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

The diffusion coefficients (D) of implanted radioisotopes -Ge-71, Si-31, and C-11 - in the polymer-derived amorphous (a-)Si-29B9C41N21 ceramic have been measured as functions of temperature (T) in different as-pyrolysed states (Ge-71) or after pre-diffusion annealing at 1600 degreesC for 2 h (Ge-71, Si-31, and C-11) by means of radiotracer techniques, in which serial sectioning was done by ion-beam sputtering. In the cases of Si-31 (half-life t(1/2) = 2.6 h) and C-11 (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(Ge-71) to previous data on diffusion in (B-free) a-Si28C36N36, it is concluded that the diffusion of Ge-71 in the a-Si29B9C41N21 ceramic is controlled by vacancies in its a-Si3N4-yCy phase which become increasingly smeared out when the C-content y decreases as a result of pre-annealing. The coincidence of D(Si-31) and D(C-11) in pre-annealed a-Si29B9C41N21 at all temperatures investigated is enforced by vacancy-mediated diffusion through SiC crystallites embedded in the a-Si3N4-yCy phase.