Understanding the main phase of violet tungsten oxide (VTO) has great impact on producing good properties tungsten powder. Because violet tungsten oxide main phase will influence tungsten particle grains morphology. Violet tungsten oxide in hydrogen reduction process undergone two major phase transition, γ-tungsten oxide→tungsten oxide (WO_2.72) → tungsten dioxide (WO₂) → tungsten powder.
After study found during the restore reaction the first phase transition is violet tungsten oxide to tungsten dioxide. At this stage with the reduction development the oxygen atoms decrease and oxygen vacancy increase of violet tungsten oxide, tungsten dioxide particles become denser than γ- tungsten oxide. But this is not easy to make hydrogen gas into the interior of the particles, the reduction process of the water molecules are also difficult to discharge, which is not conducive to the produce ultrafine or nano tungsten powder.
In the second stage of phase transition is tungsten dioxide restore to tungsten powder. According to the phase data, with the degree of reduction depth, tungsten dioxide’s oxygen atoms decrease and vacancies increased, resulting denser particles of tungsten powder, but little different with tungsten dioxide. Obviously, the morphology change of tungsten powder happen before produce tungsten dioxide.



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After many observed violet tungsten oxide the researchers found violet tungsten oxide has special structure which is good for producing fine or submicron tungsten powder.
After detailed studies of these three powders found loose structure violet tungsten oxide is conducive to speed up the reaction proceeds, and powder particle size is small than using other tungsten oxide to produce.
Using violet tungsten oxide as raw material to produce tungsten powder and the tungsten powder particle shape is kind of short rod or a chain particles which form by many particles, uniform size, particle size between 0.1 ~ 1um. Tungsten carbide powder particles shape is mainly equiaxed particle, uniform size, particle size is also between 0.1 ~ 1um. But the size is larger than the tungsten powder. This is mainly due to when carbonization the tungsten atoms does not occur large migration.
So using violet tungsten oxide to produce submicron tungsten oxide and tungsten carbide powder the particle shape are complicate and also has little single particle exist.



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Violet tungsten oxide (VTO) has a special structure and properties, is one of the most suitable materials for producing fine tungsten powder. Using violet tungsten as raw material by hydrogen reduction to obtain tungsten powder, tungsten powder and carbon mixture obtained submicron tungsten carbide powder, and then mixed with carbide powder to form submicron tungsten powder which has great significance for producing outstanding performance superfine alloy. Through the study of tungsten powder and tungsten carbide powder properties so that they can be more widely used in the different fields.

Through data analysis, violet tungsten oxide has special structure which is different form other tungsten oxide. Violet tungsten oxide is combined with rod-like and needle-like crystal, there are non-uniform of thickness, but it has big length-diameter ratio. So the kind of this structure has a lower bulk density which is good for hydrogen reduction reaction. So that the tungsten powder and tungsten carbide powder which produce by violet tungsten oxide can obtain submicron level particle size. 



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After many researches, the researchers found activated tungsten oxide is good for producing fine tungsten oxide than blue tungsten oxide. So compare the different between blue tungsten oxide and blue tungsten oxide.
The maximum content of the impurity element of activated tungsten oxide is similar with blue tungsten oxide. The mine phase of activated tungsten oxide phase is bronze (ATB) and the main phase of blue tungsten oxide is WO_2.9. The physical properties of activated tungsten oxide are: bulk density of it is 2.3 ~ 2.8g • cm^-1, -100 mesh sieve, specific surface area bigger than 3cm ² • cm^-3. Blue tungsten oxide physical properties is small different with activated tungsten oxide.

The study showed that in the same process, using activated tungsten oxide to produce tungsten powder there are little coarse crystalline grain and particle-piled which compare to blue tungsten oxide, besides tungsten powder are more uniformity and concentrated. So using activated tungsten oxide to produce tungsten powder is finer than using blue tungsten oxide.



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The quality of tungsten powder will directly affect the quality of tungsten carbide powder, so in order to produce high-quality carbide. The most important thing is to produce high quality tungsten powder. According to the actual situation of industrial production shows that using traditional blue tungsten oxide to produce tungsten powder by reduction reaction can no longer meet the requirements of some special products such as: FWG-4, FW1F and so on.
Using traditional blue tungsten oxide to produce tungsten powder by reduction reaction has some shortages. First, the distribution of tungsten powder particle size is too wide. Second, there are many coarse grains and particle-piled during producing fine tungsten powder. In order to meet the requirements of modern industrial production, the researchers use a fine powder, surface energy developed, high activity, and single phase’s activated tungsten oxide instead of blue oxide blue tungsten oxide to produce fine tungsten powder. 



