Research On Preparation And Electromagnetic Protection Performance Of Entropy-Stabilized Carbides

Entropy-stabilised carbide ceramics have important potential applications in aerospace,machinery and other fields due to their better overall performance compared to single metal component carbides due to the high mixed entropy effect.In recent years,the growing demand for communication capabilities has stimulated the development of microwave engineering,leading to large-scale electromagnetic wave pollution,which poses a threat to human health and information security.Traditional organic or metallic electromagnetic protection materials suffer from insufficient thermal stability and oxidation resistance in high-temperature applications,while entropy-stabilised carbides as electromagnetic protection materials have good mechanical properties and excellent high-temperature performance,etc.,and can meet the requirements for use in high-temperature service environments.Furthermore,the"cocktail"effect opens a new door for the design of materials with electromagnetic protection properties.In this paper,the structural stability of different systems of entropy-stabilised carbides is calculated and theoretically analysed using the first-nature principle,and the entropy-stabilised carbides are prepared by a combination of hydrothermal and molten salt-assisted methods.The main studies and results of this thesis are as follows:（1）In this thesis,three entropy-stabilized carbide systems,of（WMoCrZr）C_x,（WMoV）C_x,and（WMoVTi）C_x were modeled,and the Gibbs free energies of the three systems were-14.508 k J·mol-¹,-2.407 k J·mol^-1 and-5.845 k J·mol^-1 through first-principles calculation,which can exist stably at room temperature and provide theoretical support for the preparation of three entropy-stabilized carbides.（2）The hydrothermal synthesis process of（WMoCrZr）O_x was investigated.The optimum hydrothermal synthesis process parameters were 180°C and 8 h.The（WMoV）O_x and（WMoVTi）O_x entropy-stabilized oxides were synthesized with these hydrothermal process parameters.The phase composition,microscopic morphology and structure,and elemental composition of the products were analysed by XRD/SEM/EDS,and it was found that the entropy-stabilised oxides with a uniform and nearly equal distribution of metal elements were obtained.The entropy-stabilized oxides were then used as the metal source and graphene as the carbon source,and three entropy-stabilized carbides,（WMoV）C_x,（WMoVTi）C_x and（WMo Cr）C_x-ZrO₂,were prepared by the molten salt-assisted method,and the analysis of the phase composition,microstructure and elemental composition of the products by XRD/SEM/EDS revealed that:The microscopic morphology of（WMo Cr）C_x-ZrO₂ shows small particles of ZrO₂ adhering to the coral-like morphology of（WMo Cr）C_x,and the distribution of W,Mo and Cr elements is uniform and close to equal proportions,but the experimental results differ from the theory,probably because the absolute value of the mixing enthalpy of the system is large,resulting in a large trend of phase separation of（WMoCrZr）C_x,which is not easy to synthesize.The microscopic morphology of（WMoV）C_x and（WMoVTi）C_x are both flocculent structures composed of particles,the content of element V in（WMoV）C_x is high,and the content of V and Ti in（WMoVTi）C_x is high,but each metal element is uniformly distributed,and no obvious bias aggregation phenomenon occurs.（3）The electromagnetic shielding performance of the three entropy-stabilized carbide systems was studied by vector network analyzer,and the results showed that:（WMoVTi）C_x system has the best electromagnetic shielding effectiveness,with the highest SE_Tvalue reaching 70 d B,and the introduction of Ti elements improves the SE_Tvalue of the material;（WMo Cr）C_x-ZrO₂ complex-phase entropy-stabilized carbide has the best wave absorption performance,with the reflection loss reaching-30.88 d B,which can absorb 99%of electromagnetic waves.The introduction of Cr and Zr elements increases the RL value of the material,resulting in better microwave absorption performance.However,the effective absorption bandwidths of all the system entropy-stabilised carbides are narrow and the matching thicknesses are relatively thick.