Kinetic effect of boron on the crystallization of Si3N4 in Si–B–C–N polymer-derived ceramics

The amorphous Si–B–C–N ceramics with a similar Si/C/N atomic ratio and various boron contents of 3.7 and 6.0 at.% B were synthesized and then isothermally annealed at temperatures ranging from 1550 to 1775 °C. The course of crystallization for the modifications of Si3N4 was examined by quantitative analysis of the corresponding x-ray diffraction patterns. Additionally, recent results of similar investigations on the ceramic with 8.3 at.% B were also considered. The kinetic analysis demonstrates that the controlling mechanisms of the Si3N4 crystallization, continuous nucleation and diffusion-controlled growth, are independent of the boron content. Nevertheless, the estimated activation energy of the crystallization significantly increases from 7.8 to 11.5 eV with the amount of boron ranging from 3.7 to 8.3 at.%. It is concluded that the role of boron in the crystallization kinetics is mainly due to the effect of boron on the nucleation process. Beside the kinetic analysis, the correlation between the boron content and the Si3N4 crystallite size has been discussed.