Studies On The Preparation Of Novel Multi-component Refractory Materials By Direct Electrochemical Process In Molten Salt

Developing high-performance novel materials is vitally significant both for technology innovation and society evolvement.As new structural and functional materials,high-entropy alloys,solid solution carbides,MAX phases and metal/carbide composites have exhibited excellent performance in extensive application aspects.However,multi-component and high melting point of the elements increases both of production energy consumption and capital investment.Moreover,the structure of the material still needs to be optimized.The scale-up production of above-mentioned materials with controllable structures applied in cleaner and energy-efficient process benifits its commercial utilization.The solid state electrochemical reduction process in molten salt is exclusively advantageous in smelting refractory metals due to its clean and short process with lower energy consumption.Various conventional metals,simple-component alloys and ceramics etc.,have been successfully prepared using this process.However,the studies on the preparation of complex multi-component materials have not been reported.Herein,the synthesis of novel multi-component refractory materials via solid state electrochemical reduction process was investigated,and the effects of reaction condition and precursor ratio on the composition,morphology and performance of the product were also studied;the differences in formation processes and mechanisms between multi-component refractory materials and conventional materials were also discussed.The main work and corresponding results are described as follows:(1)The refractory high-entropy alloy Ti Zr Hf Nb Ta has been successfully prepared by direct electrochemical reduction of solid mixture transition metal oxides MO_x(M=Ti,Zr,Hf,Nb,Ta)in molten Ca Cl₂.Since graphite was used as anode,the O^2-will react with C to form CO₂ which can be captured rapidly as CO₃^2-and dissolved in molten salt during anode discharge process,the formed CO₃^2-then transform to C at the cathode and result in the contamination of the product.Based on the thermodynamic calculation,at the higher temperature,the formation of CO₃^2-can be inhibited and the transformation from CO₃^2-to CO is much favorable than to C,reducing carbon content in the product.The results show that the carbon content is0.686,0.466 and 0.28 wt%at 900,950 and 1000?,respectively.The higher reaction temperature also results in the increment of current efficiency and decrement of energy consumption.The corresponding current efficiency and energy consumption is27%and 40.06 k Wh kg^-1at 900?,42.7%and 25.27 k Wh kg^-1 at 950?,47.9%and22.52 k Wh kg^-1 at 1000?.Using SOM based anode can isolate graphite from molten salt and greatly avoid the carbon contamination.XRD and EDX demonstrate the high purity of product.SEM shows that the obtained products are porous bulk metal,which is beneficial for decreasing elasticity modulus.The measured elasticity modulus is94.8 GPa,which is lower than the reported value of the dense bulk metal(103 GPa).Overall,the preparation of uniform porous high-entropy alloy via solid state electrochemical reduction process is more advantageous than other methods.(2)Based on the strong affinity between Ti,Zr,Hf,Nb,Ta and C in high temperature and the same crystal structure of metal carbide,a series of multi-component solid solution carbides(Nb Ta C₂,Ti Nb Ta C₃,Ti Zr_0.3Nb Ta C_3.3,Ti Zr_0.3Hf_0.3Nb Ta C_3.6)have been synthesized by direct electrochemical reduction of solid mixture precursors composed of transition metal oxides and graphite powder in molten Ca Cl₂.When the metal content in precursor is of equi-atomic ratio,the solid solution of Nb Ta C₂ and Ti Nb Ta C₃ can be obtained,however,neither for Ti Zr Nb Ta C₄and Ti Zr Hf Nb Ta C₅.This observation is attributed to the slow deoxidation rate of Zr O₂ and Hf O₂ and slow migration rate of Zr and Hf.The effect of temperature on composition of the product show that the higher temperature,the lower degree of solid solution.The optimized reaction temperature and metal ratio of Zr,Hf to obtained solid solution of Ti Zr_xNb Ta C_3+x and Ti Zr_xHf_xNb Ta C_3+2x is 850?,x=0.3.SEM and TEM images show that the particle size of solid solution is less than 200 nm.The oxygen content of the product is 2.3 wt%even after 48 h of electrolysis.The longer deoxidation time is due to the extremely stability of the intermediate product(high-entropy metal oxycarbide).It is found that the high-entropy metal oxycarbide is the unique product during the electrolysis of metal oxides and carbon as hybrid cathode,enriching the species of high-entropy materials.