Nanoscale mechanism of microstructure-dependent thermal diffusivity in thick graphene sheets

The rapid advancement in the integration density of electronic components has led to a pressing need for effective thermal management solutions. Among the promising materials in this regard, graphene stands out due to its exceptional thermal conductivity properties. Currently, the production of ultra-high thermally conductive thick graphene sheets primarily involves the reduction of graphene oxide. However, despite significant progress, the impact of defects on thermal properties remains inadequately understood, limiting the achievement of thermal conductivity exceeding 1500 ​W m⁻¹ K⁻¹. During the preparation process of reduced graphene oxide-based graphene sheets, hole structures are inevitably formed, reducing the overall density and thus decreasing thermal conductivity. However, the influencing factors on thermal diffusivity, one of the determining factors of thermal conductivity, have not been reported. Thus, we defined the intrinsic thermal diffusivity specific to materials with internal holes and further investigated the correlation between the intrinsic thermal diffusivity of thick graphene sheets and microstructure through various electron microscopy characterization, thermal diffusivity measurements, and simulations. We aim to elucidate the factors and mechanisms affecting the thermal diffusivity and hence thermal conductivity. Our research reveals subtle insights, particularly regarding the impact of holes of different sizes and quantities on thermal diffusivity. Notably, our simulation results show that a real dense-small-holes structure in graphene sheets can reduce thermal diffusivity by 39.4 ​%, more than twice the reduction caused by a single-large-hole structure (16.1 ​%). Statistical conclusions obtained through three-dimensional reconstruction also perfectly match these computational results. We emphasize that the presence of dense-small-holes structures disrupt the original high-speed heat transfer paths more severely, while the effect of single-large-hole structures are relatively weaker, primarily reducing overall density and thus thermal conductivity. Additionally, we found that the out-of-plane crystallinity has a significant impact on thermal diffusivity, further enhancing our understanding of microstructural factors affecting thermal diffusivity. By elucidating these mechanisms, our findings make significant contributions to the technological advancement of producing ultra-high thermally conductive thick graphene sheets. A deeper understanding of the interaction between microstructure and thermal performance brings hope for the development of next-generation electronic device thermal management solutions. Through continued research in this field, we anticipate further improvements in the performance and efficiency of graphene thermal management systems, ultimately driving innovation in electronic device design and manufacturing.