Cotton-fiber-derived tree-ring-like Cu/C frameworks: A sustainable strategy to prepare phase change composites with anisotropic thermal conductivity, efficient solar-to-thermal conversion, and excellent thermal management capability

To address the inherent bottlenecks of low thermal conductivity and poor shape stability in paraffin wax (PW)-based phase change materials (PCMs), a novel tree-ring-like copper/carbon (Cu/C) composite scaffold with oriented hierarchical fibrous structure was successfully fabricated using medical absorbent cotton as the biomass precursor and tetra-ammine-Cu ion solution (TACIS) via a facile process combining vacuum impregnation and high-temperature inert calcination. With increasing TACIS concentration, interconnected continuous Cu nanoparticle networks are in-situ constructed on residual carbon fibers inside the tree-ring-like scaffold. PW-based phase change composites (PCCs) were further prepared by loading PW into the tree-ring-like Cu/C scaffold. The results demonstrate that the scaffold forms a continuous Cu nanoparticle network with increasing TACIS concentration, endowing PCCs with pronounced anisotropic thermal conductivity. The optimal PW/CR-Cu3.5 sample achieves an axial thermal conductivity of 9.31 W·m −1 ·K, which is 37.2 times that of pure PW, along with an ultra-low leakage rate without the need for additional binders, a 99% solar-thermal conversion efficiency under 1500 W/m 2 irradiance, and effective thermal management performance for CPU devices. This cost-effective and sustainable strategy provides a promising solution for high-performance PCMs in practical applications of thermal energy storage and electronic cooling.