Carbon nanotube (CNT) foams have unmatched energy absorption properties derived from their complex hierarchical structure. The control of the micro-scale geometry of these foams allows tuning their behavior to specific application-driven needs. Geometrical structures in CNT foams are obtained by synthesizing CNTs on substrates patterned with different growth templates: circles, lines and concentric rings. To study the effects of the microstructural geometry on the bulk mechanical response of the foams, the samples are tested under cyclic quasi-static compressive deformation (up to 50% strain). The geometry of the patterns plays a fundamental role on the samples' macroscopic energy absorption capability, maximum stress, and strain recovery. Patterned CNT structures demonstrated mechanical properties comparable or improved over non-patterned, bulk CNT foams, but with much lower density. Quasi-static compressive tests performed on different patterned structures with the same effective density (ρ = 0.02 g cm−3) exhibit considerably different responses. For example, the stress reached by foams patterned in concentric rings is ≈15 times higher than that observed for pillars and lines. The results show how the mechanical response of CNT foams can be tailored by varying the CNT microstructural architecture
Vertical arrays of carbon nanotubes (VACNTs) show unique mechanical behavior in compression, with a highly nonlinear response similar to that of open cell foams and the ability to recover large deformations. Here, we study the viscoelastic response of both freestanding VACNT arrays and sandwich structures composed of a VACNT array partially embedded between two layers of poly(dimethylsiloxane) (PDMS) and bucky paper. The VACNTs tested are ∼2 mm thick foams grown via an injection chemical vapor deposition method. Both freestanding and sandwich structures exhibit a time-dependent behavior under compression. A power-law function of time is used to describe the main features observed in creep and stress-relaxation tests. The power-law exponents show nonlinear viscoelastic behavior in which the rate of creep is dependent upon the stress level and the rate of stress relaxation is dependent upon the strain level. The results show a marginal effect of the thin PDMS/bucky paper layers on the viscoelastic responses. At high strain levels (ɛ = 0.8), the peak stress for the anchored CNTs reaches ∼45 MPa, whereas it is only ∼15 MPa for freestanding CNTs, suggesting a large effect of PDMS on the structural response of the sandwich structures
We present the design, fabrication, and dynamic characterization of micropatterned vertically aligned carbon nanotube (VACNT) foams. The foams' synthesis combines photolithographic techniques with chemical vapor deposition to create materials with an effective density up to five times lower than that of bulk VACNT foams. The dynamic response of these lightweight materials is characterized by performing impact tests at different strain rates. Results show that the dynamic stress-strain behavior of the micropatterned foams is governed by the patterns' geometry and has negligible dependence on their bulk density. The energy absorption of the micropatterned foams is higher than most other energy absorbing materials, such as honeycombs, foams, and composites of comparable density. Highly organized CNT microstructures can be employed as lightweight material for protective applications.
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