Lightweight, flaw-tolerant, and ultrastrong nanoarchitected carbon

Abstract

It has been a long-standing challenge in modern material design to create low-density, lightweight materials that are simultaneously robust against defects and can withstand extreme thermomechanical environments, as these properties are often mutually exclusive: The lower the density, the weaker and more fragile the material. Here, we develop a process to create nanoarchitected carbon that can attain specific strength (strength-to-density ratio) up to one to three orders of magnitude above that of existing micro- and nanoarchitected materials. We use two-photon lithography followed by pyrolysis in a vacuum at 900 °C to fabricate pyrolytic carbon in two topologies, octet- and iso-truss, with unit-cell dimensions of ∼2 μm, beam diameters between 261 nm and 679 nm, and densities of 0.24 to 1.0 g/cm³ Experiments and simulations demonstrate that for densities higher than 0.95 g/cm³ the nanolattices become insensitive to fabrication-induced defects, allowing them to attain nearly theoretical strength of the constituent material. The combination of high specific strength, low density, and extensive deformability before failure lends such nanoarchitected carbon to being a particularly promising candidate for applications under harsh thermomechanical environments.

Keywords: iso-truss; nanolattices; octet-truss; pyrolytic carbon; specific strength.

Fig. 1.

Fabrication and microstructural characterization of pyrolytic carbon nanolattices. (A) Schematic illustration of the fabrication process of pyrolytic carbon nanolattices. (B and C) CAD rendition of an octet- and iso-truss unit cell. (D and E) SEM images of an octet nanolattice with a strut diameter of d = 435 nm and an iso-truss nanolattice fabricated with a vertical strut diameter of d₁ = 460 nm and a slanted strut diameter of d₂ = 523 nm. (F) An HRTEM image of pyrolytic carbon extracted from the nanolattice, which indicates an amorphous nature of the pyrolytic carbon. Initial detectable structural imperfections caused by fabrication process are circled in D and E.

Fig. 2.

In situ uniaxial compression experiments on pyrolytic carbon nanolattices. (A and B) Typical mechanical response of pyrolytic carbon octet- and iso-truss nanolattices with different relative densities obtained from in situ compressions. (C and D) SEM images of an octet-truss nanolattice with relative density of 37.5% at different strains during compression. (E and F) SEM images of the iso-truss nanolattice with relative density of 39.4% at different strains during compression.

Fig. 3.

Mechanical properties versus density maps of pyrolytic carbon nanolattices. (A and B) Young’s modulus and compressive strength of pyrolytic carbon nanolattices plotted versus density on a log-log scale. For comparison, these charts include several micro- and nanoarchitected materials reported so far, such as alumina hollow nanolattices (11), alumina–polymer nanolattices (16), glassy carbon nanolattices (18), monolithic carbon aerogels (22), graphene aerogel microlattices (23), vitreous carbon nanolattices (24), cellular carbon microstructures (25), and SiOC microlattices (26).

Fig. 4.

FE simulations of uniaxial compression of pyrolytic carbon nanolattices with different unit cells. (A–C) Simulated configurations of octet-, iso-, and tetrahedron-truss nanolattices with preexisting defects introduced by imposing an initial deflection of struts. The insets are zoomed-in views of local structures with initial deflection of struts. The color represents the extent of initial deflection. (D–F) Compressive stress–strain curves from simulations (with beam elements) of octet-truss, iso-truss, and tetrahedron-truss nanolattices with different relative densities and initial specific deflection. (G–I) Compressive stress–strain curves from simulations (with solid elements) of octet-truss, iso-truss, and tetrahedron-truss nanolattices with different relative densities and initial specific deflection.

Fig. 5.

Comparison of the specific strength between our pyrolytic carbon nanolattices and other micro- and nanolattices that have been reported to date.