The physics of radar absorption is deceptively simple to state and notoriously difficult to engineer. When an electromagnetic wave strikes a surface, it can be reflected, transmitted, or absorbed. For a target to disappear from a radar screen, the material must first let the wave enter rather than bounce off—an impedance-matching problem, where the material’s effective permittivity and permeability must be tuned to resemble free space. Once inside, the wave must then be dissipated, typically through dielectric losses such as interfacial polarization, electron hopping, and aggregation-induced charge transfer, or through magnetic losses in ferrite and carbonyl-iron fillers. If either step fails, the wave either reflects away or passes through unattenuated. The review emphasizes that performance metrics such as reflection loss in decibels, absorption bandwidth, matching thickness, and radar cross section reduction must be evaluated together, because optimizing one often degrades the others.

The historical arc traced by the authors begins with the Salisbury screen, a simple resistive sheet placed a quarter-wavelength in front of a conducting backing, patented in 1952. That elegant but narrowband concept dominated for decades, alongside iron-ball paints and ferrite coatings. But modern threats operate across an ever-wider spectrum—from high-frequency over-the-horizon radar in the megahertz range, through the S-, C-, X-, and Ku-bands used by tracking and fire-control systems, up to millimeter-wave, terahertz, infrared, and even laser detection. Single-layer absorbers simply cannot cover this span, which has driven the evolution toward multilayer stacks, graded structures, and resonant architectures that combine broadband and narrowband absorption in one design.

Among the most significant architectural innovations highlighted in the review are frequency-selective surfaces and their absorbing cousins, the so-called rasorbers. These are engineered periodic patterns—printed circuits, resistive meshes, or coated fabrics—that allow certain frequencies to pass through while absorbing others. When integrated into composite laminates, they enable radomes that protect an antenna yet suppress radar returns at other frequencies. The authors describe designs ranging from resistive frequency-selective surfaces embedded in glass/epoxy composites to switchable, dual-polarized absorbers that can flip between absorbing and reflecting states using active elements such as PIN diodes and varactors. Active frequency-selective surfaces take this a step further, allowing both the frequency and the amplitude of absorption to be tuned in real time, a capability demonstrated with graphene-based capacitor structures.

Metamaterials represent another leap in design freedom. By arranging subwavelength resonant elements into carefully patterned arrays, researchers can create perfect absorbers with responses that nature does not offer in bulk materials. The review covers metamaterial absorbers spanning from the optical regime to extremely high frequencies, including polarization-insensitive terahertz absorbers, infrared and laser-compatible stealth structures, and reconfigurable metasurfaces based on phase-change materials such as vanadium dioxide. Shape-memory polymers and kirigami metastructures add mechanical tunability, stretching or deforming to shift absorption bands on demand. The authors note that information metamaterials—programmable surfaces whose electromagnetic behavior is controlled digitally—are pushing the field toward absorbers that can adapt to changing threat environments in real time.

Yet the heart of the review is polymer matrix composites, and for good reason. Aerospace structures are increasingly built from carbon-fiber-reinforced polymers, which are lightweight and strong but electrically conductive in ways that can produce strong radar reflections. The multifunctional solution is to design the structural material itself to absorb. Glass or polyimide fabrics reinforced with carbon black, carbon nanotubes, graphene, or nickel-coated fibers can be tailored layer by layer so that impedance rises gradually from the surface inward, drawing waves deep into the laminate where they are dissipated. Sandwich structures with honeycomb or foam cores add a third dimension: the cellular geometry provides mechanical stiffness while the coated cell walls create multiple internal reflections that extend the effective electrical path length. Graded honeycomb designs, in which the effective permittivity varies across the core, have demonstrated broadband absorption suitable for aircraft inlet applications, one of the most challenging scattering hotspots on a stealth platform.

