Pressureless Welding of Temperature‐Invariant Multifunctionality Body Based on Hydroxyl‐Functionalized Boron Nitride Nanosheets and Bifunctional Monoethanolamine Cross‐linker

Abstract Bulk hexagonal boron nitride (h‐BN) ceramics with structural integrity, high‐temperature resistance and low expansion rate are expected for multifunctional applications in extreme conditions. However, due to its sluggish self‐diffusion and intrinsic inertness, it remains a great challenge to overcome high‐energy barrier for h‐BN powder sintering. Herein, a cross‐linking and pressureless‐welding strategy is reported to produce bulk boron nitride nanosheets (BNNSs) ceramics with well‐crystalized and dense B–N covalent‐welding frameworks. The essence of this synthesis strategy lies in the construction of >B─O─H₂C─H₂C─H₂N:→B< bond bridge connection structure among hydroxyl functionalized BNNSs (BNNSs‐OH) using bifunctional monoethanolamine (MEA) as cross‐linker through esterification and intermolecular‐coordination reactions. The prepared BNNSs‐interlaced ceramics have densities not less than 1.2 g cm⁻³, and exhibit exceptional mechanical robustness and resiliency, excellent thermomechanical stability, ultra‐low linear thermal expansion coefficient of 0.06 ppm °C⁻¹, and high thermal diffusion coefficient of 4.76 mm² s⁻¹ at 25 °C and 3.72 mm² s⁻¹ at 450 °C. This research not only reduces the free energy barrier from h‐BN particles to bulk ceramics through facile multi‐step physicochemical reaction, but also stimulates further exploration of multifunctional applications for bulk h‐BN ceramics over a wide temperature range.