The challenge of creating lightweight materials with high strength is a central theme in materials science. Traditional methods often involve trade-offs between these properties; for instance, polymers might be lightweight but lack the strength of steel. This inherent limitation stems from the general proportionality between material strength and density. However, the introduction of nanoporous structures offers a potential solution. By creating a material with nanoscale ligaments, the overall density can be reduced while maintaining or even enhancing strength due to the so-called "Smaller is Stronger" effect. This effect, observed in nanowires, nanopillars, and nanoparticles, arises from the significant reduction in the probability of encountering defects at the nanoscale. Nanoporous materials, characterized by pore sizes typically less than 100 nm, have demonstrated unique properties due to their high surface area and tunable pore topologies. Applications range from ion exchange and sensors to energy storage and tissue engineering. Their mechanical properties, particularly the combination of lightweight and high strength, make them promising candidates for structural applications in various demanding fields. Existing nanoporous metals and ceramics, while exhibiting reasonably high strength (10–200 MPa), still face limitations. Conventional fabrication methods struggle to create ligaments thinner than 50–100 nm, limiting the exploitation of the size effect. Additionally, the inherent mass density of metals and ceramics restricts their specific strength. Therefore, developing nanoporous structures from high specific strength materials, such as carbonaceous materials, is crucial. While nanoporous amorphous carbon materials have been fabricated using various methods, including dealloying and templating, these often lack control over porosity and structure length scale and can be expensive and time consuming. This research aims to address these issues by developing a scalable and cost-effective method to produce nanoporous amorphous carbon nanopillars with precisely controlled porosity and a comprehensive investigation into their mechanical properties.

The literature extensively discusses the fabrication and properties of nanoporous materials. Various methods such as dealloying, templating (soft and hard), microwave irradiation, additive manufacturing, ion-beam processing, and laser processing have been employed. Studies on nanoporous metals and ceramics show reasonably high strength-to-weight ratios but limitations exist in ligament thickness and the mass density of the base materials. The 'smaller is stronger' effect has been documented, illustrating how nanoscale features can significantly enhance material strength due to a reduced likelihood of critical flaws. A number of studies have explored methods to create nanoporous amorphous carbon materials including sequential chemical dealloying, hard and soft templating, and two-photon laser lithography. These studies have primarily focused on electrochemical, capacitance, and biocompatibility properties of the materials, with limited investigation into their mechanical properties. Two-photon laser lithography, while effective in producing intricate nanostructures, presents challenges in terms of cost and scalability. This study sought to develop an economical and easily scalable method to produce nanoporous amorphous carbon materials while maintaining the precise control over the porosity and structure length scale needed for a systematic study and comparison to existing structural materials. The high breaking strength of C-C bonds and the mechanical size effects were understood to be fundamental to the properties of these materials.

This research employed a novel fabrication technique combining self-assembled polymeric carbon precursor materials and nano-imprinting lithography (NIL). The precursor material consisted of a nanocomposite of phenol-formaldehyde (PF) resin and bottlebrush block copolymers (BBCP). The BBCP acted as a sacrificial template, controlling the porosity of the final nanoporous carbon structure. The use of BBCP enabled precise control over porosity (0% to 59%) with minimal shrinkage (less than 20%) during carbonization. Nano-imprinting lithography provided a scalable and efficient method for patterning the nanopillars, offering a significant advantage over techniques like focused-ion-beam (FIB) milling which are extremely time-consuming. The NIL process, combined with the self-assembled BBCP and PF resin composite, ensured the formation of nanopillars with a homogenous porous structure after carbonization. This homogeneity was attributed to the relatively small domain size of the nanocomposite compared to the nanopillar dimensions. The process produced nanopillars approximately 300 nm in diameter and 1 µm in height with pore sizes of ~50 nm and ligament thicknesses of 20 nm or less. High-resolution transmission electron microscopy (HRTEM) confirmed the amorphous nature of the carbon. The fully dense and nanoporous nanopillars were subjected to several types of mechanical testing: in situ uniaxial compression tests to measure yield strength, fracture strength, and fracture strain, nanoindentation to determine Young's modulus, and cyclic compression tests to assess the damping capability. Electron energy loss spectroscopy (EELS) and Raman spectroscopy were used to analyze the atomic-scale changes in carbon bonding during deformation. Atomistic simulations using molecular dynamics (MD) were performed to gain a deeper understanding of the deformation mechanisms, particularly the sp²-to-sp³ transition and the role of the nanoscale dimensions of the ligaments.

