2018年10月19日星期五

How To Control Tungsten Powder Particle Size With Impurities

The properties of tungsten powders may be seriously affected by the third element.

doped tungsten powder image

In production, these characteristics can be used to add third elements to control the particle size of tungsten powder to produce some special kinds of tungsten oxide. This method is called doping.

When doped, the tungsten oxide is added to the aqueous solution of the element compound. The most suitable for doping is not yellow tungsten nor purple tungsten, but blue tungsten. The commonly doped elements are sodium, lithium, chromium, and rare earth elements.

1. Sodium-doped: The addition of sodium compounds is a common method in preparing coarse tungsten powders, the amount of sodium is usually between 50-200 and m/g. At 1000°C under wet hydrogen conditions, tungsten powders with an average particle size of 10-25 um can be obtained according to the thickness of the material layer, high temperature ensures strong volatilization of sodium at the end of the reduction process, it is only necessary to reduce the residue by acid leaching or immersion in the case of special needs.

2. Lithium-doped: For a variety of tungsten powder applied to grain size, the addition of lithium is better than that of sodium. Due to lithium has a high boiling point, it can not be like sodium which has self volatilization in the reduction, therefore, after the reduction it must be washed with hydrochloric acid and water, and fine particles are separated by sieve.

3. When the ball milling process is adopted, a small amount of organic solvent, alcohol or gasoline can be added to make the tungsten powder and carbon black mixed evenly in the ball crusher.

4. Chrome-added: The addition of chromium compounds favors the production of fine tungsten powder, which produces chromium oxide during the reduction process, it acts as a grain refiner. This fine tungsten powder is then used to carbonize into ultrafine tungsten carbide, because the ultrafine metal powder has a strong spontaneous combustion, in the production process using inert gas protection.

5. The addition of rare earth elements: rare earth lanthanum or yttrium tungsten oxide thin film can be formed on the surface of La crystal lanthanum or yttrium Y thereby preventing gas hydrate tungsten oxide on the surface of tungsten particles precipitate or into the local solid phase chemical reaction. Thereby, the growth of tungsten powder particles is suppressed, and the preparation and reduction of ultrafine tungsten powder are greatly helpful.



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【Know Tungsten】What Is Violet Tungsten Oxide?

Violet tungsten oxide is a kind of tungsten oxide product developed in China in recent years. Its phase composition is WO2.72 or W18O49.


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Hydrogen Reduction Tungsten Powder Particle Aggregation Degree

The hydrogen reduction process of tungsten oxide is a complex system engineering, in the process, the concentration of tungsten powder is also an important physical parameter.

tungsten powder image

It determines the plasticity of the tungsten products, and the good agglomeration is the characteristic of tungsten powder for the production of plastic tungsten products.

Hydrogen reduction tungsten powder particle aggregation types can be divided into three kinds:

1. Under low temperature and dry hydrogen conditions, the sponge metal is reduced and the particle aggregate has the shape of tungsten oxide precursor, very small crystals are bonded together in relatively compact or looser ways, usually the aggregation is called pseudo crystalline, and the smaller the size of the crystal, the higher the concentration of the particles.

2. Particles grown by chemical vapor migration are welded together with larger tungsten crystals. This type of particle occurs mainly during the reduction process and may be affected by hydrogen dew point and less aggregation when dew point is high.

3. Strong crystals of strong intergrowth are obtained by reduction at wet hydrogen and high temperature.

Aggregation is inversely related to the apparent density of tungsten powder, the dew point of hydrogen can influence the apparent density of tungsten powder to a certain extent. Therefore, the proper aggregation degree of tungsten powder is essential to hydrogen reduction of tungsten powder.

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Hydrated Tungsten Oxide

In the process of hydrogen reduction of tungsten oxide, the vapor pressure of tungsten oxide increases obviously when there is water vapor at high temperature.

electron microscopy hydrated tungsten oxide image

This increase in vapor pressure is due to the reaction of various tungsten oxides with water vapor to form a volatile hydrated tungsten oxide. In the W - O - H system, the hydrated tungsten oxide is much more volatile than other volatile tungsten compounds. Most scholars at home and abroad believe that it is the volatile hydrated tungsten oxide that causes the chemical vapor transfer (CVT) process of tungsten. Moreover, the CVT process runs through the whole stage of the reduction of tungsten oxide and hydrogen. It has a decisive influence on the physical and chemical properties of tungsten powder.

In the CVT process, tungsten and its oxides are contacted with water vapor to produce hydrated tungsten oxide , which is volatilized to the gas phase and undergoes a homogeneous reduction reaction with H2. The reduced product is deposited on the nucleated metal tungsten grains. The reduced product is deposited on the nucleated metal tungsten grains so that the tungsten powder particles grow.

The reaction rate of CVT process is fast, and the morphology of reduction products will change significantly compared with that of raw materials. The actual partial pressure of hydrous tungsten oxide WO2(OH)2 during hydrogen reduction is related to the reduction temperature and the actual humidity of the atmosphere. With the progress of reduction process, the partial pressure (humidity) of oxygen decreases continuously, and at the end of the reduction process, the partial pressure of hydrated tungsten oxide WO2(OH)2 is getting lower and lower.

In addition, hydrous tungsten oxide also occurs during the equilibrium annealing of hydrogen reduction, which causes small particles of oxide to evaporate and large particles grow, which eventually makes the annealed oxide markedly coarser.

To sum up, the key to prepare ultrafine tungsten powder is to reduce the formation of volatile hydrated tungsten oxide WO2(OH)2, and to inhibit the process of chemical vapor migration, under certain temperature conditions, the equilibrium partial pressure of WO2(OH)2 depends mainly on hydrogen humidity, and the greater the humidity is, the greater the equilibrium partial pressure of WO2(OH)2. Therefore, the key to the preparation of ultrafine tungsten powder is to rapidly remove water vapor from the reaction atmosphere.

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