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.
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