Tsinghua University News

Prof. Xiaoyan Li’s group published article in Nature Nanotechnology, reporting fabrication and mechanics of micro-sized pyrolytic carbon with low-density, large deformability and high strength

Prof. Xiaoyan Li’s group published article in Nature Nanotechnology, reporting fabrication and mechanics of micro-sized pyrolytic carbon with low-density, large deformability and high strength


On July 9, Prof. Xiaoyan Li’s research group published a an article entitled with “Theoretical strength and rubber-like behaviour in micro-sized pyrolytic carbon” in the journal Nature Nanotechnology, in collaboration with Prof. Huajian Gao’s group at Brown University and Prof. Julia Greer’s group at California Institute of Technology. This paper reported the fabrication, microstructural characterization and unique mechanical behaviors of micro-sized pyrolytic carbon.

For nearly all structural materials, the trade-offs between high strength and large deformability/ductility and high strength and low density have been recognized as two pairs of inevitable conflicts. For example, metals and alloys are ductile, but their strengths are usually about several hundred MPa. The ceramics have higher strength (up to about several GPa), but their fracture strains are below 5%. Polymers and porous materials like foams are lightweight and deformable, but their strengths are only on the order of ~10 MPa. Existing materials that come close to “optimizing” all three of these properties are Mg alloys, but their ductility is still lower than the requirements of applications for forming and performance of structural components. Designing and synthesizing materials that are simultaneously strong and stiff, substantially deformable (i.e. >10%), and lightweight represents the Holy Grail of materials sciences, and remains a long-standing and unsolved challenge.

Figure 1. Mechanical behaviors of micro-sized pyrolytic carbon. (a) Variation of compressive strength with diameter of pyrolytic carbon specimens, (b) Force-displacement curve from cyclic loading with a maximum compressive strain of ~23%; (c) Typical stress-strain curve of pyrolytic carbon micropillars with diameter less than 2.3 ?m, and (d) SEM images of tested specimens during in-situ compression.

In this work, the researchers created a new kind of small-scale carbon by pyrolyzing micro-sized cylindrical samples sculpted out of a photosensitive polymer using two-photon lithography. Careful atomic-level microstructural characterization revealed that these micro-sized pyrolytic carbon specimens consist of 1-1.5 nm curved graphene layers. The experimental measurements and detailed analyses showed the overall density of micro-sized pyrolytic carbon to be 1.4 g/cm3, i.e. similar to those of some polymers. In-situ SEM nanomechanical experiments revealed an average tensile strength of up to ~1.6 GPa and a compressive strength approaching theoretical limit of 13.7 GPa (Fig. 1a), with an ultra-large elastic limit of 20-30% (Fig. 1b). The combination of close-to-theoretical strength and low density of these micron-sized pyrolytic carbon samples points to their ultra-high specific strength of 9.79 GPa cm3/g, which surpasses the specific strengths of all existing structural materials, including polycrystalline diamond, which has the highest specific strength of ~5.61 GPa cm3/g in bulk materials reported to date. Further, pyrolytic carbon samples with diameters below 2.3 ?m sustained compressive strains that exceed 50% without catastrophic fracture, deforming like rubber (Fig. 1c). Large-scale atomistic simulations uncovered the underlying deformation mechanisms, i.e. the compressive deformation is dominated by slipping/shear of the graphene layers and densification of the structure, while the tensile deformation is governed by the nucleation, growth and coalescence of nanoscale cavities or by the extension of initial flaws (Fig. 2). These mechanisms enable the unexpected combination of low density, substantial deformability, high elastic limit and high strength of micro-sized pyrolytic carbon samples. These findings provide potential routes for designing and synthesizing new high-performance carbon materials.

Figure 2. Large-scale atomistic simulations of pyrolytic carbon under compression and tension. (a) Snapshots of a deformed sample at different compressive strains, (b) Snapshots of a deformed sample at different tensile strains.

In recent years, Prof. Xiaoyan Li’s research group worked on design, fabrication and mechanics of novel nanostructured materials with excellent mechanical properties and performance, and published some influential papers on high-profile international journals, including Nature Materials, Nature Communications, Science Advances, Advanced Materials and ACS Nano.

Dr. Xuan Zhang, Lei Zhong (PhD student) at Tsinghua and Arturo Mateos (PhD student) at CalTech contributed equally to this work. Prof. Xiaoyan Li, Prof. Huajian Gao and Prof. Julia Greer are the co-corresponding authors of this paper. This work was supported by the National Natural Science Foundation of China and the National Basic Research of China.

Links to the publication:
https://www.nature.com/articles/s41565-019-0486-y


Source: School of Aerospace Engineering
Editor: Guo Lili