Designing Pyrolyzed Nanofibers at the Atomic Level: Toward Synthesis of Ultra-high-strength Nano-carbon

Non-technical Summary:

Supported through the Solid State and Materials Chemistry Program within the Division of Materials Research, the principal investigators and their research groups investigate the scientific foundation required for developing strong fibers and nanofibers, which are predominantly made of carbon and can surpass the load bearing capacity of existing fibers. The fibers are prepared via extreme heating of polymers in a process referred to as pyrolysis or carbonization.

The researchers study the emergence of the most likely strength-compromising defects and devise methodologies to mitigate the adverse effect of defects on fiber strength. The project consists of a series of experiments investigating the fibers at the very small length scales, the nanoscale, coupled with computational modeling that is enhanced with artificial intelligence.

The findings of this research may transform how super-strong fibers are made via pyrolysis. These carbon nanofibers could be used as reinforcements in composites or other assemblies such as yarns in order to develop structural components in applications in which weight is a premium such as aeronautics and space missions. The project also has a strong educational component with a workshop to train the next generation of engineers on this topic.

Technical Summary:

The emergence of nanomaterials such as carbon nanotubes (CNTs) raised hopes for materials with mechanical strength far exceeding that of the industry standard, carbon fibers (CFs). However, this hope in nanotechnology has remained largely unfulfilled in the past two decades, due to limitations in controlling defects and limited understanding of their impact on strength.

Therefore, the focus of this project, supported through the Solid State and Materials Chemistry Program in the Division of Materials Research, is to unravel the effect of defects on the strength of partially graphitic carbon nanofibers (CNF) made via pyrolysis. The CNFs consist of amorphous carbon and stacked graphitic-like nanoribbons, i.e., turbostratic (TB) domains.

With this project, the PIs investigate a nearly 50-year-old scientific dilemma: strength predictions for carbonized fibers deviate greatly from experiments; while theory predict strength of over 30 GPa, experiments peak at ~10 GPa; moreover, unlike theoretical predictions, experiments show that the strength does not monotonically increase with graphitic content. To investigate this mismatch, the project is subdivided into three aims targeting one of three features: nanoscale voids, cleaved crosslinks, and stress concentration at boundaries of misaligned TB domains.

Aim 1 evaluates the effect of nanovoids that form due to competitions between material loss and consolidation. Continuum models with the aid of Machine Learning (ML) approaches are used to resolve stress fields around the. In Aim 2, the PIs study correlations between strength and atomic crosslinks between TB domains, and ion bombardment is used to assess the reversibility of atomic crosslinks.

In Aim 3, they investigate the impact of residual stresses and failure along misoriented TB domains via a novel Raman-based approach. The distribution of residual stresses is measured via Raman-based method, and correlation between TB alignments, residual stress distributions, and mechanics model predictions is used to evaluate the effect of domain alignment on strength.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.