Molten Salt Assisted Preparation Of Transition Metal High Entropy Alloy And High Entropy Oxide Powders

High-entropy materials are featured with the thermodynamic high entropy,structural lattice distortion,kinetic hysteresis diffusion and"cocktail"effects,which have attracted a wide attention in the fields including the high pressure,high temperature,wear resistant as well as energy due to their excellent thermal stability,corrosion resistance,diversity of active site and interatomic synergy.However,the synthesis and preparation of single-phase high-entropy powders still suffer from the extreme process condition,complex equipment,low product purity,and expensive raw materials.Molten salt synthesis has the advantages of the simple process,no special equipment,low costs and high purity of product.Quenching method enables the sample to be quickly cooled in a micro area and a very short time,and avoids the segregation of phase in the final product.To address these issues,transition-metal high entropy alloy（HEA）and oxide（TM-HEO）powders were synthesized by a quenching-assisted molten salt method with using the transition metals and metal oxides as raw materials,and NaCl,KCl and NaF as reaction media.The effects of reaction temperature,holding time and salt/reactant ratio on the phase and morphology of the as-prepared powders were investigated,and the lithium-storage property of the(Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2)O HEO powders were studied.On the basis of this,we further developed a molten salt-briquetting method,which could be used to synthesize the high-entropy rare-earth zirconate powder（RE-HEO）in a large scale.The resistance to CMAS erosion of the as-synthesized(La_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)₂Zr₂O₇HEO powder was also investigated.The results are as follows:（1）The optimal synthesis parameters for preparing single-phase FeCoNiCuMo HEA powders by a quenching-assisted molten salt method with transition metal nitrates（Fe,Co,Ni and Cu）,ammonium molybdate and citric acid as raw materials,KCl-LiCl as reaction medium and Mg as reducing agent were as follows:reaction temperature of 700°C,holding time of 3 h,magnesium powder dosage of n（TM/Mg）=1.0/0.6 and salt/reactant ratio（m_s/m_r）of 3.0/1.0.The synthetic of HEA particles followed a"dissolution-precipitation"mechanism:The KCl-LiCl complex salt melted at high temperature,and provided a liquid phase environment for the reaction system;The highly reactive Mg atoms reduced the Fe,Co,Ni,Cu and Mo ions,which solidly dissolved and precipitated to form a solid solution;And finally,the high-temperature solid solution was"transiently"solidified by liquid-nitrogen to form the single-phase HEA particles with a uniform distribution of elements.（2）The optimal synthesis parameters for preparing the rock-salt structured(Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2)O HEO powders using transition metal oxides（Mg,Co,Ni,Cu and Zn）as raw materials and KCl-NaCl as reaction medium by a quenching-assisted molten salt method were as follows:reaction temperature of 850°C,holding time of 3 h and salt/reactant ratio（m_s/m_r）of 3.0/1.0.The as-synthesized TM-HEO powders with a particle size of 1.16±0.33μm and a BET surface area of 0.953 g/m²,was used as a lithium-ion anode,which exhibited a first charge/discharge specific capacity of 963m Ah/g,and still had a specific capacity of 257 m A h/g at a high current density of 2.0 A/g.In addition,the single-phase HEO powders including the(Fe_0.2Co_0.2Ni_0.2Cu_0.2Mn_0.2)₃O₄,(Fe_0.2Co_0.2Ni_0.2Cr_0.2Mn_0.2)₃O₄and(Fe_0.2Zn_0.2Ni_0.2Cr_0.2Mn_0.2)₃O₄with a spinel structure and(La_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)₂Zr₂O₇with a pyrochlore structures were also synthesized using the quenching-assisted molten salt method,indicating that this method has a good applicability for synthesis of HEO ceramic powders.（3）Using the rare-earth oxides（La,Nd,Sm,Eu and Gd）and ZrO₂as raw materials and KCl-NaCl-NaF as salt medium,the single phase(La_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)₂Zr₂O₇HEO powder with uniform elemental distribution was synthesized in a large quantity by a molten salt-briquetting method at 1200°C for 2 h with a salt/reactant ratio（m_s/m_r）of3.0/1.0.when the diameter of briquette was（?）50 mm,26.38 g of RE-HEO powders could be obtained in a laboratory-scale synthesis.By using this powders as raw material,(La_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)₂Zr₂O₇ceramic was prepared from at 1600°C/10 h,and it exhibited a density of 95.3%,a Vickers hardness of 1244.80±226.50 HV,a nano-hardness of 15.74±0.45 GPa,a elasticity modulus of 200.33±8.50 GPa and a wear rate of 3.1×10^-12m³/（N·m）,while showing a superior resistance to CMAS erosion over the La₂Zr₂O₇ceramic.On the basis of this,the six-elements(La_1/6Nd_1/6Sm_1/6Eu_1/6Gd_1/6Y_1/6)₂Zr₂O₇andseven-elements(La_1/7Nd_1/7Sm_1/7Eu_1/7Gd_1/7Y_1/7Ce_1/7)₂Zr₂O₇HEO powders were also synthesized using the molten salt-briquetting method,which further verified the wide applicability of current method for the large-scale synthesis of HEO ceramic powders.