Higher Recovery and Better Energy Dissipation at Faster Strain Rates in Carbon Nanotube Bundles: An in-Situ Study

Abstract

We report mechanical behavior and strain rate dependence of recoverability and energy dissipation in vertically aligned carbon nanotube (VACNT) bundles subjected to quasi-static uniaxial compression. We observe three distinct regimes in their stress-strain curves for all explored strain rates from 4 × 10(-2) down to 4 × 10(-4)/sec: (1) a short initial elastic section followed by (2) a sloped plateau with characteristic wavy features corresponding to buckle formation and (3) densification characterized by rapid stress increase. Load-unload cycles reveal a stiffer response and virtually 100% recoverability at faster strain rates of 0.04/sec, while the response is more compliant at slower rates, characterized by permanent localized buckling and significantly reduced recoverability. We propose that it is the kinetics of attractive adhesive interactions between the individual carbon nanotubes within the VACNT matrix that governs morphology evolution and ensuing recoverability. In addition, we report a 6-fold increase in elastic modulus and gradual decrease in recoverability (down to 50%) when VACNT bundles are unloaded from postdensification stage as compared with predensification. Finally, we demonstrate energy dissipation capability, as revealed by hysteresis in load-unload cycles. These findings, together with high thermal and electrical conductivities, position VACNTs in the "unattained-as-of-to-date-space" in the material property landscape.

Local Relative Density Modulates Failure and Strength in Vertically Aligned Carbon Nanotubes

“…Here the first buckle generally nucleates close to the substrate, and each subsequent lateral collapse event initiates only after the preceding one was completed, thus sequentially collapsing the entire structure. 3,[5][6][7] We are unaware of any literature report that suggests a different buckling sequence (such as top-to-bottom or otherwise) for VACNT pillars under compression. VACNT micro-pillar 6 and (d) 50 µm × 60 µm (diameter × height) VACNT micro-pillar.…”

Compressive response of vertically aligned carbon nanotube films gleaned from in situ flat-punch indentations

“…Using both on-edge in situ and inbulk ex situ flat-punch indentations, we examine the combined quantitative information (load-displacement) along with the concomitant morphological changes. To explore the effect of lateral boundary constraints, these findings are compared with our previous report 9 on uniaxial compressions of the same APCVD-grown VACNTs patterned into cylindrical bundles.…”

“…[10][11][12] A wide range of mechanical behavior has been reported in VACNTs as a result of such variations in their microstructure. These include modulus and buckling strengths ranging from sub-MPa 13,14 to tens of MPa [15][16][17] to GPa, as well as a complete gamut of recoverability-from nearly 100% recovery even after large deformations 5,9,13,19,20 to permanent distortion even at modest strains. 2,11,17,21,22 Such drastic differences in properties make it impractical to compare VACNTs synthesized by nonidentical growth techniques.…”

“…These results are contrasted with our previous work on uniaxial compressions of 30 lm diameter Â30 lm tall cylindrical bundles. 9 The APCVD VACNT bundles are known to exhibit three distinct regimes in their compressive stress-strain curves: (i) a short initial elastic section, followed by (ii) a sloped plateau with characteristic wavy features corresponding to localized buckle formation, and (iii) a densification characterized by a rapid stress increase-all of which liken their deformation to that of a typical foam. 5,9,11 Unlike foams, however, the compressive strain in VACNTs is accommodated entirely via the formation of localized folds or buckles forming progressively along their lengths.…”

“…9 The APCVD VACNT bundles are known to exhibit three distinct regimes in their compressive stress-strain curves: (i) a short initial elastic section, followed by (ii) a sloped plateau with characteristic wavy features corresponding to localized buckle formation, and (iii) a densification characterized by a rapid stress increase-all of which liken their deformation to that of a typical foam. 5,9,11 Unlike foams, however, the compressive strain in VACNTs is accommodated entirely via the formation of localized folds or buckles forming progressively along their lengths. 5,11,16,22,24,25 During compression of our particular APCVD VACNT system, such localized buckles fully recovered upon load release even from relatively large strains of 60-80% (i.e., the onset of densification) under fast displacement rates of 1000 nm/s.…”