Average particle size and particle size distribution: "As supplied" tungsten powder is always more or less agglomerated, depending on reduction conditions. Therefore, the measured value for the average particle size in this stage does not always correspond to the real particle size and can even be misleading. For example, certain submicron powders show "as supplied" grain sizes of 1-2 µm, but after deagglomeration these values drop to 0.4-0.5µm.
For production control in the range of 1-10µm, the "as supplied" value mostly suffices, because production conditions are kept constant within close limits. However, for submicron-sized powders as well as for a correct characterization of coarse powder, only the "lab milled" powder should be used.
Normally, the particle size distribution is a function of particle size. The bigger the particle size, the broader the distribution. For a given particle size, the distribution can be made narrower during production by application of wet hydrogen and/or doping the oxide with alkali compounds.
Particle size distribution measurements should also be made from "lab milled" samples only. The particle size distribution influences the compacting behavior and green density as well as the sintering properties.
Much effort was expended in the past to elucidate the dependence of the compactability of tungsten powders on grain size and grain size distribution. Low grain sizes result in too low green densities due to high friction between the particles. The closer the grain size distribution, the poorer the particle packing. So it is evident that broader grain size distributions, or even blends of powders having different average grain sizes, result in better packing and higher strength of the green compacts.
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