Fabrication of large-sized high density bulk isotropic pyrocarbon materials of a special composite microstructure by fixed-bed chemical vapor deposition

Abstract

Large-sized high density bulk isotropic pyrocarbon materials of homogeneous microstructure up to Φ 75 mm × Φ 15 mm × H 15 mm are successfully prepared by fixed-bed chemical vapor deposition at 1550 °C and under ambient pressure. The isotropic pyrocarbon has a special composite microstructure composed of highly graphitized spherical nanoparticles tightly wrapped by tangled ribbon-like pyrocarbon. Detailed microstructural characterization shows that these spherical particles have an isotropic core but a highly graphitized shell composed of conical segments, which is rarely observed in studying pyrocarbons, and they also have been demonstrated to be originated from the gas phase through analyzing the components of the reactive gas extracted from the deposition zone. These particles provide the curvature needed for the formation of isotropic pyrocarbon through the thickness. Gas flow simulation is conducted to study the effects of deposition conditions on the formation of pyrocarbons at different deposition stages, indicating that a homogeneous laminar flow with a hyperbolic profile over the substrate where there is an optimal ratio of nanoparticles and gaseous carbon species is beneficial for depositing the desired high density isotropic pyrocarbon with superior comprehensive physical and mechanical properties.