The Synthesis Of High-entropy Transition Metal Nonoxide Ceramic Powders

Hig-entropy transition metal non-oxide ceramics,including high-entropy diborides,carbides and disilicides of transition metals IVB,VB and VIB groups,are expected to be used in aerospace engines,thermal protection of space and space vehicles,nuclear energy and high-speed cutting tools in extreme environments due to their high hardness,high modulus,low thermal conductivity and good oxidation resistance and other excellent properties.However,the developed high-entropy transition metal non-oxide ceramics generally have the disadvantages of low density,uneven distribution of elements and high content of oxygen impurities,which seriously limit the basic research and performance of this kind of materials.The synthesis of high-quality powders materials is the fundamental way to solve the above problems,however,there are few reports on the synthesis of high-entropy transition metal non-oxide ceramic powders.Therefore,this work aims to synthesize three kinds of high-quality high-entropy transition metal non-oxide powders with different systems.High-entropy diboride,carbide and alumino-silicide ceramic powders were synthesized by molten salt assisted Na BH₄ reduction,low temperature molten salt assisted combustion and element direct reaction method,respectively.Firstly,the synthesis possibility of the three kinds of high-entropy transition metal non-oxide ceramic powders was analyzed from the aspect of reaction thermodynamics,and then the corresponding high-entropy transition metal non-oxide ceramic powders were synthesized by the corresponding experimental methods.The influence of the ratio of raw materials,reaction temperature,and other factors on the morphology and phase composition of the products was systematically studied,the particle size,crystal structure,and element distribution uniformity of the as-synthesized powders were characterized in detail,the corresponding synthesis mechanism were also revealed.The main research contents and results of this work were as follows:Ultrafine(Hf_0.25Zr_0.25Ta_0.25Nb_0.25)B₂(HEB-1)high-entropy diboride nano-powders were sucessfully synthesized by molten salt assisted Na BH₄ reduction method.Firstly,the analysis results of Gibbs free energy of the reactions show that,the target products can be synthesized by using molten salt assisted Na BH₄ reduction method,then,the ultrafine HEB-1 high-entropy diboride nano-powders were successfully synthesized at1173 K for 24 h by using transition metal chloride and Na BH₄ powders as reactants,and Li Cl/KCl salts as molten salt medium.The results showed that the as-synthesized powders was composed of a single HEB-1 phase,in which all the metal elements were uniformly distributed in the micron and nanometer scale without segregation or aggregation.Meawhile,their average particle size was about 8.0 nm,the oxygen impurity content of the powders was 8.08 wt.%.The synthesis mechanism of the powders was mainly that Li Cl/KCl liquid molten salt medium provided mass transfer and reaction environment for the reactants,and accelerated the reaction process.The reactants underwent three processes in the liquid molten salt medium:dissociation rearrangement,reaction,and nucleation growth.High-entropy carbide nano-powders were synthesized by low temperature molten salt assisted combustion.Taking(Nb_0.25Ta_0.25Ti_0.25Zr_0.25)C(HEC-1)as the synthesis target,the analysis results of Gibbs free energy of the reactions and adiabatic temperature show that,the target products can be synthesized by low temperature molten salt assisted combustion,then,the HEC-1 high-entropy carbide nano-powders were successfully synthesized by low temperature molten salt assisted combustion when the Mg content was 1.55Mg by using the transition metal oxide,C powders and Mg powders as reactants,and the Na F salts as the molten salt medium.The results showed that the average particle size of the as-synthesized powders was 19.0 nm,all the metal elements are uniformly distributed in micron and nano scale,and the content of oxygen impurities in the synthesized powders is 2.98 wt.%.In addition,using this method,we can successfully synthesize high-entropy carbide nano-powders composed of any five or four elements in the five elements of Nb,Ta,Ti,Zr and Cr,any high-entropy carbide powders composed of six or more elements can not be synthesized in the eight elements of Nb,Ta,Ti,Zr,Hf,W,Mo and Cr under present conditions.The(Mo_0.25Nb_0.25Ta_0.25V_0.25)(Al_0.5Si_0.5)₂(HEAS-1)high-entropy alumino-silicide powders were synthesized by element direct reaction method.By adjusting the element ratio of Si and Al in the sublattice of anion site,the high-entropy alumino-silicide powders with multi-principal cation and cation structure was prepared,firstly,combining first principles calculation,the analysis results of Gibbs free energy of the reactions show that,the target products can be synthesized by direct reaction of elements,then,the HEAS-1 high-entropy alumino-silicide powders were successfully synthesize at 1573 K for 30 min by using the transition metal,Si and Al elemental powders as reactants.The results showed that the as-synthesized powders is mainly composed of HEAS-1 target phase and a small amount of Al₂O₃ and Si O₂ impurity phase,the particle size of the powders was 0.5 to 3?m,the structure is hexagonal crystal structure,all metallic or nonmetallic elements were uniformly distributed on the micron and nanometer scales.