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→Precious Metal Flakes
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]]
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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).
== Precious Metal Preparations==
<figure id="fig:Solar cell with print pattern of ARGONOR N920">
[[File:Solar cell with print pattern of ARGONOR N920.jpg|right|thumb|Figure 2: Solar cell with print pattern of ARGONOR N920]]
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While in the past mostly glass ware and ceramics (table china) were coated for decorative purposes with gold or platinum, precious metals have since quite a few years been applied to non-metallic substrates such as ceramics, glass or plastics to make their surfaces electrically conductive. To coat these surfaces, fine powders of the precious metal are dispersed in a carrier, containing a paint basis and organic solvents. Such preparations can be applied by screen or tampon printing, by spraying, immersion or with a paint brush.
The firing preparations in liquid or paste form are widely used in electrical and electronic engineering and especially in the thick-film technology (<xr id="tab:Liquid Silver Preparations for Firing Application (ARGONOR)"/><!--(Table 8.2)-->). The precious metal filler material is mostly pure silver because of its high electrical conductivity. During firing in an oxidizing atmosphere at temperatures between 400 and 850°C a well adhering and highly conductive surface layer is formed.
When utilizing screen printing techniques any shapes of conductive patterns can be created (<xr id="fig:Solar cell with print pattern of ARGONOR N920"/><!--(Fig. 8.2)-->) resulting in conductive paths with good electrical properties and high temperature stability.
Conductive adhesives are used mostly for mechanical bonding with low thermal impact. As the adhesive components high-polymer organic substances such as epoxy resins and mixed polymers are mostly used. They are made of electrical filler materials such as flake shaped silver powders (70 – 80 wt%). Silver based conductive adhesives are available as single or two component adhesive systems. Both types are hardening without the application of pressure.
<figure id="fig:Flexible keyboard contact pattern printed with AUROMAL 170">
[[File:Flexible keyboard contact pattern printed with AUROMAL 170.jpg|left|thumb|Figure 3: Flexible keyboard contact pattern printed with AUROMAL 170]]
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Conductive paints and adhesives have broad applications in electrical and electronic engineering. They are used for example for the contacting of film resistors, mounting of terminal wires, conducting electrostatic electricity or contacting components at low temperatures.
The mechanical strength of the bond connections depends mostly on the selected hardening temperature (<xr id="fig:Shear force of an adhesive joint"/> <!--(Fig. 8.4)-->).
<figure id="fig:Flexible keyboard contact pattern printed with AUROMAL 170Shear force of an adhesive joint">[[File:Flexible keyboard contact pattern printed with AUROMAL 170Shear force of an adhesive joint.jpg|rightleft|thumb|Figure 34: Flexible keyboard contact pattern printed with Shear force of an adhesive joint (silver adhesive AUROMAL 170K 20) as a function of the hardening temperature]]
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<figure idbr style="figclear:Shear force of an adhesive jointboth;">[[File:Shear force of an adhesive joint.jpg|right|thumb|Figure 4: Shear force of an adhesive joint (silver adhesive AUROMAL K 20) as a function of the hardening temperature]]</figure>
=== Precious Metal Flakes===
To obtain certain desired physical properties of preparations, the dispersed precious metals in flat flake-like particles (generally called "flakes") are needed. These are produced by milling fine metal powders in the presence of milling additives or agents. The properties of these metal flakes, i.e. silver flakes (ability to disperse easily, flow characteristics, electrical conductivity) are strongly dependent on the particle shape and size as well as on the type of milling agents used. (<xr id="fig:SEM photos of silver flakes a fine grain b large flat"/><!--Fig. 8.5-->) illustrates through SEM photos a type of rather fine silver flake (medium particle size 4 – 6 µm) (a) and another one with relatively large flat but thin flake shapes (particle size 8 – 11 µm) (b). Typical commercial silver flake types are listed with their respective properties in (<xr id="tab:Typical Commercial Silver Flake Types"/><!--(Tab. 8.4)-->). Gold and platinum can also be produced as powder flakes. However, in terms of the quantities used, they are of lesser economic importance.
<figure id="fig:SEM photos of silver flakes a fine grain b large flat">
[[File:SEM photos of silver flakes a fine grain b large flat.jpg|rightleft|thumb|Figure 5: SEM photos of silver flakes (a) fine grain (b) large flat]]
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