2018年12月3日星期一

Tungsten Powder Specific Surface Area

The specific surface area is the total area of the unit mass material, the unit is m2 / g. Generally refers to a specific surface area of a solid material, such as powders, fibers, particles, and other bulk materials.

tungsten powder image

It is difficult to measure the surface area of powder or porous material. They not only have irregular outer surface, but also complex inner surface. They have large specific surface area and large activity, so they have strong adsorption ability.

Tungsten powder specific surface area is usually 0.01m2/g (coarse tungsten powder) to 12m2/g (fine tungsten powder), sintering active surface area is a measure of tungsten powder (solid state sintering), dissolution characteristics (liquid phase sintering) and reaction ability in the carbonization process and gas solid material between the main index.

According to the specific surface area value, the average particle size of tungsten powder can be calculated, and the reduction effect of tungsten powder has been judged. It is worth mentioning that, the finer the size of tungsten powder, the powder surface is often not "smooth", but "rough", and even a certain degree of micropores, this is different from what we expected, so the specific surface area of ultrafine tungsten powder will be 2-4 times larger.

tungsten powder image

The large specific surface area of ultrafine tungsten powder means that the high specific surface energy is stored in the developed surface, therefore, ultrafine tungsten powder is easy to oxidize when exposed to air and is prone to spontaneous combustion when subjected to friction or mechanical vibration.

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Why Does X-Ray Machines Require Tungstate Materials?

Tungstate is a typical self-activated luminescent material. Its luminescence spectrum is very stable, and its intrinsic luminescence spectrum is very wide and occupies most of the visible region.

X-ray machine image

The cation in the tungstate strongly affects the position of the emission band. With the increase of atomic number of the cation, the peak shift of the emission band is blue, and the bandwidth is from 250 to 300 nm.

It was not long before Roentgen discovered X-rays in 18 century, people use the penetrating power of the rays to check the internal condition of the human body or objects, and to observe or photograph the image directly by the enhancement of the ray emitting material. In 1896 found that CaWO4 can be used as phosphor, in 1906, Puppin first used CaWO4 as X-ray luminescent material. In 1916, Patterson made it a luminescent material for X-ray intensifying screen. In recent years, a great deal of research work has been done on the luminescence properties of single tungstate doped with rare earth ions, especially for doping rare earth ions.

Some scholars have found that tungstate luminous brightness increases with the purity of the compound. Tungstate can be activated by some impurities, which are doped into the tungstate lattice and then give it a special property of luminescence. There are not many kinds of impurities, almost limited to rare earth elements.

In 1948, some scholars studied the tungstate materials doped with rare earth powder. It was found that Ba, Ca, Cd and Zn tungstate showed good scintillation at room temperature, shortly thereafter, American scholars used the Vemuil method to grow CaWO4 and CdWO4 single crystals, although their light output is lower than that of halide crystals, the density is large and the afterglow is weak, in particular, CdWO4 single crystal has high absorption coefficient of X-ray, good light transmission performance and strong radiation resistance, and is soon used as X-ray fluorescence intensifying screen. With the high energy physics electronic collider accelerator, scintillation detector is widely used to explore the new scintillation crystal and its luminous characteristics once again become a hot spot.

It is worth mentioning that ZnWO4 crystal is a promising scintillator in this field. It has the advantages of high density, high light output and short attenuation time, not easy to deliquescence. It can make up for the shortcomings of traditional scintillator NaI and CsI. At present, ZnWO4 single crystals have been used as scintillation crystals, Cambridge University Cavendish Laboratory, the American Cancer Institute and the German company Siemens are used to make radiometric detector and X-CT probe.

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How To Purify Water by Nano Tungsten Trioxide

With the rapid development of industry, while bringing great economic benefits, the environmental pollution caused by the waste water is also becoming more and more serious.

nano tungsten trioxide water quality purification image

Organic industrial wastewater with high concentration and low biodegradability, deep color, complex composition, containing oil, acids, amines, demulsifier and other pollutants, organic wastewater is difficult to degrade, bring unpredictable effects and harm to the ecological system.

The photocatalytic purification of organic compounds in water is a new technology, which uses the sun to degrade organic matter, energy saving and no pollution to environment, has great application prospect.

Nano tungsten trioxide is a high quality n-type semiconductor material, as photocatalyst, n-type semiconductor has many kinds, such as TiO2, ZnO, Fe2O3, CdS and WO3. The results show that WO3 has good photocatalytic stability and is an ideal catalyst for photocatalytic degradation of pollutants in water.

