Lightweight, Flaw-Tolerant, and Ultrastrong Nanoarchitected Carbon
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Lightweight, flaw-tolerant, and ultrastrong nanoarchitected carbon Xuan Zhanga, Andrey Vyatskikhb, Huajian Gaoc,1, Julia R. Greerb,1, and Xiaoyan Lia,d,1 aCenter for Advanced Mechanics and Materials, Applied Mechanics Laboratory, Department of Engineering Mechanics, Tsinghua University, 100084 Beijing, China; bDivision of Engineering and Applied Science, California Institute of Technology, Pasadena, CA 91125; cSchool of Engineering, Brown University, Providence, RI 02912; and dCenter for X-Mechanics, Zhejiang University, 310027 Hangzhou, China Contributed by Huajian Gao, February 9, 2019 (sent for review October 8, 2018; reviewed by Yonggang Huang and Christopher M. Spadaccini) It has been a long-standing challenge in modern material design to Most work on micro/nanoarchitected materials to date has create low-density, lightweight materials that are simultaneously been focused on hollow-beam-based architectures, which offer robust against defects and can withstand extreme thermomechan- exceptionally light weight with a concomitant high compliance ical environments, as these properties are often mutually exclusive: [e.g., nickel-based hollow-tube microlattices with a Young’s The lower the density, the weaker and more fragile the material. modulus of 529 kPa and a compressive strength of ∼10 kPa at a Here, we develop a process to create nanoarchitected carbon that density of ∼0.010 g/cm3 (7) and ceramic hollow-tube nanolattices can attain specific strength (strength-to-density ratio) up to one to with Young’s moduli of 0.003 to 1.4 GPa and compressive three orders of magnitude above that of existing micro- and strengths of 0.07 to 30 MPa at densities of 0.006 to 0.25 g/cm3 nanoarchitected materials. We use two-photon lithography followed (10–14)]. These micro/nanoarchitected materials have a common by pyrolysis in a vacuum at 900 °C to fabricate pyrolytic carbon in feature of length scale hierarchy, that is, relevant dimensions of two topologies, octet- and iso-truss, with unit-cell dimensions of their structural elements span three to five orders of magnitude, ∼2 μm, beam diameters between 261 nm and 679 nm, and densities from tens of nanometers to hundreds of micrometers and even 3 of 0.24 to 1.0 g/cm . Experiments and simulations demonstrate that greater. Structural features of nickel-alloy hollow-tube nanolattices 3 for densities higher than 0.95 g/cm the nanolattices become insensi- fabricated using large-area projection microstereolithography span tive to fabrication-induced defects, allowing them to attain nearly seven orders of magnitude in spatial dimensions, from tens of theoretical strength of the constituent material. The combination of nanometers to tens of centimeters. These nanolattices attain ten- ENGINEERING high specific strength, low density, and extensive deformability be- sile strains of >20% with a low modulus of 125 kPa and a low fore failure lends such nanoarchitected carbon to being a particularly tensile strength of ∼80 kPa at a density of ∼0.20 g/cm3,which promising candidate for applications under harsh thermomechanical corresponds to the relative density of 0.15% (17). The deform- environments. ability of these nanolattices is attributed to a combination of bending- and stretching-dominated hierarchical architectures nanolattices | pyrolytic carbon | octet-truss | iso-truss | specific strength distributed over successive hierarchies and shell buckling, an elastic instability characteristic of thin-walled hollow cylinders ightweight porous materials, such as wood, bone, Euplectella (17). Among the thin-walled architectures, 3D periodic graphene Lsponges, diatoms, and bamboo, are ubiquitous in nature. aerogel microlattices have been synthesized via direct ink writing; These natural structural materials have been extensively in- – vestigated (1 5) and have been shown to be resilient against Significance externally applied loads and powerful in absorbing and dissi- pating impact energy. Such properties have been enabled by two i A long-standing challenge in modern materials manufacturing and design principles: ( ) a multiscale hierarchy of constituent ma- design has been to create porous materials that are simultaneously terials and length scales, which generally consist of complex lightweight, strong, stiff, and flaw-tolerant. Here, we fabricated multilevel architectures with characteristic dimensions from pyrolytic carbon nanolattices with designable topologies by a two- ii nano- to macroscale (5) and ( ) their tolerance of flaws when the step procedure: direct laser writing and pyrolysis at high temper- characteristic material length scale falls below a critical value (4). ature. The smallest characteristic size of the nanolattices Both principles have been applied to engineering advanced approached the resolution limits of the available 3D lithography materials to various degrees of success (5, 6). technologies. Due to the designable unit-cell geometries, re- A general guideline for a material to be considered “light- duced feature sizes, and