Magnetic field enhancing preferred orientation of nickel-cobalt plated carbon fibers in cement paste, with relevance to compression self-sensing

The arrangement of carbon fibers in the cement matrix affects the stability and sensitivity of the composite. This study investigated the effects of bath pH and electroless plating time on the magnetic metallization (nickel coating, cobalt coating, and nickel-cobalt alloy coating) on carbon fibers and successfully prepared the CFRCs in which the metal-plated carbon fibers could be oriented by magnetic fields. The results showed that a pH of 10 and a plating time of 6 min were favorable for reaction kinetics, with the thickness gain, conductivity, and saturation magnetization of metal coatings all being the largest. The compressive strength of the prepared composites was increased by 26.5% by adding nickel-cobalt coated carbon fibers, which is attributed to the alignment of fibers refining pore structure. The resistivity of CFRCs with metal-plated carbon fibers is one order lower in magnitude than that with uncoated carbon fiber, due to that the alignment of fibers increases the conduction connectivity by decreasing the distance of fiber-fiber and the “quantum tunneling”. The cyclic compressive loading tests showed that the resistance changed reversibly with strain, such that the degree of reversibility is higher for CFRCs with pre-magnetic treatment than those without the treatment. The piezoresistivity (strain sensitivity) of CFRCs was stronger with magnetic field-induced orientation treatment, showing that the strain sensitivity increased by 114% after pre-magnetic treatment for CFRCs containing nickel-cobalt coating. This is attributed to that the well-aligned metal-plated carbon fibers form more conduction paths along the alignment direction, such that the resistivity along this direction is more sensitive to the compressive strain parallel to this direction.