Preparation Of(Hf,Zr,Ta,Nb)C Based High-entropy Carbides For Tuning Microwave Absorbing Performance

With the rapid development of communication technology and electronic countermeasure technology,the new generation of warplanes are required to have faster flight speed and better radar stealth ability,which requires the materials of warplanes to have high temperature resistance,wide frequency wave absorption,excellent mechanical properties and so on.（Hf,Zr,Ta,Nb）C high-entropy carbide（HEC）is an important candidate for thermal protection materials,due to its high temperature resistance and excellent mechanical properties.However,there are three problems:1.the preparation temperature of single-phase homogeneous HEC ceramic powder is high;2.Width of effective absorption band of HEC ceramic narrow;3.the sintering temperature of HEC ceramic block is high.Based on this,firstly,this project introduces Ni or Co into HEC,and then reduces the preparation temperature of HEC powder.Secondly,by doping Ni or Co into HEC,the electromagnetic parameters of HEC are regulated,and then the absorption performance of HEC is regulated.Finally,the low temperature densification of HEC-based ceramic blocks was achieved by liquid phase sintering with Ni as the sintering agent.Specific research results are as follows:（1）Ceramic powders were prepared by liquid precursor method and high temperature heat treatment.Single-phase HEC ceramic powders（1900-HEC）were prepared at 1900℃.Then,Ni doped HEC（1500-HEC/Ni）and Co doped HEC（1500-HEC/Co）ceramic powders were prepared at 1500℃,and the phase composition,microstructure,element distribution and wave absorption properties of the powders were systematically studied.The experimental results show that:a)the introduction of Ni can significantly reduce the formation temperature of HEC powder;b)Residual Ni or Co elements can regulate the electromagnetic parameters of HEC powder,and then regulate the absorption properties of HEC/Ni and HEC/Co.The minimum reflection loss(RL_min)of the 1900-HEC sample with a thickness of 1.68 mm is-30.06 d B at12.7 GHz.When the sample thickness is 1.50 mm,the maximum effective absorption bandwidth（EAB）reaches 4.0 GHz.In addition,when the thickness of the 1500-HEC/Ni sample is 4.45 mm,the RL_min at 4.4 GHz is-55.14 d B.When the thickness is 1.50 mm,the maximum EAB reaches 3.6 GHz.Secondly,when the thickness of the 1500-HEC/Co sample is4.67 mm,the RL_min at 4.20 GHz is-52.68 d B,and the maximum EAB reaches 4.3 GHz when the thickness is 1.50 mm.The absorption performance of 1500-HEC/Co sample was improved compared with 1900-HEC sample,and the effective doping of Co element improved the absorption performance of HEC powder.（2）Using 1500-HEC/Ni ceramic powders as raw materials,ceramic blocks were prepared by spark plasma sintering process（SPS）at different sintering temperatures（1300-1600℃）.The experimental results show that Ni,as a sintering agent,can promote the liquid phase sintering,thus significantly reducing the sintering temperature of HEC ceramics.In addition,the Young’s modulus and Vickers hardness of 1500-HEC/Ni ceramic blocks obtained at1500℃are the best,which are 490.75 GPa and 17.96±0.78 GPa,respectively.