TUNGSTEN METAL POWDER PRODUCTION - Reduction of Doped Tungsten Oxides- 3

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.

Please visit our website www.tungsten-powder.com

TUNGSTEN METAL POWDER PRODUCTION - Reduction of Doped Tungsten Oxides- 2

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.

Please visit ourwebsite www.tungsten-powder.com

TUNGSTEN METAL POWDER PRODUCTION - Reduction of Doped Tungsten Oxides- 1

5. Reduction of Doped Tungsten Oxides

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.

TUNGSTEN METAL POWDER PRODUCTION - Tungsten Powder - 5

Specific surface area. The specific surface area is an important criterion for the sintering activity ( solid state sintering), dissolution processes (liquid phase sintering), and reaction with gaseous or solid substances during carburization. Commonly, it is in the range of 0.01m2/g (coarse powders) up to 12m2/g (very fine powders).

Based on the specific surface area roughness of the particles. In particular, very fine powder particle do not exhibit “smooth” but “rough” surface, or even exhibit a certain degree of microporosity, hence increasing the specific surface area by factor of 2-4.

Apparent and tap density. Both densities increasing with increasing average grain size, but are additionally influenced by the grain size distribution, particle shape, and degree of agglomeration.

Compressibility. This is the ratio of green density to apparent density. The ratio increasing with increasing pressure until a limit is reached. Compressibility of a powder is an important criterion for press and die design.

Please visit our website www.tungsten-powder.com

TUNGSTEN METAL POWDER PRODUCTION - Tungsten Powder - 4

Agglomeration. The different between “as supplied” and “lab milled” (deagglomerated) particle sizes is measure of the degree of agglomeration. Agglomeration is very important for the strength of the green compacts and is therefore a necessary property for powder going into ductile tungsten production.

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.

Compaction and green strength is also influenced by the particle morphology. More irregular shapes, which cause interlocking, improve the strength.

Please visit our website www.tungsten-powder.com if you have any inquiry of tungsten powder.

TUNGSTEN METAL POWDER PRODUCTION - Tungsten Powder - 3

Physical properties. The relevant physical properties are average particle size, particle size distribution, apparent, tap, and compact or green density, specific surface other and can be influenced by the oxide properties and reduction conditions. They represent the important criteria for further processing and are responsible for the compactability, sintering behavior, dissolution reactions during liquid-phase sintering, and carburization reaction.

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.

Please visit our website www.tungsten-powder.com to get more information.

TUNGSTEN METAL POWDER PRODUCTION - Tungsten Powder - 2

Chemical properties: Purity. The purity of tungsten powder is of particular importance in PM manufacturing of tungsten metal, since during subsequent sintering further purification through evaporation is only possible to a certain extent. The demand for purity of tungsten powder has increased steadily during the last three decades. Considerable improvements in hydrometallurgy have led to concentrations fairly below 10 µg/g for most of the elements. This trend with time can be demonstrated by comparing today’s usual specifications with those given in the last book on “Tungsten” by Yih and Wang, published in 1979.

The reason for this enhance demand for powder purity originates in the fact that remaining impurities after sintering greatly affect the workability and properties of the final product.

By considering the impurity-to-tungsten ratio on the long path from ore concentrates to compact tungsten metal, one observes a constant increase in purity up to the stage of APT crystallization. At this stage, the maximum purity is more or less reached. Consequently, the purity of the tungsten powder depends mainly on the APT cleanliness. During APT processing to tungsten powder, the purity already decreases again. Sources of contamination are contacts with metallic tubes or boats in the respective furnaces. Slightly enhance, overall concentrations of the elements, such as Fe, Ni, Cr, and Co, are the consequence. This type of impurity occurs heterogeneously and represents small areas of locally high concentrations of foreign elements. If they are big enough, they might act as the origin of sintering defects.

Moreover, volatile elements or compounds present in the hydrogen atmosphere can be adsorbed by the tungsten powder during reduction. Typical examples are alkali metals.