Hao Wu

Hexagonal boron nitride (h-BN) has excellent thermal conductivity and dielectric properties, which shows great potential for low-dimensional devices. However, mechanical properties of h-BN have not been comprehensively investigated through experiments. In this work, we conduct in situ direct tensile tests on freestanding single-crystal few-layer h-BN nanosheets with various layer numbers from 3 to 8, with an elaborate sample transfer and characterization protocol. Young's modulus of 573.8 ± 101.4 GPa and a tensile fracture strain up to 3.2% are revealed, which are comparable to its monolayer counterpart. Moreover, we find a tough-to-brittle transition in few-layer h-BN with the increase in layer number, which is attributed the interplay between the van der Waals interaction and in-plane covalent bonding. These findings could open up new possibilities in mechanical research of van der Waals materials and provide guidance for the design of h-BN-based devices and composites.

The applications of silica-based glass have evolved alongside human civilization for thousands of years. High-precision manufacturing of three-dimensional (3D) fused silica glass objects is required in various industries, ranging from everyday life to cutting-edge fields. Advanced 3D printing technologies have emerged as a potent tool for fabricating arbitrary glass objects with ultimate freedom and precision. Stereolithography and femtosecond laser direct writing respectively achieved their resolutions of ~50 μm and ~100 nm. However, fabricating glass structures with centimeter dimensions and sub-micron features remains challenging. Presented here, our study effectively bridges the gap through engineering suitable materials and utilizing one-photon micro-stereolithography (OμSL)-based 3D printing, which flexibly creates transparent and high-performance fused silica glass components with complex, 3D sub-micron architectures. Comprehensive characterizations confirm that the final material is stoichiometrically pure silica with high quality, defect-free morphology, and excellent optical properties. Homogeneous volumetric shrinkage further facilitates the smallest voxel, reducing the size from 2.0 × 2.0 × 1.0 μm³ to 0.8 × 0.8 × 0.5 μm³. This approach can be used to produce fused silica glass components with various 3D geometries featuring sub-micron details and millimetric dimensions. This showcases promising prospects in diverse fields, including micro-optics, microfluidics, mechanical metamaterials, and engineered surfaces.

Biological materials such as conch shells with crossed-lamellar textures hold impressive mechanical properties due to their capability to realize effective crack control and energy dissipation through the structural synergy of interfacial modulus mismatch and lamellar orientation disparity. Integrating this mechanism with mechanical metamaterial design can not only avoid the catastrophic post-yield stress drop found in traditional architectural materials with uniform lattice structures but also effectively maintain the stress level and improve the energy absorption ability. Herein, a novel bioinspired design strategy that combines regional particularity and overall cyclicity is proposed to innovate the connotation of long-range periodicity inside the metamaterial, in which the node constraint gradient and crossed-lamellar struts corresponding to the core features of conch shells are able to guide the deformation sequence with a self-strengthening response during compression. Detailed in situ experiments and finite element analysis confirm that the rotated broad layer stacking can shorten and impede the shear bands, further transforming the deformation of bioinspired metamaterial into a progressive, hierarchical way, highlighted by the cross-layer hysteresis. Even based on a brittle polymeric resin, excellent specific energy absorption capacity [4544 kJ/kg] has been achieved in this architecture, which far exceeds the reported metal-based syntactic foams for two orders of magnitude. These results offer new opportunities for the bioinspired metamaterials to substitute the widespread syntactic foams in specific applications required for both lightweight and energy absorption.

To meet the evolving requirements of material designability, sustainability, and eco-friendliness, the development of syntactic foams puts great emphasis on filler optimization and matrix selection. Here, we present a novel microfluidic expansion coupled with the thermal contraction method to improve the fabrication of syntactic foam fillers (SFFs), highlighted by the independently regulated parameters in contrast to the hydrodynamic regulation approach. The eccentricity of droplets can be reduced via osmotic swelling under a hypotonic circumstance and further preserved through real-time UV polymerization. The eccentricity of shrunken SFFs after heat treatment is found to inherit the regulated configurations of droplets and this homogeneity difference of shell thickness can result in the mechanical performance deviation. Then, we choose greener material, geopolymer, as the binder matrix and prepare geopolymer syntactic foams (GSFs) by mold casting. The inclusion of SFFs has increased the atomic volume ratio of Si : Al in the matrix, thereby forming a strong interfacial bonding. The compressive yield strength of GSFs is inversely proportional to the volume fraction of SFFs, and the specimens after thermal exposure can still maintain their shape integrity and exhibit similar mechanical performance even with the formation of cracks. Moreover, the fire-resistant performance of GSFs has been visually validated by the combustion comparison experiment, characterized as flame retardancy, smoke-free, and basically intact morphology, which should give insights into the design and fabrication of functional composites.