Temperature. Temperature influences the rate of all reactions occurring during reduction, hence the dynamic and partial pressure of the volatile [WO2(OH)2] which forms during reduction and which is responsible for the chemical vapor transport (CVT) of tungsten. Temperature and tungsten particle size are directly proportional while temperature and time required for final reduction are inversely proportional.
Oxide Feed. The tungsten mass flow determines the amount of H2O liberated during the entire reduction process. The higher the flow, the larger the grain size.
Tungsten Powder Layer Height. During reduction and accompanying water formation, the powder layer exerts a considerable diffusion resistance against water removal from the layer. The higher the layer, the greater the diffusion resistance and the more slowly the reaction water will be removed. The local humidity is higher at the bottom of growth conditions of the metal particles formed at a particular temperature. The layer height is directly proportional to powder grain size.
Porosity of the Powder Layer. The porosity of the powder layer, and thus its permeability, is determined by the macroporosity (intermediate space between the oxide particles) and by microporosity (porosity of the individual oxide particles). The higher the porosity of the powder layer, the better the material exchange H2O→H2 during reduction and the less the grains of the tungsten particles will grow, resulting in a smaller particle size.
Hydrogen Flow Rate. A higher hydrogen flow enhances the material exchange due to the more rapid removal of water vapor. Therefore, the flow is inversely proportional to the average grain size.
Hydrogen Flow Direction. Concurrent hydrogen flow with respect to the tungsten flow generates a higher dynamic humidity at the later part of the reduction, while counter current flow (which is the standard condition) provides higher humidity during the early reduction stages.
Hydrogen Dew Point. The dew point of the incoming hydrogen influences the overall humidity during reduction. More "wet" hydrogen enhances the tungsten particle growth.
Grain Size Distribution. Grain size distribution is to a great extent the consequence of powder layer height. The growth conditions for the individual particles are different and depend on their position within the powder layer. The humidity is higher in the interior and decrease as one approaches the surface. This gradient results in large grain-sized particles inside and smaller grain-sized particles at the surface-neighboring areas. It is easy to understand that the distribution is broader for high powder layers and closer for lower layer. In any case, the distribution can be improved (made closer) by using "wet" hydrogen, since the water vapor gradient from inside to outside the layer will be decreased.
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.
Morphology. As noted earlier, low temperature and dry conditions largely suppress any CVT of tungsten and lead to the formation of metal sponges, which are pseudomorphous to the oxide precursor (APT, H2WO4). They consist of very fine, polygonal and polycrystalline metal particles. With increasing temperature and humidity, individual tungsten grains form by CVT over comparably large distances. The particles are faceted and commonly exhibit the characteristic shape of the cubic metal. Well-faceted crystals, showing growth steps and being partly intergrown, are characteristic for very humid conditions (high temperature, large powder layer height).
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1.Tungsten Oxide |
