TUNGSTEN METAL POWDER PRODUCTION-GENERAL-6

Agglomeration. One may distinguish between three different types of agglomeration:

Reduction performed at low temperature and dry conditions results in the formation of metal sponges, which are pseudomorphous to the oxide precursor. The very fine crystals(some tenths of a micron or less) stick together in a more or less loose manner. Usually, this type of agglomeration is called pseudomorphology. The finer the crystal size, the higher the strength of these agglomerates.

Bigger tungsten crystals(from approximately 1 to several μm), grown individually grown by CVT, are welded together. This type of agglometeration is looser than the above-described pseudomorphology and occurs mainly under dry reduction conditions. It can be influenced by the dew point of the incoming hydrogen. Higher dew point results in less agglomeration. The strength of this agglomeration type increases with temperature.

Reduction under wet conditions and high temperature (high concentration of [WO₂(OH)₂] in the vapor phase) results in strongly intergrown crystals .

Agglomeration is closely related (inversely proportional) to the apparent density of the tungsten powder. Correspondingly, the apparent density can be influenced within certain limits by the hydrogen dew point. Agglomeration is a prerequisite for good compactability of the tungsten powder.