Ultrafast synchrotron X-ray imaging and multiphysics modelling of liquid phase fatigue exfoliation of graphite under ultrasound

Ultrasound-assisted liquid phase exfoliation is a promising method for manufacturing of 2D materials in large scale and sustainable manner. A large number of studies using ex-situ nano/micro structural characterization techniques have been made to investigate the underlying mechanisms, aiming to understand the exfoliation dynamics. Due to the complex multiphysics and multi-length nature of the process, those ex-situ methods cannot provide the real-time and in-situ dynamic information for understanding how exactly layer exfoliation starts and grows under ultrasound. Here, we used the ultrafast synchrotron-X-ray phase-contrast imaging (a combined temporal resolution of 3.68 mu s and a spatial resolution of 1.9 mu m/pixel) to study the exfoliation dynamics in real time and operando condition. We revealed, for the first time, the fatigue exfoliation phenomenon at the graphite surface caused by the imploding ultrasonic bubbles occurring cyclically in line with the ultrasound frequency. A multiphysics numerical model was also developed to calculate the shock wave produced at bubble implosion and the resulting cyclic and impulsive tensile and shear stresses acting on the graphite surface. Our research reveals that the graphite layer exfoliation rate and efficiency are predominantly determined by the number of imploding bubbles inside the effective cavitation bubble zone. The findings are valuable for developing industrial upscaling strategies for ultrasound processing of 2D materials. (C) 2021 The Authors. Published by Elsevier Ltd.