Research On Vanadium And Chromium Nitride Nanocomposites’ In-situ Synthesis Using Microwave And Its Applications

In this paper,starting from the innovation of raw materials,the traditional micronmter sized raw materials was replaced by nanometer sized vanadiumu oxide,chromium oxide and carbon black.Inaddition,nano-vanadium nitride/chromium nitride composite powders were prepared using traditional carbothermal reduction method and relatively novel microwave in-situ synthesis methods,respectively.The experiments mainly studied the reaction temperature,holding time,and carbon content in the traditional carbothermal reduction method,and the effect of microwave power,heating time,and the amount of carbon in the microwave in-situ synthesis method on the phase composition and microstructure of the synthesized product.In terms of phase characterization,the synthesized products were characterized by XRD,TG-DSC,SEM,BET,and TEM.The experimental and characteration results show that:1.When the vanadium nitride/chromium nitride composite powder was synthesized by the traditional carbothermal reduction method,VN/CrN nanocomposite powders with an average particle size of 300 nm were synthesized at 1200℃for 2 h.The as-prepared nanocomposite powders with a surface area of 21.09 m²/g were mainly composed of VN,CrN and VCrN₂.2.When using microwave heating to synthesize a single vanadium nitride powder,the optimum process conditions were20 wt.%of carbon,microwave power of 900 W,and heating for 2 h.The prepared VN powders were mostly spherical,and the average size of the powder particles is about 48 nm.The preparation of VN powder by microwave as a heat source can significantly reduce the synthesis temperature.3.When using microwave heating to synthesize single component of chromium nitride powders,the optimum process conditions were 22 wt.%of carbon,microwave power of 1100 W,and heating for 2 h.The CrN powder particles were mostly polygonal,and the particles in some regions were sintered.The sintered necks appeared between the particles.The average size of the particles was about 500 nm.4.The material transformation sequence that occurs during the gradual reduction and nitridation of Cr₂O₃ is:Cr₂O₃→（Cr₃C₂）→Cr₂N_0.39C_0.61→Cr_6.2N_3.5C_0.3→CrN→Cr₂N.4.When synthesis of vanadium nitride/chromium nitride composite powders using in-situ microwave-assisted method,the Chromium nitride/vanadium nitride composite powder with a nitrogen mass fraction of16.45%,a specific surface area of 7.22 m²/g and average particle size of approximately 400nm were achieved using carbon content of 22 wt.%,microwave power of 1100 W,and heating for 2 h in N₂ atmosphere.Most of the powder particles were polygonal,and there were sintering necks between a small number of particles and hard agglomeration occured.In the preparation process,the lowest formation temperature of chromium nitride/vanadium nitride is 929°C.Compared with the traditional carbon reduction method（>1200°C）,microwave heating method can significantly reduce the reaction temperature.When the composite powder was added to the ceramic binder,the strength of the ceramic binder was increased by a maximum of 188%and the fluidity was increased by a maximum of 90.2%.5.Through the combination of thermodynamic analysis,it can be seen that V₂O₅ has undergone stepwise reduction to generate V₂O₃ in the reaction process,and V₂O₃ directly generates VN without carbonization.The reaction process can be expressed as V₂O₅→VO₂→V₂O₃→VN;On the contrary,Cr₂O₃ is directly carbonized to Cr₃C₂ during the reaction,and then Cr₃C₂ is nitrided to CrN.The process can be expressed as:Cr₂O₃→Cr₃C₂→CrN.Increasing the reaction temperature and increasing the holding time facilitates the diffusion of nitrogen atoms into the crystal,which can increase the nitrogen content of the resulting VN/CrN composite powder.