Diffusion in polymer-derived Si–(B–)C–N ceramics, particularly amorphous Si29B9C41N21

Abstract

Abstract The diffusion coefficients (D) of implanted radioisotopes – 71Ge, 31Si, and 11C – in the polymer-derived amorphous (a-)Si29B9C41N21 ceramic have been measured as functions of temperature (T) in different as-pyrolysed states (71Ge) or after pre-diffusion annealing at 1600 °C for 2 h (71Ge, 31Si, and 11C) by means of radiotracer techniques, in which serial sectioning was done by ion-beam sputtering. In the cases of 31Si (half-life t 1/2 = 2.6 h) and 11C (t 1/2 = 20.4 min), implantation, diffusion annealing, and sputter-sectioning were done on beamline and in situ of a novel set-up specially constructed for diffusion studies of short-lived radiotracer atoms. In all cases, the T-dependencies of D are of Arrhenius type. Comparing the Arrhenius parameters of D(71Ge) to previous data on diffusion in (B-free) a-Si28C36N36, it is concluded that the diffusion of 71Ge in the a-Si29B9C41N21 ceramic is controlled by vacancies in its a-Si3N4 –y C y phase which become increasingly smeared out when the C-content y decreases as a result of pre-annealing. The coincidence of D(31Si) and D(11C) in pre-annealed a-Si29B9C41N21 at all temperatures investigated is enforced by vacancy-mediated diffusion through SiC crystallites embedded in the a-Si3N4 –y C y phase.