high quality of pyrolytic carbon, the weight” is for its density to be less than that of water (i.e., ρ ≤ created nanoarchitected carbon structures are lightweight, can 3 1.0 g/cm ) (1, 7). Recent breakthroughs in material processing be made virtually insensitive to fabrication-induced defects, at- techniques, especially in 3D microfabrication and additive tain nearly theoretical strength of the constituent material, and manufacturing, provide a particularly promising pathway to achieve specific strength up to one to three orders of magnitude fabricate lightweight materials, which often possess a suite of above that of all existing micro/nanoarchitected materials. other beneficial properties such as high specific stiffness, high specific strength, and good resilience/recoverability (7–27). A Author contributions: H.G., J.R.G., and X.L. designed research; X.Z. and A.V. performed penalty for the ultralight weight of such nano- and micro- research; X.Z., A.V., H.G., J.R.G., and X.L. analyzed data; and X.Z., H.G., J.R.G., and X.L. wrote the paper. architected materials is a severe reduction in their stiffness and σ ∼ ρ ρ m E ∼ ρ ρ n Reviewers: Y.H., Northwestern University; and C.M.S., Lawrence Livermore strength through power law scaling: y ( / s) , ( / s) , National Laboratory. where σ is the yield strength, E the Young’s modulus, ρ the y The authors declare no conflict of interest. density, and ρs the density of the fully dense constituent solid (1). m n This open access article is distributed under Creative Commons Attribution-NonCommercial- The exponents and are generally greater than 1, which NoDerivatives License 4.0 (CC BY-NC-ND). renders developing methodologies to create materials that are 1To whom correspondence may be addressed. Email: [email protected], jrgreer@ simultaneously lightweight and strong/stiff while maintaining caltech.edu, or [email protected]. their other properties (i.e., thermal stability, electrical conduc- This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. tivity, magnetism, recoverability, etc.) a grand unsolved challenge 1073/pnas.1817309116/-/DCSupplemental. because of restricted material choices and limited architectures. Published online March 18, 2019. www.pnas.org/cgi/doi/10.1073/pnas.1817309116 PNAS | April 2, 2019 | vol. 116 | no. 14 | 6665–6672 Downloaded by guest on September 28, 2021 these materials are exceptionally lightweight (with a density of rolyzing a UV-mask patterned polymer template, reported a 0.031 to 0.123 g/cm3), compliant (with a modulus of 1 to 10 MPa), modulus of 1.1 GPa, a compressive strength of 10.2 MPa, and a and weak (with a low strength of 0.10 to 1.6 MPa) and exhibit fracture strain of only ∼3% at a density of 0.19 g/cm3 (24). Glassy nearly complete recovery after compression to 90% strain (23). carbon microlattices with rhombic dodecahedron unit-cell and Some efforts have also been dedicated to the synthesis and beam diameters of 50 to 150 μm, fabricated using stereo- development of mechanical properties of micro- and nano- lithography and pyrolysis, had densities of 0.03 to 0.05 g/cm3, architected materials that are composed of nonhollow beams of moduli of 5 to 25 MPa, compressive strengths of 0.08 to 0.35 MPa, various materials, achieving greater stiffness and higher densities and fractured at a strain of ∼5% (25). Glassy carbon nanolattices compared with their hollow-beam counterparts. Most of these with tetrahedral unit cells created via TPL and pyrolysis had studies have been on architectures composed of core-shell types of smaller dimensions (0.97- to 2.02-μm unit cells and beam diame- beams, usually with an acrylic polymer core and a thin (from tens ters of ∼200 nm), a modulus of 3.2 GPa, and a compressive of nanometers to several hundred nanometers), rigid outer coat- strength of ∼280 MPa at a density of ∼0.35 g/cm3 (18). These ing. For example, triangular-truss microlattices with polymer-core- advances highlight a strong coupling between the density and alumina-shell beams have been synthesized by combining two- compliance of architected materials: The lower the density, the photon lithography (TPL) direct laser writing (DLW) and atomic softer and the weaker the material. layer deposition and sustained a modulus of ∼30 MPa at a low We developed an approach to fabricate nanoarchitected py- fracture strain of ∼4 to 6% and a density of 0.42 g/cm3 (16). Octet- rolytic carbon and to demonstrate two prototype unit-cell ge- truss nanolattices made up of 262- to 774-nm-diameter polymer ometries, octet- and iso-truss, shown in Fig. 1, using TPL and beams with sputtered 14- to 126-nm-thick high-entropy alloy (HEA) coatings were reported to have a Young’smodulusof16to pyrolysis. The octet-truss architecture has cubic anisotropy and 95 MPa and a compressive strength of 1 to 10 MPa at densities superior overall properties compared with other conventional between 0.087 and 0.865 g/cm3 (20). Samples with HEA thick- lattices, such as triangular, tetrahedral, or cubic trusses and nesses less than 50 nm completely recovered after being com- foams (28), whereas the iso-truss structure is isotropic and has pressed for >50% (20).