The Synthesis Of High-entropy Carbide Ceramic Powders

High-entropy carbides,as a new type of ultra-high temperature ceramic materials,possess high melting point,high hardness,strength and wear-resistance,and outstanding high-temperature physical and chemical stability.In addition,large configurational entropy contributes to their improved mechanical and chemical properties,such as the hardness,the modulus and high-temperature physical and chemical stability,which makes them become one of the most promising candidates for the aerospace,military industrial and the other extreme fields.However,the as-reported high-entropy carbides generally have a series of problems such as large grain size,low relatively density,uneven element distribution,oxygen impurities,and so on,which severely limits the basic research and the performance of high-entropy carbides.The synthesis of the high-quality high-entropy carbide powders is crucial to cope with these concerns.To date,the researches about the synthesis of high-entropy carbide powders have been rarely reported.Therefore,in this study,three different methods were proposed to synthesize high-entropy carbide powders.Based on the first-principles calculation,the formation possibilities of the corresponding systems were firstly analyzed from the thermodynamic aspect.Then,the various high-entropy carbide powders were successfully prepared by carbothermal reduction,molten-salt method and carbothermal reduction and nitridation method respectively.We systematically investigated the effects of reaction temperature,reaction systems and the ratio of reactants on the phase composition of products.Afterwards,the morphology,crystal structure,particle sizes and composition uniformity of the as-synthesized products were characterized in detail.The formation mechanism and growth process of high-entropy carbides powders synthesized by different methods were also revealed.Detailed studies and results are shown as follows.(Zr_0.25Ta_0.25Nb_0.25Ti_0.25)C high-entropy carbide powders were prepared via carbothermal reduction.The mix enthalpy was calculated to be-0.232 k J/mol by first principle calculation and the formation possibility of product was investigated by thermodynamical analysis.And then(Zr_0.25Ta_0.25Nb_0.25Ti_0.25)C high-entropy carbides powders were successfully synthesized via carbothermal reduction method at 2473 K for 60 min by using metal oxides and carbon as starting materials.The results showed that the as-synthesized powders had a single rock-salt structure with the particle sizes of 0.5-1μm.All metal elements showed a homogeneous distribution from nanoscale to microscale without any segregation or aggregation.Besides that,they exhibited the distinguished coral-like morphology with the hexagonal step surface,whose growth was governed by a classical screw dislocation growth mechanism.In addition,the results showed that the other single-phase quaternary systems from Hf,Zr,Ta,Nb,and Ti elements could not be synthesized by carbothermal reduction at 2473 K.(Ta_0.25Nb_0.25Ti_0.25V_0.25)C high-entropy carbide nanopowders were synthesized by molten-salt assisted method.The mix enthalpy was calculated to be-0.193 k J/mol by first principle calculation.Based on the theoretical analysis of the formation possibility of different starting material systems by thermodynamical analysis,we successfully prepared(Ta_0.25Nb_0.25Ti_0.25V_0.25)C high-entropy carbide nanopowders by using metal powders to react with carbon black in KCl molten salt media at 1573 K for 60 min.The results showed that the as-synthesized products were a single rock-salt structure and exhibited angular particles with an average particle size of 80nm.All metal elements distributed uniformly from nanosacle to microscale without any segregation or aggregation.Molten salt medium can accelerate the reaction process through providing mass transfer and reaction environment for the reactants.In the molten salt liquid medium,the reactants went through three processes:dissociation and rearrangement of reactants,reaction of reactants,and nucleation and growth of products.(Hf_0.25Zr_0.25Nb_0.25Ti_0.25)(C_0.5O_0.5)were successfully fabricated via carbothermal reduction method.Namely,introducing oxygen atoms to anionic sublattice by adjusting the ratio of carbon in starting materials,aims to prepare multi‐anionic sublattice structural high-entropy oxycarbide powders.Firstly,the formation possibility of high-entropy oxycarbide powders via carbothermal reduction were investigated by thermodynamical analysis,Then,(Hf_0.25Zr_0.25Nb_0.25Ti_0.25)(C_0.5O_0.5)were successfully prepared by using metal oxides reacting with carbon black at 2473K for 60 min.The results indicated that the as-synthesized products showed a single-phase rock-salt structure and a granular morphology formed by helical stacking of hexagonal platforms.The average particle sizes were in range of 1-4μm.All metal elements and non-metal elements showed a highly uniform distribution from microscale to nanoscale without any segregation or aggregation.Furthermore,the experimental results showed that the single-phase(Hf_0.25Zr_0.25Nb_0.25Ti_0.25)（C,O）products with different ratios of anion were all prepared successfully at 2473 K,but the other single-phase quaternary systems from Hf,Zr,Ta,Nb and Ti elements could not be synthesized at the same temperature.(Zr_0.25Ta_0.25Nb_0.25Ti_0.25)(C_0.5N_0.5)high-entropy carbonitrides powders were successfully synthesized via carbothermal reduction and nitridation.The formation possibility of high-entropy carbonitrides powders via carbothermal reduction and nitridation reaction was firstly analyzed from thermodynamical aspect.Then,(Zr_0.25Ta_0.25Nb_0.25Ti_0.25)(C_0.5N_0.5)and(Hf_0.25Ta_0.25Nb_0.25Ti_0.25)(C_0.5N_0.5)were successfully fabricated by using the melamine and metal oxides as precursor at 2473K for 60 min.The results showed that the as-synthesized products exhibited a good single-phase rock-salt structure with stacking hexagonal platform morphology,whose particle size ranged from 1 to 2μm.All metal and nonmetal elements distributed uniformly from nanoscale to microscale without any segregation or aggregation.