The fabricated nanoporous carbon nanopillars demonstrated exceptional mechanical properties. Their yield and fracture strengths were higher than those of most engineering materials with similar mass density. The nanopillars displayed both elastic and plastic behavior. The fracture strength of the nanopillars ranged from approximately 1.7 GPa to 4 GPa, and the yield strength ranged from approximately 1.3 GPa to 3 GPa, depending on the porosity. Fracture strain was surprisingly large, up to 35%. This high deformability was linked to the reversible and irreversible portions of the sp²-to-sp³ transition in the carbon structure. The reversible portion contributed to large elastic strain, while the irreversible portion accounted for the plastic deformation. Moreover, the nanopillars displayed high damping capability, with a loss factor up to 0.033, comparable to Ni-Ti shape memory alloys. The mechanical properties of the nanopillars correlated with their porosity and pore size, and a transition in fracture strain was observed as porosity increased from 51% to 59%, or as pore size decreased. This transition was associated with changes in the plastic strain and attributed to the flaw tolerance effect at the nanoscale in ligaments of approximately 10 nm or less. EELS analysis revealed a 4% reduction in sp² bonds after compression to 35% strain, confirmed by Raman spectroscopy data which showed an increase in D peak intensity and a positive shift of G peak post-compression, suggesting a local sp²-to-sp³ transition. Atomistic simulations confirmed the reversible and irreversible portions of the sp²-to-sp³ transition, consistent with experimental observations and providing an explanation for the high damping capability and large plastic strain. The high fraction of Suquet upper bound exhibited by the nanopillars (50–75%) suggested that the nanoscale size effects and the absence of critical flaws dominated the material's mechanical properties. The absence of significant flaws was further supported by FIB-milled micropillar compression tests which showed similar mechanical properties to the nanoimprinted nanopillars.

The findings demonstrate the successful fabrication of nanoporous amorphous carbon nanopillars with an exceptional combination of lightweight, ultrahigh strength, large fracture strain, and high damping capability—a combination not observed in existing structural materials. The superior mechanical properties are attributed to the nanoscale ligament size effect, minimizing the presence of critical flaws, and the reversible and irreversible sp²-to-sp³ transitions. The combination of scalable nanoimprinting and the use of self-assembled polymeric precursors provide a route to producing these materials. The tunable porosity also provides a means of controlling the density and other mechanical properties of the material, offering a degree of design freedom not previously available. These results highlight the potential of amorphous carbon-based nanoporous structures as advanced structural materials, particularly in applications requiring high strength-to-weight ratios, good damping, and high deformability. The study's findings are consistent with atomistic simulations, confirming the role of the sp²-to-sp³ transition in the observed mechanical behavior. The transition from sp² to sp³ bonds occurs because the sp³ bond is energetically preferred under compression, and local tensile stresses in the nanoporous structures prevent the sp²-to-sp³ transition in those regions.

This research successfully demonstrated the fabrication and characterization of nanoporous amorphous carbon nanopillars with unprecedented combinations of lightweight, ultrahigh strength, large fracture strain, and high damping capability. The combination of scalable nano-imprinting lithography and self-assembled precursors enabled the production of these materials with precisely controlled porosity and structure. The unique mechanical properties stem from the nanoscale size effect, minimizing the probability of flaws, and from reversible and irreversible sp²-to-sp³ transitions. Future research could explore further optimization of the nanoporous structure to approach the Suquet upper bound, investigate the effects of different pore geometries and distributions, and explore applications of these materials in diverse fields. The tunability of porosity and incorporation of other elements present opportunities for tailoring the properties for specific applications.

While the study provides compelling evidence for the exceptional mechanical properties of the nanoporous amorphous carbon nanopillars, some limitations exist. The study's focus on compression testing might not fully capture the material's behavior under other loading conditions (e.g., tension, shear). The high strain rates used in the simulations could influence the results and may not accurately reflect the behavior under slower loading conditions. Further research is needed to fully understand the long-term stability and durability of these materials under various environmental conditions. The effects of different pore geometries on the mechanical properties require further investigation.