===8.1 Precious Metal Powders===Precious metal powders are used as raw materials for many technical products as well as for medical and decorative applications. This includes the production of silver composites for electrical contacts (Ag/Ni, Ag/metal oxides, Ag/C, Ag/W, etc.), catalysts, electrodes or dental products. Besides these, precious metal powders are used as the base material in preparations as well as conductive paints and adhesives.

Precious metal powders are used as raw materials for many technical products as well as for medical consist of small particles of approx. 1 – 100 µm diameter, which distinguish themselves by particle shape, particle size and decorative applicationsparticle size distribution. Among these are Depending on the manufacture of composite manufacturing process, silver materials for electrical contacts (Ag/Ni, Ag/metal oxides, Ag/C, Ag/W, etc), catalysts, electrodesparticles may be spherical, crystalline or dental productsdentritic. Besides these precious metal powders are used as the base material in preparations as well as conductive paints and adhesivesSmaller particle size typically leads to a larger surface area.

Precious metal <figure id="fig:Different shapes of silver powders consist "> [[File:Different shapes of small particles of approxsilver powders. jpg|right|thumb|Figure 1 – 100 µm diameter which distinguish themselves by particle shape, particle size and particle size distribution. Depending on the manufacturing process, : Different shapes of silver particles may be powders a) spherical, crystalline, or dentritic. Smaller particle size typically leads to a larger surface area.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/cm3 cm³ depending on the morphology of the particles and their tendency to agglomeration. Precious metal powders can be compacted by pressing and then sinteredsintering 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 – havedifferent 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).

Fig<figtable id="tab:DifferentTypesofSilverPowders"><caption>'''<!--Table 8. 811: -->Different Types of Silver Powders'''</caption><table class="twocolortable"><tr><th><p class="s6">Powder type</p></th><th><p class="s6">GE</p></th><th><p class="s6">GN1</p></th><th><p class="s7">ES</p></th><th><p class="s6">V</p></th></tr><tr><td><p class="s6">Manufacturing Process</p></td><td><p class="s6">chemical</p></td><td><p class="s6">chemical</p></td><td><p class="s7">electrolytic</p></td><td><p class="s6">atomized</p></td></tr><tr><td><p class="s6">Particle shape</p></td><td><p class="s6">agglomerated</p></td><td><p class="s6">agglomerated</p></td><td><p class="s7">dentritic</p></td><td><p class="s6">spherical</p></td></tr><tr><td><p class="s6">Avg. particle diameter</p><p class="s6">(median) [µm]</p></td><td><p class="s6">10 - 15</p></td><td><p class="s6">20 - 40</p></td><td><p class="s7">-</p></td><td><p class="s6">32 - 60</p></td></tr><tr><td><p class="s6">Medium particle size</p><p class="s6">(FSS - Fisher Sub Sieve Size) [µm]</p></td><td><p class="s6">-</p></td><td><p class="s6">-</p></td><td><p class="s7">4.0 - 6.0</p></td><td><p class="s6">-</p></td></tr><tr><td><p class="s6">Tap density</p><p class="s6">(DIN/ISO 3953) [g/cm³]</p></td><td><p class="s6">0.7 - 1.1</p></td><td><p class="s6">2.0 - 2.5</p></td><td><p class="s7">2.0 - 3.0</p></td><td><p class="s6">4.0 - 6.7</p></td></tr><tr><td><p class="s6">Specific surface area</p><p class="s6">(B.E.T.) [m²/g]</p></td><td><p class="s6">0.5 - 0.9</p></td><td><p class="s6">-</p></td><td><p class="s7">-</p></td><td><p class="s6">-</p></td></tr></table></figtable> == 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]]</figure> 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. ===Precious Metal Firing Preparations===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.  <figtable id="tab:Liquid Silver Preparations for Firing Application (ARGONOR)"><caption>'''<!--Table 8.2:-->Liquid Silver Preparations for Firing Application (ARGONOR)'''</caption><table class="twocolortable" style="width:80%"><tr><th><p class="s6">Preparation</p></th><th><p class="s6">Substrate</p><p class="s6">Material</p></th><th><p class="s6">Application by</p></th><th><p class="s6">Firing Temperature [°C]</p></th><th><p class="s6">Properties</p></th><th><p class="s6">Silver Content [wt%]</p></th></tr><tr><td><p class="s6">Argonor N92</p></td><td><p class="s6">glass, ceramics</p></td><td><p class="s6">paint brush, spray gun</p></td><td><p class="s6">530 - 650</p></td><td><p class="s6">Viscosity</p><p class="s6">500 – 1.000 mPa·s, good solderability</p></td><td><p class="s6">65</p></td></tr><tr><td><p class="s6">Argonor</p></td><td><p class="s6">glass,</p><p class="s6">ceramics</p></td><td><p class="s6">screen</p><p class="s6">printing</p></td><td><p class="s6">530 - 650</p></td><td><p class="s6">Viscosity</p><p class="s6">10 – 15.000 mPa·s, good solderability</p></td><td><p class="s6">65</p></td></tr></table></figtable> === Conductive Paints and Adhesives=== Conductive paints are precious metal preparations in liquid or paste form. They contain the metal filler material, fine silver powders particles as conductive pigments mostly in flake form, apaint compound on artificial resin basis and an organic solvent (<xr id="tab:Silver Paints, Conductive Pastes, and Conductive Adhesives"/><!--(Table 8.3) spherical b-->). The solvent evaporates during drying in air or by aging at slightly elevated temperatures. This allows the silver particles to connect metallically and form conductive paths (<xr id="fig:Flexible keyboard contact pattern printed with AUROMAL 170"/><!--(Fig. 8.3)-->). 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%) rounded crystal applomerates. 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]]</figure><br style="clear:both;"/>