The filler toolbox described in the review is remarkably diverse. Carbon nanotubes, discovered in 1991, remain a workhorse because their high aspect ratio drives electrical percolation at low loadings, enabling both absorption and mechanical reinforcement. Graphene and its derivatives add two-dimensional conductive networks and enormous interfacial area for polarization loss. MXenes—two-dimensional transition metal carbides first reported for electromagnetic interference shielding in 2016—combine metallic conductivity with low infrared emissivity, making them attractive for radar-infrared compatible stealth. Magnetic fillers such as ferrites, carbonyl iron, and MOF-derived amorphous magnetic powders contribute permeability-based losses that complement dielectric mechanisms, and hybridizing the two is a central strategy for broadband performance. Conducting polymers like polyaniline and polypyrrole, first synthesized in their conductive forms in the 1970s, offer processable, corrosion-resistant alternatives for coatings and textiles.

The review also surveys manufacturing routes, and here the comparison is critical because processing determines whether laboratory performance survives in a real structure. Solution casting and spray coating are simple but struggle with filler dispersion and scale. Electroless plating can metallize fabrics and fibers uniformly, producing nickel- or copper-coated textiles with strong shielding effectiveness. Additive manufacturing emerges as a transformative route: 3D printing enables bio-inspired concentric infill patterns, triply periodic minimal surface meta-structures in polymer-derived ceramics, and seamless meta-sandwich composites that are simultaneously lightweight, load-bearing, and broadband-absorbing—geometries that would be impossible to fabricate by conventional lamination. Nanoimprint lithography is bringing scalable production of multilayer metasurface devices within reach.

Beyond stealth, the authors point to a widening set of applications. Electromagnetic interference shielding is now essential for dense electronics, 5G and 6G hardware, and electric vehicles, and the same materials that absorb radar can suppress unwanted emissions. Flexible and wearable systems—knitted and woven fabrics with stainless steel, silver nanowires, or carbon nanostructures—combine shielding with joule heating, breathability, and durability. Thermal management is becoming a co-design requirement, since absorbed electromagnetic energy converts to heat; recent composites are engineered to absorb waves and dissipate heat simultaneously. Multispectral compatibility is another frontier, with transparent metamaterials and low-emissivity coatings working together to defeat radar, infrared, and laser detection in a single stack.

The review closes with an honest assessment of the challenges ahead. Low-frequency absorption, particularly in the S-band and below, still demands thick or heavy solutions, and achieving it with lightweight structures remains an open problem. Impedance matching across wide bandwidths, environmental durability, scalable manufacturing, and the integration of active components into structural laminates all require further work. The authors point toward high-entropy materials for extreme environments, intelligent electromagnetic materials designed with machine learning, and deeper synergy between material chemistry and structural architecture as the most promising directions. What emerges from their synthesis is a field in transition: radar absorption is no longer a coating problem but a materials-design problem, in which the airframe itself becomes the absorber. If that vision matures, the next generation of stealth platforms may owe their invisibility not to what is painted on them, but to what they are made of.

Subject of Research: Multifunctional radar-absorbing materials integrating structural and electromagnetic performance

Article Title: Review: multifunctional radar-absorbing materials—integration of structural and electromagnetic performance

Article References: Kishor, K., & Sheikh, J. (2026). Review: multifunctional radar-absorbing materials—integration of structural and electromagnetic performance. Journal of Materials Science, 61(43), 33398-33430. https://doi.org/10.1007/s10853-026-13614-x

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13614-x

Keywords: radar-absorbing materials, polymer composites, stealth technology, metamaterials, frequency-selective surfaces, electromagnetic wave absorption, carbon nanotubes, graphene, MXenes, honeycomb structures, additive manufacturing, electromagnetic interference shielding

The defining trick of a high entropy alloy lies in its name. Where a conventional alloy is dominated by a single base metal doped with trace additions, an HEA dissolves five or more elements at concentrations of roughly 5 to 35 atomic percent, typically with atomic radii differing by less than 15 percent. The authors note that the field has converged on a second, thermodynamic definition as well: a genuine HEA must possess a configurational entropy of at least 1.5R, where R is the gas constant, meaning the sheer number of possible atomic arrangements is large enough to stabilise simple solid solution phases instead of brittle intermetallic compounds. The result is a material whose structure and properties can be tuned not by tweaking a trace element but by redesigning the entire elemental recipe.

