AgN03 NaC02H Ags NaN03 C02g 2 H2g

These powders are coarse in size, resembling those made by electrochemical methods. Average particle sizes vary from less than 1 to 20 .i,!m. These powders exhibit good flow and good sintering characteristics. Chemically precipitated silver powders can be made in a range of sizes and shapes. Examples are shown in Fig. 1, 2, and 3.

Fig. 1 Irregular-shaped silver powder
Fig. 2 Small-sized silver powder
Fig. 3 Spherical-shaped silver powder

Electrochemical Processes. Several electrochemical processes are used to manufacture silver powders. Galvanic reduction precipitates silver powder from a solution by a more anodic metal such as zinc, copper, or iron (Fig. 4). This process produces an irregularly shaped powder with a free bulk density ranging from 1.5 to 4.0 g/cnr. Particle sizes of 100 /' m or more are produced by galvanic reduction. Some galvanically reduced powders exhibit good flow characteristics.

Fig. 4 Galvanic reduction

In electrolytic reduction, an electrical current passes through an electrolyte at room temperature to produce a crystalline silver. Current densities vary from 215 to 2150 A/m2 (20 to 200 A/ft2), depending on the desired particle size. Silver anodes, usually as cast silver bars or granulated silver, are dissolved. Crystallized silver is then grown on a cathode in an experimental setup such as that shown in Fig. 5. This crystalline silver powder is dendritic and can be very large, with particle sizes ranging from 1 /'m to as large as several millimeters, depending on processing conditions and current densities (Ref 8).

Fig. 5 Electrochemical reduction anodes

Fig. 5 Electrochemical reduction

Atomization and Aerosol Processes. Atomization of a molten stream of silver with gas or liquid is used to produce silver powder. The atomized powder generally approaches a spheroidal shape, with a very smooth surface, minimal porosity, and a very high powder density. Average particle sizes generally are 10 to 50 i'm. and physical classification is usually required to provide a narrow size distribution. The average particle size can be shifted by adjustment of the gas velocity, design of the atomization die, and by control of the metal feed (Ref 5).

Aerosol routes can be used to produce silver powders. Solid, spherical, dense silver powders can be produced by spray pyrolysis (aerosol decomposition). An aerosol of a silver salt solution such as silver nitrate is heated to temperatures as low as 600 °C in nitrogen to decompose the silver salt and produce a fully dense silver powder. Spherical silver powders of different sizes and density can be made by controlling reaction temperature, carrier gas, solution concentration, residence time, and droplet size. Particle size of the silver powders range from 5 to smaller than 0.5 /'m (Ref 9).

Another spray pyrolysis technique for making silver powders involves the spraying of a silver nitrate solution into a flame formed by a coaxial burner. Control of the flame temperature and the spray conditions allow for the control of particle size and shape. By adjusting the flame temperature, particles can be made nonspherical (temperatures below the melting point), spherical (temperatures above the melting point), and ultrafine particles by having the flame temperature sufficiently hot enough to evaporate the silver (Ref 10).

Mechanical Milling Processes. Mechanical comminution can be used to alter most types of silver powders with processes such as ball milling, vibratory milling, or attrition milling. These methods are time consuming and care needs to be taken to keep from introducing significant levels of impurities from the equipment and media. Many types of milling media can be used including glass balls, zirconia balls, and metal balls. Sizes of media range from 10 to less than 1 mm depending on the size of the starting silver powder and the size of the resulting silver flake desired. Initially, larger silver particles and silver agglomerates are broken down during the milling process. Continued milling produces flat, two-dimensional silver flake as shown in Fig. 6. Usually an organic surfactant is introduced during the milling process to prevent the cold welding of the particles to each other during the grinding process and to control the size and shape of the silver flake. Typical surfactants include saturated and unsaturated fatty acids and long-chain aliphatic molecules. By altering the surfactant, solvent, milling media, and milling conditions, specific silver flakes can be designed for particular applications (Ref 11).

Fig. 6 Silver flake

Thermal Reduction. Heat is used to reduce silver salts to produce silver powders. Silver oxide and silver carbonate can be reduced to silver powder by thermal reduction in an inert atmosphere at temperatures greater than 250 °C (480 °F) as follows:

2Ag20 + heat ■ >4Ag(s) + 02(g) Ag2C03 + heat ■ >4Ag(s) + 2C02(g) + 02(g)

These powders tend to be spongy, agglomerated, and very porous and therefore require subsequent mechanical treatment to provide usable silver powder.

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