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

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.