3D reconstruction and nanoscale simulation of boron/silicon modified phenolic matrix using FIB-SEM imaging and lattice Boltzmann models

Fiber-reinforced phenolic composites are essential for thermal protection in hypersonic vehicles. However, the mechanisms governing their nanoscale pores, particle networks and heat transfer remain insufficiently understood for various modified phenolic. This study systematically investigated boron- (B-Ph) and silicon-modified (Si-Ph) phenolic matrices, focusing on their synthesis, chemical composition, and thermomechanical properties. The nanoscale morphology was analyzed using two-dimensional scanning electron microscopy (SEM) and three-dimensional focused ion beam-scanning electron microscopy (FIB-SEM), while a diffusion-limited cluster–cluster aggregation (DLCA) algorithm reconstructed realistic structures. A lattice Boltzmann model incorporating Knudsen effects predicted thermal conductivity. Results indicate that boric acid modification increases crosslinking density, yielding denser particle packing and narrower pore distribution, whereas silane modification forms a finer gel network with broader pores. This more complex nanoscale morphology of Si-Ph samples requires a larger bulk representative elementary volume of 4 × 4 × 5.04 սm (400 voxels) for accurate thermal conductivity estimation, compared to 1.78 × 2.04 × 2.1 սm (200 voxels) for B-Ph samples. Compared to the FIB-SEM imaging, structural models generated by the DLCA approach yielded porosity and pore size distributions in better agreement with experimental observations, enabling accurate prediction of effective thermal conductivity while significantly reducing experimental and computational demands. Simulation results revealed that smaller mean pore sizes intensify the influence of Knudsen diffusion, thereby rendering pore size a dominant parameter in thermal transport. These findings offer a mechanistic understanding of nanostructure-governed thermal conductivity in modified phenolic resins and provide guidance for the design of advanced thermal protection materials.