Improved conductivity and piezoresistive properties of Ni-CNTs cement-based composites under magnetic field

A novel type of cement-based composite with preferentially orientated nickel-coated carbon nanotubes (Ni-CNTs) was fabricated by induction of magnetic field (MF) method for piezoresistive sensor in structural health monitoring system. The microstructures of Ni-CNTs and their aligned features in the as-fabricated composites were observed by scanning electron microscope (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD), respectively. The four-electrode method was applied to measure the conductivity and piezoresistive properties of as-fabricated composites. Besides, the effects of several influencing factors, including Ni-CNTs orientation and content, loading rate on the conductivity and piezoresistive properties of Ni-CNTs cement-based composites were determined. The results indicate that the CNTs coated with crystalline nickel are orientated preferentially along the MF direction, of which the orientation angles are concentrated between −30°-20°, presenting a Gaussian distribution. The conductivity and piezoresistive properties of oriented Ni-CNTs cement-based composites are anisotropic. The electrical resistivity and percolation threshold of composites have a sequence: perpendicular to MF > without MF > parallel to MF. Under cyclic compressive loading, Ni-CNTs cement-based composites parallel to MF direction exhibit the best piezoresistive properties. The optimum content level of CNT for the piezoresistivity is 1.20 vol%, which is slightly higher than the percolation threshold. Also, the strain sensitivity of composites parallel to MF is most sensitive to the loading rate.