Preparation Of (Ta_0.2Nb_0.2Ti_0.2Zr_0.2Hf_0.2)C High-Entropy Ceramics And Its Thermophysical Properties

High entropy carbide ceramics(HECs),as a new type of ultra-high temperature ceramics,have great research and application value in aerospace and other fields because of their high melting point,high strength,good thermal insulation and high temperature stability.Under the influence of high entropy effect,solid solution high entropy ceramics(Ta0.2Nb0.2Ti0.2Zr0.2Hf0.2)C have better high temperature oxidation properties,mechanical properties,thermal protection properties and ablation resistance than any single-phase transition metal carbides TaC,NbC,TiC,ZrC and HfC.In this thesis,(Ta0.2Nb0.2Ti0.2Zr0.2Hf0.2)C highentropy ceramic powders were successfully synthesized by a solvent-thermal combined with carbothermal reduction process using TaCl5,NbCl5,TiCl4,ZrCl4,and HfCl4 as the metal sources,furfuryl alcohol as the carbon source,and ethanol as the solvent.Porous(Ta0.2Nb0.2Ti0.2Zr0.2Hf0.2)C high-entropy ceramics with high compressive strength as well as low thermal conductivity were prepared by a pressureless sintering process by adding a porogenic agent to the precursor powder.The(Ta0.2Nb0.2Ti0.2Zr0.2Hf0.2)C-SiC composites with gradient structure were prepared by melt infiltration silicon method to provide new ideas for the research of ablation-resistant thermal insulation materials.The main research contents and conclusions are as follows:(1)The effects of heat treatment temperature and carbon content on the microstructure and oxidation behavior of(Ta0.2Nb0.2Ti0.2Zr0.2Hf0.2)C high-entropy ceramics were investigated.The results show that the increase of heat treatment temperature can promote the solid solution reaction,and finally the single-phase solid solution of the high-entropy ceramics was formed at the heat treatment temperature of 1800℃.With the increase of carbon content,firstly,the carbon thermal reduction reaction was gradually complete,the metal oxide content was reduced,and the oxidation resistance was improved.However,when the carbon content was further increased,the enriched carbon increased and the antioxidant properties decreased.It is found that when high entropy solid solution is formed,some carbon atoms will be enriched from their original position to the position with larger gap,thus forming carbon vacancies and enriched carbon.The increase of carbon vacancies can accelerate the mass transfer,which is beneficial to the solid solution reaction and the aggravation of lattice distortion.The excessive enriched carbon is easily oxidized to CO and CO2 gas at lower temperatures,which reduces the oxidation resistance of high-entropy ceramics.In a comprehensive comparison,the samples had the highest onset oxidation temperature(610.3℃)and the highest peak oxidation temperature(714.4℃)when the carbon content is slightly below the random ratio requirement.(2)The effects of SiO2 porogen content on the pore structure,compression properties and thermal conductivity of porous(Ta0.2Nb0.2Ti0.2Zr0.2Hf0.2)C highentropy ceramics were investigated.The results show that with the increase of SiO2 porogenic content from 0 wt.%to 20 wt.%,the small pores(0.4～3 μm)gradually disappear and the large pores(20～50 μm)gradually increase,and the porous(Ta0.2Nb0.2Ti0.2Zr0.2Hf0.2)C high-entropy ceramics have a double pore structure.As the porosity increased from 23.08%to 59.34%,the thermal conductivity of porous(Ta0.2Nb0.2Ti0.2Zr0.2Hf0.2)C high-entropy ceramics decreased from 4.12 W·m-1 K-1 to 1.11 W·m-1 K-1 and the compressive strength decreased from 133.1 MPa to 41.9 MPa.the aggravation of lattice distortion on phonon scattering reduced the thermal conductivity of the material,and the solid solution strengthening improves the mechanical properties of the material.At the same time,the double porosity structure makes several small pores in the high porosity ceramics.These small pores reduce the thermal conductivity of the material through phonon scattering and prolong the crack expansion path,which contributes to the improvement of the thermal and mechanical properties of porous ceramics.(3)The influence of the pore structure of high-entropy ceramics on the microstructure and ablation behavior of the gradient-structured(Ta0.2Nb0.2Ti0.2Zr0.2Hf0.2)C-SiC composites prepared by the melt-silicon infiltration method was investigated.The results show that the thickness of the dense layer gradually increases from 33.3 μm to 121.2 μm with the increase of porosity and pore size.The(Ta0.2Nb0.2Ti0.2Zr0.2Hf0.2)C-SiC composites with the largest dense layer thickness forms the uniform oxide layer with the lowest mass ablation rate of 0.33×10-4 g·cm-2·s-1 and the best ablation resistance.The densification of the composite surface layer can reduce the chemical erosion and mechanical stripping during ablation.Among them,the transition metal carbides form transition metal oxides and dissolve in molten SiO2 under the effect of ablation to form a high temperature resistant composite dense oxide layer,which can effectively impede the diffusion of oxygen and play a self-healing effect and effective protection during the ablation process.