2,315
edits
Changes
→Precious Metal Powders
Precious metal powders consist of small particles of approx. 1 – 100 µm diameter, which distinguish themselves by particle shape, particle size and particle size distribution. Depending on the manufacturing process, silver particles may be spherical, crystalline or dentritic. Smaller particle size typically leads to a larger surface area.
<figure id="fig:Different shapes of silver powders">
[[File:Different shapes of silver powders.jpg|right|thumb|Figure 1: Different shapes of silver powders a) spherical b) rounded crystal applomerates]]
</figure>
The measured densities of powders – both, the apparent density and the tap density – are low, compared to the wrought metals because of the gaps between the particles. They vary in a wide range between 0.5 and 6 g/cm<sup>3</sup> depending on the morphology of the particles and their tendency to agglomeration. Precious metal powders can be compacted by pressing and sintering afterwards; a certain amount of porosity is however always retained.
Precious metal powders are produced by various methods, such as for example electrolysis, atomizing from the molten phase, chemical precipitation
or by cementation with non-precious metals. Depending on the manufacturing process, silver powders – as the by far largest volume precious metal powder used – have different properties as shown in (<xr id="tab:DifferentTypesofSilverPowders" /><!--(Tab. 8.1)--> and [[Silver_Based_Materials#label-tab:Quality_Criteria_of_Differently_Manufactured_Silver_Powders|Table Quality Criteria of Differently Manufactured Silver Powders]]<!--(Tab. 2.12)-->). Atomizing from a melt results in a powder with high tap density composed of spherical particles. Using electrolytic deposition from a silver salt solution, creates randomly shaped dentritic to crystalline particle structures. Chemical processes can result in rather fine particles with a large specific surface area. <xr id="fig:Different shapes of silver powders"/><!--Fig. 8.1--> shows typical SEM photographs of atomized silver powder in spherical shapes (a) and a cementation powder composed of rounded crystal agglomerates (b).
