2018年7月27日星期五

Self Propagating High Temperature Synthesis Method

Self Propagating High Temperature Synthesis (SHS) is a new technology which uses the high reaction heat produced between reactants and synthesis of materials in a short time.

ultrafine tungsten powder image

In 1967, Soviet scientists began to study the SHS technology system and apply it to industry. Self propagating high temperature synthesis is the reaction with certain gases in vacuum or filled (N2, H2, Ar2 etc.) combustion reaction container, then the combustion products were crushed and grinded by different specifications of powder technology. The technological process is as follows: material preparation, briquetting, preheating, ignition, self propagating, high temperature synthesis, mechanical fragmentation, product performance test.

The process of preparing tungsten powder by SHS is carried out in a home-made reactor and argon atmosphere, and tungsten filament parallel to the surface of the sample is used as the ignition source. After combustion, the product was soaked in 10% HCl (excess 30%) solution for a certain period of time and then washed to neutral by deionized water, At the same time, no Ca2+ and Mg2+ were identified in the filtrate, and then dried to remove MgO and CaO. The resulting filter residue was leached with 5% NaOH and then filtered, washed with water to neutral and rinsed with 10% HCl. After repeated drying, the tungsten powder was obtained. The XRD, SEM and particle size distributions of the products after combustion and after leaching showed that: By means of self propagating reduction method, the average particle size was 0.87 um and the specific surface area was 1.09m2/g, the purity bigger or equal to 99.0% tungsten powder.

By self propagating high temperature synthesis method to produce ultrafine tungsten powder, it has the advantages of high efficiency, energy saving, economy and environmental protection. However, there are some shortcomings such as fast reaction speed and difficult to control, but there is no doubt that SHS is very promising.

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Ultrafine Tungsten Powder Preparation -- Gas Evaporation Method

In an inert gas of a certain pressure (<1.013x10--5Pa), it is heated by resistance, laser heating, electron heating, arc heating, high-frequency heating, etc. 

gas evaporation method image

This method is the most effective way to prepare ultrafine particles with a particle size less than 100nm by melting and evaporating materials and condensing them into ultrafine particles. Gas evaporation was first proposed by the Japanese in 1963.

In the process of studying the process of preparing nanometer tungsten powder by gas evaporation, Chinese scholars evacuated the vacuum chamber to 2.5 * 10 - 5Pa and filled the high-purity argon with purity of 99.99% to 100~500Pa, heating to make the purity of 99. 9%, diameter of 1mm tungsten wire evaporation. The vaporized gas atoms, W, collide with an inert gas atom, lose energy, agglomerate into nanometer sized clusters, and then condense in a sample grid cooled by liquid nitrogen. The nano tungsten powder structure as the body centered cubic (B.C.C) structure and A-15 alpha - W beta - W coexist, crystalline structure with different positions without condensation. The heat treatment at 700 °C can make the beta - W structure of A-15 structure irreversibly transform into B.C.C structure, this method can be used to prepare nanocrystalline tungsten powders with a grain size ranging from several nanometers to several hundred nanometers.

ultrafine tungsten powder image

The preparation method of ultrafine tungsten powder were of high purity, no other impurities in the process of pollution, fast reaction speed, good crystalline structure, but in the field of China's tungsten powder production does not seem to get a wide range of popularity, the main reason is that this method requires high technical equipment, personnel requirements are also higher.

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Ultrafine Tungsten Powder Preparation -- High Energy Ball Milling

High energy ball milling method is used in low temperature, in the protective atmosphere, by using the rotation or vibration of the ball mill.

simoloyer mill image


The hard sphere is subjected to strong impact, grinding and stirring, and the metal is crushed into nanometer particles. At present, high energy ball milling mainly uses vibrating ball mill, planetary ball mill and stirred ball mill to prepare nanometer powder.

In the nano structure formation mechanism study, that high energy ball milling is a process in which the particle circulation shear deformation, in this process, a large number of lattice defects ever produced within the great grain particles, leading to the re-combination of particles in the grain boundary. In the system of cells, the formation of nanocrystals is only the evolution of structures under mechanical driving. In the multivariate system, low temperature solid state reaction components will occurred during ball milling grinding, crushing, cold welding effect and mechanical effect, the grain size decreased with the increase of milling time, and the strain increased with the increase of milling time.

