Nonhomogeneous morphology and the elastic modulus of aligned carbon nanotube films

Abstract

Carbon nanotubes (CNT) arrays are interesting because of the combination of their small size and outstanding physical properties. Vertically aligned carbon nanotubes (VACNTs), also named CNT forests or CNT turfs, show high heat conductivity and sufficient mechanical compliance to accommodate thermal expansion mismatch for use as advanced thermal interface materials. This paper reports measurements of the in-plane moduli of single-walled CNT (SWCNT) and multiwalled CNT (MWCNT) films. The mechanical response of these films is related to the nonhomogeneous morphology of the grown nanotubes, such as entangled nanotubes of a top crust layer where growth begins. Aligned nanotubes in the middle region, and lower density nanotubes in the bottom layer where growth terminates. To investigate how the entanglement layer governs the mechanical moduli of CNT films, we eliminate the effect of the crust layer by etching the CNT films from the top. A microfabricated cantilever technique shows that crust removal reduces the resulting moduli of the etched SWCNT films by as much as 40%, whereas the moduli of the etched MWCNT films does not change significantly, suggesting a minimal crust effect on the film modulus for thick films (>90 μm). This improved understanding will allows us to engineer the mechanical properties of CNT films for future applications.

Wall Shear Stress Sensors Based on Carbon Nanotube Pillars

“…A similar measurement performed on the 10× thicker and, hence, 1000× stiffer reference element (Figure S9) also provided a value in this range. The Young’s modulus is near the lower end of the 0.5–100 MPa range reported in the literature, ,, which is desirable for high sensitivity.…”

“…Such mild oxygen plasma treatments are similarly used to remove the entangled crust layer atop vertically aligned CNT pillars that form during growth and to induce defects to improve precursor adhesion during atomic layer deposition . Oxygen plasma treatment has been reported to insignificantly alter the Young’s modulus of multiwalled CNT forests …”

“…However, CNTs possess other attractive properties that could be exploited for flow sensing. Multiwalled CNTs are electrically conductive, can be integrated into electronic devices, − and can be grown into complicated three-dimensional pillar microstructures from simpler two-dimensional patterns. ,, CNT pillars offer relatively low Young’s modulus in bending (0.5–100 MPa), ,, for high responsiveness to flow, and can withstand substantial normal loads before detaching (40–350 kPa), contributing to durability . Furthermore, the mechanical properties of CNT pillars can be tuned over orders of magnitude through coatings or adjusting chemical vapor deposition process parameters. , …”