Hydrodynamic tuning thermal conductivity and microstructure of wood-based nanocellulose through hydrochloric acid concentration

The continuous increase in power density of flexible electronic devices underscores the critical need for developing efficient thermal management solutions. Cellulose nanofibril (CNF) has emerged as an ideal base material owing to its advantageous properties, including biodegradability, high mechanical strength, and electrical insulation. However, conventional methods for enhancing thermal conductivity often adversely affect processability or increase interfacial thermal resistance, making the maximization of CNF's intrinsic thermal transport potential without additives a significant ongoing challenge. This study developed a novel approach combining microfluidic hydrodynamic focusing with hydrochloric acid-induced gelation, successfully fabricating CNF filaments with a highly aligned structure. The results demonstrate a non-monotonic dependence of thermal conductivity on HCl concentration, with an optimal value of 1.013 W m−1 K−1. Raman and micro-Fourier transform infrared spectroscopy analyses confirmed that this optimum performance stems from the synergistic enhancement of crystallinity and the hydrogen-bonding network. Notably, a radial crystallinity gradient structure, governed by H+ diffusion during non-equilibrium gelation, was identified. This heterogeneous structure simultaneously imparts high thermal conductivity, outstanding mechanical properties, and remarkable flexibility. By integrating chemical regulation with fluidic assembly techniques, this work elucidates the physical mechanism underlying the enhancement of intrinsic thermal transport in CNF, providing a new design strategy for developing high-performance flexible thermal management materials.