Electromagnetic materials modify electromagnetic fields that interact with them in specific and intentional ways. Typically, the purpose of electromagnetic materials is to redirect, absorb, attenuate, or block electromagnetic radiation. As coatings and sealants are concerned, two key applications of electromagnetic materials are the control of radar cross section (RCS) and the reduction of electromagnetic interference (EMI). In either case there are two basic material objectives: absorption and conduction.
Conductive coatings consist of a conductive filler—usually a flake or powder—dispersed in a polymeric binder at high enough loading that percolation (a network of conductive pathways) occurs. Fillers may be metallic, ceramic, or polymeric. Binders must provide adequate adhesion, strength, and flexibility to stand up to stresses that will be placed on the vehicle surface.
Conductive gap fillers can supplement or substitute for conductive coatings. These materials, having the consistency of putty or caulk, also comprise a conductive filler in a polymer binder. They form electrical connections across gaps, such as between body panels or other joined parts; and they also smooth the geometry of a conductive skin (ground plane), thereby reducing diffuse scattering.
ARS has a history of developing conductive coatings and gap fillers to address a range of applications and requirements, among which are:
- Surface electrical continuity
- Reflective ground plane for resonant absorbers
- Low-temperature flexibility
- High temperature strength and adhesion
- Fluid and chemical resistance
- Static charge dissipation
Radar absorbing materials (RAM) attenuate the specular reflection of electromagnetic waves and diminish the strength of surface currents near edges and electrical discontinuities. They are also of key importance in countering the amplifying effects of cavity ringing. Tailored impedance composites are created by dispersing electrically or magnetically lossy fillers in a polymer-bound matrix. Examples of electrically lossy fillers are carbon black, graphite, intrinsically conductive polymers (ICPs), and filamentary metals; magnetic fillers include iron and ferrite powders.
Using standard and proprietary design tools, ARS can predict the complex impedance of loaded materials and use the computed properties to design a tuned resonant absorber. The graph to the right predicts complex relative permittivity and permeability for an iron-loaded polymer slab.
The next chart is a modeled reflectivity curve from the iron-loaded slab on a conductive ground plane. ARS can formulate the absorber recipes with a
variety of binders and spray or cast the materials into single or multi-layer films. Realized performance typically shows excellent agreement with design predictions.
The third chart is an indication of broad band performance achieved with a 2-layer absorber stack.
The third chart is an indication of broad band performance achieved with a 2-layer absorber stack. For higher frequency performance, ARS can design single or multi-layer absorbers that cancel incident radiation to 100 GHz and higher. Our design tools allow us to place absorption nulls at specific frequencies for 2, 3, or 4-band performance. ARS has a long and very successful history developing lightweight, metal-free, high-frequency absorbers in spray coating and sheet form.
To treat areas where spray or sheet RAM is not feasible, such as in seams or channels, or to make repairs in existing RAM treatments, gap fillers may be engineered to match the complex impedance properties of the neighboring RAM. This is accomplished generally by loading a binder with the same filler and concentration used in the surrounding RAM. This matches impedance between materials and suppresses scattering.
Once an absorber or conductive material is designed and fabricated, ARS can evaluate its microwave performance in a near normal-incidence arch, pictured at right, or in waveguide. The HP network analyzer plots reflection loss as a function of frequency. The chart for a C-Band RAM appears below.