Effects of Electrolyte Composition

The composition of the electrolyte is a major factor in the production of powder. Copper concentration in the electrolyte must be sufficiently low to prevent adherent deposits. In the desired range, current efficiency rises with increasing copper ion concentration, as shown in Fig. 7, with a maximum of 92% in the range of 23 to 33 g/L (3 to 4.4 oz/gal) copper. Above approximately 33 g/L (4.4 oz/gal), current efficiency decreases, and a hard deposit is produced instead of powder. Apparent density and particle size also increase with a rise in copper ion concentration.

Copper concentration in electrolytic bath, oz/gal 2 3 4

-

T

Current

/

. efficiency

/

/

V

\ X 1

paren

: dens

tv

Copper concentration in elecirolytic baih, g/L

Fig. 7 Effect of copper concentration on current efficiency and apparent density. Source: Ref 1

Acid Concentration. A high acid concentration favors the formation of powder. As shown in Fig. 8, current efficiency increases to a maximum at a concentration of 120 g/L (16 oz/gal) of sulfuric acid, then gradually falls as the acid concentration increases. Continued rise in acid concentration leads to passivity. Apparent density decreases with increasing acid concentration.

Fig. 8 Effect of acid concentration on current efficiency and apparent density. Source: Ref 1 and 14

Addition Agents. The copper sulfate/sulfuric acid electrolyte occasionally is modified to alter powder characteristics. The addition of colloidal materials, such as glue or glucose, leads to the formation of fine powder deposits, possibly because the colloid retards the evolution of hydrogen at the cathode. Table 4 indicates the effects of a number of additions to the electrolyte.

Table 4 Effect of addition agents on current efficiency and particle size

Test No.

Addition Agent

Solution

Voltage,V

Current

Sieve analy

sis, %

strength, %

efficiency, %

-200 mesh

-300 mesh

1

1.0

95.9

74.6

55.0

2

Boric acid

0.5

1.0

95.2

100

3

Glucose

0.5

1.2

85.4

100

4

Glycerine

0.5

1.9

94.7

100

5

Glue

0.5

1.5

94.5

100

The addition of surfactants is reported to yield a powder with controllable particle size at a current density of 215 A/m2 (20 A/ft2), in contrast with the 700 to 1100 A/m2 (64 to 100 A/ft2) normally used, which results in considerable reduction in power cost (Ref 2). Small quantities of copper chloride have been added to the electrolyte to increase the dendritic character of the powder particles and to increase the yield of fine powder due to the polarizing effect of the chloride ions (Ref 3). The addition of sodium sulfate reportedly reduces the cathode current density, and as the sulfate content is increased, the powder becomes finer (Ref 4). By contrast, replacement of the normal sulfuric acid electrolyte by a sulfamate electrolyte favors the formation of a coarse copper powder (Ref 5).

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