Cutting-edge technologies such as hypersonic vehicles and reusable launch systems impose stringent requirements on thermal protection system, demanding for ultra-lightweight materials with extremely low thermal conductivity and high-temperature stability. Conventional SiO₂ aerogels suffer from rapid sintering and densification at elevated temperatures, leading to pore collapse and decline in thermal insulation performance. Although introducing high-melting-point oxides or increasing particle size can delay the sintering process, it often results in a trade-off between thermal stability and insulation performance. To overcome this bottleneck, a hollow-core-shell synergistic design strategy is proposed. This approach is expected to simultaneously maintain low density and low thermal conductivity, while significantly enhancing the material's thermal stability via coupling the effects of micro-nano pore regulation and interfacial phonon scattering. In this work, the influence of Al₂O₃ coating thickness on the density, thermal conductivity, and thermal stability of large hollow particles of SiO₂ coated with Al₂O₃ (SiO₂@Al₂O₃) was investigated. The evolution behavior of this composite structure at high temperatures was elucidated, and the mechanism by which the Al₂O₃ shell could suppress a liquid-like atomic diffusion was revealed, providing some insights and theoretical guidance for the development of next-generation high-performance thermal insulation materials.

A hard-template-assisted precipitation method was employed to synthesize SiO₂@Al₂O₃ hollow particles. Polystyrene microspheres were used as templates, on which SiO₂ was deposited to form large hollow structures. After calcination to remove the PS cores, SiO2 hollow particles were obtained. The SiO₂ particles were dispersed in an aqueous phase, and Al(OH)₃ was in-situ precipitated on their surfaces via the dropwise addition of AlCl₃ and NaOH solutions with cetyltrimethyl ammonium bromide (CTAB) as a dispersant. Subsequent high-temperature calcination converted the hydroxide layer into an Al₂O₃ coating, yielding SiO₂@Al₂O₃ core-shell hollow particles. A series of samples with different Al/Si mass ratios (i.e., 1%, 5%, and 10%) were prepared via adjusting the amount of AlCl₃. The morphological evolution of the materials at 25-1200 ℃ was monitored in real time by in-situ transmission electron microscopy. The effective thermal conductivity of the samples at 25-1100 ℃ was measured by a Hot Disk thermal constants analyzer and the laser flash method. Furthermore, the molecular dynamics simulations were performed by Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS), and a "liquid-like atom" cluster identification algorithm was applied to reveal the mechanism by which the Al₂O₃ shell could suppress surface diffusion and particle sintering in an atomic scale.

The results of in-situ TEM sintering experiments reveal that uncoated SiO₂ hollow particles exhibit significant shrinkage and rupture at 1200 ℃ for only 2 min. In contrast, the sample coated with 3.9% Al₂O₃ (the sample with Al/Si of 5%) maintains its hollow structure after 30 min under the same conditions, with a linear shrinkage ratio of below 9%. However, the excessive coating (e.g., 10%) leads to the formation of a continuous Al₂O₃ layer between particles, increasing the effective contact area and promoting high-temperature mass transport and densification. The thermal property measurements further demonstrate that the 3.9% Al₂O₃-coated sample exhibits an ultralow effective thermal conductivity of 0.036 W·m^-1·K^-1 at 25 ℃, and a moderate increase to 0.074 W·m^-1·K^-1 at 1100 ℃, indicating the excellent high-temperature thermal insulation performance. The initial density of this sample is 0.268 6 g·cm^-3, which is 14% lower than that of SiO₂ solid nanoparticles with the size of 10 nm and shows a minimal variation (i.e., 0.268 6-0.298 8 g·cm^-3) at 25-1200 ℃. Its effective thermal conductivity remains below that of various fiber-reinforced aerogel systems in the literature. The molecular dynamics simulations based on LAMMPS reveal the atomic-scale mechanism underlying the high-temperature stability of the Al₂O₃ shell. During heating, the number of "liquid-like atom" clusters increases continuously, while effectively suppressing their size growthes. This behavior significantly delays the kinetics of cluster coalescence, elevating the surface fusion completion temperature from approximately 1000 K in the uncoated system to 1500 K. The Al₂O₃ shell effectively delays the transition from surface diffusion-dominated sintering to volume diffusion-dominated sintering, providing the related mechanism and theoretical foundation for overcoming the intrinsic trade-off between low thermal conductivity and high-temperature stability.

This study proposed a "large hollow structure with Al₂O₃ shell" synergistic design, enabling the controllable fabrication of SiO₂@Al₂O₃ particles that could combine a structural stability at 1200 ℃ with ultralow thermal conductivity and low density. At 3.9% Al₂O₃ coating, the particles maintained a shrinkage ratio of below 9% after 30-min high-temperature exposure. The effective thermal conductivity increased modestly from 0.036 W·m^-1·K^-1 at 25 ℃ to 0.074 W·m^-1·K^-1 at 1100 ℃, while the density was 14% lower than that of conventional SiO₂ solid nanoparticles with the size of 10 nm. The in-situ TEM spectra and molecular dynamics simulations confirmed that the Al₂O₃ shell significantly delayed sintering via suppressing cluster coalescence and the surface diffusion of liquid-like atoms, thus elucidating the high-temperature stabilization mechanism across multiple scales. This strategy could provide both experimental and theoretical foundations for controllable preparation of lightweight and high-temperature thermal insulating materials, offering a substantial engineering value for thermal protection in extreme environments and for additive manufacturing of complex lightweight thermal structures.