STRUCTURE OF PYROCARBON INFILTRATED BY PULSE-CVI

Pulse chemical vapor infiltration (P-CVI) was developed to study both the texture and density of infiltrated pyrocarbon. In this work model pores were machined in graphite with various diameters. Compared to classical chemical vapor infiltration (I-CVI) where the deposition of pyrocarbon is mainly controlled by temperature, pressure and precursor type, P-CVI provides an additional controlling parameter: the residence time (t(R)). For same pressure, temperature and mother molecules, t(R) variations were seen to produce texture changes. The comparison of the infiltration within the different pores has shown that two deposition mechanisms exist with a continuum in between, one of which was favored by very low residence time (heterogeneous reactions). In this case infiltration was pore-size-insensitive with an homogeneous texture in-depth. But the anisotropy was never found to be very high. The second is supposed to occur by diffusion and ''condensation'' of higher molecular weight species (resulting from homogeneous reactions). The deposited carbon is then characterized by a texture gradient: texture is highly anisotropic at the entrance of the pore and is increasingly disorganized in depth (pore-size-sensitive). Between these two extremes, (i.e. at intermediate t(R)), it was possible to combine the interest of the heterogeneous mechanism with a gentle maturation (homogeneous reactions). This was obtained at low pressure and low temperature, producing a highly anisotropic pyrocarbon that was infiltrated without gradient. In this case pyrocarbon structure is characterized by a large layer size (L(2)>10 nm), a high preferred orientation and, surprisingly, a reduced turbostratic coherency.