Table 8.1: Different Types of Silver Powders<figtable id="tab:DifferentTypesofSilverPowdersSilver Paints, Conductive Pastes, and Conductive Adhesives"> Different Types of Silver Powders<table border="1" cellspacing="0" style="bordercaption>'''<!--collapseTable 8.3:collapse" value="<caption-->Different Types of Silver PowdersPaints, Conductive Pastes, and Conductive Adhesives'''</caption>"><tr><td><p {| class="s6twocolortable">Powder type</p></td><td><p classstyle="s6text-align: left; font-size: 12px">GE|-!Preparation !Substrate<br /p>Material!Application by!Drying<br /td>[°C]!Properties!Usage Amount<td><p class="s6">GN1<br /p><[g/td>100 cm<tdsup><p class="s7">ES2</psup>]!Area Resistance<br /td><td><p class="s6">V<[Ω/p>m</tdsup>2</tr><tr><td><p class="s6">Manufacturing Process</p></td><td><p class="s6">chemical</p></td><td><p class="s6">chemical</p></td><td><p class="s7">electrolytic</p></td><td><p class="s6">atomized</p></td></tr><tr><td><p class="s6">Particle shape</p></td><td><p class="s6">agglomerated</p></td><td><p class="s6">agglomerated</p></td><td><p class="s7">dentritic</p></td><td><p class="s6">spherical</p></td></tr><tr><td><p class="s6">Avg. particle diameter</psup><p class="s6">(median) [µm]</p></td><td><p class="s6">10 |- 15|AROMAL 38|glass, plastics|spraying, immersion,<br /p>paint brush|RT,<br /td><td>30 min<p class="s6">20 - 40<br /p>100°C|hard well conducting<br /td><td><p class="s7">Ag layer for broad applications|0.5 -2|</p></td><td><p class="s6">32 0.1|- 60|AROMAL 50|glass, wax, plastics|spraying, immersion,<br /p>paint brush|10 min<br /td>RT|very flat surface,<br /tr><tr><td><p class="s6">Medium particle size</p><p class="s6">(FSS especially for electrolytic build- Fisher Sub Sieve Size) [µm]</p></td><td><p class="s6">up|0.5 -2|</p></td><td><p class="s6">-</p></td><td><p class="s7">4.0 - 6.0</p></td><td><p class="s6">2|-|AROMAL 70T|plastics|tampon printing|60 min<br /p>RT|hard and well conductive coating||</td></tr><tr><td><p class="s6">Tap density</p><p class="s6">(DIN/ISO 3953) [g/cm<span class="s8">3</span>]</p></td><td><p class="s6">0.7 1|- 1.1|AROMAL 141|plastics,<br /p></td><td><p class="s6">2.0 paper- 2.5based plastics|screen printing|45 min<br /p>120°C|mechanically <br /td>very strong coatings||<td><p class="s7">20.0 05|- 3.0|AROMAL 170|plastics|screen printing|30 min<br /p>100°C|flexible layers,<br /td>well suited for foil materials||<td><p class="s6">40.0 05|- 6.7|AROMAL K 5 A+B|metal, glass|dispenser,<br /p>screen printing|24h RT,<br /td>3h<br /tr>80°C|mechanically very strong<tr><td><p class="s6">Specific surface area<br /p>bond connection<p class="s6">(B.E.T.) [m<span class="s8">2<br /span>/g]</p></td><td>as alternative to soldering||<p class="s6">0.5 1|- 0.9|AROMAL K 20|metal, plastics,<br /p>ceramics|dispenser,<br /td>screen printing|15 min<td><p class="s6">-<br /p>150°C|flexible bonds which help<br /td>decrease thermal stresses||<td><p class="s7">0.1|-</p></td><td><p class|DOSILAC|colspan="s66">-</p></td></tr></table>|Silver conductive paints in spray cans; can be spray painted; properties similar to those of AUROMAL 50|}

