| Molybdenum(Mo)is one of the most common-used refractory metal that has high melting point,high strength,high creep and corrosion resistance,low thermal expansion coefficient,and excellent thermal and electronic conductivities.Therefore,Mo alloys are very attractive for many criticatory fieldss Normally,a powder metallurgy approach is employed to prepare refaractory materials and their alloys.Compared to the micron-sized particle,ultrafine/nano particles can be sintered to dense materials with small grain sizes at the temperatures much lower than the micron particles.It is well known that grain refinement of materials can significantly improve their mechanical properties such as strength,hardness and wear resistance.Therefore,the preparation of both refractory metal ultrafine/nano particles and ther ultrafine-grained nanostructured materials have been attracted great attentions in recent years.Nowadays,hydrogen reduction of commercial molybdenum oxide is an important and well-established industrial process for the manufacturing of high-quality Mo powder.However,it is hard for this method to produce ultrafine/nano powder.Even though a lot of methods for synthesizing ultrafine/nano particles have been developed,nowadays,the cost,production efficiency and powder properties are still the main challenges for the production of ultrafine/nano Mo powder in a large scale,and many additional efforts are still needed.Therefore,in this study,two kinds of methods were developed for the preparation of ultrafine/nano Mo powder with low cost and high efficiency.One kind is the hydrogen reduction of M0O2 with aissitence of nucleation agents or Mo nanoseeds.Another kind is the reduction of MoO3 with carbon black followed with deep reduction by H2.The sintering behavior of the prepared Mo nanopowder was also investigated.Additionally,the preparation of ultrafine-grained nanostructured oxide dispersion strengthened(ODS)Mo alloys by sintering of nanopowder at low temperatures was also investigated.The followed progresses were abtained:1)The salt-assisted(chloride salt,0.1-0.5wt%)hydrogen reduction of MoO2 was developed to controll the nucleation and growth of Mo,and Mo particles with average particle size ranging from nanometer to micrometer were successfully prepared at 805-1000? It was found that the crucial issue of preparing ultrafine/nano Mo powder by hydrogen reduction of MoO2 is the controlling of nucleation and growth processes.With the assistence of dispersed chlorine salt particles,a large number of dispersed molybdenum nuclei can be formed,and then these molybdenum nuclei can grow by chemical vapor transport(CVT,based on the formation of gaseous MoO2(OH)2).Finally,large MoO2 particles were transformed into ultrafine/nano molybdenum particles.2)With the assistance of 0.1%NaCl,the decrease of particle size of MoO2 can not only increase the number of formed Mo nuclei(particles),but also increase the reaction rate,which can reduce the particle size and improve the dispersion of Mo particles.Under isothermal reduction condition,ultrafine/nano-Mo powders with average particle size of 100 to 800 nm were successfully prepared by adjusting the reaction temperature(840-1000?)and particle size of MoO2.It was found that the hydrogen reduction reaction of MoO2 was controlled by interfacial chemical reaction.For the case of without NaCl,the activation energies of hydrogen reduction of MoO2 with different particle sizes were in the range of 54.89-62.23 kJ/mol,while in the presence of 0.1%NaCl,the activation energies were increased to 67.05-73.76 kJ/mol.The nucleation and growth of Mo in hydrogen reduction process can be further optimized by nucleating at lower temperatures and growing at higher temperature,and smaller Mo particles with better dispersion can be successfully prepared.The added NaCl was easy to be removed by evaporating at above 900?.3)Mo nanopowder was successfully prepared by the growth of Mo nanoseeds in the hydrogen reducation MoO2("nanoseeds+CVT growth"),which could have great potencial to solve the problem of preparing nanosized Mo in industrial hydrogen reduction process.MoO2 powders with different amounts of Mo nanoseeds were successfully prepared by the reaction between carbon black amd commercial MoO3 with the different C/MoO3 molar ratios from 0.5 to 1.5.Then,Mo nanopowder was successfully prepared by hydrogen reduction of the MoO2 containing Mo nanoseeds at 750-900?,and the minimum average particle size can reach to about 70 nm.This strategy is also suitable for the preparation of W nanopowder and W nanopowders with the average particle size ranging from 35 to 180 nm were successfully prepared.4)A efficient and industrially feasible method was developed to parepare Mo nanopowder via the reduction of commercial MoO3 with carbon black followed with deep reduction with hydrogen.It was found that for the reduction of MoO3 with carbon black,the reaction process was determined by the C/MoO3 molar ratio.For the pareparation of Mo,the theoritical C/MoO3 molar ratio is 2.3,and the reaction process was MoO3(s)?MoO2?MoO2+Mo2C?Mo.When the molar ratio of C/MoO3 was 2.3,the particle size of prepared MoO2 nanosheets can reach to 90 nm,while for the case of 2.8,the particle size of MoO2 nanosheets was reduced to 60 nm.The average particle size of the prepared Mo and Mo2C can reach to about 67 and 30 nm respectively.A small amount of MoO2 remained in the prepared Mo nanopowder can effectively reduce the residual carbon content.After deep reduction with hydrogen,the residual carbon content can be reduced to about 0.02%,which can be further reduced to 0.008%when hydrogen containg 2%water vapor was used.The crucial issues for the preparation of MoO2,Mo and Mo2C nanoparticles via the reduction of MoO3 with carbon black were the great number of nucleation points spplied by carbon black and the migration of MoO3.When other reductant,such as large activated carbon,large granular graphite or gas reductants,was used,the particles size of the prepared MoO2,Mo and Mo2C reached to a few microns.5)A two-staged carbothermic reduction of MoO3 at 600? and 1050 0C,respectively,followed by deep hydrogen reduction at 800?,were conducted to produce Mo nanopowder in a relatively large-scale(60 g).Due to the very high sintering activity of Mo nanopowder,it can be sintered to near-fully densified compact with a relative density of about 95.8%at 1200?,which was much higher than the value of 70%for commercial micron-sized Mo powder.The hardness of sintered nanopowder at 1200? was about 254 HV,which was much higher than the value of 182 HV as sintering commercial micron-sized Mo at 1600?.The addition of Mo nanopowder into micron-sized Mo powder can noticeably activate its sintering and improve its hardness.6)A simple and efficient pathway was developed to prepared Mo nanopowders doped with oxides nanoparticles by reducing the doped MoO3(doping by sparying method)with carbon black at 600? and 1050?,and hydrogen at 800?.After sintering in H2 at around 1300?,nanostructured ODS Mo alloys with Mo and La2O3(or Al2O3)grain sizes of about 0.5 ?m and 50-75 nm were successfully prepared(relative density,about 95%).The obtained nanostructured ODS Mo alloys had much high hardness values.Benefiting from the fine-grain strengthening and dispersion strengthening mechanisms,for the Mo-La2O3 alloys,the highest value of hardness can reach to 338 HV(1%La2O3),while the highest hardness of Mo-Al2O3 alloys reached to 385 HV(1%Al2O3). |
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