Preparation of superhard BxCy fibers by microvortex-flow hyperbaric laser chemical vapor deposition

A novel class of freestanding BxCy fibers was prepared by hyperbaric-pressure laser chemical vapor deposition. Utilizing mixtures of diborane and helium with hydrocarbons, such as methane, ethene, and pentane, BxCy-alloy fibers were prepared at axial rates of up to 12.2 μm/s. Regions of kinetically-limited and transport-limited growth were identified, and the activation energy for deposition from B2H6 + C5H20 + He mixtures (at relative concentrations of 1:25:10) was found to be 197 ± 27 kJ/mol, while the rate constant was approximately 1810 μm/s. Compositions ranged from B0.4C0.6 to B0.03C0.97 depending on the gas mixture and laser powers employed; axial and radial fiber compositions/microstructure were explored using Auger spectroscopy and electron microscopy. Glassy BxCy fibers with Vickers hardnesses of 42–45 GPa were grown at laser powers below 150 mW. The growth kinetics of pure boron fibers was also investigated from BCl3 + H2, BF3 + H2, and B2H6 + H2 gas mixtures, producing fine-grained α-boron and large single-crystals of β-boron. Micro-scale vortices in the gas flow emanating from the reaction zone were observed using particle image velocimetry; such vortices enhance axial fiber growth rates through rapid gas mixing. Arrays of fibers were also grown in-parallel using diffractive optical elements.