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Violet tungsten oxide (VTO) production generally based on ammonium paratungstate (APT) as raw materials in industrial rotary kiln calcination conducted by NH₃, NH₃ decomposition H₂, so that APT in a mild reducing atmosphere calcination, gradually reduced to the violet tungsten oxide.

During the reaction, the furnace temperature, feed rate and speed of the furnace have some influence on the violet tungsten oxide producing. In certain conditions, the furnace temperature is too high, the feed rate is too small or the material staying in the furnace are too all of these factors are likely to cause APT not completely dissolved to violet tungsten oxide(W₁₈O₄₉). This will result in product clumping or knot furnace, thereby affecting the quality of the product.
And after study found when the furnace temperature is too low, while the violet tungsten oxide is needle crystal morphology, while the furnace temperature was higher, and the VTO is rod crystal morphology. Needle-like and rod-like morphology are basic crystalline morphology of VTO. After many studies, researchers found using needle-like crystal‘s VTO to produce tungsten powder are finer. Using rod-like crystal‘s VTO to produce tungsten powder is coarser. At the same time ammonium amount also will impact on VTO production, in order to ensure the reducing atmosphere furnace, for ammonium control is also very important.



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Cracked tungsten tube prepared by chemical vapor deposition is investigated by ring machine cutting method and X-ray stress analysis system. The results shows that crack initiating at two neighboring columnar crystal grain boundaries of the internal surface with the action of large residual tensile stress in the internal surface of the tungsten tube, and the propagation expended from columnar crystal grain boundaries to the outer surface of the tungsten tube. The existence of a large residual compressive stress on the outer surface of tungsten tube can arrest propagating cracks.

 

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During the preparation of tungsten tube in CVD(Chemical Vapor Deposition) process, in order to improve the strength and toughness of CVD tungsten tube, one or several layers of very fine tungsten wire can be wound on deposition matrix to obtain CVD tungsten tube wound by tungsten wire (the tungsten wire-CVD tungsten composite materials). CVD tungsten tube wound by tungsten wire allows the tungsten tube to change the direction of growth, and the crystal grains will also be refined, so the strength and toughness of CVD tungsten tube is improved.

Because of the presence of tungsten wire, the density and purity of CVD tungsten tube wound by tungsten wire will decline slightly. But it still owns more than 99% of purity; and still has higher density than 19g / cm3

Therefore, CVD tungsten tube wound by tungsten wire owns a high purity and density, a significant increase in crush strength, and fracture morphology of significantly change.


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Controlling the deposition matrix removal process is one way of suppressing the generation of tungsten tube surface transverse cracks.

If adopting vacuum melting method to remove the deposition matrix, the temperature rising rate should be slow, especially when the temperature is over the deposition temperature to the melting point of copper. This method is preferred options, because this method can make the removal of the matrix and the deposited tungsten tube’s stress annealing process going on simultaneously. In one hand, the transverse cracks of tungsten tube surface can be suppressed; on the other hand, the deposited matrix material can be recycled directly.

Using nitric acid, then the concentration of nitric acid should be less than 35%. Because, if the corrosion rate of deposition matrix is excessive, then stress release rate of tungsten tube will be too fast, furthermore, it will also makes the corrosion deposit matrix uneven, resulting in t tungsten tube will generate cracks in deposit matrix removal process.

 

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Suppression method of tungsten tube surface transverse cracks:

Rate of temperature increase of annealing treatment should be carried out slowly instead of too fast; especially the temperature ranges below tungsten’s brittle transition temperature, in order to release deposition tungsten tube’s internal stress slowly.

Underspeed the cooling rate after the end of the deposition, especially the temperature ranges above tungsten’s brittle transition temperature. This is an important thing, particularly for the deposition preparation of products which has big size, thick wall, and complex shape and is easy to produce concentrated stress.

In the condition of meeting the strength requirements and processing conditions of the deposition matrix, the matrix thickness should be uniform and as thin as possible to reduce the deformation stress during cooling process.

Selecting the matrix material whose expansion coefficient is close to the base material, by this way, the residual stress within deposited products can be significantly reduced.

 

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