Radiative modeling of hollow-fiber-doped silica aerogel composites enabled by transmittance-based complex refractive index inversion

The complex refractive indices of infrared-functional additives are pivotal for accurately predicting radiative heat transfer and designing high-temperature-resistant aerogel composites. However, existing datasets for specific materials frequently exhibit substantial discrepancies and lack comprehensive spectral coverage across the critical infrared range. In this study, a transmittance-driven inverse model was developed to retrieve broadband complex refractive indices spanning 2-25 μm, eliminating the need for a known real refractive index at a reference wavelength. Integrating low-dimensional parameterization, the Kramers-Kronig (K-K) relation, and a residual-adaptive hybrid optimization framework that combines a genetic algorithm with sequential quadratic programming (GA→SQP), the model was validated through application to three distinct infrared opacifier additives. The retrieved complex refractive indices not only precisely replicate experimental transmittance measurements but also demonstrate strong overall consistency with literature data. Leveraging these refined refractive index datasets, numerical simulations revealed that radiative thermal conductivity increases as the incidence angle decreases, with this angular sensitivity becoming more pronounced at elevated temperatures. Furthermore, silica aerogel composites doped with hollow fibers outperform their solid-fiber counterparts in high-temperature radiative insulation, attributed to the enhanced short-wavelength (∼2-5 μm) scattering capabilities of hollow fibers. Specifically, at 1300 K, three-dimensionally randomly oriented silica aerogel composites doped with SiC hollow fibers (hollow ratio 0.6, volume fraction 5%) exhibit a 9.93% reduction in radiative thermal conductivity compared to composites doped with solid SiC fibers, alongside a 22.3% decrease in bulk density. This work establishes a generalized inverse model for deriving complex refractive indices from transmittance spectra, while also providing actionable design guidance for the development of lightweight, high-performance thermal insulation aerogel composites tailored to high-temperature environments.