A lightweight carbon sponge absorbs over 99.9999 percent of radar signals and insulates against heat, offering dual stealth performance without added metals or coatings.
(Nanowerk Spotlight) Modern radar systems can detect objects the size of a bird from hundreds of kilometers away. Infrared sensors can track heat signatures through clouds, darkness, and camouflage. In a world of increasingly sophisticated surveillance technologies, invisibility is no longer just a tactical advantage. It is a technical challenge that is growing harder to meet. Whether for military aircraft avoiding detection or communication devices trying to reduce signal interference, the demand is the same: materials that can absorb electromagnetic energy across a wide range of frequencies while remaining light, durable, and effective.
The requirements are precise and often in conflict. Materials must be thin but still absorbent. They need to cover broad frequency ranges without becoming heavy. And they must handle not only radar waves but also infrared radiation, two fundamentally different parts of the electromagnetic spectrum. The search for a single material that can do all of this without layering or hybridization has remained largely unsolved.
Carbon-based materials have long been candidates for this task. They are stable, conductive, and easy to manufacture. But they come with a built-in flaw. Their high conductivity tends to reflect, rather than absorb, incoming waves. Engineers have tried various fixes, from adding magnetic particles to etching in pores, but each solution introduces new trade-offs such as weight, complexity, or fragility.
What if a single piece of carbon could be structured internally to overcome these limitations? What if, like the layered anatomy of a leaf, it could guide waves inward at its surface and trap them at its core?
The structured carbon material, called O-CMS-150, is fabricated using a process known as gradient oxidation-assisted calcination. The researchers began with a commercial melamine formaldehyde sponge, a porous polymer that is easy to shape and handle. They then introduced oxygen-containing chemical groups to the sponge by heating it in air under controlled conditions. These groups were more concentrated at the surface and less so in the interior.
During carbonization, this gradient led to a structure with a disordered outer layer and a more graphitized, or structurally ordered, core. The outer layer helps electromagnetic waves enter the material. The inner region is better suited for absorbing and dissipating the energy once it is inside.
This architecture gives the material its most critical properties. At a thickness of just 2.37 millimeters and a filler loading of 5 percent, O-CMS-150 achieves a minimum reflection loss of minus 65.3 decibels. That corresponds to over 99.9999 percent absorption of incoming microwave energy. The effective absorption bandwidth extends from 11.38 to 18.00 gigahertz, covering the entire Ku-band used in radar and satellite communications. These results are notable because they were achieved using only carbon, without the addition of metallic or magnetic particles.
The schematic illustration of the preparation process and forming mechanism of a) non-gradient structure of CMS and b) gradient structure of O-CMSs. (Image: Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
Microscopy revealed that the internal structure contains layers of carbon with lattice dislocations and atomic vacancies. These imperfections behave like tiny electric dipoles. As they respond to incoming electromagnetic fields, they convert energy into heat through polarization loss. The continuous carbon framework inside the sponge also creates a conductive network that enhances energy attenuation. The outer layer, being less ordered, improves impedance matching by reducing the contrast between the material and the incoming wave. This balance allows the wave to enter the sponge more efficiently, instead of bouncing off the surface.
The pore structure adds another level of function. Channels and cavities inside the sponge cause the wave to scatter and reflect internally. Each time the wave interacts with the material, energy is lost. This multi-reflection mechanism is important for achieving strong absorption in a compact form. Tests confirmed that electrical permittivity changes smoothly from the outer layer to the core, which is a key feature in achieving both entry and attenuation of electromagnetic energy.
Alongside electromagnetic performance, the material also addresses the problem of thermal visibility. Infrared detection works by identifying heat emitted from objects. A material that absorbs radar waves but radiates heat is only a partial solution. To be fully stealth-compatible, it must also insulate against heat transfer.
The researchers tested thermal performance by placing the sponge on a hot plate set to 80 degrees Celsius. After two hours, the top surface of the sample remained below 38 degrees. Its thermal conductivity was measured at 32.5 milliwatts per meter-kelvin, indicating that it effectively slows the transfer of heat.
This insulating ability results from the same internal features that support microwave absorption. The pores trap air, which is a poor conductor of heat. The presence of lattice defects and disordered carbon at the surface causes heat-carrying vibrations, called phonons, to scatter. These effects combine to reduce the overall thermal signature of the material.
To assess radar stealth capabilities, the team simulated the radar cross-section of a metal plate with and without the O-CMS-150 coating. The coated version reflected far less microwave energy. The radar cross-section reduction reached 31.88 square decibels, a figure that compares favorably with many existing radar-absorbing materials. This metric quantifies how detectable an object is by radar. The lower the value, the less visible it becomes.
What sets this approach apart is the simplicity of its ingredients. There are no exotic dopants, no metallic fillers, and no multi-layered composites. The performance arises from carefully controlled oxidation and carbonization steps that determine the distribution of pores, graphitic domains, and chemical functional groups. This method could be scaled for manufacturing and may be applied to other porous materials beyond melamine-based sponges.
The team found that the temperature used during pre-oxidation was critical. At 150 degrees Celsius, the oxygen groups formed in just the right concentration to encourage pore formation, enhance internal order, and maintain structural integrity. Lower temperatures did not generate enough oxygen functionalization. Higher temperatures led to excessive gas evolution, disrupting the formation of ordered carbon layers.
The carbon sponge developed in this study demonstrates that electromagnetic and thermal stealth are not necessarily competing goals. With the right structural design, both can be achieved in a single material phase. The combination of ultralow weight, wide frequency coverage, and dual-domain performance points to a new direction in stealth material design. By taking inspiration from natural structures and focusing on microstructural control, the researchers have created a material that meets the demands of radar and infrared avoidance without sacrificing manufacturability or durability.
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