Production of Silver Powders

Silver powders are used in a variety of electrical, electronic, and industrial applications. In powder metallurgy, silver powders are used in the manufacture of electrodes for primary batteries and storage cells, and as mixtures with other metals that are used in the preparation of electrical contacts and other P/M parts containing silver. These mixtures include silver-tungsten, silver-iron, silver-tungsten carbide, silver-molybdenum, silver-cadmium oxide, silver-graphite, and silver nickel (Ref 5). These electrical applications generally require a dense, coarse silver powder of high purity with good compressibility. Average particle sizes vary from approximately 10 to 100 /'m.

In the electronics industry, silver powders are used in conductive inks, pastes, and adhesives. Conductive silver inks were probably the first thick-film inks to be developed and are used in the manufacture of capacitors, potentiometers, printed circuits, and other electronic parts. Silver pastes for glass substrates are heated up to 650 °C and are used in such applications as fluorescent character display tubes, plasma displays, heaters for kettles, hot trays, automobile antennas, and heated automobile windows (defoggers). Lower-temperature silver pastes containing cured polymers are used as conductive films for electromagnetic shields, undercoat for copper plating, membrane switches, flexible substrates, and amorphous solar cells. Silver pastes are also used as conductive adhesives for connecting leads to terminals and components on substrates or components to components. Examples include die-attach materials, lead wire outlets of quartz oscillators, silicon die bonding, and bonding of sliding carbon brushes to the lading springs (Ref 2).

The value of using silver is that it is the least expensive precious metal that is compatible with the normal thick-film process and can be made to have good bond strength with very high conductivity. Although its leach resistance to solder can be poor, slight modifications to the solder can help overcome this problem. Silver compositions are also easily compatible with resistor and dielectric systems. The major disadvantage of silver is its strong tendency to migrate over the surface of insulators and resistors when subjected to electrical fields under conditions of higher humidity. The rate of migration is considerably reduced in silver alloys, but migration is always a concern with silver-bearing conductors (Ref 6).

Generally, silver powders used in electronic processes are chemically precipitated, exhibit high purity, and are small in size. Average particle sizes range from 10 /'m to below 1 /'m. Silver powders that are mechanically milled into flat powders or silver flakes are also used to provide more surface coverage of the substrate. These flakes are well suited for conductive coatings and shielding materials and are mixed with silver powders in conductive ink systems.

Other industrial applications of silver powders include water purification, catalyst manufacture, dental amalgams, and preparation of silver alloys. In addition, silver powders are used as a protective coating against hostile industrial environments and as a source of very-high-purity silver (99.99+% pure).

Most of the methods currently used to manufacture metal powders can be applied to the production of silver powders. Chemical, physical (atomization and mechanical), thermal decomposition, and electrochemical processes are used industrially to make silver powders. These powders can be characterized by physical properties including surface area, tap density, particle size, particle size distribution, and shape.

Chemical Processes. Most silver salts are easily reduced from solution, with limitations imposed only by their solubilities. The most common salts are silver nitrate and silver oxide. Organic reducing agents such as alcohol, sugar, or aldehydes are used to manufacture silver powder from silver oxide. For example, dextrose can be used to produce a very fine silver powder:

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