Hierarchical Co‐Assembly Achieves Shape‐Programmable All‐Boron‐Nitride Monoliths with Excellent Thermophysical Performances

ABSTRACT Despite boron nitride's (BN) exceptional physical performance and durability, the current lack of a systematic methodology for BN processing, which stems from its extreme robustness, often necessitates the use of additive materials, thereby frequently sacrificing its desirable properties. Here, we report binder‐free BN monoliths derived from a suspension with tunable rheology and long‐term colloidal stability. The control of solvent affinity allows the production of two distinct BN morphologies: (1) physically exfoliated, large‐size BN flakes (p‐BN) and (2) mechanochemically produced, small‐size BN particles (m‐BN) with hydroxyl‐functionalized edges. Crucially, the interfacial interactions and aspect ratio complementarity between the two BN components enable spontaneous co‐assembly into a long‐term stable, binder‐free suspension with programmable rheology. The resulting binder‐free BN films exhibit a 19‐fold enhancement in cohesive energy (3.8 J·m − 2 vs. 0.20 J·m − 2 for p‐BN), high in‐plane thermal conductivity (>40.6 W·m − 1 ·K − 1 ), and a neutron absorption coefficient of 28.3 cm − 1 , offering a promising solution for advanced aerospace, nuclear, and optoelectronic systems operating under severe environmental constraints.