Jizhe Cai

This study explored the use of boron nitride nanotubes (BNNTs) as reinforcing fillers to enhance the mechanical properties of polymer-derived carbon matrix composites. BNNT-reinforced carbon matrix composites containing 0.5–5 wt% BNNTs were fabricated with pyrolysis conducted at different temperatures. X-ray diffraction and Raman spectroscopy revealed enhanced crystallinity and reduced defects in carbon matrix composites with BNNT addition. At 1200 °C pyrolysis temperature, sample shrinkage decreased from 28% in the control sample without BNNT addition to 12% with 5 wt% BNNTs, demonstrating BNNTs’ significant influence on the matrix. The density increased by 20.1% with 5 wt% BNNTs. Mechanical testing demonstrated an enhancement in the failure strain from 0.7% to 0.8% and an 87.8% increase in the work of fracture with 5 wt% BNNTs. Furthermore, the flexural strength and modulus improved by 68.7% and 55.6%, respectively, at this BNNT concentration. Increasing the pyrolysis temperature to 1500 °C further boosted the mechanical properties, with the flexural strength increasing by 283.7% and the flexural modulus by 528.6% when comparing samples containing 5 wt% BNNTs to those without BNNT reinforcement. Samples processed at 1500 °C with 5 wt% BNNT composition exhibited optimal performance.

A clear understanding of the dynamic behavior of metals is critical for developing superior structural materials as well as for improving material processing techniques such as cold spray and shot peening. Using a high velocity (from 120 m/s to 700 m/s; strain rates >10^7 1/s) micro-projectile impact testing and quasistatic (strain rates: 10^-2 1/s) nanoindentation, we investigate the strain-rate-dependent mechanical behavior of single-crystal aluminum substrates with (001), (011), and (111) crystal orientations. For all three crystal orientations, the dynamic hardness initially increases with increasing impact velocity and reaches a plateau regime at hardness 5 times higher than that of at quasistatic indentations. Based on coefficient of restitution and post mortem transmission Kikuchi diffraction analyses, we show that distinct plastic deformation mechanisms with a gradient dislocation density evolution govern the dynamic behavior. We also discover a distinct deformation regime-stable plastic regime-that emerge beyond the deeply plastic regime with unique strain rate insensitive microstructure evolution and dynamic hardness. Our work additionally demonstrates an effective approach to introduce strong spatial gradient in dislocation density in metals by high-velocity projectile impacts to enhance surface mechanical properties, as it can be employed in material processing techniques such as shot peening and surface mechanical attrition treatment.

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.