Electrolytic Synthesis Of Niobium Carbide And Niobium Carbide Based Composite Materials In Molten Salt

Transition-metal carbides present a series of advantages,such as high melting point,high hardness,good thermodynamic stability,and excellent acid alkali inertness,which not only inherits the the electromagnetic properties but also overcome the drawbacks of low melting point and boiling point of the metal matrix.In addition,carbides are widely used in high temperature atmosphere,friction,chemical corrosion resistant fields,etc.However,there are still some challenges for carbides when used alone,such as brittleness,easy cracking,low wettability and poor sinterability.The insufficient features could be enhanced by adding more ductile metals or intermetallic compounds to form metal-carbide cemented composite materials.Herein,NbC was employed as ceramic phase to prepare carbide-metal composite materials.During this procedure,the combination modes between NbC and low melting point metal(Sn),high melting point metal(Fe)and intermetallic compound(Nb5Si3)were respectively investigated,as well as the features of the composites.NbC-Sn,NbC-Fe and NbC-Nb5Si3 were directly prepared by molten salt electrolysis,in which CaCl2-NaCl mixed molten salt was served as the electrolyte.The mixture of metal oxides and carbon powders were ball milled and pressed to a cylindrical pellet as the cathode.A graphite rod was employed as an anode.In addition,the synthesis mechanism and the effects of process parameters on the structure and performance were investigated.The main results are summarized as following:(1)Nb2O5 and carbon were sintered as the cathode.Electrolysis was carried out in a molten eutectic mixture of CaCl2-NaCl at 900℃ under 3.0 V for 11 h.The average size of the NbC particles is 100 nm with a bulk density of 6.07 g/cm3.(2)In order to prevent nano-sized NbC particles from aggregation,the introduction of Sn could not participate in the reaction and Sn can be separated from NbC nanoparticles by acid etching.The liquid Sn would flow between Nb2O5 and carbon powder during the electrolysis because of its low melting point of 232℃.After 12 h electrolytic reduction,NbC-Sn composite powder can be obtained,which was subsequently leached by 18%mass fraction HCl aqueous solution to remove Sn.Meanwhile,NbC particles with good dispersion and uniform size were obtained.(3)The core-shell structured NbC-Fe composite material was prepared by electrochemical reduction of Fe2O3,Nb2O5 and carbon powder at 900℃ in the molten salt for 10 h,which could overcome the unsatisfactory wettability between NbC and liquid steel.NbC particles were served as the core,and uniformly wrapped by a ductile shell of Fe.The diameter of composite particle is 100 nm.Then the NbC-Fe composite was added to the low-carbon steel to smelt.The specimens were characterized via SEM,metallographic and hardness analysis.The results show that the core-shell structure of NbC-Fe composite not only presents much better wettability and dispersivity in the steel,but also significantly refines grains.The grain size of steel with NbC-Fe is only half of that without adding NbC-Fe.Moreover,NbC-Fe particles also can help increase the hardness of steel effectively.(4)Nb5Si3 was prepared in the molten salt by electrochemical reduction.The results show that a-Nb5Si3 and γ-Nb5Si3 were formed when the mole ratio of Nb/Si is larger than 1.67,and when the ratio is lower than 1.67,a-Nb5Si3 was formed at 900℃in molten salt.A multicore-shell structured NbC-Nb5Si3 is available after the electrolytic reduction.This kind of multicore-shell structure is made by four or five NbC particles as the core,coated with Nb5Si3 metal shell.The in-situ prepared mulcore-shell structural composite material is stable,avoiding the phase pollution between metal and ceramic.