2018年6月26日星期二

Ultrafine Tungsten Powder Preparation -- Plasma Method

Plasma is considered to be the fourth basic form of solid, liquid, gas and other substances.  It is a collection of charged particles and neutral particles that are freely moving and interact with each other.

plasma reactor image

Macroscopically, it is electrically neutral. When there is enough energy added to the gas, the gas molecules and high-energy electrons will occur between the collision or absorption of electromagnetic control and ionization process occurred, and then the formation of plasma state.

The basic principle of plasma preparation of ultrafine tungsten powder is in an inert atmosphere or reactive atmosphere, the DC discharge (or otherwise) to produce high temperature gas ionization plasma, so that the molten metal evaporation, metal vapor, quenching or chemical reaction to form ultrafine particles.

At present, the main methods to obtain plasma are thermal ionization method, shock wave method, photoionization method, ray radiation method, and DC, low-frequency, radio frequency, microwave gas discharge method and so on. A tungsten alloy powder with an average grain size of 10nm can be prepared by mixing plasma with DC plasma and DC arc.

The tungsten powder prepared by plasma method has the advantages of good quality, cleanness, less pollution, small size and narrow size distribution range. In addition, plasma technology is considered to be the most effective way to prepare spherical powder.

The disadvantage of the plasma method is that it is not easy to maintain the continuous stability of the plasma flow, so that the evaporation and reaction of the raw material can not be ensured. Although the method of preparing the nanometer spherical tungsten powder and tungsten carbide powder has been reported recently, there is still a great distance from industrial production.

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Ammonium Metatungstate Preparation Equipment -- Microwave Heating Furnace

The thermal degradation method is a process for the preparation of ammonium metatungstate based on ammonium paratungstate (APT).

This method is also the mainstream process of preparing ammonium metatungstate (AMT). At present, China's industrial use of pyrolysis equipment is mainly rotary calciner, at the same time, there are a number of applications in other countries for AMT patented pyrolysis equipment, microwave heating furnace is one of them.

Because the microwave energy level is close to the vibrational level of the water molecule, the hydrous material can dehydrate rapidly under the microwave field, which is also the working principle of the microwave heating furnace.

microwave heating furnace image

Microwave heating furnace operating characteristics are: 1. Uniform and fast, which is the main characteristics of microwave drying. As the microwave has a large penetration capacity, heating can be generated directly within the medium heat, and the drying speed; 2. Selective, microwave heating is related to the nature of the material itself, in a certain frequency of the microwave field, because of water dielectric loss larger than other materials, so the moisture content of other dry material than much more heat; therefore, moisture content is much more endothermic than other dry materials; 3. High thermal efficiency, responsive, because the heat directly from the dry material inside, the heat in the surrounding medium loss is minimal, coupled with the microwave heating chamber itself does not absorb heat and the microwave, all the emission effect on the material, high thermal efficiency; 4. Drying thoroughly, the degree of drying materials can be adjusted according to customer requirements, the limit of drying can dry the material to less than 0.5% moisture content. Adopting microwave drying equipment, the drying process is clean and will not cause pollution to the required drying materials.

The pyrolysis time of APT in a single chamber furnace is 6 hours at 150 °C - 250 °C, but the pyrolysis time in the microwave furnace is only 6, 12 minutes, which shows that the pyrolysis rate is fast.

Obviously, the advantages of microwave heating furnace is uniform heating, fast speed, but this technology is difficult to promote the production, the main reason is the large microwave furnace is expensive, the effect of improvement is limited but the cost has increased a lot, so people are more willing to choose lower cost, better effect of other multiple hearth furnace.

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【Know Tungsten】What Is Blue Tungsten Oxide?

Blue Tungsten Oxide is one of the important raw materials for making tungsten powder. As an important intermediate of tungsten in modern industry, it has been popular for many years. 


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Blue Tungsten Oxide Preparation -- Internal Reduction Method

Blue tungsten oxide is one of the important raw materials for making tungsten powder. 

