Reflection and Absorption of Microwaves (2–100 GHz) by Carbon-Fiber-Reinforced Composite Surfaces

Abstract Millimeter waves of 35 GHz was demonstrated to be applicable for thermal loading of carbon-fiber-reinforced composite structures, e.g., in thermographic testing. To optimize the heating efficiency, the frequency dependence of the reflectance for woven and multidirectional fiber reinforcement was examined in the range 2 to 100 GHz. It is shown that maximum absorptance, corresponding to minimum reflectance, occurs in the range 60–100 GHz depending on the fiber-reinforcement structure. The polarization of microwaves must be orientated perpendicular to the (dominant) surface fiber direction. Compared to 35 GHz, the use of the higher frequencies increases the absorptance of microwaves from about 10 (40) to about 60% (70%) for woven (multidirectional) fiber reinforcement. This increase results from the interference of reflections at the laminate surface and the interfaces between layers of changing fiber direction. Woven plies have been represented by two layers of mutually perpendicular fiber direction (warp and weft).