Preparation of metal powder

Published: 27th January 2012
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According to the changing functional principle, it is usually divided into 2 categories: mechanical method and physical-chemical method. The metal powder can also be obtained directly from the solid, liquid, gaseous metal refining. It is also from reduction, pyrolysis and electrolysis of the metal compounds in different states by changing the system. When meeting the refractory metal carbides, nitrides, borides and silicides, the combination and restoration approach can be used directly to obtain the metal powder. Because the preparation methods are different, a powder’s shape, structure and size and other characteristics are often varied greatly. In the preparation for the powder, the most widely used methods are the reduction, atomization and electrolysis.
Using reducing agents to seize the oxygen in the metal oxide powder can reduce the metal to powder. Gas reluctant includes hydrogen, ammonia, gas, natural gas and so on. There are solid carbon reluctant and sodium, calcium, magnesium and other metals. Reduction of hydrogen or ammonia commonly used to produce tungsten, molybdenum, iron, copper, nickel, cobalt and other metal powders. Carbon reduction is often used to produce iron. Strong reducing agent with the metal sodium, magnesium, calcium, can produce tantalum, niobium, titanium, zirconium, vanadium, beryllium, thorium, uranium and other metal powder (see metal thermal reduction). High-pressure hydrogen reduction of metal salts with the aqueous solution can be obtained nickel, copper, cobalt and their alloys or coated powder (see hydrometallurgy). Restore the rule of law into the powder particles are mostly irregular shape of the sponge structure. Powder particle size reduction depends on temperature, time, and the granularity of raw materials and other factors. Reduction can make metal powder from the most of metal. It is a widely used method.
Atomizing molten metal into tiny droplets and solidifying the cooling medium into powder).Second-rate (melt flow and high-speed fluid medium) atomization method is to use high-pressure air, nitrogen, argon (gas atomization) and high-pressure water (water spray) for the liquid jet stream media to crush the metal . Also the use of crushed and melt spinning disk itself (from the power pole and crucible) rotation of the centrifugal atomization method, and other methods, such as dissolved hydrogen vacuum spray atomization, ultrasonic atomization, etc. As the small droplets and heat exchange conditions are good, the general rate of condensation droplets can reach 100 ~ 10000K / s, higher than several orders of magnitude when the ingot. Therefore, the alloy composition uniformity, a small organization, it is made of alloy with no macro-segregation, and excellent performance. Generally nearly spherical aerosol powder, water spray system Debu rules can shape. Powder characteristics such as particle size, shape and crystalline organization depends on the melt properties (viscosity, surface tension, superheat) and atomization process parameters (such as melt flow diameter, the nozzle structure, blasting media pressure, flow rate , etc.). Almost all are available by molten metal atomization method of production, particularly suitable for production of alloy powder. The method is of high efficiency, and easy to expand the industrial scale. Currently , it is not only used for industrial mass production of iron, copper, aluminum and various alloys powder, but also used to produce high-purity (O2 <100ppm) high-temperature alloys, high-speed steel, stainless steel and titanium alloy powder. Moreover, the chilling technology system to take rapid cooling powder (condensation rate> 100,000 K / s) and more attention. It can be produced with a high-performance ceramic materials.
Using direct current through the metal salt aqueous ,metal ions on the cathode discharge is precipitation,the powder which is easily broken forms. Metal ions from the same general dissolution of the metal anode, and current from the anode to the cathode under the effect of migration. The main factors affecting powder size is the electrolyte composition and electrolysis conditions (see electrolytic solution). Mostly dendritic general electrolytic powder, high purity, but this method consumes a large, high cost. The application of electrolysis is also very wide, used to produce copper, nickel, iron, silver, tin, lead, chromium, manganese and other metal powders; under certain conditions, also can be prepared from alloy powders. For tantalum, niobium, titanium, zirconium, beryllium, thorium, uranium and other rare and refractory metals, often used as a composite molten salt electrolyte (see molten salt electrolysis) to the system to take powder.
Because of homogeneous, fine grain and great activity, the fine powder which is the less than 10μm has a special status in the manufacture materials (such as dispersion strengthened alloys, ultra-hole metal, metal tape) and direct applications (such as solid rocket fuel and magnetic fluid seal, magnetic ink, etc.) In addition to the carbonyl and electrolysis in the manufacturing , the vacuum arc evaporation condensation method , spray, a total decomposition of double salt precipitation, gas reduction and other methods can be used too.
The thermal spray powder plays an important role on coating, atomic energy engineering material and other special uses. Using the two chemical milling deposition methods of gas and liquid phase ,such as thermal dissociation of hydrogen reduction, high-pressure hydrogen reduction, replacement, electro-deposition and other methods, can produced metal and powder coating of metal, a mixture of various metal and non-metallic. Source:

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