Comparison of diffusion in amorphous Si–C–N and Si–B–C–N precursor-derived ceramics

We carried out silicon self-diffusion studies in amorphous precursor-derived ceramics of composition Si ₃BC _4.3N ₂ and Si _2.6C _4.1N _2.3 to compare the atomic transport properties of a boron containing and of a boron free material in the system Si–(B)–C–N. Ion implanted stable 30 Si isotopes were used as tracers and secondary ion mass spectrometry (SIMS) was used for depth profiling. The experimentally determined diffusivities are lower by a factor of 10 for Si ₃BC _4.3N ₂ than for Si _2.6C _4.1N _3.3 in the whole temperature range investigated. This might be a hint that the mobility of the constituent elements plays an important role in the stabilization of the amorphous state of Si ₃BC _4.3N ₂ at high temperatures. Both ceramics obey an Arrhenius behaviour with activation enthalpies and pre-exponential factors of ΔH=5.7 eV and D 0 =0.001 m 2 / s for Si ₃BC _4.3N ₂, and ΔH=5.55 eV and D 0 =0.003 m 2 / s for Si _2.6C _4.1N _3.3 , respectively. These results are consistent with a diffusion mechanism mediated by vacancy like defects operating in these amorphous ceramics, which is analogous to the related crystalline silicon based non-oxide ceramics Si ₃N ₄ and SiC.