Dynamic Strengthening of Carbon Nanotube Fibers under Extreme Mechanical Impulses

Carbon nanotube fibers with dynamic strength up to 14 GPa

High dynamic strength is of fundamental importance for fibrous materials that are used in high-strain rate environments. Carbon nanotube fibers are one of the most promising candidates. Using a strategy to optimize hierarchical structures, we fabricated carbon nanotube fibers with a dynamic strength of 14 gigapascals (GPa) and excellent energy absorption. The dynamic performance of the fibers is attributed to the simultaneous breakage of individual nanotubes and delocalization of impact energy that occurs during the high-strain rate loading process; these behaviors are due to improvements in interfacial interactions, nanotube alignment, and densification therein. This work presents an effective strategy to utilize the strength of individual carbon nanotubes at the macroscale and provides fresh mechanism insights.

Fabricating Ultrastrong Carbon Nanotube Fibers via a Microwave Welding Interface

Liquid crystal wet-spun carbon nanotube fibers (CNTFs) offer notable advantages, such as precise alignment and scalability. However, these CNTFs usually suffer premature failure through intertube slippage due to the weak interfacial interactions between adjacent shells and bundles. Herein, we present a microwave (MW) welding strategy to enhance intertube interactions by partially carbonizing interstitial heterocyclic aramid polymers. The resulting CNTFs exhibit ultrahigh static tensile strength (6.74 ± 0.34 GPa) and dynamic tensile strength (9.52 ± 1.31 GPa), outperforming other traditional high-performance fibers. This work provides a straightforward yet effective approach to strengthening CNTFs for advanced engineering applications.

Micron-Thick Interlocked Carbon Nanotube Films with Excellent Impact Resistance via Micro-Ballistic Impact

The highest specific energy absorption (SEA) of interlocked micron-thickness carbon nanotube (IMCNT) films subjected to micro-ballistic impact is reported in this paper. The SEA of the IMCNT films ranges from 0.8 to 1.6 MJ kg-1 , the greatest value for micron-thickness films to date. The multiple deformation-induced dissipation channels at the nanoscale involving disorder-to-order transition, frictional sliding, and entanglement of CNT fibrils contribute to the ultra-high SEA of the IMCNT. Furthermore, an anomalous thickness dependency of the SEA is observed, that is, the SEA increases with increasing thickness, which should be ascribed to the exponential growth in nano-interface that further boosts the energy dissipation efficiency as the film thickness increases. The results indicate that the developed IMCNT overcomes the size-dependent impact resistance of traditional materials and demonstrates great potential as a bulletproof material for high-performance flexible armor.

Fabricating strong and tough aramid fibers by small addition of carbon nanotubes

Synthetic high-performance fibers present excellent mechanical properties and promising applications in the impact protection field. However, fabricating fibers with high strength and high toughness is challenging due to their intrinsic conflicts. Herein, we report a simultaneous improvement in strength, toughness, and modulus of heterocyclic aramid fibers by 26%, 66%, and 13%, respectively, via polymerizing a small amount (0.05 wt%) of short aminated single-walled carbon nanotubes (SWNTs), achieving a tensile strength of 6.44 ± 0.11 GPa, a toughness of 184.0 ± 11.4 MJ m^-3, and a Young's modulus of 141.7 ± 4.0 GPa. Mechanism analyses reveal that short aminated SWNTs improve the crystallinity and orientation degree by affecting the structures of heterocyclic aramid chains around SWNTs, and in situ polymerization increases the interfacial interaction therein to promote stress transfer and suppress strain localization. These two effects account for the simultaneous improvement in strength and toughness.

Microparticle Impact Testing at High Precision, Higher Temperatures, and with Lithographically Patterned Projectiles

In the first decade of high-velocity microparticle impact research, hardly any modification of the original experimental setup has been necessary. However, future avenues for the field require advancements of the experimental method to expand both the impact variables that can be quantitatively assessed and the materials and phenomena that can be studied. This work explores new design concepts for the launch pad (the assembly that launches microparticles upon laser ablation) that can address the root causes of many experimental challenges that may limit the technique in the future. Among the design changes contemplated, the substitution of a stiff glass launch layer for the standard elastomeric polymer layer offers a number of improvements. First, it facilitates a reduction of the gap between launch pad and target from hundreds to tens of micrometers and thus unlocks a reproducibility in targeting a specific impact location better than the diameter of the test particle itself (±1.75 µm for SiO₂ particles 7.38 µm in diameter). Second, the inert glass surface enables experiments at higher temperatures than previously possible. Finally-as demonstrated by the launch of thin-film Au disks-a launch pad made of materials standard in microfabrication paves the way to facile microfabrication of advanced impactors.

