Research On Preparation Of Rare Earth Oxide Doped Tungsten Nanopowders And Their Properties

Tungsten(W)is widely used in the fields of military industry,atomic energy industry,electric vacuum,light source et.al,due to its series of excellent performance.Powder metallurgy has been intensively employed to produce W,while dispersion strengthening and grain refinement are effective means to improve materials properties under the premise of obtain high density.In this study,we successfully fabricated ultrafine-grained W alloys using La2O3 doped W nanopowders prepared by solution combustion synthesis,The main research contents are listed as follows;(1)The influence of La2O3 source(lanthanum nitrate)added on the phase,morphology and reduction process of tungsten oxide synthesized by solution combustion has been studied in detail.W nanopowders with different La2O3 doping amount were successfully prepared.When the amount of La2O3 added is Owt%,0.5wt%and 1.0wt%,the product synthesized by solution combustion is needle-like W18O49,and the complete reduction temperature is 700?.When the amount of La2O3 added was 2.0wt%an d 5.0wt%,the product is a mixed oxide of needle-like W18O49 and granular H0.53WO3,and with the amount of La2O3 added increases,H0.53W.O3 phase increases.The complete reduction temperature is raised to 750?and 850?,respectively.The size of all W nanopowders is less than 50nm.The distribution of La2O3 in W powders is uniform,and even the W powder with 5.0wt%La2O3 added doesn't have significant La2O3 aggregation.During the solution combustion synthesis process,as the exothermic redox reaction between the oxidant and the fuel proceeds,the metatungstate is first pyrolyzed to form WO3,and then the NH3 decomposed by the fuel and the oxidant reduces WO3 to W18O49.The product of the combustion synthesis becomes H0.53WO3,while the La2O3 formed by the pyrolysis of lanthanum nitrate is not reduced by NH3 and still exists in the form of La2O3.(2)The densification behavior of the La2O3 doped W nanopowders prepared by solution combustion synthesis was studied.The ultrafine-grained W alloys were obtained,while theirs microstructure and physical properties were investigated.The Owt%,0.5wt%,1.0wt%,2.0wt%and 5.0wt%La2O3 doped W nanopowders can reach a relative density of more than 95%at the sintering temperature of 1350?,1500?,1500?,1650? and 1650?,respectively.And the grain size of the obtained W alloys were 4.92?m,0.66?m,0.57?m,0.47?m and 0.72?,respectively.The La2O3 particles in W-0.5wt%and W-1.0wt%alloys are evenly distributed,and the large-sized particles are rare.The average size of the intragranular and intergranular La2O3 particles in the W-1.0wt%alloys is 27nm and 57nm,respectively.The W-2.0 wt%and W-5.0wt%alloys have many large-sized La2O3 particles distributed on the grain boundaries,and the average size of intergranular La2O3 particles of the W-2.Owt%and W-5.Owt%alloys is 147nm and 210nm,respectively.Pure W,W-0.5wt%La2O3 and W-1.0wt%La2O3 all reached the maximum microhardness at 1500?,which were 587.1HV0.2,646.0HV0.2 and 684.1HV0.2,while W-2.0wt%La2O3 and W-5.0wt%La2O3 reached a maximum microhardness of 739.3HV0.2 and 660.0HV0.2 when sintered at 1650?.(3)The densification and grain growth kinetics of the as-synthesized W nanopowders were studied,as well as the evolution of La2O3 particles and the inhibition mechanism of grain growth.The sintering activation energy of pure W and W/1.0wt%La2O3 powder is only 505kJ/mol and 650kJ/mol,respectively,while the sintering activation energy of W/5.0wt%La2O3 powder rapidly increases to 1625kJ/mol.The grain growth behavior of W alloy with different La2O3 additions during high temperature calc:ination is quite different.The grain growth of W-2.0wt%La2O3 alloy is the slowest.After calcination at 1900? for 6h,the grain size of W-2.0wt%La2O3 alloy is only 1.23 ?m,and its grain boundary mobility at 1900°C is only 7.5×10-22-3.0×10-21m3·N-1·s-1.In the as-synthesized powders,the nano-sized W particles and the La2O3 particles are uniformly mixed,and durin g the sintering process,the La2O3 particles enter the W particles by the consolidation of W particles.Increasing the number of La2O3 particles in the unit space in the W matrix can effectively increase the effect of La2O3 particles on suppressing grain growth.But when the amount of La2O3 particles added is too large,the amount of La2O3 particles in the unit space is reduced due to the aggregation of La2O3 particles,weakening the inhibition of grain growth by the second phase particles.