In the wastewater treatment, in 1985, some scholars first reported to WO3 as a photocatalyst treatment of printing and dyeing wastewater experiments, the results show that: When the visible light is irradiated onto the semiconductor powder suspended in the aqueous solution, the dye is decomposed into CO2, H2O, N2, etc., thereby reducing COD and chroma. In order to improve the COD and chroma removal rate of printing and dyeing wastewater, there are three kinds of hollow spherical WO3H2O prepared by using citric acid as structural adjusting agent, there are scholars using citric acid as a structural regulator to prepare three-dimensional hollow spherical WO3H2O. When applied to photocatalytic degradation of organic dyes, visible light irradiation, can be decomposed 94.8% RHB dye molecules.

Nano tungsten trioxide have several key factors photocatalys: (1) Nanostructured materials provide the appropriate surface area to volume ratio; (2) The band gap energy of the lower valence band can be effectively utilized in the visible region of the solar light; (3) The appropriate band gap structure can make the sample more favorable to the redox reaction; (4) The stability of the material, so as to endure the harsh operating environment, therefore, in recent years, more and more researches have been done on the degradation of organic pollutants by WO3 photocatalytic oxidation at home and abroad.

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How Does Nano Tungsten Trioxide Purify The Air

With the growing awareness of environmental protection and recycling of resources, more and more researches are being made on the degradation of organic pollutants.

nano tungsten trioxide air purification image

The photocatalytic degradation of organic compounds is a new technology, which uses sunlight to degrade organic compounds, saves energy and does not pollute the environment. Therefore, it has great application prospects.

Nano tungsten trioxide is a high quality n-type semiconductor material, as a photocatalyst, WO3 photocatalytic stability is good, photocatalytic degradation of air pollutants also has an ideal catalytic effect, and China is rich in tungsten reserves, ranking first in the world, WO3 has a wide range of sources.

In the role of purification of air, the main goal of nano tungsten trioxide is to improve the energy utilization efficiency of hydrogen and oxygen.

Some scholars have summarized the nano tungsten oxide as a catalyst for several key factors: (1) Nanostructured materials provide the appropriate surface area to volume ratio; (2) A relatively low valence band energy forbidden band gap, so as to effectively use sunlight in the visible region; (3) The appropriate band gap structure can make the sample more favorable to the redox reaction; (4) The stability of the material, so as to endure the harsh operating environment.

In the field of air purification, nano tungsten trioxide can be directly used in the air as oxygen oxidants, effectively decompose indoor and outdoor organic pollutants, and oxidation to remove nitrogen oxides in the atmosphere, sulfides and various types of odor, etc., the reaction condition is gentle, is a very convenient air purification technology, so tungsten trioxide is an ideal choice for air purification machine raw materials.

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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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2018年9月25日星期二

How Rare Earth Elements Affect Tungsten Oxide Hydrogen Reduction

In the process of hydrogen reduction of tungsten oxide, the effect of the reduction is not only the metal elements, there are some trace elements, such as rare earth elements.

The effects of trace elements are related not only to quantity, but also to the shape and location of individual elements and the entry stage, for example, the elements that form fine grain particles at grain boundaries are different from the same elements in solid solutions. They are soluble in the bonding phase and different from the tungsten carbide grains.

Some scholars believe that the addition of lanthanum or yttrium (La) (Y) in hydrogen reduction of tungsten oxide, can play a similar role to that of Al.

fine tungsten oxide image

In the process of research, scholars used the doping method of wet mixing and flow drying to prepare the blue tungsten powder, and push the boat by reverse hydrogen, the decomposition process is performed in two stages, namely at 500 °C, 850 °C, 900 °C, 950 °C, 1000 °C temperature point five 4h reduced with hydrogen. Study found that while maintaining the hydrogen flow rate, the amount of loading the boat, and the holding time, the higher the temperature, the tungsten blue powder particles increases. But with the addition of La2O3, tungsten powder particles gradually became thin, the surface gradually increased, in addition, the addition of 2% Y2O3 is better than the effect of adding 3% La2O3.

