Numerical Investigation of Using Mechanical Metamaterial as a Broadband Microwave Absorber

<br/>A porous structure composed of lossy materials enables advanced microwave absorption compared with non-porous material owing to its improved impedance matching and multiple internal reflection. Optimizing the internal geometry of lossy material-based porous structures leads to the active control of microwave absorption for desired target operating ranges while the mechanical strength and stiffness can be secured for practical applications. It has been shown that introducing porous, strong, and stiff mechanical metamaterial with a low density as a microwave absorber has a high potential to create a new kind of broadband absorber satisfying both the electromagnetic and mechanical requirements. So far, empirical approaches were employed to investigate the wave absorption characteristic of materials.There is currently a lack of understanding of the underlying physics of geometric features such as lattice structure, volume fraction, or unit cell length. Furthermore, there are challenges in exploring myriad input variables and visualizing the electromagnetic fields interacting with the structure.<br/>Here, we investigate the effect of the geometric feature of the mechanical metamaterial on microwave absorption, using the microwave simulation using finite element analysis. By varying geometric features such as volume fraction, unit cell length, and lattice type of different mechanical metamaterials (octet-truss, body-centered cubic, cubic foam, and octet foam), we define different absorption mechanisms depending on the structural adjustments. The central finding is that low-density unit-cell (5-20% relative density) absorbs microwaves better than high-density structure, because microwaves going through more porous material enables impedance matching and improved internal multiple scattering effects. When the thickness of the whole structures is varied by changing the unit cell length of the mechanical metamaterial, different lattice structures show a similar response regardless of the lattice types at the subwavelength scale. Meanwhile, when the size of the total thickness of the structure is larger than the incident wavelength, it results in major variance in absorbance with different mechanisms based on the lattice type. Lastly, broadband microwave response (4-18 GHz) under the fixed unit cell length is examined to elucidate different microwave absorption mechanisms depending on lattice structures such as strut-based and facet-based geometries. This work can pave the way for expanding mechanical metamaterial as a broadband microwave absorber by defining the correlation between the geometric feature to the microwave absorption.