Nano-particles doped carbon nanotube films for in-situ monitoring of temperature and strain during the processing of carbon fiber/epoxy composites

Carbon nanotube (CNT) film is favored in structural health monitoring of advanced composite materials, primarily due to its commendable mechanical properties and piezoresistive properties. Nonetheless, floating catalytic chemical vapor deposition (FCCVD) is an attractive method for fabrication of CNT films, and the electrical response to strain of FCCVD-prepared CNT films is impeded by high aspect ratio and lamellar packing structure. For this purpose, FCCVD CNT films were modified by HCl dissolving Fe impurities, nano-SiO₂ particles doping and freeze-drying in combination to increase the spacing between CNTs and its networks as well as their strain sensitivities. It showed that the gauge factor (GF) according to the variation of resistance (ΔR/R₀) of the co-modified film (CNT-HCl-SiO₂ film) was up to 15.6 for the tensile strain at the bottom surface of unidirectional carbon fiber reinforced plastic (CFRP) laminates during the process of bending tests. The bending cycle experiment of the CFRP showed relatively stable changes of ΔR/R₀ with the strains for CNT-HCl-SiO₂ film, while that of the pristine CNT film (CNT-HCl-0 film) displayed unstable non-monotonic changes and that of HCl purified CNT film (CNT-HCl-10 film) revealed a gradual declining tendency. Moreover, the ΔR/R₀ of CNT-HCl-SiO₂ film exhibited excellent sensitivity to the strains of multiple bistable-deformations of cross-ply CFRP laminates. Strain gauge analysis indicated that a 51% increase of ΔR/R₀ of CNT-HCl-SiO₂ film at the 90° layer surface corresponded to the average strain of 434 με, meanwhile a 37% increase of ΔR/R₀ of the CNT film at the 0° layer surface corresponded to the strain of averagely −173.9 με, and both exhibited super high GFs of 1175 and 2108, respectively. Based on this high sensitivity, CNT-HCl-SiO₂ film also had the ability to predict the release of residual stress during the demoulding process of CFRP.