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Posted: Jun 30th, 2010
Carbon nanotubes turn glass fibers into multifunctional sensors
(Nanowerk Spotlight) Glass fibers are a widely used reinforcing agent for many materials, from polymers to concrete. The most prominent glass fiber composite is fiberglass, a glass-reinforced plastic. The performance of the glass fiber composite over time depends on the durability of the polymer matrix and the fiber fracture behavior of the material.
Since a conventional glass fiber is electrically insulating, traditionally, the monitoring for composite damage has been conducted by external sensors – a technique that degrades the mechanical properties of the material's structure and increases the cost. A common technique for measuring strains on the surface of a structure is the use of commercially available resistance-type strain gauges. However, these gauges can only measure strains at relatively large scale, specific locations and directions.
Researchers have therefore been working on the development of electrically conductive glass fiber plastics by adding conductive particles such as carbon blacks and carbon nanotubes to a polymer matrix or by treating the composite surface with antistatic or metallic coatings. This provided a noninvasive way to measure to monitor damage via the electrical resistance method (a fracture of carbon fibers or nanotubes results in changes in electrical resistance).
With regard to damage monitoring, however, these approaches are less sensitive to the fracture of the load-carrying fibers and provide less information on the development of very early cracks in the fiber/polymer matrix interphase, where the microscale damage is usually initiated.
Researchers in Germany have now demonstrated a simple approach to deposit multiwalled carbon nanotube (MWCNT) networks onto glass fiber surfaces, thereby achieving semiconductive MWCNT–glass fibers.
Schematic of a single MWNTs-glass fibre and AFM topography image of fibre surface. (Reprinted with permission of Wiley-VCH Verlag)
"Our work demonstrates that it is possible to develop a semiconductive interphase between a glass fiber and an epoxy matrix by coating carbon nanotubes on the glass fiber," Prof. Edith Mäder and Dr. Shang-lin Gao tells Nanowerk. "The local concentration of these carbon nanotubes within the interphase at the nanoscale provides multifunctional sensibility, i.e. stress/strain, temperature, and relative humidity, especially, early warning of fiber composite damage."
The researchers at the Leibniz Institute of Polymer Research in Dresden, Germany, have demonstrated that the techniques of electrical resistance measurements usually performed with conducting carbon fibers, is also applicable to electrical insulating glass fibers for in situ sensing of strain and damage at the micrometer scale, which, unlike other attempts, does not require additional sensors and dispersion of carbon nanotubes in the whole polymer matrix.
They note that the single MWCNT-glass fiber and corresponding epoxy matrix composites show stress/strain, temperature, and relative humidity dependence in their electrical conductivity. "As in situ multifunctional sensors, they are capable of detecting piezoresistive effects as well as the local glass transition temperature" they say. "This means that the conducting electrical resistance measurements previously limited to conductive carbon fibers for sensing strain and damage could be a more widely applicable method. Based on our approach, they are also applicable to any nonconductive fibers such as glass or polymer fibers."
They also note that, compared with previously reported CNT/polymer nanocomposites, the composites of the team's CNT-coated glass fibers in an epoxy matrix exhibit ultrahigh anisotropic electrical properties and an ultralow electrical percolation threshold; they are capable of detecting piezoresistive effects and early warning of fiber composite damage, as well as the local glass transition temperature.
The development of these novel, glass-fiber-reinforced plastics with electrical conductivity has opened up new opportunities in which unique functionalities can be added to existing material systems for a broad range of applications, including electrostatic dissipation, electric field shielding, and damage detecting before catastrophic failure of the fibers.
Further investigation into this functional glass fiber system will focus on improving the dispersion homogeneity of the carbon nanotubes on the fiber surface as well as improving the ability to tailor sensitivity based on coating design and durability towards cyclic loading of thermoplastic composites.