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.
We investigated carbon aerogel samples with super low densities of 0.013 g cm-3 (graphite is 2.5) and conducted compression experiments showing a very low yield stress of 5-8 kPa. To understand the atomistic mechanisms operating in these super low density aerogels, we present a computational study of the mechanical response of very low-density amorphous carbonaceous materials. We start from our previously derived atomistic models (based on the DynReaxMas method) with a density of 0.16 g cm-3 representing the core regions of carbon aerogels. We considered three different phases exhibiting either a fiber-like clump morphology interconnected with string-like units or a more reticulated framework. We subjected these phases to compression and shear deformations and analyzed the resulting plastic response via an inherent-structure protocol. Strikingly, we find that these materials possess shear plastic relaxation modes with extremely low values of yield stress, negligible with respect to the finite values predicted outside this "zero-stress" region. This is followed by a succession of two additional regimes with increasing yield stress values. Our analysis of the atomistic relaxation mechanisms finds that these modes have a collective and cooperative character, taking the form of nanoscopic shear bands within the clumps. These findings rationalize our experimental observations of very low-stress plastic deformation modes in carbon aerogels, providing the first steps for developing a predictive multi-scale modeling of the mechanical properties of aerogel materials.
Poly(p-phenylene-benzimidazole-terephthalamide) (PBIA) fibers with excellent mechanical properties are widely used in fields that require impact-resistant materials such as ballistic protection and aerospace. The introduction of heterocycles in polymer chains increases their flexibility and makes it easier to optimize the fiber structure. However, the inadequate orientation of polymer chains is one of the main reasons for the large difference between the measured and theoretical mechanical properties of PBIA fibers. Herein, carbon nanotubes (CNTs) are selected as an orientation seed. Their structural features allow CNTs to orient during the spinning process, which can induce an orderly arrangement of polymers and improve the orientation of the fiber microstructure. To ensure the complete 1D topology of long CNTs (≈10 µm), PBIA is used as an efficient dispersant to overcome dispersion challenges. The p-CNT/PBIA fibers (10 µm single-walled carbon nanotube 0.025 wt%) exhibit an increase of 22% in tensile strength and 23% in elongation, with a maximum tensile strength of 7.01 ± 0.31 GPa and a reinforcement efficiency of 893.6. The artificial muscle fabricated using CNT/PBIA fibers exhibits a 34.8% contraction and a 25% lifting of a 2 kg dumbbell, providing a promising paradigm for high-performance organic fibers as high-load smart actuators.
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.
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.
Establishing scaling laws for amorphous alloys is of critical importance for describing their mechanical behavior at different size scales. In this paper, taking Ni2Ta amorphous metallic alloy as a prototype materials system, we derive the scaling law of impact resistance for amorphous alloys. We use laser-induced supersonic micro-ballistic impact experiments to measure for the first time the size-dependent impact response of amorphous alloys. We also report the results of molecular dynamics (MD) simulations for the same system but at much smaller scales. Comparing these results, we determined a law for scaling both length and time scales based on dimensional analysis. It connects the time and length scales of the experimental results on the impact resistance of amorphous alloys to that of the MD simulations, providing a method for bridging the gap in comparing the dynamic behavior of amorphous alloys at various scales and a guideline for the fabrication of new amorphous alloy materials with extraordinary impact resistance.
Effect of highly-porous and lightweight carbon nanotube sponges on the high-power continuous wave laser ablation resistance of the sandwich panel was investigated experimentally. As a comparison, thermal responses of monolithic plate, carbon nanotube film filled sandwich panel, unfilled sandwich panel and carbon nanotube sponge filled sandwich panel subjected to continuous wave laser irradiation were analyzed. Experimental results showed that the laser resistance of the carbon nanotube filled sandwich panel is obviously higher than the unfilled structure. The added failure time of the sandwich panel by filling the cores with the carbon nanotube sponge of unit mass was about 18 times and 33 times longer than that by filling with the conventional ablative and insulated material. It could be understood by the high thermal diffusion coefficient and latent heat of sublimation of the carbon nanotube sponge. During ablation by the continuous wave, the carbon nanotube sponge not only fast consumed the absorbed laser energy through phase change of a large-area material due to its high latent heat of sublimation, but also quickly dispersed the heat energy introduced by the continuous wave laser due to its high thermal diffusion coefficient, leading to the extraordinary laser ablation resistance.
