Super-Strong, Micrometer-Thin Carbon Nanotube Yarns
Xavier Lepro Chavez | 20-ERD-023
Micrometer-thin commercial fiber-like materials that are flexible and strong are conspicuous in their absence. In most cases, this is due to inherent limitations of the manufacturing approach. Even highly ductile metals that can be thinned down to wires several micrometers thick become brittle once they are in the single-digit micron range. Natural fiber alternatives already at the micrometer scale (like spider silk), suffer from low reproducibility given their environment (e.g., relative humidity) and biology-dependent (e.g., spider species, age, diet, etc.) mechanical properties. To fill this gap in material availability, nanomaterials offer a great promise given their material properties in terms of strength, weight, and our ability to design and tune their properties according to the desired application.
Under this project, we developed a process to manufacture carbon nanotube (CNT) fibers well beyond the previous state of the art, down to 1 μm in diameter. Carbon nanotubes, despite possessing exceptional mechanical properties (surpassing stiffness and strength metrics of other common materials like steel alloys by 100× at the nanoscale), when bundled together into macroscopic ensembles result in materials exhibiting a 100-fold drop in strength. We studied a strategy aimed to close this gap in property scaling that resulted in a 10× improvement in the resulting material's mechanical properties, as measured by their Young's modulus. To reach this goal, we reinforced the weak physical interactions among individual CNTs bundled together within dry-spun yarns by using vapor-phase polymerization of a crosslinkable polymer to improve the transference of the mechanical load across the structure. Unlike liquid infiltration, this vapor-phase approach retains the yarn's inherent internal porosity, and thus a larger internal area fraction is accessible for polymer reinforcement. Synchrotron X-ray scattering (RSoXS) revealed that polymer-reinforced yarns undergo limited CNT bundle rearrangement when subjected to tensile loads compared to pristine yarns. This evidence supports the hypothesis that the polymer hinders CNTs' slippage, the root cause of the poor scaling of mechanical properties in these materials. Another synchrotron technique, 30-nm resolution scanning transmission X-ray microscopy (STXM) showed that, after reinforcement, the spatial mass distribution of the polymer within the yarns is homogeneous and contributes to a total mass density increase of 30%. Finally, the effects of polymer reinforcement on the mechanical performance of yarns at extreme cryogenic conditions (to temperatures as low as 5 K) were evaluated and, for the first time, their stress–strain curves were obtained with a specially designed, in-house assembled tensile tester. The development of strong micron-sized fibers is of interest in Target Fab to fabricate alternative (also known as "tetracage") capsule supports to advance in the progress of ICF technology and to support Stockpile Stewardship mission at LLNL. Moreover, this project also generated new characterization capabilities and techniques that are not specific to CNTs and could be used to study the behavior of other micrometer-sized materials under extreme conditions.
In this project we developed micrometer-sized, synthetic fiber-like flexible carbon nanotube threads of tunable strength that are of interest to LLNL's HED and ICF programs for a new generation of one-dimensional target-support tetracage architectures. Minimal (or "barely there") support materials are of current interest in the NIF to reduce the induced perturbations derived from the contact area between the capsule ablators with their supports during ICF experiments and advance the program needs under the Stockpile Stewardship Main Mission Area of LLNL and NNSA. The recently hired technical staff supported and characterization capabilities developed under this project bring to Target Fabrication and LLNL new capabilities and state-of-the-art technologies to advance in the development and understanding of the new micromaterials to support the HED, Advanced Materials and Manufacturing core competencies.
Publications, Presentations, and Patents
Lepró, X., et al. 2022. "Liquid-Free Covalent Reinforcement of Carbon Nanotube Dry-Spun Yarns and Free-Standing Sheets." Carbon 187 (2022/02): 415-24. LLNL-JRNL-818731.
Schwartz, A., et al. 2022. "Mechanical Properties of Carbon Nanotube Yarns at Cryogenic Conditions for Tetracage Targets." 24th Target Fabrication Specialist Meeting, June 6-9, 2022. LLNL-POST-835519.
Lepró, X., et al. 2022. "Micron-Thick Carbon Nanotube Yarns for Alternative Capsule Support." 24th Target Fabrication Specialist Meeting, June 6-9, 2022. LLNL-PRES-835485.
Jean-Remy, P., et al. 2022. "Interfacial Chemical Functionalization Enhances Load Transfer in Hierarchical Carbon Nanotube Assemblies." Lawrence Berkeley Molecular Foundry User Meeting, Berkeley, CA, August 18-19, 2022. LLNL-POST-838845.
Lepró, X., et al. 2022. "Micron-Thick Carbon Nanotube Yarns for Alternative Capsule Support." Target Fabrication R&D Innovative Initiative, January 25, 2022. LLNL-PRES-830931.