Brief introduction to seven new preparation technologies of metal powders

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Preparation of iron powder that has low oxygen content by atomization method

This production is carried out under oxygen-free atmosphere and under the existence of some paraffin. In the production process, carbon will react with iron and generate very thin carbon-rich surface layer. Besides, carbon will reduce the ductility of particles and will improve the surface sintering activity. In the powder compact, as carbon is easy to spread into the particle center and adjacent particles, it is very suitable for producing powder metallurgy steel that do not need to be added with graphite.

Regarding the application of powder metallurgy, this kind of oxygen-free powder allows to replace nickel and copper with cheap alloying elements (chrome, manganese and so on). Nickel, to be served as strategic resources, it not only has high price, but also is a kind of carcinogens, which should be used as little as possible. This kind of powder is also suitable for producing high-strength parts by warm-pressing and hot isostatic pressing.

Preparation of soft magnetic metal composite powder

Currently, soft magnetic composite materials have been widely used, which are produced after insulation by cladding a layer of oxide or heat reactive resin on pure iron powder particles. In the low frequency application of soft magnetic composite materials, high permeability and low iron loss materials can be made by mixing coarse grain of iron powder and heat reactive resin. In the high frequency application of soft magnetic composite materials, as particles need to have more effective insulation, the particle size is required to be smaller so as to further reduce eddy current loss. It can be made into isotropic soft magnetic composite parts without high temperature sintering. When the powder grain size increases, the permeability will also increase, but the coercive force will reduce.

metal powder for magnetic composite partsPreparation of nanopowder by combustion flame-chemical vapor deposition

Nanopowder can be produced by combustion flame-chemical vapor deposition. In this technology, the stable flat flame is generated in the combustion of low pressure fuel and oxygen. Chemical matrix and fuel will be guided into combustion chamber together and will have thermal rapid thermal decomposition in the hot spots of flame. As the surface temperature of combustion chamber is distributed well, the gas phase residence time is short, the chemical matrix concentration is even as well as it can have thermal decomposition in very narrow hot spots, it can be used in producing high quality nanopowder that has concentrated size distribution. At present, this method has been used in producing SiO2, TiO2, Al2O3, SnO2, V2O5, ZrO2 and other oxide nanopowders. Nanopower that produced by this method only needs a small cost. According to the estimation of annual output of 100 tons of nanopowder, the cost of each kilogram of nanopowder will not exceed 50 dollars.

Preparation of nanopowder by laser

Americans use ordinary blender, laser and cheap reaction materials so as to rapidly, cheaply and neatly produce 1-100nm silver powder and nickel powder. For example, after guiding silver nitrate solution and a kind of reducing agent in the blender, the mixture will be irritated with laser for a short time and will be stirred at same time. When the laser pulse is shoot on the liquid, it will form an extremely small “hot spot”, thus the silver nitrate will react with reducing agent and generate tiny silver particles. By changing laser strength, rotate speed of blender and reaction composition, the silver powder particles can be controlled as well as the particle shape can be controlled in some degree.

This method, of which the production speed is 0.5-30g/min, has higher productivity than other nanopowder preparation methods. Compared with hydrazine, a kind of carcinogens that used in producing silver powder previous, the reaction materials that used in this method will not pollute environment. Silver powder that produced by this method can be used in manufacturing solder, dental packing, circuit board, high speed photographic film and so on.

Preparation of nanopowder by electrical explosion of metallic wire

High power electric pulse will be applied on the metal wire that is protected by argon and will be restrained by the special field generated by high power pulse. As plasma column will be heated to more than 15000K, the resistance will increase sharply and will lead to the debacle of special field. Afterwards, high pressure that generated by metal vapor will cause explosion and form the shock wave, thus the metal aerosol will have rapid adiabatic cooling and the nanopowder can be formed eventually. This method can be used in producing aluminum, nickel, silver, copper, zinc, platinum, molybdenum, titanium, zirconium, indium, tungsten and alloyed powders that can be used in propellant, explosive, firework, the cementation of metal and ceramic, sintering aid, catalyst, synthesis of organometallic compounds and so on.

Preparation of low-cost nanopowder by mechanochemical method

Australians have developed a kind of mechanochemical method that can be used in producing low-cost nano metal powder and ceramic powder. In this method, ball mill is used in activating the chemical reaction, in which the superfine nano-metal or compound grain will be formed; afterwards, separating and extracting the micro grain. For example, in the mechanical lapping of FeCl3, it will be reduced into the mixture of iron and chloride by sodium, calcium or aluminum. After removing the chloride with appropriate washing method, nanometer-scale iron powder can be obtained.

This method can be used in successfully producing 10-20nm powders that will have high chemical purity and the surface oxide will be lower than 10%-15%. It can be also used in producing oxide powder that has the particle size of less than 5nm. Its potential high technology applications are as follows: it can be used in cutting tools, advanced ceramics, high density magnetic recording media, magnetofluid, catalysts and so on.

Preparation of nano metal powder by sonochemistry

American scientists made nano metal powder by using sonochemistry technology. Sonochemistry is a kind of subject that studies the formation, growth, implosion and other phenomenon of bubbles that are generated by the high strength ultrasonic wave in the liquid. Due to the rupture of these ultrasound bubbles, it will generate strong local heating and will form “hot spot” in the cold fluid, the transient temperature will reach 5000 degrees Celsius, the pressure will reach about 1GPa and the duration time will be about one over a billion second.

Rough and figuratively speaking, the above mentioned data is equal to the surface temperature of sun, the pressure of ocean bottom and the time of lightning. When the bubbles are fractured, the metal volatile compounds that are contained in the bubbles will be resolved into single metal atom and soon be gathered into cluster which will contain hundreds of atoms and have the diameters about 2-3nm.

This kind of small magnetic metal atom clusters will be like paramagnetic materials, in which the magnetic moment is composed by the atomic spin of clusters, besides, all the spin are in the same direction, therefore, its magnetic moment is 100 times higher than common materials. Stable ferric colloid can be formed by cladding this kind of particles, in which the particles will be permanently in the suspended state. Currently, ferric colloid has been used in the industrial production of “magnetofluid”, which can be used in loudspeaker, magnetic ink, magnetic fluid seal, lubricant, bearing and so on.