The research shows that in the process of blue tungsten oxide hydrogen reduction, the study shows that in the process of blue tungsten oxide hydrogen reduction, the morphology of tungsten powder is changed into quasi regular polyhedron without quasi La or Y by adding rare earth element La or Y; La or Y is present in the form of La2O3 or Y2O3. The La and Y contents determine the distribution of La2O3 or Y2O3 in the crystal. When the content of La2O3 or Y2O3 is high, the La2O3 or Y2O3 particles are dispersed distributed.

The results show that the oxides and tungstate forms in the blue tungsten hydrogen reduction tungsten powder exist in the tungsten blue hydrogen reduction tungsten powder, the addition of an oxide film of La or Y on the surface of the tungsten crystal can prevent the precipitation of the vapor phase tungsten oxide on the surface of the tungsten powder particles Or the chemical reaction of the solid phase, and thus inhibit the growth of tungsten powder particles, it is of great help to the preparation and reduction of ultrafine tungsten powder.

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How Impurities Affect Tungsten Powder Production

In the production of tungsten metal by powder metallurgy, the chemical purity is especially important because the subsequent sintering process can only be achieved by evaporation to a certain extent.

With the development of industry, the requirement of the purity of tungsten powder is increasing day by day, because the impurities left after the sintering of tungsten powder have a great influence on the processability and properties of the final products.

In the production process of tungsten concentrate to tungsten powder, tungsten purity is constantly improved, and the purity is high when APT is made. However, when APT is processed into tungsten powder, the purity decreases. Why?

tungsten powder image

According to the study, tungsten powder impurities mainly from the material and the reduction of the furnace tube and metal boat contact, as a result, the impurity content is increased, and these impurities are present in a heterogeneous form, the result is the increase in impurity content. These impurities exist in heterogeneous form, forming a region with a high concentration of impurities, and if the size of these cells to a certain extent, it may become the source of sintering defects.

There are mainly three kinds of impurities in the reduction process of tungsten powder:

The first kind of impurity is the alkali metal meter, which is a volatile element or compound in hydrogen, regardless of the amount of it, which causes the adverse effects of particle growth; In the process of reduction, tungsten powder may also be adsorbed, which is the biggest enemy to prepare ultrafine tungsten powder.

The second kind is represented by Fe, S and As, mainly from the contact with reduction furnace or metal equipment. When the content is low, it has no great influence, but when the content increases to a certain extent, the particle size of tungsten powder will become larger;

The third kind, represented by Al, Mo and P, can inhibit the growth of tungsten powder particles. A little amount can be used to assist the preparation of ultrafine tungsten powder. But from the purity of tungsten powder requirements, the lower the better.

tungsten powder image

In addition, the preparation of tungsten powder is also affected by some trace elements, such as tungsten oxide reduction process of excessive moisture, the process of reducing the concentration of water vapor in the furnace, will lead to tungsten powder particles and particle size distribution uneven, it can add appropriate amount of rare earth elements, can play a similar role in Al, effectively inhibit the hydrogen reduction process of tungsten powder particles grow.

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Nano Tungsten Trioxide

Nano tungsten trioxide is a high quality n-type semiconductor material, with excellent gas sensitivity to H2S, NH3, H2O3 and other gases are sensitive.

In modern industry is often used to produce gas sensors and electrochromic device. A gas sensor is a device that converts the type of gas and its associated information into electrical signals so that it can detect, monitor, and alarm. Tungsten trioxide (WO3) is a n-type semiconductor with wide band gap, its excellent sensitivity and selectivity makes it becomes the main material of the sensor elements. In the field of coal, petroleum, chemical, power and other production, using tungsten trioxide as a component of the sensor can be timely and accurate to flammable, explosive, toxic, harmful gas detection and automatic control.

Tungsten trioxide sensor was first used to detect H2S and H2, Japanese scholars in 1992 for the first time reported WO3 sintered coating on the very low concentration of NOX has a good selectivity, other scholars have gradually expanded the market by adding precious metals (Pd, Pt, Au) to tungsten trioxide to give them a higher sensitivity and selectivity for nanometer tungsten trioxide sensors.

nano tungsten trioxide image

In recent years, through a large number of domestic and foreign researchers work together to improve the WO3-based gas-sensitive materials, gas sensitivity, selectivity, reduce the working temperature as well as different doping and improvement processes has made varying degrees of progress.

At present, the main components of nano tungsten oxide on gas sensing mainly has the following applications:

1. For leakage of the combustible gas alarm and humidity sensor, the measured gas is mainly CO, CH4, H2, NH3, NO2, SO2 etc. it is mainly used in environmental monitoring.

