One-Pot Coating of Ceramic Powders by Exfoliated Boron Nitride Layers with a Dense CO₂ Medium and Ultrasound-Aided Mixing

Abstract

The coating or doping of substrates by twodimensional (2D) materials to impart superior functional properties (such as improved thermal conductivity, bacterial resistance, and reduced friction) is receiving increased attention. Environmentally benign, rapid, and scalable coating techniques are desirable for this purpose. We report a novel process for coating alumina, silicon carbide, and boron carbide substrates with hexagonal boron nitride (h-BN) layers. The process consists of two sequential steps. First, h-BN layers are exfoliated from bulk h-BN in supercritical carbon dioxide (scCO 2 ) using ultrasound-aided mixing. This step is followed by self-assembly (i.e., coating) of the exfoliated h-BN on the substrates in liquid CO 2 aided by ultrasound. The liquid CO 2 state is achieved by simply lowering the pressure and temperature of the first step below the critical point of the CO 2 state (P c = 72.8 atm; T c = 31.1 °C). Scanning electron microscopy and energydispersive spectroscopy micrographs clearly reveal the exfoliation of h-BN under scCO 2 conditions as well as the assembly of h-BN on various substrates under liquid CO 2 conditions. X-ray diffraction patterns confirm the structural integrity of the coated h-BN layers. It was also confirmed that, without a transition to the liquid CO 2 phase following exfoliation in scCO 2 , there was a negligible coating of h-BN on the substrates. Researchers in the field could consider this benign process to rapidly coat or dope materials with h-BN and other 2D materials to impart improved functional properties in myriad applications.