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The wide application of high-frequency electromagnetic (HFEM) technology has promoted the research and realization of efficient EMW absorbing materials. Complex electromagnetic environments require efficient EMW absorber materials with high absorption capacity, thin thickness, light weight, and absorption bandwidth. Although conventional metal-based absorbers exhibit high magnetic loss and strong absorption capacity to some extent, the shortcomings of high density and low stability seriously hinder their further development.
High-entropy materials (HEMs) are a class of materials that exhibit a high degree of compositional complexity, typically characterized by the presence of multiple principal elements in near equimolar concentrations. High-entropy absorbing materials have received much attention in the EMW absorption community due to their properties such as high conformational entropy, lattice distortion, delayed diffusion and cocktail effect. For example, Zhao et al. prepared high-entropy TiO3 oxides by electrostatic spinning and heat treatment.TiO3 has a wide absorption bandwidth, which is almost twice that of single-phase BaTiO3.The improvement of the EMW absorption properties is attributed to the lattice distortions and oxygen vacancies induced by the doping of multiple cations. Similarly, Sun et al. synthesized the high-entropy alloy FeCoNiMn0.5Al0.2 using mechanical alloying.The alloying process improved the magnetic loss capability of the HEA and efficiently optimized the impedance matching.The optimum absorption peak of the HEA absorber was - 44 dB with an absorption bandwidth of 3.8 GHz.Although the current HE absorbers have shown good EMW absorption performance, the several key challenges have hindered their practical application. These challenges include low absorption efficiency, high density, high filler load, and poor impedance matching. Addressing these issues is critical to improving the practical suitability of hem for EMW absorption applications.
The main point of this paper
Advantages of metal and carbon component integration:
Integration leads to favorable physical, optical and electrical properties and enhanced electromagnetic wave (EMW) absorption.
Carbon nanofiber (CNF) properties:
Porous structure, high specific surface area, high electrical conductivity and low manufacturing cost for EMW absorption studies.
Role of electrospun polymer Membrane:
As a structural substrate to promote uniform dispersion of metal ion precursors and achieve uniform distribution of nanoparticles in the CNF matrix.
Importance of porous CNF design:
Properly designed porous structured CNFs can induce interfacial polarization and generate multiple interfaces and voids that effectively scatter EMW.
Challenges of High Entropy Alloys (HEA):
Thermodynamic differences between the constituent elements of nano-HEA may lead to phase separation, posing a challenge to the preparation of single-phase HEA by Joule heating.
Advantages of Joule heating method:
Generating huge Joule heating by applying high currents at both ends of the conductor, shortening the processing time to the order of seconds, and controlling the composition and microstructure of sintered HEA.
Innovative point of this study:
Honeycomb porous carbon nanofiber (HCNF)/HEA composites were prepared by electrostatic spinning-Joule heating method.
Preparation process of the composites:
Electrostatic spinning of PVA-M-PTFE nanoparticles in sol, followed by ultra-fast Joule heating process to form honeycomb porous structure and HEA nanoparticles.
Properties of the composites:
The dielectric properties of the composites were gradually enhanced with increasing entropy, and the synergistic effect of the honeycomb fully open pore structure and HEA improved the EMW absorption performance.
The optimum reflection loss (RL) of HCNF/HEA was -65.8 dB and the effective absorption bandwidth (EAB) was 7.68 GHz under the ultra-low filling condition of 2 wt %.
High Entropy Alloys (HEA) and Electromagnetic Wave Absorption:
HEAs have attracted much attention in the field of electromagnetic wave (EMW) absorption due to their efficient synergistic interactions between multiple components and tunable electronic structure
Challenges of HEA/carbon composites:
Synthesis of stabilized HEA/carbon composites is challenging, especially during conventional heat treatments that are prone to phase separation
Advantages of the Joule heating method:
The Joule heating method effectively controls the composition and microstructure of HEA, optimizes the internal electronic structure, and improves the EMW absorption performance by applying a large electric current at both ends of the conductor to generate enormous Joule heat.
Preparation of HCNF/HEA composites:
HCNF/HEA composites were prepared by successfully integrating HEA into honeycomb porous carbon nanofibers (HCNF) using electrostatic spinning and Joule heating methods
Properties of HCNF/HEA composites:
Under the condition of ultra-low filling (2 wt%), the HCNF/HEA composites exhibited a minimum reflection loss of -65.8 dB and a maximum absorption bandwidth of 7.68 GHz
Microstructure of the composites:
The fibers of the prepared HCNF/HEA composites were uniformly distributed with alloy nanoparticles without significant agglomeration
Electromagnetic wave absorption mechanism:
The proposed absorption mechanisms include strong polarization loss, three-dimensional conductive network, honeycomb porous structure, and complementary properties of magnetic and dielectric losses
Potential of Joule heating synthesis method:
The Joule heating synthesis method shows potential in the development of HEA-based lightweight absorbers
Electrical and thermal conductivity of the composites:
HCNF/HEA composites exhibit high electrical and thermal conductivity, which is significantly better than low entropy alloy (LEA) and medium entropy alloy (MEA) composites.
Surface chemical structure:
XPS analysis showed that the elements Fe, Co, Ni, Cu and Mn were uniformly distributed in the HEA nanoparticles without obvious elemental aggregation phenomenon
In this study, the potential of cobalt iron cobalt nickel copper manganese hydroxide (HCNF/HEA) nanoparticles on porous carbon fibers for the absorption of electromagnetic waves was investigated. It was shown that the desired HCNF/HEA composites could be successfully prepared by combining electrostatic spinning and Joule heating techniques.The HCNF/HEA composites possessed the essential characteristics of an excellent electromagnetic wave absorber, including high porosity, electrical and thermal conductivity, and strong electromagnetic energy conversion.The RLmin of the HCNF/HEA was as high as -65.8 dB at 10.9 GHz, with an optimum EAB of 7.68 GHz. The optimum EAB is 7.68 GHz. most excitingly, the filling level of the absorber is only 2 wt %, the highest ever recorded for a wave absorbing material in HEA. the excellent performance of HCNF/HEA composites is attributed to their honeycomb porous structure and the high entropy effects of the alloy. These effects enhance polarization and conduction losses, impedance matching and electromagnetic wave attenuation properties. In summary, this study provides valuable insights into the potential of HEA for electromagnetic wave absorption and offers exciting avenues for the development of advanced HEA-based electromagnetic wave absorbers using Joule heating technology.
