Apparent negative electrical resistance in carbon fiber composites

Apparent negative electrical resistance was observed, quantified, and controlled through composite engineering. Its mechanism involves electrons traveling in the unexpected direction relative to the applied voltage gradient, due to backflow across a composite interface. The observation was made in the through-thickness direction of a continuous carbon fiber epoxy–matrix two-lamina composite, such that the fibers in the adjacent laminae were not in the same direction and that the curing pressure during composite fabrication was unusually high (1.4 MPa). At a usual curing pressure (0.13 MPa), the resistance was positive. At an intermediate curing pressure (0.33 MPa), the apparent resistance was either positive or negative, depending on the current direction, due to non-uniformity in the thickness within a junction. The magnitude of the apparent negative resistance decreased with increasing temperature. Appropriate apparent negative and positive resistances in series, as provided by more than two laminae, allowed tailoring of the total apparent resistance. Apparent negative resistance was also observed in carbon fiber cement-matrix composites and in bare carbon fibers held together by pressure. Relevant applications are electrical