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

摘要

A novel class of freestanding B_2C_y fibers was prepared by hyperbaric-pressure laser chemical vapor deposition. Utilizing mixtures of diborane and helium with hydrocarbons, such as methane, ethene, and pentane, B_2C_y-alloy fibers were prepared at axial rates of up to 12.2 mu m/s. Regions of kinetically-limited and transport-limited growth were identified, and the activation energy for deposition from B_2H_6 + C_5H_(20) + He mixtures (at relative concentrations of 1:25:10) was found to be 197 + - 27 kJ/mol, while the rate constant was approximately 1810 mu m/s. Compositions ranged from B_(0.4)C_(0.6) to B_(0.03)C_(0.97) depending on the gas mixture and laser powers employed; axial and radial fiber compositions/microstructure were explored using Auger spectroscopy and electron microscopy. Glassy B_xC_y 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 BCl_3 + H_2, BF_3 + H_2, and B_2H_6 + H_2 gas mixtures, producing fine-grained alpha-boron and large single-crystals of beta-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.