Large size (and high mobility) molybdenum powder is mainly used for precision devices welding and spraying, the physical indicators are: large size (size greater than 10¼m), large bulk density (particle size is 3.0 ~ 5.0 g/cm3), good flowability (10 ~ 30s/50g).
Relative fisher particle size is generally under 5¼m, the particle size distribution is basically showed normal distribution, bulk density is between 0.9 ~ 1.3 g/cm3, the morphology of molybdenum powder is irregular particles, for poor mobility (Hall flow meter can not be measured) conventional molybdenum powder, there are mainly have three difficulties on preparing such molybdenum powder: large size, high density, good fluidity. The ideal molybdenum powder appearance which meets the requirements of these three points is large diameter solid sphere, this is different from non-specifications loose particles morphology of conventional molybdenum powder. In general, increases retrofit technology of molybdenum powder can be divided into chemical method and physical method.
Chemical methods prepare large particle size ammonium crystal block, in accordance with the hereditary principle, through subsequent calcination, reduction, prepared large-grained particles of molybdenum powder (conventional molybdenum powder particles are actually the aggregates of many small particles), followed by some mechanical treatment to obtain rounded shape, great density, large size molybdenum powder particles. This method is feasible theoretically, but prepares large single crystals of ammonium molybdate particles are more difficult, and the follow molybdenum powder size and morphology hereditary quantitative rule are not clear, process is longer.
Mechanical granulation technology, mechanical pressing the mixed molybdenum powder which plus binder in the mold or granulation equipment to obtain a certain size, then remove the binder, sintered into a certain intensity rule particles group. This method is simple in principle, but the experiments show that it is relatively simple to increase molybdenum powder particle size, no significant improvements on liquidity.
Plasma granulation technology, which applied in the metal powder change shape for a long time, the principle is that in protective atmosphere, through a certain way to put the powder into the plasma flame core, melting the powder particles at several thousand degrees Celsius, and then self-balling by the surface tension during the free fall, after cooling, the spherical droplet become large size, high- density spherical powder. The powder obtained in this way has good physical indicators, broad market prospects, but has greater technical difficulty, particularly in the powder delivery and keeping a protective atmosphere, cooling and collecting the finished products are more difficult, large equipment investment, difficult to keep in good repair.
Fluidized bed reduction method, the fluidized bed reduction method of molybdenum powder is proposed by the American researchers, through two phases of fluidized bed reduction, directly reduce the granular or powder form MoO3 into molybdenum powder. Phase 1 reduce the MoO3 into MoO2 at 400 ~ 650 by using ammonia as fluidized reducing gas; Phase 2 reduce MoO2 to Mo at 700 ~ 1400 by using hydrogen as fluidized reducing gas. Since in the fluidized bed, the gas and the solid can be obtained the fullest contact, the bed temperature is uniform, so that the reaction speed is fast, can effectively control the molybdenum powder size and shape, molybdenum powder produced by this method is equiaxed particles, with good powder fluidity, the subsequent sintering has high density. This approach has not found information about its specific production applications.
Article Source:http://www.mhcmp.com
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