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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