Graphite forms via annihilation of screw dislocations

TL;DR

This study reveals that screw dislocations hinder graphite formation by preventing layer stacking, and their annihilation during heating promotes crystallite growth, offering insights to improve synthetic graphite production.

Contribution

It identifies screw dislocations as key defects in graphite formation and demonstrates their spontaneous formation and annihilation during heating through combined microscopy, simulations, and XRD analysis.

Findings

Screw dislocations inhibit graphite layer stacking.

Heating causes screw annihilation and crystallite growth.

Screw defects are confirmed by HRTEM and simulations.

Abstract

Graphite is the thermodynamically stable form of carbon, and yet is remarkably difficult to synthesise. A key step in graphite formation is the removal of defects at high temperature ($>$2300~$^{\circ}$C) that allow graphenic fragments to rearrange into ordered crystallites. We find the critical defect controlling graphitisation is a screw dislocation that winds through the layers like a spiral staircase, inhibiting lateral growth of the graphenic crystallites ($L_a$) and preventing AB stacking of Bernal graphite. High-resolution transmission electron microscopy (HRTEM) identifies screws as interdigitated fringes with narrow focal depth in graphitising polyvinyl chloride (PVC). Molecular dynamics simulations of parallel graphenic fragments confirm that screws spontaneously form during heating, with higher annealing temperature driving screw annihilation and crystallite growth. The time evolution of graphitisation is tracked via X-ray diffraction (XRD), showing the growth of $L_a$ and reduction of the interlayer spacing consistent with molecular dynamics of screw annihilation. This mechanistic insight raises opportunities to lower the barrier for graphitisation as well as broadening the range of carbonaceous materials that can turn into graphite, thereby lowering the cost of synthetic graphite used in lithium-ion batteries, carbon fibre, and electrodes for smelting.