Research On Preparation Of Ultrafine Rhenium Powder By Chemical Vapor Deposition

With excellent properties of high temperatures mechanical,thermal fatigue resistance, wear resistance, corrosion resistance, as well as catalytisis,rhenium and its alloys are widely used in national defense, aerospace, electronics industry as well as petrochemical industry. Methods of producing rhenium powder by hydrogen reduction of ammonium perrhenate and electrolysis are of simple technology and easy to realize large-scale production, but, the properties of these products are with irregular morphology, Coarse and uneven-distributed of particles, low crystallinity, low tap density and poor liquidity etc.Preparation of rhenium powder by plasma is also reported, but its equipment is complicated, and energy consumption is quite high. In addition, good quality rhenium or alloy coating can be obtained from rhenium halide by CVD,however, this kind of method has the problems of strong corrode and high requirements of material of equipment.In order to solve these problems,lessons have been drawn from the preparation of rhenium or alloy coating by CVD in this thesis.Product of extraction metallurgy of rhenium was used as raw material, the low-temperature thermal decomposition of Re2O7 was utilized, and ultra-fine rhenium powder was obtained by hydrogen reduction for the fist time.The technology of decomposing of NH4Re04 and transporting of Re2O7 in oxygen atmosphere was invented, which can avoid the further decomposition of Re2O7 broken down into low rhenium oxide, and solve the problem of volatile.To produce rhenium powder by CVD through using NH4ReO4 as raw material has the advantages of simple raw materials,less demand of material of equipment, environmental friendly, as well as high performance of product.Results from calculation of thermodynamic show that, within the temperature 700K, Re2O7, which is the product of NH4Re04 decomposition, can not break down into low rhenium oxide, through controlling Po2 greater than 10-5.884Pa, which can ensure the complete decomposition of NH4Re04 and volatile to the CVD reactor.theΔrGΘof hydrogen reduction of Re2O7 is less than 0,and when the temperature becomes high and theΔrGΘbecomes small,the trend becomes obvious. The effects of technological conditions such as temperature of decomposition of ammonium perrhenat, carrier gas (N2) flow, temperature of reduction, oxygen gas flow, partition wall position, and hydrogen gas flow on the products were analyzed systematically through the experiment of single factor condition. The morphology, particle size, and structure of the products were characterized by scanning electron microscope(SEM), laser particle analyzer, x-ray diffraction (XRD).The results indicated that the optimal conditions are decomposition temperature 400℃,carrier gas (N2) flow rate 60L/min, reduction temperature 1000℃,oxygen flow rate 60ml/min, no partition, and hydrogen stoichiometric ratio 6.The desirable powder, prepared under optimal conditions.,prepared under optimal conditions, is with spherical or similar spherical morphology, clear outline of particles. It also has high purity, high crystallinity, narrow particle size normal distribution, with average partical size of 0.308μm(D50),oxygen content of 0.45%,specific surface area of 4.37m2/g, which meets the basic needs of modern powder metallurgy.