High-performance fiber-reinforced composites (FRCs) are widely used in bulletproof structures, in which the mechanical properties of the single fibers play a crucial role in ballistic resistance. In this paper, the quasi-static and dynamic mechanical properties of three commonly used fibers, single aramid III, polyimide (PI), and poly-p-phenylenebenzobisoxazole (PBO) fibers are measured by a small-scale tensile testing machine and mini-split Hopkinson tension bar (mini-SHTB), respectively. The results show that the PBO fiber is superior to the other two fibers in terms of strength and elongation. Both the PBO and aramid III fibers exhibit an obvious strain-rate strengthening effect, while the tensile strength of the PI fiber increases initially, then decreases with the increase in strain rate. In addition, the PBO and aramid III fibers show ductile-to-brittle transition with increasing strain rate, and the PI fiber possesses plasticity in the employed strain rate range. Under a high strain rate, a noticeable radial splitting and fibrillation is observed for the PBO fiber, which can explain the strain-rate strengthening effect. Moreover, the large dispersion of the strength at the same strain rate is observed for all the single fibers, and it increases with increasing strain rate, which can be ascribed to the defects in the fibers. Considering the effect of strain rate, only the PBO fiber follows the Weibull distribution, suggesting that the hypothesis of Weibull distribution for single fibers needs to be revisited.
It has been a key issue for photovoltaic (PV) cells to survive under mechanical impacts by tiny dust. In this paper, the performance degradation and the damage behavior of PV cells subjected to massive dust impact are investigated using laser-shock driven particle impact experiments and mechanical modeling. The results show that the light-electricity conversion efficiency of the PV cells decreases with increasing the impact velocity and the particles' number density. It drops from 26.7 to 3.9% with increasing the impact velocity from 40 to 185 m/s and the particles' number densities from 35 to 150/mm2, showing a reduction up to 85.7% when being compared with the intact ones with the light-electricity conversion efficiency of 27.2%. A damage-induced conversion efficiency degradation (DCED) model is developed and validated by experiments, providing an effective method in predicting the performance degradation of PV cells under various dust impact conditions. Moreover, three damage modes, including damaged conducting grid lines, fractured PV cell surfaces, and the bending effects after impact are observed, and the corresponding strength of each mode is quantified by different mechanical theories.
Residual stresses play a crucial role in both light-electricity conversion performances and the lifespan of photovoltaic (PV) cells. In this paper, the residual stress of triple junction cells (i.e. GaInP/GaInAs/Ge) induced by laser-driven massive micro-particle impact is analyzed with a novel method based on backscattering Raman spectroscopy. The impact process, which induces damage to the PV cells and brings the residual stress, is also investigated by optical microscopy (OM) and Scanning Electron Microscopy (SEM). The results show that the PV cells would exhibit various damage patterns. At the same time, strong residual stresses up to hundreds of MPa introduced in the damaged PV cells after impact have been analysis, providing an effective perspective to better understand the damage behavior and residual stress features of PV cells during their service life.
Despite the increasing popularity of photonic Doppler velocimetry (PDV) in shock wave experiments, its capability of capturing low particle velocities while changing rapidly is still questionable. The paper discusses the performance of short time Fourier transform (STFT) and continuous wavelet transform (CWT) in processing fringe signals of fast-changing low velocities measured by PDV. Two typical experiments are carried out to evaluate the performance. In the laser shock peening test, the CWT gives a better interpretation to the free surface velocity history, where the elastic precursor, main plastic wave, and elastic release wave can be clearly identified. The velocities of stress waves, Hugoniot elastic limit, and the amplitude of shock pressure induced by laser can be obtained from the measurement. In the Kolsky-bar based tests, both methods show validity of processing the longitudinal velocity signal of incident bar, whereas CWT improperly interprets the radial velocity of the shocked sample at the beginning period, indicating the sensitiveness of the CWT to the background noise. STFT is relatively robust in extracting waveforms of low signal-to-noise ratio. Data processing method greatly affects the temporal resolution and velocity resolution of a given fringe signal, usually CWT demonstrates a better local temporal resolution and velocity resolution, due to its adaptability to the local frequency, also due to the finer time-frequency product according to the uncertainty principle.
