Nano tungsten powder is widely used in modern industry and science area, so good performance nano tungsten powder can improve application in the field of modern industry and science which has great significance. But good properties nano tungsten powder producing requirements is quite high. The violet tungsten oxide (VTO) has different properties with other tungsten oxides. It is the most suitable material for produce nano powder, so using VTO as material producing nano tungsten powder not only the tungsten powder has good performance and the process is relatively simple. But in order to producing good properties tungsten oxide in addition to control the raw martial but also need to control the production process which is temperature, push boat speed and the amount of hydrogen, only in gentle heating process, moderate reaction, smooth gas migration so can effectively control the water vapor to escape which is an important factor for producing good properties nano tungsten powder.

By controlling the raw materials and process conditions the nano tungsten powder has high purity and crystallinit as well as has stable properties and loose structure. What’s more the appearance of it is sphere and distribution uniform which has large specific surface area, pore structure simple.

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Violet tungsten oxide (VTO) restore to tungsten powder in the first and second redox reactions the particle size of these two reduction has not big different.

Continue to analyze the third redox reaction found the third time tungsten trioxide restore to tungsten powder and the specific surface area of it is 1.082 m² • g^-1 in the third time reaction which is the 6.56 times to second reduction tungsten powder’s specific surface area. Apparently,   third reduction tungsten powder particle size is very small, and most concentrated in the 0.21-1.05um, but there is significant agglomeration, so that the measured particle size is agglomeration but not tungsten powder. According oxidation - reduction mechanism shows that the majority of the oxide reduction process is carried out simultaneously, but some tungsten oxide first reaction and others after reaction then the after reaction tungsten powder will cover in the first reaction tungsten powder so the tungsten powder particle size will group up, so that the tungsten powder particle size distribution is relatively concentrated and uniform.

VTO after third times reduction and second time oxidation can produce submicron tungsten powder particle size between 0.21-1.05um and specific surface area is 1.082 m² • g^-1 which has not coarse particle. 

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After analyzing tungsten powder which restore from violet tungsten oxide for first time researcher found tungsten powder particle size are smaller and more uniform, but has reunion phenomenon.

Then the researcher analysis the second redox reaction found tungsten trioxide restore to tungsten powder in hydrogen and the specific surface area of tungsten powder is 0.165m² • g^-1 which is the1.71 times to first reduction tungsten powder’s specific surface area. From this data analysis the researchers found comparison the first time and the second time reaction of tungsten powder particle size they didn’t have large different. The second reduction tungsten oxide particle size is mainly between 4.24 and 11.11um, centralized distribution and uniform.

The tungsten powder in the air second oxidize to tungsten trioxide and the specific surface area of it is 0.719 m² • g^-1, is the 4.96 times to first oxidized tungsten trioxide’s specific surface area which is more finer than first time but the distribution is very scattered and not concentrated area. This is mainly because the tungsten trioxide has significant agglomeration, the particle size which measuring is agglomeration not the tungsten trioxide particles.

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Because violet tungsten oxide (VTO) producing process and process parameters are simple as well as equipment are easy to control and many advantages so many manufactures are like to use VTO to producing tungsten powder. VTO cycle reduction method can producing excellent performance submicron or micro tungsten powder particles, to further understand tungsten powder in the reactions process which is used cycle reduction method will conducive to the future development and maturity of this method.

After studies, VTO in first hydrogen reduced to tungsten powder and the specific surface area of tungsten oxide is 0.141m² • g^-1 and particle size mainly between 4.72-12.13um. Compared with the original VTO particle size distribution, after first reduction the particle of VTO is significantly thinner, but still existing coarse particles. The tungsten powder in the air oxidation to tungsten trioxide for the first the specific surface area of it is 0.145m² • g^-1, which is 1.13 times of VTO. It can be seen, the first time oxidized to tungsten trioxide the particle are smaller than VTO particle size which is more uniform distribution, but still significant reunion.

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Fine and uniformity tungsten powder are key point for producing carbide, and micron and submicron tungsten carbide powder is useful to producing carbide. According to the relevant academic report found that single tungsten oxide can produce submicron or micron tungsten powder. Using violet tungsten oxide (VTO) producing fine tungsten oxide not only the process of it is simple as well as process parameters are easy to control and also equipment are not complex etc. Producing process as follows:
1. VTO were first reduction in hydrogen, along a first temperature of 640 ℃, the second temperature of 760 ℃, third temperature of 860 ℃, when the temperature reaches 860 ℃ heat preservation for 20min.
2. in the air at 500 ℃ making tungsten powder to oxidation with slowly heated for the first time, then heat preservation for 10min.
3. Replace the first step and second to continuous cycle reduction reaction.
4. Using alcohol on the analytical results above steps were dispersed pattern, then using BT-9600 laser particle size analyzer and JXA-840A electron probe scanner to analysis particle size and morphology of samples.

