgy, that's a whole different story, and a topic for a whole other thread.