Investigation On The Synthesis And Characteristics Of Novel MAX Phase Nb₄AlC₃

MAX phase is a big family of tenary transition metal compounds.The co-existence of various kinds of chemical bonds endows it excellent performance of both ceramics and metals,which provides it wide application prospect.As a member of various MAX phases,Nb₄AlC₃ maintains excellent high-temperature mechanical performance,and is seen as one of the most hopeful MAX phases in the application of high-temperature structural materials.However,its flexural strength and hardness at room-temperature are relatively low.On the other hand,the Al element cannot form Al₂O₃ scale in advance during the oxidation process of Nb₄AlC₃,which causes poor anti-oxidation performance of Nb₄AlC₃.Therefore,it would be more favorable to realize its high-temperature applications if its mechanical performance and anti-oxidation performance were further improved.Furthermore,taking the advantage of weak bonding between MX layers and Al atoms,a new kind of two-dimensional（2D）carbide materials called MXene can be fabricated by etching the Al atoms.The MXenes are widely studies as electrode materials for lithium-ion batteries.Their stable layered structure can provide large amounts of sites for lithium-ion insertion.Sodium-ion batteries share similar working mechanism with lithium-ion batteries.Compared to lithium resources,the storage of sodium resources is more abundant.Besides,the price is much cheaper.Hence,sodium-ion battery is seen as the one of the candiates for the most hopeful next-generation secondary batteries.It is favorable to discover the performance of 2D MXene Nb₄C₃T_x（T_x represents-O,-OH or-F,etc.）as electrode materials of sodium-ion batteries.In this thesis,firstly,the Nb₄AlC₃ ceramic with relative density of 97.6%was fabricated by in-situ reactive/hot-press sintering,using the Nb,Al and C powders were used as original precursors.The mechanical performance of Nb₄AlC₃ was improved by doping with Ta or W elements.After analyzing of the relationship between mechanical performance and phase compositions,together with microstructure evolution,the results showed that there were core-shell-like(Nb_1-xTa_x)₄AlC₃ crystal grains forming in the samples at first.Then,as the amount of Ta doing increased,the core-shell-like crystal grains of second phase TaC/(Nb_1-xTa_x)C further formed in the samples.This structure can strengthen the interior or the periphery of the crystal grains.The sample with 50 mol%Ta doping maintained the best comprehensive mechanical performance.For comparison,the limit of W forming solid solution with Nb₄AlC₃ was rather low.There were crystal grains of second phase(Nb_1-xW_x)C in the sample when the doping amount was merely 1.25 mol%.The comprehensive mechanical performance of 2.5mol%W doping is the best among the W doping samples.The mechanism of improving the mechanical performance of Nb₄AlC₃ is mainly the synergistic strengthening effect of both solid solution and second phase grains.Ta or W doping has little effect on the thermal expansion coefficient of Nb₄AlC₃.The formation of large amount of second phases TaC/(Nb_1-xTa_x)C and(Nb_1-xW_x)C lowered the thermal conductivities of Ta or W doped samples.Secondly,the anti-oxidation performance of Nb₄AlC₃ was studied.The result showed that severe oxidation weight gain was observed at later stage of oxidation at600°C.This was mainly because that the oxide scale consisted of Nb₂O₅ and NbAlO₄,which could not protect the base phase.The Ta or W doping maintained very limited improvement in the anti-oxidation performance of Nb₄AlC₃.Thus,to further improve the anti-oxidation performance,the dense silicide layers with thickness of 53.3⁵6.5μm on the surfaces of Nb₄AlC₃ and its Ta or W doping samples were fabricated by molten-salt Si-permeation method.The layers mainly consisted of（Nb,Me）Si₂ and SiC,where Me represented Ta or W elements.At the initial stage of oxidation or low oxidation temperature,a continuous oxide scale SiO₂ will form on the surface of the Si-permeated sample,which could largely improve its anti-oxidation performance.Since the oxidation resistance of the TaSi₂ is superior to that of the NbSi₂,the anti-oxidation performance of the Si-permeated sample with 50 mol%Ta doping was better.Their oxidation rate constants at 1000¹100°C（0-20 h）shared the same magnitude with Ti₃AlC₂ of good anti-oxidation performance.However,at 1200°C,a large amount of transition metal oxides were formed,prohibiting the sample from forming a continuous SiO₂ scale,which leads to a rapid increase in oxidation weight.As for the sample with 2.5 mol%W doping,the WO₃ resulting from oxidation of W in the silicide film will evaporate to some extent at high temperature,which makes its oxidation rate constant a little lower than that of the Nb₄AlC₃ after Si permeation.The formation of silicide could increase the anti-oxidation temperature of Nb₄AlC₃ to 1100°C,which can be better combined with its good high-temperature mechanical performance,making Nb₄AlC₃ more hopeful for application as high-temperature structural materials.Lastly,the preparation and performance of the Nb₄C₃T_x-based 2D materials were preliminarily studied.Using the Nb₄AlC₃,Nb_3.5Ta_0.5AlC₃ and Nb_3.9W_0.1AlC₃ ceramic powders as precursors,the MXenes were fabricated after etching with HF aqueous solution.The transformation rate was higher than 87 wt.%.When tested as anode materials of sodium-ion batteries,the Nb₄C₃T_x exhibited good electrochemical performances.After 100 cycles at the current density of 100 mA g^-1,it could still deliver a discharge capacity of 69 mA h g^-1,which was comparable to sodium storage capacity of the widely studied Ti₃C₂T_x,which could demonstrate the potential of Nb₄C₃T_x for application in electrode materials.For comparison,the smaller distances of interlayers,together with the defects caused by solid solution,have led to worse electrochemical performances of these two samples.