Modern manufacturing heavily relies on efficient and precise structural material processing to create durable, customized components for diverse applications. However, existing techniques often struggle with internal stress and defects caused by molecular displacement during processing, significantly impacting mechanical properties and precision. Two-dimensional materials like graphene offer potential solutions through bottom-up self-assembly. While self-assembly methods have shown promise in creating porous, layered, or array structures, controlling the final shape and minimizing defects remains a significant hurdle. Random interlayer slippage frequently leads to unpredictable structural changes. This paper presents a new approach using curvature gradient structures to harness inherent internal stress during material forming, resulting in a more controlled and robust shaping process. This strategy, inspired by the layered cells of a bean pod, utilizes the asymmetrical shrinkage of graphene layers during dehydration to induce internal stress and enhance structural strength. This approach overcomes limitations of existing methods by using internal stress as a driving force for precise shaping, leading to the creation of high-strength structural components.

The literature review highlights existing challenges in material forming technologies and the potential of two-dimensional materials like graphene for addressing these issues. Previous research demonstrates the use of graphene self-assembly to create porous, layered, and array structures. However, these methods lack control over shape and are prone to defects due to random interlayer slippage. Gradient structures, characterized by a gradient of functional groups or pores, have emerged as a promising approach to dissipate internal stress, but their application to shaping processes is still in its early stages. The authors draw inspiration from biological systems, specifically the curvature junctions in bean pod cell layers, which demonstrates how asymmetrical shrinkage during dehydration can generate internal stress for enhanced structural strength. This biological inspiration informs the design of the graphene superstructures.

The study details the fabrication of graphene superstructures with curvature gradients (cg-G) via an alkaline-assisted hydrothermal synthesis using graphene oxide (GO) as a precursor. The graphene walls are aligned around a geometric axis, which also serves as the curvature axis. Laser processing allows for precise shaping with micrometer accuracy. The evaporate-casting process is key: water molecules act as plasticizers, enabling graphene nanosheet mobility. Capillary forces during dehydration tighten the assembly, inducing local bending via a curvature gradient. The axis-center distance and tilt angle are precisely controlled to manipulate the final shape. Characterization techniques include SEM, STEM, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, energy dispersive spectroscopy, and X-ray diffraction to analyze the morphology, structure, composition, and mechanical properties of the cg-G. Finite element analysis (FEA) is used to computationally predict the deformation of samples with different curvature gradients. Coarse-grained molecular dynamics (CG-MD) simulations help understand the deformation mechanisms during the evaporate-casting process. Finally, the self-tightening effect of the cg-G is investigated through various structural designs, including a gasket structure and a mortise and tenon joint. Mechanical tests evaluate compressive and tensile strength.

The research successfully fabricates cg-G with a controllable curvature gradient. SEM images show a clear curvature gradient, with alignment near the curvature center and parallel alignment further away. The evaporate-casting process creates protrusions and a compact structure, leading to a density of 1.5 g cm⁻³ and high compressive strength (70 MPa/(g/cm³)). This surpasses the strength of various metals and doubles the strength of traditional graphene-based materials. Precise control over the curvature gradient is achieved by adjusting the axis-center distance (d). In-plane and out-plane evaporate-casting experiments demonstrate a strong correlation between bending angle and curvature gradient, confirmed by FEA predictions. The lateral dimensions remain consistent despite varying curvature gradients. The self-shaping mechanism is attributed to capillary forces during water evaporation and the hierarchical deformation of graphene walls. A mathematical model relating radial strain and curvature radius is proposed and validated by FEA and experimental results. The application of external forces demonstrates the self-plastic capability of cg-G, allowing for the creation of complex shapes. The self-tightening effect is demonstrated using a gasket structure that withstands a significant load, and a mortise and tenon structure exhibiting exceptionally high specific tensile strength (143 MPa cm³ g⁻¹), exceeding that of welded metals and various connectors. The energy utilization of mechanically interlocked cg-G structures is significantly higher than other materials.

The findings demonstrate a highly controlled and efficient method for fabricating strong, precisely shaped structural materials using graphene. The ability to harness internal stress through curvature gradients addresses the limitations of conventional material forming technologies. The exceptional mechanical properties of the resulting cg-G structures, surpassing those of many metals and existing connectors, open up new possibilities in various engineering applications. The developed mathematical model enhances the understanding of the self-shaping process, allowing for predictable and programmable fabrication of complex structures. This research bridges the gap between material science and engineering design, enabling the creation of advanced, high-performance components. The combination of bottom-up self-assembly with precise control over shape and structure represents a significant advancement in materials processing.

This study presents a novel evaporate-casting method for creating graphene superstructures with a controlled curvature gradient, leading to ultra-high strength structural materials. The method provides exceptional control over the final shape through precise manipulation of the axis-center distance and tilt angle. The resulting materials exhibit superior mechanical properties, exceeding those of various metals and other connector types. The findings open up exciting avenues for designing and manufacturing high-performance components across various fields. Future research could explore the scalability of this method, investigate its adaptability to other two-dimensional materials, and explore applications in diverse engineering fields.

While the study demonstrates the effectiveness of the evaporate-casting method, several limitations should be noted. The current study focuses primarily on small-scale structures; further research is needed to explore the scalability of the method for larger-scale applications. The long-term stability of the cg-G structures under various environmental conditions needs further investigation. The influence of different types of graphene and the optimization of the alkaline-assisted hydrothermal synthesis process warrant further exploration. Finally, a more extensive investigation into the effects of different types of alkali on the resultant structures and properties is also required.