Silica-Inspired Aerogel Thermal Metamaterials with Gradient Porosity: High-Temperature-Induced Pore Sintering...

Localized densification of nanoporous silica under combined mechanical compression and elevated temperature involves coupled pore collapse, skeletal rearrangement, and thermally activated sintering. Clarifying how local pre-compression regulates these processes is important for understanding the surface and near-surface densification of nanoporous silica and related porous materials. In this study, the microscopic sintering behavior of a silica-inspired aerogel-like nanoporous model under the coupling of non-uniform local stress and high-temperature fields (indentation depths of 50-150 Å and temperatures of 298-1800 K) was systematically investigated using molecular dynamics simulations combined with a three-dimensional (3D) topological recognition algorithm (probe sphere method and DBSCAN clustering). The results indicate that the sintering densification of the silica-inspired aerogel model exhibits significant pore-size dependence and a "depth-temperature inverse relationship": the local pre-compression induced by the 150 Å indentation facilitates thermally activated atomic rearrangement and shifts the onset of densification to a lower temperature, leading to an early bimodal splitting of the pore size distribution at 1300 K, accompanied by a significant jump in the elastic modulus from 3.0 to 10.07 GPa. In contrast, the 50 Å shallow region requires heating to 1800 K to achieve an equivalent densification effect. Furthermore, topological analysis quantitatively reveals the phase transition process of the pore network from connected to isolated: taking 1300 K as an example, the number of connected pore clusters decreases from the initial 86 to 70 (at 1000 ps), marking the fracture of the connected network; subsequently, the number of isolated pores surges to 4861, and the residual connected framework is severely fragmented into 136 micro-clusters. Based on the above microstructural and topological evolution data, a four-stage thermo-mechanical synergistic evolution process of the silica-inspired aerogel model is summarized. These findings provide quantitative fundamental data that conceptually supports the design of functional gradient structures with alternating "dense-thermally-conductive" and "porous-thermally-insulating" layers within a single continuous aerogel matrix; such structures may be realized in the future through strategies such as arrayed nanoindentation combined with high-temperature sintering.