2. Used to detect natural gas, liquefied petroleum gas and urban gas and other civilian gas leakage, gas automatic control of microwave cooking food, it can also detect unpleasant gases from some buildings and other public places.

3. Used to detect the petrochemical industry in the carbon dioxide, nitrogen oxides, sulfur oxides and other highly toxic gases; detection of power transformer oil during the process of deterioration of hydrogen; detection of meat and other volatile food freshness.

4. It can also detect the concentration of alcohol in the breath of drivers during road traffic, prevent drunk driving and reduce traffic accidents.

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Ultra Coarse Tungsten Powder Preparation

With the development of alloy industry, there is also a polarizing demand for tungsten powder.

Not only has ultrafine tungsten powder, but also has ultra coarse tungsten powder. In industrial production, ultrafine tungsten powder is widely used, and the preparation of ultrafine tungsten powder is complicated. Therefore, it has been studied more. However, ultra coarse tungsten powder also has a specific use. 


Ultra coarse tungsten powder is a coarser tungsten powder with a particle size greater than 20 microns. It has the advantages of good strength and toughness. It is widely used in the field of oil drilling, mining tools, stamping die, hard material etc. How to prepare ultra coarse tungsten? Just reverse the thinking of preparing ultrafine tungsten powder.

The traditional scheme of preparing ultra coarse tungsten powder is based on the preparation of ultrafine tungsten powder as negative teaching material. 1. Try to improve fisher particle size of tungsten raw material tungsten trioxide, which means use yellow tungsten oxide or blue tungsten oxide, prevent the purple tungsten oxide. 2. Using low hydrogen flow, large boat load, slow pushing speed, high temperature reduction to produce ultra coarse tungsten powder. 3. Using high temperature, slow pushing production to produce block ultra coarse tungsten carbide powder, volatilizing organic solvents. 4. Ultra coarse tungsten carbide powder was prepared by breaking the alloy ball.

The traditional mode of production is relatively complicated, difficult to solve two major problems. The large-scale production of ultra coarse tungsten carbide is restricted. 1. In the production process of the tungsten powder, charge capacity is large, the size of tungsten powder between surface and bottom is different, crystalline grain uniformity is poor, it affects alloy function. Sometimes even grading and screening methods are needed to obtain ultra coarse tungsten carbide powder. It leads to low yield and high energy consumption. Usually obtain tungsten carbide powder of fisher particle size around 20um. 2. In the process of carbon blending, the uniformity of ultra coarse tungsten powder mixed with carbon black is poor. Resulting in segregation during carbonization, the phenomenon of black heart and interlayer appeared after the powder carbonization. Carbon black is not completely permeable, it leads to low combined carbon content and high free carbon content, re-carbonization is required once or for many times. it has low efficiency and high energy consumption. In several broken ball-milling it reduces ultra coarse tungsten carbide fisher particle size, not up to standard for ultra coarse tungsten carbide powder.

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2018年8月11日星期六

Ultrafine Tungsten Powder Preparation -- Hydrogen Reduction Method (1)

Ultrafine tungsten powder is superior to common coarse grain material in mechanical, therefore, the mechanical properties, physical properties and chemical properties are better than those of ordinary coarse-grained materials. 

However, the preparation of ultrafine tungsten powder is more stringent than ordinary tungsten powder, not only have better quality raw materials, but also more suitable methods and equipment are required.

Since 1950s, a great deal of research work has been done on tungsten smelting industry both at home and abroad, and many years of systematic research have been carried out on the preparation of ultrafine tungsten powder, and some effective methods have been developed. Hydrogen reduction is the main method of preparing ultrafine tungsten powder. It is simple, advanced, reasonable, stable and reliable, low cost and so on.

With the development of tungsten powder manufacturing, great progress has been made in the selection of tungsten oxide raw materials, from the early tungstic acid and yellow tungsten into blue tungsten and purple tungsten as the main material. The structure, composition and physicochemical properties of the raw material have great influence on the particle size and uniformity of tungsten powder.

hydrogen reduction method image

The hydrogen reduction process of tungsten oxide is a rather complicated physical and chemical process, including chemical transformation, phase change process, solid phase diffusion and chemical vapor migration.

