محاضرة رقم 6

2.2. Physical Methods
Under such methods the electrolytic method and atomization are prevalent.
2.2.1 Electrolytic Method
It is used extensively in the preparation of copper, beryllium, iron and nickel powders. Adjustment of the chemical and physical conditions during electrodeposition makes it possible to cause the metal to deposit loosely on the cathode of the cell either as a light cake or in flake form. Both are readily crushed to a powder. The method yields a high purity metal with excellent properties for conventional powder metallurgy processing. The process involves the control and manipulation of many variables and in some cases is significantly more costly than other techniques. For example, electrolytic iron powder is more costly than reduced or atomized powder with the same characteristics, while electrolytic copper powder is quite competitive with reduced and precipitated types.
The following are the factors promoting powdery deposits: (a) high current densities; (b) weak metal concentrations; (c) additions of colloids and acids; (d) low temperature; (e) high viscosities; (f) avoidance of agitation; (g) suppression of convection.
It may be very difficult to produce a high purity powdery deposit at relatively fast rates economically. Consequently, in many cases the deposit is a solid and must be pulverized, for example iron. Electrolytic deposits, powders or solids are usually very reactive and brittle. For both these reasons the material may be given a special annealing treatment. Powders formed during electrolysis have a characteristic dendritic shape; however, this could be changed substantially due to subsequent processing.