EMI Shielding Effectiveness of Copper/Epoxy Composites
Samples of shaped and treated copper (Cu) fibre/brittle matrix composites have been fabricated with a variety of fibre shapes, treatments, diameters, and powder inclusions and tested for shielding effectiveness (SE) using the ASTM standard D4935-99 method over the frequency range 200 to 1500 MHz. A small amount of nickel (Ni) powder was included in some specimens to evaluate the effect of magnetic permeability on SE. Most tests were conducted at constant (15 vol%, “v%”) fibre volume fraction. Results show that a smaller fibre diameter results in significant SE improvements. SE was also substantially improved by relatively minor fibre shaping (which changes along the fibre length) or by fine-scale roughening via chemical treatment. Addition of small amounts of Cu powder leads to major improvements in SE. Most of the improvements are presumably due to the establishment of conductive networks, although fibre shaping or treatment allows greater scattering and thus more internal reflections with attendant absorption. The smaller diameter fibres consistently gave better SE. When a continuous fibre mesh was used, high SE resulted, showing the dominance of fibre connectivity for such materials; the surface condition of the mesh is relatively unimportant. The improvements in SE can be achieved with essentially identical mechanical properties, leading to an effective multifunctional material, although most of the SE values achieved so far are modest. Ni powder results in overall improvement in SE as well as a cyclic dependence of SE on frequency, which indicates phase interference at a scale of particle size not previously seen.