In the process of ball milling, due to the repeated deformation of the sample and the defect density reached the critical value in the local strain band, the grain began to break, and the process continued to repeat, and the grain was refined until the nanostructure was formed.

Some scholars have shown that metal tungsten powders with grain size 5nm can be prepared by high energy ball milling. The main factors affecting the ball mill technology are: raw material characteristics, milling time, ball mill media, ball material ratio, ball mill atmosphere.

High energy ball milling is an important method for preparing ultrafine tungsten powder. The preparation of nano tungsten powder with high yield, simple process and equipment, making the powder fine grain size and other advantages, is a promising technology in recent years has attracted more and more attention of materials scientists.

The main disadvantage is that the ball milling process will introduce impurities to make the tungsten powder contaminated, and the resultant powder is easy to agglomerate and not easy to disperse, and the energy consumption is higher. Therefore, this method is more suitable for the preparation of powder with little requirement of purity.

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Ultrafine Tungsten Powder Preparation -- Hydrogen Reduction Method

The hydrogen reduction method 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. The reaction pathway of tungsten oxide reduction process is different, and the reaction rate is different. However, in order to obtain the best indicators, WO2 should be avoided in the reaction process to achieve the direct reduction of ultrafine tungsten powder from blue tungsten or purple tungsten. Although the quality of blue tungsten or purple tungsten is very important, other details should not be neglected. The key to obtain the desired effect is to master the key parameters such as temperature, humidity, operation and impurities.

1. Temperature is the key factor to determine the preparation of ultrafine tungsten powder. The production has proved that, when the temperature is greater than 800, faith particle size of blue tungsten or purple tungsten began to fluctuate, the higher the temperature is, the reduction reaction will experience intermediate stage of WO2, WO2 is easy to sinter and re-crystallization at high temperature, which leads to the thickening of tungsten powder. Therefore, in industrial production, the total effect of elevated temperature on particle size is to thicken the W powder.

ultrafine tungsten powder image

2. Humidity is the main factor affecting the preparation of ultrafine tungsten powder: Because tungsten oxides form WO2 (OH)2 with water vapor, this compound has high vapor pressure at 500~700 °C, H2 will be reduced in the reduction product, so that the particles become coarse. The main method of controlling humidity is to control the hydrogen flow in the reduction furnace, In general, it takes more H2 flow than ordinary tungsten powder to remove water vapor to prevent WO2 generation.

3. The amount of loading will also affect the size of the reduced tungsten powder. When the loading amount is large, the thickness of the material layer increases, which is not conducive to the diffusion of water vapor during the reduction. The tungsten oxide and water vapor have a greater chance of action, thus causing the powder to be thickened. The larger the loading capacity, the thicker the material layer, the coarser the size of the tungsten powder produced.

4. The influence of impurities on the particle size of tungsten powder is also very large. The impurities of tungsten powder are mainly three kinds: alkali metals Fe, S and As, regardless of the metal content, all of which cause the adverse effects of particle growth; The second kind, represented by Fe and AS, has no great influence on its content, but when the content increases to a certain extent, tungsten powder will grow; The third kind, represented by Al, Mo and P, can inhibit the growth of tungsten powder particles.

5. Hydrogen reduction reduces the pre-reduction of tungsten oxide in dryer hydrogen, which facilitates the refinement of tungsten powder; Reverse hydrogen reduction is the usual way to pass hydrogen. It makes the humidity of hydrogen higher at the beginning of the reduction process, and then reduces the particle size of the tungsten powder after reduction.

It is worth mentioning that, in the production process, the purple tungsten because of its special pore structure, which has good air permeability, can rapidly discharge water vapor from the reducing atmosphere, so the influence of particle size by boat loading weight and flow rate of hydrogen is very small. Compared to blue tungsten, purple tungsten is the best raw material for the production of ultrafine tungsten powder.

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