Properties of powdered metals

Question

I have seen certain metals such as Aluminium used in a situation where it is in a powdered form and "compacted" into a shape under intense pressure (in a mould), and it ends up looking mostly like solid metal. It is also heated afterwards, I assume to form a complete metallic bond.
I have a few questions on this:

1. How are oxide layers on the powder affected when it is compacted? Eg. Can the $Al_2O_3$ layer, or at least the O atoms from them, be "inside" the moulded object when it is compacted?
   
2. Metals in a powdery form aren't as reflective as in their "solid" form (also please let me know the right word for that). Why is this?
   
3. Are most metals in their powdered form pure, eg. Not like a salt, but a single element?
   
4. I know gold powder, obtained from chloroauric acid, becomes gold in its metal form when melted down. It looks brownish and takes up far more volume than the gold it contains. Is this pure Au or a compound/salt?
   
5. Also, in 4. with the gold powder, it actually forms rather large clumps from the chloroauric acid. How is this possible assuming it's a pure element?

Thought I better get all the questions done in one go. Thanks in advance

Answer 1

Let me try to answer the part 1. of your question.
The process of compacting powdered Aluminium is also called Sintering. And as you can guess, the thin oxide layer of Alumina inhibits the binding of Aluminium grains between the powder particles.

So, while sintering, we need to either:
1) Break part of the Alumina film or,
2) Reduce part of it to Aluminium,
in order to facilitate formation of a metallic bond between the powder particles.

For 1), we use a nitrogen atmosphere as it enhances the destruction of Alumina.
For 2), we use because magnesium as a alloying element because magnesium, even in small amounts, concentrates on the surface of powder particles and supports sintering by local reduction of Alumina.

In conclusion, I would say that compacted Aluminium does have Alumina inside the molded object. But metallic bonds between powders are formed after partly removing the oxide film by the above mentioned methods.