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Write a note on powder preparation methods.
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[A] Metal powder formation: Metal powders are generally pure products. Their degree of purity, particle size and shape depends on the method by which the metal powder is manufactured.

The following methods have been used in powder production.

1) Mechanical Communication (Processes) - This method is applied to metals like antimony which is comparatively brittle in nature such brittle metals can be crushed to a size of 1x 10-4 mm,as they are in angular in shape they are most suitable for powder metallurgy.

Ductile metals give powder on crushing and milling, but the resulting products is in the form of flakes which is not suitable for powder metallurgy.

Mechanical pulverization involves mechanical pulverizes consisting of either counter rotating plate or rapidly moving hammer followed by milling e.g. Method is used for metals like Al, Mg & Pb & Zn

2. Atomization:

Steps- Melting ā€“ Solidification ā€“ Disintegration of melt ā†“ Capillary drop, Liquid atomization & Gas atomization

In this method, liquid metal is forced through a small orifice or nozzle in a stream of air or water & jet of liquid is broken down by blast of compressed gas. Now a day the metal is atomized by striking a rapidly rotating disc. E. g. Al, Mg, Pb Zn Cd & Sn

For example: Ferrochrome which is an alloy of chromium and iron is prepared in alloy formed by reducing ferrochrome iron ore with carbon FeO.Cr2O3 + 4Cā†’ Fe +2Cr +4CO

3. Electrolytic process: When conventional electrolysis is carried out, a powder gets deposited on the cathode surface. The powder is scraped first then washed, dried & then pulverized further to get powder of desired size e.g. Fe, Cu, Ag

4. Decomposition: A typical precipitation method includes preparation of a carbonyl vapors by passing carbon monoxide at a high temperature over the heated metal and producing powder by precipitation. e.g. Fe(CO)5 or Ni(CO)4 on decomposition yield very pure Fe & Ni powders.

5. Electrolytic process: This method involves electroplating principle to obtain metal all conditions are adjusted in such a manner that spongy deposits are obtained at cathodes. These deposits are subsequently ground to powder having particular size. Brittle deposits give angular particles where as spongy deposits are flattened.

6. Decomposition: A typical precipitation method includes preparation of a carbonyl vapours by passing carbon monoxide at a high temperature over the heated metal and producing powder by precipitation. e.g. Fe(CO)5 or Ni(CO)4 on decomposition yield very pure Fe & Ni powders.

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