Commercial Practice

Liquid copper is superheated to approximately 1150 to 1200 °C (2100 to 2200 °F), utilizing flow rates of 27 kg/min (60 lb/min) or more. Generally, to produce a predominantly -100 mesh powder, water pressures of 10 to 14 MPa (1500 to 2000 psi) are used. Atomization may be conducted in an air or inert (nitrogen gas) atmosphere.

During melting of the copper, impurity content must be controlled to achieve good fluidity and high conductivity (Fig. 1). The need for oxygen control at this stage of the process depends on subsequent processing and end use of the powder. In conventional refining of copper, poling and steam-generated hydrogen keep copper oxidation under control in accordance with the equilibrium curves shown in Fig. 19. High oxygen content tends to produce a more irregular powder, and subsequent reduction of the atomized powder further improves compactibility through agglomeration and pore generation. If the powder is used in the as-atomized condition, lower oxygen contents are generally preferred, because of the detrimental effects of oxygen in many applications.

3S

5 lorr

1 23

1 torr.

1

\\

92.5 torr

Fig. 19 Effect of partial pressures of hydrogen and steam on oxygen content of liquid copper at 1150 °C (2100 °F). Cu + H2 < >2Cu + H20 reaction at various water vapor pressures

The apparent densities of -100 mesh gas-atomized copper powders, as a consequence of their spherical particle shape, range between 4 and 5 g/cm3. Oxygen picked up during atomization is present partly as surface oxide and partly as copper oxide throughout the bulk of a copper particle. Removal of oxygen requires reduction temperatures of approximately 700 °C (1290 °F) or higher. At these temperatures, considerable sintering occurs, which in turn requires substantial milling of the sinter cake.

During reduction, hydrogen readily diffuses through solid copper to react with oxygen and form steam. The large steam molecules, unable to diffuse through solid copper, force their way outward through grain boundaries--phenomenon known as hydrogen embrittlement of copper that manifests itself in the formation of blisters or cracks. Figure 20 illustrates the grain boundary widening of air-atomized copper particles due to this phenomenon. These defects improve both compactibility and sintering rate during liquid phase sintering of copper mixed with tin.

Fig. 20 Hydrogen-embrittled, air-atomized copper after reduction in hydrogen Alloying Additions

Some applications of copper powders require apparent densities lower than those attainable with water atomization of pure copper. These powders can be produced by addition of small amounts, up to 0.2%, of certain elements (for example, magnesium, calcium, titanium, and lithium) to the liquid copper prior to atomization (Fig. 21 and 22). These metals are believed to decrease the surface tension of copper and/or to form thin oxide films on the particle surface during atomization. Magnesium additions are most frequently used to produce compacting-grade copper powders for applications such as bronze bearings, filters, structural parts, and additives for iron powders. These powders may have apparent densities as low as 2 g/cm3.

Addilive, wt%

Fig. 21 Effect of additions to the molten copper on apparent density of atomized copper powder

Fig. 22 Scanning electron micrograph of water-atomized copper containing 0.5% Li

The addition of small amounts (0.1 to 0.3%) of phosphorus to the liquid copper, prior to atomization, allows the production of a powder that is very spherical and very low in oxygen. During atomization, even with air, the phosphorus oxidizes preferentially and forms protective gaseous phosphorus pentoxide (P2O5). Such powders have apparent densities up to approximately 5.5 g/cm3. Spherical copper powders with closely controlled particle size ranges are used in applications such as thermal spray coatings, metal impregnated plastics, and heat exchangers. Irregular copper powders are used in compacting applications such as bronze mixes for self-lubricating bearings, additions to iron mixes, friction materials, electrical brushes, diamond cutting wheels, and electrical parts requiring high strength and electrical/thermal conductivity. Irregular copper powders are also used in copper brazing pastes and various chemical applications such as catalysts and in the production of copper compounds. Table 6 lists the properties of typical commercial atomized copper powders. The specific surface areas of these powders are from 0.02 m2/g for coarse spherical gas atomized powder, to 0.2 m2/g for fine water atomized powder.

Table 6 Properties of typical commercial grades of water and gas-atomized copper powders

Chemical

properties

Physical properties

Copper,

Hydrogen

Acid

Apparent

Sieve analysis, % Tyler

% min

loss, % max

insoluble, % max

density, g/cm3

60 +80

+100

+150

+200

+325

+325

99.0(a)

NA

NA

4.5-5.5

5 max 30-60

30-60

15 max

99.0(a)

NA

NA

4.5-5.5

. . . 2 max

20-50

50-75

10 max

trace

98.5(a)

0.7

NA

4.5-5.5

. . . trace

0.2 max

5 max

2 max

bal

60-90

98.5(a)

0.7

NA

4.5-5.5

0.5 max

bal

95 min

99.3(b)

0.3

0.1

2.5-2.7

0.8 max

35 max

70 max

bal

5 max

99.3(b)

0.3

0.1

2.5-2.8

1 max

20 max

25 max

40 max

30-45

99.3(b)

0.3

0.1

2.5-2.8

0.5 max

10 max

20 max

bal

42-55

99.3(b)

0.3

0.1

2.8-3.0

trace

1 max

15 max

bal

55-65

99(c)

0.35

NA

2.1-2.4

5 max

15-25

10-20

15-35

20-40

99(c)

0.35

NA

2.3-2.6

1 max

10 max

5-20

15-30

60-70

99(b)

0.5

0.1

2.1-2.5

1 max

3 max

14 max

85 min

(a) Air atomized.

(b) Water atomized/annealed.

0 0

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