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Experimental material: high-purity hydrogen, high purity argon gas and high-purity tungsten hexafluoride and deposition matrix: copper tube.

Experimental procedure:

Decontaminating, acetone washing and drying the surface of deposition matrix, which would be placed in the reaction chamber then.

To avoid the explosion due to hydrogen mixed with air, argon should be purged into the reaction chamber and all the pipelines to empty the air. After emptying air, open the hydrogen flow needle valve, pass over the hydrogen and then light the exhaust gas discharged, then close the valve;

Heating the deposition matrix by current to make it achieve the required deposition temperature;

After mixing the WF₆ and H₂ according to a certain proportion, passing them through the reaction chamber through the flow control device;

WF₆ and H₂ react chemically on the surface of deposition matrix, to obtain deposition layer of refractory metal;

After a certain reaction time, turn off the heating power and reactive gas control valve, so the reaction chamber will be cooling;

Removing the sample after the temperature drop to below 100 ℃, and obtaining tungsten tube by means of cutting, and the removal of the deposition matrix.

By changing the deposition temperature and gas flow rate to get tungsten tube in different processes, and then test its properties.

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Experimental materials and reagents are as follows:

H₂: High Purity(99.999％),it is reduction gas;

WF₆: High Purity(99.9％),it is reactant gas;

Ar: High purity(99.999％), it is used in the reaction system for evacuating air;

Deposition Substrate: copper (there are different diameters: 6mm, 10mm, 16mm, and 35mm);

Tungsten tube is processed by CVD (Chemical Vapor Deposition) method.

Under the following conditions: the deposition temperature is the same (600 ℃), and the deposition matrix diameters (6mm, 10mm, 16mm, 35mm) is different. The morphology of tungsten tube obtained by the aforementioned condition is fine columnar crystals and its growth direction is almost identical to brass radial.

By comparing the microscopic morphology of tungsten tube obtained under the above conditions, what is found that the parameters of tungsten tube in the subsequent depositing process can be ignored, that is to say, you can ignore the impact of the deposition matrix diameter on tungsten tube matrix microstructure morphology.


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Under different deposition temperature (500 ℃, 600 ℃, 700 ℃ and 800 ℃), keeping the composition proportion and the inlet content of WF₆ and H₂ the same amount, the cross section metallographic microstructure of the tungsten tube processed by CVD (Chemical Vapor Deposition) is different.

When the deposition temperature is lower(500 ℃), the organization of the deposited layer presenting fine crystalline column;

When the deposition temperature reach 600 ℃, the speed of the tungsten atoms generated by the chemical reaction of the reduction will accelerate. In the deposition process, some grain grow ing occasionally get the lateral growth space and grow, which will make deposit layer of column grain structure coarsening.

Some occasional rapid growth of the grain growth to obtain the lateral growth space and growth

When the deposition temperature is raised to 700 ℃, the speed of the tungsten atoms generated by the chemical reaction of reduction has been very fast, these tungsten atoms deposited on surface of deposition will restart growth interface nucleation and further growth. In this process, morphology of the deposited layer will change significantly. Re-nucleation process will result in, firstly, columnar crystal growth pattern of the deposition layer will tend to clutter; secondly, the crystal grain of deposition layer will obviously refined.

If the deposition temperature rising to 800 ℃, re-nucleation process will be more obvious, at the same time, the microstructure of the deposited layer has lost columnar crystal growth characteristics, and its organizational form is messy and grain size is uneven.


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As the CVD (Chemical Vapor Deposition) process, by constantly absorbing tungsten atom, initially formed thin polycrystalline grain, which will grow in all directions and form the randomly oriented growth area of deposition microstructure.

After a further depositing and growing, since the cross growth of these crystal grains in all directions will stop growing due to the mutual contacting and blocking among crystal grains, so only the growth in a direction which is perpendicular to the surface of copper matrix is able to obtain required crystal grains growth space, in order to continue to grow. Because there is such a competitive growth way, so a columnar grain structure is ultimately formed, whose direction is perpendicular to the growth development of the surface, whereby the region is mainly composed of deposition layer.


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CVD tungsten tube morphology:

In the CVD (Chemical Vapor Deposition) process, due to the chemical reaction with H₂, WF₆ adsorbed on the surface of the copper matrix reduce to tungsten atom, and these tungsten atoms will gather nucleation on the copper matrix surface and grow, at last, the tungsten tube is formed by cutting and other process.

The formation of the first layer of the microstructure is related to the forming process of the initial crustal nucleus of tungsten layer and nuclei initial growing up process.

Tungsten atoms will deposit and diffuse on the surface of the deposition matrix and gather on the surface of the active matrix position, these positions are usually near the grain boundary, surface level position and the crystal defect surface outcrops.

When nucleation aggregated reach a certain degree of supersaturation, the deposited layer of crystals will form initial nuclei. Because those active sites presented on the deposit surface distribution is not uniform, so, from the microscopic point of view, the initial deposition surface nuclei are not uniformly distributed.


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