The review organises the field around four so-called core effects, first outlined by Jien-Wei Yeh and completed by S. Ranganathan in 2003. The high entropy effect favours stable, disordered solid solutions; sluggish diffusion slows the rate at which atoms move and phases transform; severe lattice distortion arises because every atom is surrounded by neighbours of different sizes, straining the crystal; and the cocktail effect captures the surprising composite behaviours that emerge when many elements interact at once. Crucially, each of these effects has a direct electromagnetic consequence. Lattice distortion bends electronic energy bands, splits energy levels and creates localised states between the valence and conduction bands, modulating conductivity and dielectric response in ways conventional alloys cannot easily replicate.

That structural disorder translates into an unusually rich toolbox of electromagnetic loss mechanisms. When a microwave strikes an HEA, energy is dissipated through electronic polarisation, as electron clouds shift against their nuclei in the local electric fields created by chemically distinct atoms; through interfacial polarisation, as charges pile up at grain and phase boundaries; through magnetic loss, as magnetic domains in ferromagnetic alloys containing iron, cobalt, nickel or manganese respond and resonate; and through conductive loss, as free electrons oscillate and convert wave energy into heat via Ohmic heating. The challenge, the authors stress, is balance: too much conductivity turns an absorber into a mirror, and a large mismatch between the material’s impedance and that of free space causes most of the wave to reflect away before it can be absorbed.

Impedance matching is therefore the central design problem, and here HEAs offer unusual flexibility. Because complex permittivity and complex permeability can both be adjusted through composition, HEAs can be engineered to bring the material impedance closer to that of ambient space, minimising the reflection coefficient while maximising the attenuation constant that governs how quickly the wave’s amplitude decays inside the absorber. The quantitative payoff is striking. Reported reflection losses for HEA-based materials span from about minus 10 decibels, the threshold below which 90 percent of incident power is absorbed, down to roughly minus 70 decibels, depending on composition, thickness and frequency. The reviewed literature includes an FeCoNiMnCu0.5 alloy with a reflection loss of minus 71.0 decibels at 6.79 gigahertz from a layer just 2.93 millimetres thick, and boron-doped FeCoNiCr alloys reaching minus 62.5 decibels at 2.11 millimetres.

Getting there, however, takes careful processing. Because pure HEAs tend to be strongly metallic and conductive, the review describes several engineering strategies for taming them. Mechanical alloying, particularly high-energy ball milling, flattens spherical particles into high aspect ratio flakes that promote multiple polarisation events and tune the complex electromagnetic parameters. Annealing then sharpens crystallinity and boosts saturation magnetisation: FeCoNiAlCr0.9 powders improved from minus 26.88 to minus 47.55 decibels after heat treatment, while FeCoNiMn0.5Al0.2 annealed at 500 degrees Celsius delivered minus 44.425 decibels over a 3.825 gigahertz bandwidth. Surface modifications do impressive work too. Phosphating FeCoNiMn flakes produced a protective layer that lifted performance to minus 62.4 decibels at 10.7 gigahertz while adding corrosion and oxidation resistance, and dealloying, which leaches aluminium out of FeCoNiCuAl surfaces, enhanced impedance matching and pushed losses past minus 56 decibels at two separate frequencies.

