SIMULTANEOUS PYROLYTIC DEPOSITION OF CARBON AND BORON

Boron-containing pyrocarbons were prepared at temperatures 1400-1650°C by simultaneous pyrolytic deposition from gas mixtures containing CH4 and BCl3 on an induction heated graphite substrate. The deposits thus prepared with boron content in the range 0-3 per cent (w/o) were investigated using X-ray diffraction, optical microscopy, oxidation and apparent density determination (flotation method). The results show that boron enters into both substitutional and interstitial positions in the lattice. The degree of graphitization increases considerably with increasing boron content up to about 1 per cent and then decreases. This is interpreted by assuming that up to this concentration boron forms preferentially substitutional solid solution, and that in excess of this concentration, it is introduced interstitially. Other properties studied - apparent density, preferred orientation and apparent crystallite thickness, show similar dependence on boron content. Oxidation in a saturated solution of Ag2Cr2O7 in H2SO4 reveals the presence of two structural components (F1 and F3) with oxidation rates differing by a factor of about 103. It was found that boron causes an increase in both rate constant, and a relative decrease in the component with high oxidation rate. © 1969.