Silver powder and flakes

Figure 2.61. Silver powder and flake. Courtesy of Technic, Inc., Woonsocket, RI, USA.

Technic, Inc. Engineered Powders Division 300 Park East Drive Woonsocket, RI 02895, USA tel 1 401 769 7000 fax 1 401 769 2472 gold powder and flakes silver powder and flakes 91 147... 

A second information is provided by the market of precious metals for electronics applications. In the United States, the largest volume in 1992 was for silver powders and flakes with 249 tons, second were silver-palladium alloys with 31 tons, followed by palladium, gold, and platinum with 6.2, 3.1, and 0.8 tons, respectively. The paste and ink formulations loaded with 60-80% of precious metals included a metal value exceeding 180 million, and the added value for the producers of powders and flakes was approximately 30-32 million. The companies making the conductive pastes added approximately 100 million to the final products. This means that in the US alone the total market was over 300 million for conductive adhesives and pastes, inferring a worldwide production worth over 1 billion dollars. The most important manufacturers of multilayer ceramic capacitors and other discrete devices using large quantities of conductive materials are located in Japan and South-East Asia. 

Solid glass beads 2.46-2.54 Silver powder and flakes 10.5... 

Aluminium, copper, nickel and silver powders or flakes are used to obtain EMI (electromagnetic interference) grades. The other properties - colour, modulus, impact strength. .. are modified. 

Names silver powder, silver flakes, atomized silver powder, silver/palladium powder and flakes... 

More heat-resistant thermoplastic resins are usually preferred in polymer blends which contain metallic fillers such as iron, copper, aluminum and silver powder or flakes. At least six factors must be considered when preparing these types of polymer-metal composites (1) particle size ratio, (2) particle shape, (3) prepolymer viscosity, (4) electrostatic attraction, (5) oxide layer thickness, and (6) polymer bead shear. Another factor to be considered in blending is the difference in density between the organic polymer and metallic filler. 

Polymeric Conductive Formulations. The conductive systems used in polymerie formulations inelude C, Ag, Ni, Cu, and various mixtures. Table 8.5 illustrates typical sheet resistivities for various metal powders and flakes. Silver flakes produce the most conductive system, since silver oxide remains conductive and exhibits slow oxidation kinetics, resulting in wide processing latitude with respect to curing cycles. Base metals oxidize in air and, therefore, require very limited curing profiles. As seen in Table 8.3, polymeric conductors have reasonable corrosion resistance and can be used for organic die attachment applications. However, wire bonding and solderability are typically poor for these systems. 

Silver coated glass beads 0.6-0.8 Aluminum powders and flakes 2.7... 

Alumina particles are one of the commonly used fillers for improving the thermal conductivity of adhesives in particular insulation adhesives. Aluminum and silver powders or flakes are used to improve the thermal and electrical conductivities for adhesives intended to be an electrical or thermal path. The filler volume content level is very important to get sufficient conductivity. However, excessive filler content might cause degradation in mechanical properties of the adhesives (Kahraman and Al-Harthi 2005 Kahraman et al. 2008). 

Metallic Powders. These are usually either aluminum or bronze flakes and vary ia shades from silver to gold, depending on the composition of the metal used. The silver powders can also be toned with organic pigments to produce golds or copper shades usiag transparent yellow or red pigments. 

For many electronic and electrical appHcations, electrically conductive resias are required. Most polymeric resias exhibit high levels of electrical resistivity. Conductivity can be improved, however, by the judicious use of fillers eg, in epoxy, silver (in either flake or powdered form) is used as a filler. Sometimes other fillers such as copper are also used, but result in reduced efficiency. The popularity of silver is due to the absence of the oxide layer formation, which imparts electrical insulating characteristics. Consequently, metallic fibers such as aluminum are rarely considered for this appHcation. 

Properties Silver-white, lustrous, malleable metal. Sp. gr. 1.74. M. P. 650°. B. P. 1120°. In powdered or flake form it burns readily with a brilliant, bluish-white light and with evolution of great heat of combustion. Powder liberates hydrogen when iu contact with water. 

Metal effect pigments are flake-shaped metallic particles supplied in the form of powders, pastes, pellets, suspensions or color concentrates [5.166-5.168]. Typical metal effect pigments include aluminum ( silver bronze ) and copper- and copper/zinc-alloys ( gold bronze ). 

Dry bulk resistivity, O-cm 0.005-0.008 (silver-coated products of PQ), 0.0017 (silver coated solid and hollow glass spheres - Conduct-O-Fil), 0.004 (silver coated glass fiber - Conduct-O-Fil), 0.0005-0.0006 (silver coated copper powder - Conduct-O-Fil), 0.0012 (silver coated copper flake - Conduct-O-Fil), 0.0007 (silver coated aluminum sphere - Conduct-O-Fil), 0.003 (silver coated inorganic flake - Conduct-O-Fil), 0.006 (silver coated nickel granules - Conduct -O-Fil), 0.0000016 - pure silver... 

For EMI shielding applications, numerous processes are used, some require conductive fillers. These applications include parts molded with conductive filler and conductive paints. Conductive fillers used in commercial applications include aluminum, silver, nickel, and copper flakes and powders, stainless steel fibers, and fibers and flakes coated by nickel and silver. Thermoplastic compounds can provide up to 65-70 dB of electromagnetic noise attenuation but obtaining values over 45 dB is difficult. Static dissipative compounds (ESD) are mostly produced with carbon black which accounts for approximately 90% of the market but many other fillers are also used. 

The processing and surface preparation of silver particles is critical to their performance as a filler. The first step in manufacturing silver flakes is to produce silver powder. Powdered silver may be produced by chemical precipitation, electrolytic precipitation, or melt atomization. Most commercially available silver powders are precipitated by reducing silver from a silver nitrate solution. Particle sizes of powdered silver range from submicron to 5 pm." Silver powders are selected based on size and... 

Conductive metals are also advocated. Bekaert Fibre Technologies manufactures stainless steel fibres for vehicle fuel systems and electronic equipment housings in Europe, Bekintex of Belgium markets Beki-Shield GR stainless steel fibres containing a polymeric binder for use in EMI and ESD applications. Very low doses are said to be sufficient. Nickel-coated mica has been suggested as a conductive filler for ABS. Aluminium, copper and silver in the form of either powders or flakes have all been used. 

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