Ultrathin Freestanding Ceramic Films Made of Boron Nitride Nanosheets as Thermally Conductive Dielectrics

It has been a critical challenge to fabricate ultrathin (≤1 μm) freestanding, highly crystalline hexagonal boron nitride films that are electrically, thermally, and mechanically robust for a long time. Conventional approaches typically require polymer binders or excessive film thickness (≥10 μm) to prevent structural failure during detachment or other processes. In this work, we report the fabrication of freestanding ceramic films composed of boron nitride nanosheets with thicknesses down to 1 μm via a capillary-assisted hydrogen delamination followed by ultrahigh-temperature sintering in seconds. The resulting ultrathin freestanding films achieve a density close to the theoretical limit and exhibit an in-plane thermal conductivity of 58.6 W m^-1 K^-1 with a breakdown strength exceeding 200 kV mm^-1. This leads to a unique combination of high mechanical flexibility and microwave transparency (a signal strength reduction of ∼4 dB) with the lowest zero-drift temperature coefficient at 0.00213. The versatility of such ultrathin freestanding boron nitride films holds great promise for innovative flexible nanoelectronics in 5G communications, the Internet of Things, and other applications.