Tungsten trioxide hydrogen reduction reaction can be described as the following equation:

WO3+3H2→W+3H2O

The equation may seem simple, but the actual operation is a complex process, the process parameters of each link, such as reduction temperature, material thickness, hydrogen flow and ventilation direction are connected, the interaction of different parameters will lead to different results, a little deviation could affect the particle size of tungsten powder.

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Ultrafine Tungsten Powder Preparation -- Hydrogen Reduction Method (2)

Hydrogen reduction is the main method for preparing ultrafine tungsten powder.

It is a rather complicated physical and chemical process, including chemical transformation, phase change process, solid phase diffusion and chemical vapor transfer.

In W-O-H there are 6 solidification phase, which are: tungsten trioxide (WO3), beta tungsten oxide (WO2.9, W20O58, blue tungsten) gamma tungsten oxide (WO2.72, W18O49, Purple Tungsten) two tungsten oxide (WO2), -W alpha and beta -W. Therefore, the process of hydrogen reduction reaction of tungsten oxide is complex, and different reduction processes will occur with the change of temperature and humidity: In a high temperature and a humid reducing atmosphere, the reduction proceeds in distinct stages ( WO3 → WO2.9 → WO2.72 → WO2→ W ); In low temperature and dry hydrogen, it is possible to reduce tungsten oxide to tungsten powder directly from high valence tungsten oxide ( WO2.9 → W,WO2.72 → W ), the process is roughly: tungsten oxide → hydrogen reduction → screening → batch → tungsten powder.

hydrogen reduction method image

The reaction pathway of tungsten oxide reduction process is different, and the rate of reduction reaction is different, which leads to different particle size of tungsten powder after reduction. Some scholars have shown that tungsten powder obtained by WO2.9 → WO2 → W ( or WO2.72 → WO2 → W ) is different from tungsten powder obtained by WO2.9 → W ( or WO2.72 → W ).

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Ultrafine Tungsten Powder Preparation -- Hydrogen Reduction Method (3)

The hydrogen reduction process of tungsten oxide, is a very complex physical and chemical processes, including chemical conversion, phase transformation, solid diffusion and so on.

hydrogen reduction method image

The process generally is: tungsten oxide, hydrogen reduction, sifting, blending, tungsten powder. Different tungsten oxide reduction process of reaction pathways, the reaction speed is different, in general, the reduction reaction pathway and to avoid the simple single reaction generated in the process of WO2 is beneficial to preparing fine tungsten powder uniformly. That is to say, the tungsten powder obtained by route WO2.9 → W or WO2.72 → W would be more satisfactory.

Hydrogen reduction of tungsten oxide is usually carried out in a multiple tube furnace or a rotary furnace. Multiple tube reduction furnace has 3 to 5 containers used for loading, use artificial way in accordance with the order of loading and unloading material, excessive hydrogen or against the tungsten material movement direction into the tubular reduction furnace and material reaction; The rotary reduction furnace is heated uniformly by constant rotation so that the material is in a dynamic powder flow.

In the reduction furnace, tungsten powder particle size reduction temperature can be roughly divided into two sections: in the low temperature (<800 °C), the rate of change of particle size with temperature (i.e., slope of tangent) is more gentle; at high temperature (>800 °C), particle size begins to fluctuate with temperature. When the temperature is high, the reduction reaction will go through the intermediate stage of WO2. WO2 can be easily sintered and recrystallized at high temperature, which results in the thickening of tungsten powder. So in the actual production, taking into account the low temperature reduction reaction speed is slow, affects the production efficiency, often use the method of ladder type heating. That is, the temperature of the latter temperature zone of the reduction furnace is higher than the previous one. If the temperature gradient is too large, the particle size distribution will become wider. This is also why the use of multiple tube equipment is more than the rotary furnace equipment.

different reduction temperatures image

In addition, tungsten oxides form WO2(OH)2 with water vapor, when the compound has a large vapor pressure at 500~700 °C, it will be reduced to H2 on the reduction product, thereby thickening the particles. In addition, the water vapor will oxidize the tungsten powder, and the oxidized products will be reduced on the surface of the tungsten oxide, which will thicken the particles. Therefore, to obtain tungsten powder with finer particle size, more H2 flow is needed than usual to remove water vapor to prevent the formation of WO2.

In the process of producing ultrafine tungsten powder, the key is to control the suitable temperature and reduce the formation of volatile hydrated tungsten oxide (WO2(OH)2), and restrain the process of chemical vapor migration.

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