Precious Metal Powders and Preparations

Precious Metal Powders

Precious metal powders are used as raw materials for many technical products as well as for medical and decorative applications. Among these are the manufacture of composite silver materials 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 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.

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 depending on the morphology of the particles and their tendency to agglomeration. Precious metal powders can be compacted by pressing and then sintered; 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

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Fig. 81: Different shapes of silver powders a) spherical b) rounded crystal applomerates

different properties as shown in Table 1 and 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. Fig. 8.1 shows typical SEM photo-graphs of atomized silver powder in spherical shapes (a) and a cementation powder composed of rounded crystal agglomerates (b).

Table 1: Different Types of Silver Powders

Powder type	GE	GN1	ES	V
Manufacturing Process	chemical	chemical	electrolytic	atomized
Particle shape	agglomerated	agglomerated	dentritic	spherical
Avg. particle diameter (median) [µm]	10 - 15	20 - 40	-	32 - 60
Medium particle size (FSS - Fisher Sub Sieve Size) [µm]	-	-	4.0 - 6.0	-
Tap density (DIN/ISO 3953) [g/cm3]	0.7 - 1.1	2.0 - 2.5	2.0 - 3.0	4.0 - 6.7
Specific surface area (B.E.T.) [m2/g]	0.5 - 0.9	-	-	-

Precious Metal Preparations

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 (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 (Fig. 8.2) resulting in conductive paths with good electrical properties and high temperature stability.

Table 8.2: Liquid Silver Preparations for Firing Application (ARGONOR)

Preparation	Substrate Material	Application by	Firing Temperature [°C]	Properties	Silver Content [wt%]
Argonor N92	glass, ceramics	paint brush, spray gun	530 - 650	Viscosity 500 – 1.000 mPa·s, good solderability	65
Argonor	glass, ceramics	screen printing	530 - 650	Viscosity 10 – 15.000 mPa·s, good solderability	65

Fig. 8.2: Solar cell with print pattern of ARGONOR N920

Conductive Paints and Adhesives

Conductive paints are precious metal preparations in liquid or paste form. They contain the metal filler material, fine silver particles as conductive pigments mostly in flake form, a paint compound on artificial resin basis, and an organic solvent (Table 8.3). 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 (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 electrically 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.

Table 8.3: Silver Paints, Conductive Pastes, and Conductive Adhesives

Preparation	Substrate	Application	Drying	Properties	Usage	Area
	Material	by	[°C]		Amount [g/100 cm²]	Resistance [S /m²]
AROMAL 38	glass, plastics	spraying, immersion,	RT, 30 min	hard well conducting	0.5 - 2	< 0.1

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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 (Fig. 8.4).

Fig. 8.3: Flexible keyboard contact pattern printed with AUROMAL 170

Fig. 8.4: Shear force of an adhesive joint (silver adhesive AUROMAL K 20) as a function of the hardening temperature

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. 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 Table 8.4. Gold and platinum can also be produced as powder flakes. By volumes used they are however of lesser commercial importance.

Fig. 8.5: SEM photos of silver flakes (a) fine grain (b) large flat

Table 8.4: Typical Commercial Silver Flake Types

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]	0.7 - 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