In industrial practice, the boats are loaded with a certain oxide weight (layer height) and pushed through the furnace with a given temperature profile and hydrogen throughput. After dynamic equilibrium is reached, the particle size of the metal powder is measured. If the powder does not meet the requirements, parameter adjustments such as change in temperature, boat load, hydrogen throughput, or push time are introduced.
Subsequent to reduction, the powders are screened on 60 mesh (sometimes also on 200 mesh) to eliminate contaminants stemming from furnace or boat materials and are blended to form a homogeneous powder batch. No special atmosphere is necessary for handling, since the powder surfaces are rapidly saturated with oxygen and water vapor. However, below 1 µm, the powders may be pyrophoric and precautions are necessary, in particular below 0.5 µm. Reduction under concurrent hydrogen flow is the most effective method to avoid burning of the fine powders. Already, during the cooling stage in the furnace, the powder is contacted to “wet” hydrogen, and the surface is saturated when the powder leaves the furnace. Under countercurrent flow conditions, the powder has to be slowly saturated with oxygen. This can be achieved either by an inert gas storage (nitrogen or argon containing small amounts of oxygen) or by exposing the powder to the atmosphere in small portions in order to omit local overheating. This can be done by leaving the powder in the boat for approximately 30 min.
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