Browsing by Author "Peng, Jianqiang"

Filter results by typing the first few letters
Now showing 1 - 1 of 1
  • Thumbnail Image
    ItemOpen Access
    Thermochemistry and constitution of precursor-derived Si-(B-)C-N ceramics
    (2002) Peng, Jianqiang; Aldinger, Fritz (Prof. Dr.)
    In the present work the phase equilibria and phase reactions of Si-C-N and Si-B-C-N precursor-derived ceramics were studied by means of the CALPHAD method combined with experimental studies: e. g. DTA/TG, XRD, HRTEM and SEM. Thermodynamic calculations of different types of phase diagrams, phase fraction diagrams and phase composition diagrams in regard to the reaction behavior of Si-C-N and Si-B-C-N ceramics were carried out using software such as THERMO-CALC. Consistent Scheil's reaction schemes for the systems Si-C-N and Si-B-C-N are presented. The thermal degradation of precursor-derived Si-C-N ceramics has been characterized quantitatively by taking into account the endothermic phase reactions Si3N4+3C = 3SiC+2N2 and Si3N4 = 3Si+2N2. Because of the first reaction the thermal stability of the Si-C-N ceramics is limited to about 1550°C. The results of the thermodynamic calculations are in good agreement with the experimental results and describe very well quantitatively the high temperature behavior of the ceramics. The incorporation of boron in Si-C-N ceramics can increase the thermal stability up to 2000°C. Both effects, i. e. (1) decreased carbon activity and (2) encapsulation effect cause the high temperature stability of Si-B-C-N ceramics. A model for the phase formation was developed which can describe qualitatively the dependence of thermal stability on material composition. The thermal expansion behavior and the thermal diffusivity of some precursor-derived amorphous Si-C-N and Si-B-C-N ceramics were studied applying differential dilatometry and the laser flash method, respectively. No glass transition was detected for these amorphous ceramic materials. The thermal expansion coefficients of these ceramics are comparable with those of crystallized Si3N4. The thermal conductivities of these ceramics are much lower than those of crystallized Si3N4 and SiC. In addition, phase reactions of polymer-precursors of the system Si-C-N-H and resulting compositions were also calculated by the CALPHAD method.