The electrical and electromechanical responses of ~200 µm thick extruded nanocomposite films comprising of 4 wt.% and 5 wt.% multiwall carbon nanotubes mixed with polypropylene are investigated under an alternating current (AC) and compared to their direct current (DC) response. The AC electrical response to frequency (f) and strain (piezoimpedance) is characterized using two configurations, namely one that promotes resistive dominance (resistive configuration) and the other that promotes the permittivity/capacitive contribution (dielectric configuration). For the resistive configuration, the frequency response indicated a resistive–capacitive (RC) behavior (negative phase angle, θ), with a significant contribution of capacitance for frequencies of 10⁴ Hz and above, depending on the nanotube content. The piezoimpedance characterization in the resistive configuration yielded an increasing impedance modulus (|Z|) and an increasing (negative) value of θ as the strain increased. The piezoimpedance sensitivity at f = 10 kHz was ~30% higher than the corresponding DC piezoresistive sensitivity, yielding a sensitivity factor of 9.9 for |Z| and a higher sensitivity factor (~12.7) for θ. The dielectric configuration enhanced the permittivity contribution to impedance, but it was the least sensitive to strain.

Carbon nanotube yarns (CNTYs) are outstanding hierarchical fibers with electrical properties that depend on temperature and mechanical stimuli, which makes them attractive as smart materials for sensing applications. In order to better understand the electrical response of CNTYs under a combination of dynamic loading and variations of temperature, their monotonic and cyclic tensile mechanical response, in situ Raman spectroscopy, mechanical hysteresis, and dynamic mechanical analysis (DMA) are investigated herein. The piezoresistivity and thermoresistivity of CNTYs were characterized to correlate the contribution of temperature and strain to the effective electrical response of the CNTY under DMA testing. It was found that the tensile load bearing of the CNTYs is governed mainly by structural changes of its bundles/fibrils, rather than by stretching of the carbon bonds. The large energy dissipation capabilities of the CNTYs arise mainly from friction among their fibrils and bundles, and also due to irreversible morphological changes in their hierarchical structure. The (mechanically constrained) thermoresistive characterization showed that the electrical resistance of the CNTYs decreases with increasing temperature, yielding an average temperature coefficient of resistance of −8.63 × 10⁻⁴ K⁻¹. The electrical response of the CNTYs during DMA temperature scans is governed by their thermoresistive response.

Carbon nanostructures are used to coat engineering fibers to render electrical conductivity to smart polymer composites for applications such as self-sensing, actuation, deicing, and self-healing. Despite their great potential and wide use, specific metrics for assessing quality control of the air-spraying method used to coat the fibers are still missing. Dedicated studies comparing the advantages and disadvantages of using carbon nanotubes or multilayer graphene sheets also remain scarce and controversial. In this study, glass fiber weaves are coated with either multilayer graphene sheets or with carbon nanotubes using the air-spraying method. Key air-spraying parameters such as volumetric flow rate, spot size, and areal density are proposed as quality control metrics and systematically characterized. The resulting coatings were further evaluated for thickness, structural damage assessment through Raman spectroscopy, electrical resistance distribution on the fiber, and adhesion to the fiber surface. The findings indicate that carbon nanotube coatings produced more uniform, more densely packed, and more efficient electroconductive coatings with higher adhesion to the glass fibers at less weight content. They also require less energy density during the dispersion process to produce a uniform coating. In contrast, multilayer graphene sheets required four times more material to achieve coatings with similar or slightly lower electrical resistance distribution. These coatings were integrated into laminated polymer composites to fabricate smart composites with damage self-sensing capabilities. Both materials exhibited high sensitivity to damage detection and localization.