Ceramic Matrix Composites as Self‐Standing Monoliths and Coatings for Thermal Superinsulation and Passive Daytime Radiative Cooling

Abstract Buildings and industries account for a significant portion of global energy consumption, thus effective thermal management materials are crucial. Ceramics are an ideal candidate due to their high‐temperature resistance, corrosion resistance, electrical insulation, and flame retardance. However, traditional ceramics' brittleness, rigidity, and high densities make it a challenge in shaped‐construction applications. Herein, aluminum ceramic fiber (ACF)‐based ceramic matrix composites (CMC) have been designed and synthesized by a polymer shaping strategy, in which ACFs are stable and homogenously dispersed in aluminum sols in the presence of hydroxypropyl methylcellulose. The resulting ACF‐CMC can form a self‐standing monolith or be used as ceramic coatings with low density (≈0.13 g cm ^{− 3} ) and thermal conductivity (≈0.036 W m ⁻¹ K ⁻¹ ). It shows exceptional thermal insulation capabilities, maintaining a temperature gradient of up to 1180.5 °C under a severe 1300 °C flame test. Additionally, the ACF‐CMC possesses high solar reflectance (0.785) and infrared emissivity (0.857–0.910), resulting in passive daytime radiative cooling (PDRC) of 7.3 °C. The polymer shaping strategy provides a simple and scale‐up technology to produce lightweight, self‐standing, and shapable CMC with both robust thermal insulation in extreme temperatures and PDRC, which can be potentially used in building and industry energy savings.