Controllable Preparation and Strengthening Strategies towards High-Strength Carbon Nanotube Fibers

Carbon nanotubes (CNTs) with superior mechanical properties are expected to play a role in the next generation of critical engineering mechanical materials. Crucial advances have been made in CNTs, as it has been reported that the tensile strength of defect-free CNTs and carbon nanotube bundles can approach the theoretical limit. However, the tensile strength of macro carbon nanotube fibers (CNTFs) is far lower than the theoretical level. Although some reviews have summarized the development of such fiber materials, few of them have focused on the controllable preparation and performance optimization of high-strength CNTFs at different scales. Therefore, in this review, we will analyze the characteristics and latest challenges of multiscale CNTFs in preparation and strength optimization. First, the structure and preparation of CNTs are introduced. Then, the preparation methods and tensile strength characteristics of CNTFs at different scales are discussed. Based on the analysis of tensile fracture, we summarize some typical strategies for optimizing tensile performance around defect and tube-tube interaction control. Finally, we introduce some emerging applications for CNTFs in mechanics. This review aims to provide insights and prospects for the controllable preparation of CNTFs with ultra-high tensile strength for emerging cutting-edge applications.

Extreme Dynamic Performance of Nanofiber Mats under Supersonic Impacts Mediated by Interfacial Hydrogen Bonds

Achieving extreme dynamic performance in nanofibrous materials requires synergistic exploitation of intrinsic nanofiber properties and inter-fiber interactions. Regardless of the superior intrinsic stiffness and strength of carbon nanotubes (CNTs), the weak nature of van der Waals interactions limits the CNT mats from achieving greater performance. We present an efficient approach to augment the inter-fiber interactions by introducing aramid nanofiber (ANF) links between CNTs, which forms stronger and reconfigurable interfacial hydrogen bonds and π-π stacking interactions, leading to synergistic performance improvement with failure retardation. Under supersonic impacts, strengthened interactions in CNT mats enhance their specific energy absorption up to 3.6 MJ/kg, which outperforms widely used bulk Kevlar-fiber-based protective materials. The distinct response time scales of hydrogen bond breaking and reformation at ultrahigh-strain-rate (∼10⁷-10⁸ s^-1) deformations additionally manifest a strain-rate-dependent dynamic performance enhancement. Our findings show the potential of nanofiber mats augmented with interfacial dynamic bonds─such as the hydrogen bonds─as low-density structural materials with superior specific properties and high-temperature stability for extreme engineering applications.

Washable, Sewable, All-Carbon Electrodes and Signal Wires for Electronic Clothing

Smart wearable electronic accessories (e.g., watches) have found wide adoption; conversely, progress in electronic textiles has been slow due to the difficulty of embedding rigid electronic materials into flexible fabrics. Electronic clothing requires fibers that are conductive, robust, biocompatible, and can be produced on a large scale. Here, we create sewable electrodes and signal transmission wires from neat carbon nanotube threads (CNTT). These threads are soft like standard sewing thread, but they have metal-level conductivity and low interfacial impedance with skin. Electrocardiograms (EKGs) obtained by CNTT electrodes were comparable (P > 0.05) to signals obtained with commercial electrodes. CNTT can also be used as transmission wires to carry signals to other parts of a garment. Finally, the textiles can be machine-washed and stretched repeatedly without signal degradation. These results demonstrate promise for textile sensors and electronic fabric with the feel of standard clothing that can be incorporated with traditional clothing manufacturing techniques.

Extreme Energy Dissipation via Material Evolution in Carbon Nanotube Mats

Thin layered mats comprised of an interconnected meandering network of multiwall carbon nanotubes (MWCNT) are subjected to a hypersonic micro-projectile impact test. The mat morphology is highly compliant and while this leads to rather modest quasi-static mechanical properties, at the extreme strain rates and large strains resulting from ballistic impact, the MWCNT structure has the ability to reconfigure resulting in extraordinary kinetic energy (KE) absorption. The KE of the projectile is dissipated via frictional interactions, adiabatic heating, tube stretching, and ultimately fracture of taut tubes and the newly formed fibrils. The energy absorbed per unit mass of the film can range from 7-12 MJ kg-1, much greater than any other material.

Superior Energy Dissipation by Ultrathin Semicrystalline Polymer Films Under Supersonic Microprojectile Impacts

Distinct deformation mechanisms that emerge in nanoscale enable the nanostructured materials to exhibit outstanding specific mechanical properties. Here, we present superior microstructure- and strain-rate-dependent specific penetration energy (up to ∼3.8 MJ kg^-1) in semicrystalline poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) thin films subjected to high-velocity (100 m s^-1 to 1 km s^-1) microprojectile (diameter: 9.2 μm) impacts. The geometric-confinement-induced nanostructural evolutions enable the sub-100 nm thick P(VDF-TrFE) films to achieve high specific penetration energy with high strain delocalization across the broad impact velocity range, superior to both bulk protective materials and previously reported nanomaterials. This high specific penetration energy arises from the substantial stretching of the two-dimensionally oriented highly mobile polymer chains that engage abundant viscoelastic and viscoplastic deformation mechanisms that are further enhanced by the intermolecular dipole-dipole interactions. These key findings provide insights for using nanostructured semicrystalline polymers in the development of lightweight, high-performance soft armors for extreme engineering applications.