Enhancing Thermal Conductivity and Phase Change Stability in Polyethylene Glycol Composites via Aspect Ratio Engineering of Boron Nitride Nanosheets

Thermal management in high‐frequency electronic devices requires materials combining efficient heat dissipation with thermal energy storage. Organic phase change materials, like polyethylene glycol (PEG), offer promising thermal storage but suffer from low thermal conductivity and leakage during solid–liquid transitions. Herein, a strategic approach to maximize hexagonal boron nitride (h‐BN) nanosheet aspect ratio for enhanced thermal management in PEG composites is reported. High‐aspect‐ratio boron nitride (BN) is synthesized using melamine (C 3 H 6 N 6 )‐assisted carbothermal reduction and nitridation, followed by N , N ‐dimethylformamide‐mediated exfoliation to produce few‐layer structures with aspect ratios up to 191.79. Comparative analysis of BN nanosheets with varying morphologies in PEG matrices demonstrates that aspect ratio critically governs thermal transport efficiency. The optimized composite achieves exceptional thermal conductivity (0.92 W m −1 K −1 ) while preserving 84% of pristine PEG's phase change enthalpy (152.1 J g −1 ) at 10 wt% loading. Furthermore, the composite exhibits superior thermal cycling stability with only 4.3% leakage after 50 thermal cycles, significantly outperforming conventional systems. This work establishes filler morphological engineering as a key strategy for developing next‐generation thermal interface materials with optimized performance.