Doping and compositing broaden the palette further. Adding small amounts of non-metals such as carbon, nitrogen or boron introduces ionic character and dipole oscillations that feed dielectric loss; boron-doped FeCoNiCr alloys achieved a maximum bandwidth of 5.78 gigahertz while retaining mechanical strength and oxidation resistance. Encapsulating HEA nanoparticles in graphitic carbon shells, via routes from arc discharge in methane to metal-organic chemical vapour deposition, simultaneously improves impedance matching and reduces radar cross-section, with one carbon-coated FeCoNiCuMn composite reaching minus 65.8 decibels and an absorption width of 7.68 gigahertz. Pairing HEAs with polymers, oxides or ceramics blends the alloy’s magnetic loss and structural stability with the partner phase’s dielectric loss, as in FeNiCrMn-polylactic acid composites printable by fused deposition modelling, and FeCoNi(Si0.6Al0.2B0.2) powders embedded in paraffin, which the review singles out as among the most versatile performers, combining thin 1.2 to 2.0 millimetre layers with reflection losses down to minus 44.1 decibels in the X and Ku bands.

Measuring all this in a way that matters outside the laboratory is its own discipline, and the review devotes detailed attention to it. Vector network analysers paired with horn antennas measure scattering parameters such as S11 and S21 in free-space fixtures, from which shielding effectiveness and reflection loss are derived; HEA-reinforced carbon fibre composites tested this way achieved total shielding of about 45.9 decibels at 4 gigahertz. Open area test sites, with calibrated ground planes and antennas separated by 3 to 30 metres, serve compliance testing of large enclosures, while indoor anechoic chambers lined with pyramidal absorbers provide reflection-free environments for monostatic and bistatic radar cross-section measurements across controlled angles, polarisations and frequency sweeps. The review reports HEA composite radar cross-section reductions of roughly 10 to 30 decibels in chamber tests, and notes that such measurements bridge laboratory-grade shielding data and real-world electromagnetic detectability.

Obstacles remain before these alloys fly on actual aircraft. The authors highlight the absence of standardised testing protocols across frequency bands and environments, which makes comparing studies difficult; edge effects and sample size limits that hamper scaling VNA results to full components; interfacial stability and oxidation concerns when HEAs are coupled with polymers; and early-stage cost and manufacturability. Even the frequency coverage is lopsided: most HEA research targets the C and X bands central to military radar and communications, leaving S-band and Ku-band coverage comparatively underexplored. The review calls for quantitative assessment of absorption mechanisms, a firmer physical understanding of lattice distortion, scalable synthesis, multi-scale modelling and durability testing under extreme conditions as the priorities that will determine whether laboratory spectacles become fielded technologies.

The most transformative possibility, the authors argue, lies in merging HEA chemistry with additive manufacturing. Because 3D printing can control composition, microstructure and geometry simultaneously, it promises lightweight graded structures whose electromagnetic response is designed in from the melt, capable of surviving high temperatures and corrosive atmospheres while retaining their absorption performance. From stealth platforms and radar-visible UAV signatures to telecommunications, energy storage and even healthcare electronics, the combination of chaotic multielement chemistry and precision manufacturing offers something conventional absorbers cannot: a single material family whose dielectric and magnetic personalities can be tuned almost at will. If the remaining manufacturing and standardisation hurdles fall, the alloys once prized mainly for their strength and heat resistance may quietly become the invisible backbone of an electromagnetically cleaner, harder-to-detect world.

Subject of Research: High entropy alloys as electromagnetic wave absorbing materials for stealth and shielding applications

Article Title: A review on high entropy alloys as an absorber of electromagnetic radiation

Article References: Gowda, N. R., Setupathy, A., Raj, A. A. B., & Thangaraju, S. (2026). A review on high entropy alloys as an absorber of electromagnetic radiation. Journal of Materials Science: Metallurgy, 1(1), Article 5. https://doi.org/10.1007/s44492-026-00006-0

Image Credits: AI Generated

DOI: 10.1007/s44492-026-00006-0

Keywords: high entropy alloys, electromagnetic wave absorption, stealth technology, radar cross section, impedance matching, magnetic loss, dielectric loss, radar absorbing materials, electromagnetic interference shielding, mechanical alloying, additive manufacturing, lattice distortion