(3)The presence of high entropy effect leads to the longer electrolysis time for the formation of high-entropy carbides.In order to obtain the lower oxygen content of new ceramic material more efficiency,we synthesized highly ordered MAX phased material.For the synthesis of Ti₃Al C₂,Ti C is the main impurity due to the loss of Al and the affinity between Ti and C.Increasing the Al₂O₃ content and decreasing the C content in the precursor can suppress the formation of Ti C.The optimized preparation conditions and element ratio of Ti:Al:C in precursor is 950?,3 V,4 h and 3:1.8:1.8.SEM image shows that layered Ti₃Al C₂ particles are obtained.The formation processes of layered Ti₃Al C₂ particles can be divided into two steps:Ti O₂-Al₂O₃-C is first reduced to Ti C_xO_y and Ca-Al-O,following by Ti₃Al C₂.The oxygen content of Ti₃Al C₂can be decreased to 4300 ppm after only 4 h electrolysis,which is lower than that using Ti O₂,Ti O₂-C and Ti O₂-Al₂O₃ as precusor,suggesting a faster deoxidation rate of Ti O₂-Al₂O₃-C intermediates.The formation of layered structure of Ti₃Al C₂ is attributed to the template effect of graphite plane.Due to the presence of conductive materials,the underpotential reduction process between Ti C_xO_y and Ca-Al-O,as well as the unique layered structure of Ti₃Al C₂,the reduction rate are considerably enhanced.Besides,the typical 211 phased Ti₂Al C and Cr₂Al C have also been successfully prepared using solid state reduction process,demonstrating a applicable synthesis method for MAX phase materials.The optimized preparation conditions to obtain high purity Ti₂Al C is at 950?under 3 V for 4 h with 2:1.5:1 of Ti:Al:C molar ratio in precursor.Accordingly,the optimized synthesis condition for Cr₂Al C is at 850?under 2.8 V for 6 h with 2:1.5:1(Cr:Al:C).SEM images show that the morphologies of Ti₂Al C and Cr₂Al C are different.Ti₂Al C shows layered structure while Cr₂Al C shows nodular-like structure.However,the formation processes of Ti₂Al C and Cr₂Al C are similar in two steps,initially,the hybrids of MO_x,/Al₂O₃,/C as precursor are reduced to MC_xO_y and Ca-Al-O,following by further reduction to form M₂Al C(M=Ti,Al).SEM images show that plate-like Ti C_xO_y and Cr C_xO_y can be obtained in the initial formation process.As the deoxidation process proceeds,the plate-like Ti C_xO_y transforms into layered structure,however,plate-like Cr C_xO_y become fragmented and then agglomerate into granular structure by sintering.The different properties of Ti C_xO_y and Cr C_xO_y should be the key influence factor towards the different product morphologies.Compared to Ti C_xO_y,Cr C_xO_y has poorer thermodynamic stability,resulting in rapid removal of O^2-in the initial deoxidation process,therefore,the plate-like structure can hardly preserve and become fragmented.Furthermore,because of the low melting point of Cr C_xO_y,the fragmented Cr C_xO_ycan be sintered and transformed to granular-like structure.(4)Regarding the synthesis of metal and carbide using molten salt electrolysis,the metal/carbide composite(Ni-W_xC/C)with high activity for urea electrocatalysis was prepared by direct electro-reduction of affordable Ni O-Ca WO₄-C in molten Ca Cl₂-Na Cl at 700?.The addition of graphite can greatly decrease the particle size of Ni,introducing W_xC into catalytic Ni can reduce the overpotential for UOR.As a result,the obtained Ni-W_xC/graphite composite exhibits a high anodic current density for urea oxidation,which is about 11-fold and 52-fold higher than that of Ni/graphite and Ni(0.6 V vs.Hg/Hg O),respectively.After changing the carbon source from graphite to CNTs,the anodic current density was further increased by 43%,reaching50.31 m A cm^-2.Moreover,the cathodic catalyst W_xC/CNTs obtained by the same preparation process exhibited high performance towards HER,with a low onset potential of 131.5 m V and a tafel slope of 69.5 m V dec^-1.Constructing an electrolyzer using Ni-W_xC/CNTs as anode and W_xC/CNTs as cathode can reach 10 m A cm^-2 at merely 1.65 V in 1M KOH aqueous solution containing 0.33 M urea,with excellent long-term electrochemical durability.The environmental-friendly production process using affordable feedstock by molten salt electrolysis deepens our knowledge in the synthesis of efficient catalyst for urea oxidation and hydrogen production.