Compared with yellow tungsten, blue tungsten oxide is easy to be doped because of its good activity and large specific surface. It can maintain its original crystalline shape and has been widely used as an upgraded version of yellow tungsten oxide.  There are three main methods for the preparation of blue tungsten oxide. APT closed calcination method, APT hydrogen mild reduction method, internal reduction. Internal reduction method was proposed by scholars in Fudan University in 1981. 

Internal reduction method by means of an internal component of the tungsten compound to make reduction of W. Intrinsic reduced tungstate was synthesized with APT as raw material (Intramo - lecularly Reduching Tunnstate). It has thermal decomposition in an inert atmosphere, blue tungsten oxide with simple phase composition, large specific surface and high purity has been obtained. 

internal reduction method image

The distinguishing feature of internal reduction method is that the yellow tungstic acid is dissolved in ammonia water first, then add proper amount of urea, the intermediate, which is called the " reduced tungstate IRT ", is obtained by adjusting the acidity with oxalic acid. The dried IRT is then decomposed at 500°C to 700°C in the nitrogen stream. Using the ammonia decomposition of molecule internal component to reduce,  blue tungsten oxide can be prepared. 

The internal reduction method is carried out in a multi-tube furnace. The thermal decomposition temperature of IRT was determined to be 620 ±30°C, and the pyrolysis time was controlled at about 30 minutes.

Two different components of blue tungsten have been prepared by internal reduction method. One is the tetragonal system WO: the particles are very thin, specific surface 22一30m²/g. The other is monoclinic system W20O58, the particles are very crude, specific surface 10m²/g. The blue tungsten oxide prepared by internal reduction method has the advantages of high purity, large specific surface, uniform particle size and simple phase composition. It can be used as a new raw material for tungsten wire, carbide and other products, and its properties and practical use are good.

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Blue Tungsten Oxide and Purple Tungsten Oxide

The performance of tungsten oxide materials will directly affect the reduction behavior of tungsten powder and the final product performance.

Therefore, on which material is most suitable for the production of ultrafine tungsten powder has been the subject of much attention in material science workers.

Blue tungsten oxide and purple tungsten oxide is the main raw material for production of ultrafine tungsten powder, blue tungsten oxide originated in the last century 70's, mainly WO2.90, W20O58 and ammonium tungsten bronze (ATB) mixture. Purple Tungsten oxide is a kind of tungsten oxide product developed in recent years in China. Its phase composition is WO2.72 or W18O49.

Blue tungsten is present through hydrogen reduction of raw materials prepared by tungsten powder is the most widely used, as long as the conditions of low temperature, dry hydrogen, high hydrogen flow, thin material layer, producing ultrafine tungsten powder method can restore the blue tungsten oxide by hydrogen.

Purple tungsten is developed in recent years, because of its unique properties, it has shown its superiority in the production of ultrafine tungsten powder.

At present, in the production of ultrafine tungsten powder, purple tungsten and blue tungsten two kinds of raw materials are used simultaneously. Therefore, some scholars have compared the two kinds of raw materials and conducted hydrogen reduction experiments under the same process conditions. The difference of particle size and uniformity of tungsten powder after reduction has been compared. The following conclusions are obtained by comparison:

BTO and PTO reduction image

1. Both blue tungsten and purple tungsten can be used to produce ultrafine tungsten powder under certain reduction conditions. But the excessively high hydrogen flow of the blue tungsten and the excess material will lead to an increase in cost.

2. Purple tungsten due to its special pore structure, the water vapor can be rapidly discharged from the reducing atmosphere to produce a fine and uniform tungsten powder, and the particle size of tungsten powder is affected by the loading volume and the hydrogen flow rate.

3. Purple tungsten generating hydrogen gas in drying without going through the intermediate stage of reducing WO2, thereby obtaining a fine and uniform tungsten powder.

To sum up, the preparation of ultrafine tungsten powder from purple tungsten has more advantages than blue tungsten.


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