Powder properties can be significantly influenced by the addition of a third element. This can be used industrially for the production of special W grades, as mentioned below.
Tungsten blue oxide is treated with aqueous solutions of the respective element compounds.
Sodium Doping. Addition of sodium compounds (concentrations equal to 50-200µ g/g Na) is a usual practice in producing coarse tungsten powder. At 1000℃ and wet conditions, average grain sizes of 10-25µm (depending on the layer height) can be attained. The high temperature guarantees the extensive evaporation of sodium at the end of the reduction process. Only for special purposes must a subsequent acid or water leach be applied to decrease the residual sodium concentration.
Lithium Doping. Lithium compounds are preferred to sodium for very coarse tungsten powder grades (50-100µm). Reduction conditions are the same as above. Due to the high lithium boiling point, it is not volatilized during reduction Iike sodium . The powder has to be leached by hydrochloric acid and water subsequent to reduction. Finer fractions are separated by screening.
Chromium Doping. Additions of chromium compounds can be used to produce very fine metal powder grades (<0.5µm). Chromium oxide is formed during reduction and acts as a grain refiner.
Such powders are subsequently carburized to ultrafine WC. Special precautions have to be taken because of the high pyrophoricity of the metal powder.
NS or AKS Doping. For the production of lamp filaments (incandescent or halide lamps), a special doping procedure is carried out which is commonly called NS or AKS doping. NS stands for the non-sag properties of the coiled lamp filament, while AKS represents the doping elements used in this process: Al, K and Si.
TBO is doped with about 3000-3500µg/g K, 2000-2500µg/g Si, and 400-500µg/g Al in the form of potassium silicate and aluminum chloride or nitrate solutions.
In reduction of the doped oxide, part of the dopants is incorporated during the CVT growth of the metal particles in the form of silicates. Excess dopant, which remains on the tungsten crystal surfaces, is removed by subsequent leaching of the powder in HCI and HF .acids, while the dopants which are internally trapped are trapped. The amount of incorporated dopants, the size of the inclusions, and their chemical composition can be influenced by the reduction conditions within certain limits. About 100-150ppm K, 60 -100ppm/ Al, and 200-300 ppm Si commonly remain in the acid-washed powder.
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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