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. 8.2 Precious Metal Preparations=4)-->). <figure id="fig:Shear force of an adhesive joint">[[File:Shear force of an adhesive joint.jpg|left|thumb|Figure 4: Shear force of an adhesive joint (silver adhesive AUROMAL K 20) as a function of the hardening temperature]]</figure> <br style="clear:both;"/>

===8.2.1 Precious Metal Firing Preparations===The firing To obtain certain desired physical properties of preparations , the dispersed precious metals in liquid or paste form flat flake-like particles (generally called "flakes") are needed. These are widely used in electrical and electronic engineering and especially produced by milling fine metal powders in the thick-film technology ''(Table 8.2)''presence of milling additives or agents. The precious properties of these metal filler material is mostly pure flakes, i.e. silver because of its high flakes (ability to disperse easily, flow characteristics, electrical conductivity. During firing in an oxidizing atmosphere at temperatures between 400 ) are strongly dependent on the particle shape and 850°C a size as well adhering and highly conductive surface layer is formedas on the type of milling agents used.When utilizing screen printing techniques any shapes <xr id="fig:SEM photos of conductive patterns can be created ''(silver flakes a fine grain b large flat"/><!--Fig. 8.25--> 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)'' resulting in conductive paths (b). Typical commercial silver flake types are listed with good electrical their respective properties in <xr id="tab:Typical Commercial Silver Flake Types"/><!--(Tab. 8.4)-->. Gold and high temperature stabilityplatinum 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|left|thumb|Figure 5: SEM photos of silver flakes (a) fine grain (b) large flat]]</figure>

Fig. <figtable id="tab:Typical Commercial Silver Flake Types"><caption>'''<!--Table 8.54: SEM photos of silver flakes (a) fine grain (b) large flat-->Typical Commercial Silver Flake Types'''</caption>

Table {| class="twocolortable" style="text-align: left; font-size: 12px;width:80%"|-!Type of Flake!F56!B190!ES4|-|Main characteristics|Low tap density|Very fine|Pure, wide grain size distribution|-|Silver content [wt%]|> 99.0|> 99.0|> 99.7|-|Med. Grain size [μm] Tap density|3 - 8|4 - 6|9 - 13|-|DIN/ISO 3953 [g/cm³]|0.4: Typical Commercial Silver Flake Types7 - 1.1|2.1 - 2.7|2.7 - 3.6|-|Spec. Surface area B.E.T. [m²/g]|0.7 - 1.1|0.3 - 0.7||}</figtable>

<table border="1" cellspacing="0" style="border-collapse:collapse"><tr><td><p class="s6">Type of Flake</p></td><td><p class="s6">F56</p></td><td><p class="s6">B190</p></td><td><p class="s6">ES4</p></td></tr><tr><td><p class="s6">Main characteristics</p></td><td><p class="s6">Low tap density</p></td><td><p class="s6">Very fine</p></td><td><p class="s6">Pure, wide grain size distribution</p></td></tr><tr><td><p class="s6">Silver content [wt%]</p></td><td><p class="s6">&gt;99.0</p></td><td><p class="s6">&gt;99.0</p></td><td><p class="s6">&gt;99.7</p></td></tr><tr><td><p class="s6">Med. Grain size [µm] Tap density</p></td><td><p class="s6">3 - 8</p></td><td><p class="s6">4 - 6</p></td><td><p class="s6">9 de:Edelmetallpulver_und_- 13</p></td></tr><tr><td><p class="s6">DIN/ISO 3953 [g/cm<span class="s8">3</span>präparate]</p></td><td><p class="s6">0.7 - 1.1</p></td><td><p class="s6">2.1 - 2.7</p></td><td><p class="s6">2.7 - 3.6</p></td></tr><tr><td><p class="s6">Spec. Surface area B.E.T. [m<span class="s8">2</span>/g]</p></td><td><p class="s6">0.7 - 1.1</p></td><td><p class="s6">0.3 - 0.7</p></td><td/></tr><tr><td/><td/><td/><td/></tr></table>