During sintering, the silicates dissociate thermally. The smaller Al, Si, and O atoms ,escape via different through the tungsten crystal lattice while the large K atoms remain in the form of potassium bubbles. It is this potassium that enables the formation of rows of smaller potassium bubbles during wire fabrication, and consequently the formation of a long-grained interlocking microstructure during subsequent filament operation, which is the key to the non-sag characteristics .
The grain-growth-enhancing effect of K makes it possible to conduct the reduction at a lower temperature than normally applied for W powder of 3-5 µm average grain size . The hydrogen gas has to be kept separated from hydrogen for reduction of undoped powder, because it is contaminated by K.
NS-W powders exhibit a characteristic morphology, exhibiting holes in the crystal surface, which form during HF-washing of the powders due to the dissolution of partly intergrown silicate phases.
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