In the last decades, a new class of materials with outstanding properties at the nanometric scale was presented to the world along with promising advances in various areas of technology. Such materials showed great electrical and thermal conductivity, and remarkable mechanical properties. Among these nanoscale materials, the nanofibers were promptly foreseen as great reinforcement substitutes for macroscale particles in composite materials, especially where lightweight plays a decisive role. However, areas of research concerned with the impact of these materials on the environment began to shed light on the consequences of nanofibers that are released to the atmosphere at the manufacture or at the disposal phase, for example. These studies showed that free airborne biopersistent nanofibers having certain dimensional characteristics can provoke serious health issues when they reach the deep airways of the respiratory system through inhalation. Furthermore, the harm caused by biopersistent nanofibers was directly related to their flexural rigidity. Thus, knowing the mechanical behavior of these materials became crucial not only for the potential application in structural components, but also to control the spread of rigid nanofibers before the potential risks are assessed. The presented work proposes a novel method to evaluate the flexural rigidity of nanofibers by employing a non-complex experimental setup using the Dynamic Scanning Electron Microscopy technique. In this method, the Young’s modulus of cantilevered nanofibers was obtained through mechanical excitation and resonance detection experiments based on the Euler-Bernoulli beam theory. Upon deriving the Young’s modulus experimentally, the flexural rigidity was calculated and discussed. Multi-walled carbon nanotubes and silicon carbide nanowires were investigated. The results showed scattered values of Young’s modulus from 15 to 161 GPa and 105 to 340 GPa, respectively. The nanotubes exhibited a curvilinear morphology, which is not exactly in accordance with the Euler-Bernoulli principle. However, it was important to examine this material since it became a benchmark for toxicological studies. For that, an alternative model to determine the Young’s modulus based on the vibration of curved beams was proposed and compared to Euler-Bernoulli. In addition, the curvy shape of the nanotubes was noticed to increase uncertainties on the length measurements performed via SEM images, because it produces a projected length different from the fiber’s true length. This effect can cause error deviations in average up to 59% on the Young’s modulus. To minimize these errors, a parallax method to reconstruct the 3D model of the fiber based on the 2D images with different tilt angles is recommended. On the other hand, the silicon carbide nanowires were very straight, showing a vibration behavior very approximate to the theoretical values for perfect linear elastic beams, and an average error of 18.6% on the Young’s modulus. Both nanofibers showed flexural rigidity values above the critical rigidity threshold of 10-19 N·m2, which is the maximum permitted to prevent damages in defense cells of lungs. Therefore, these fibers were classified as potential hazard for humans. The method described is applicable to nanofibers and nanowires with known material density, exhibiting a beam-like shape and electrically conductive or semiconductive.
Development of a method to measure the flexural rigidity of nanofibers
Entwicklung einer Methode zur Messung der Biegesteifigkeit von Nanofasern
2022
Sonstige
Elektronische Ressource
Englisch
DDC: | 629 |
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