Non-monotonously tuning thermal conductivity of graphite-nanosheets/paraffin composite by ultrasonic exfoliation

Organic phase change materials (PCMs) have drown continuous attentions over time due to their large latent heat and constant-temperature solid-liquid phase transition with promising applications in thermal energy storage. Nevertheless, they suffer from the relatively low intrinsic thermal conductivity. Filling the PCMs with graphite-nanosheets (GNs) by ultrasonic exfoliating could alleviate this problem, and GNs with longer ultrasonic exfoliation time is reported to possess larger effective thermal conductivity (ETC) monotonously. In this paper, we discover a non-monotonous variation of ETC for the first time, when enhancing the ETC of paraffin with ultrasonic exfoliated GNs. The mechanism behind this phenomenon is explained by the variation of GNs morphologies over time in the paraffin. Experimental results reveal that longer exfoliation time can increase the aspect ratio and ETC, but over a critical time, the bending stiffness of GNs decrease and the particles tend to be folded with increased interfacial thermal resistance and decreased ETC. The ETC as a function of ultrasonic time shows an obvious peak value at ∼2 min, and the ETC could be increased from 0.3 to 3.0 W/(m·K) at GNs loading of 4 wt% with negligible effect on the phase change characteristics. In addition, the GNs/paraffin composite exhibits the quick thermal response and longer working time. The present non-monotonous discovery reveals the underlying mechanism and provides suggestions on the improvement of ultrasonic exfoliation process.