Thermodynamic and kinetic studies on the thermal stability of amorphous Si-(B-)C-N ceramics [article]

Amir Hossein Tavakoli, Universität Stuttgart, Universität Stuttgart
The term "Thermal Stability" is used for polymer-derived ceramics (PDCs) to describe the temperature range of stability against the crystallization and chemical degradation of these amorphous materials. Si–(B–)C–N PDCs reveal an attractive capability of high temperature stability ranging up to 2000°C. Despite the reported attempts aimed at a better understanding of the thermal stability in Si–(B–)C–N PDCs, fundamental investigations with focus on the thermodynamic and kinetic aspects of the
more » ... ctural transformations in these materials are lacking. In this Ph.D. thesis, thermodynamics and kinetics of the crystallization of amorphous Si–(B–)C–N PDCs as well as the degradation of these materials were comprehensively studied for the first time. The focus is on the influence of the boron content on the structural evolution within these materials. Thermodynamic calculations in this work were computed using the Thermo-Calc software in order to estimate the equilibrium phases together with the temperature range of their stability and the thermodynamic impact of boron on the driving energy of the crystallization. Moreover, the sequence of crystallite formation was predicted by modeling of the metastable phase equilibria. In order to carry out the experimental investigations, a polysilazane, which leads to the amorphous Si-C-N ceramic, was used as a base material. Within this polymer, different amounts of boron were introduced in order to provide amorphous ceramics with a constant atomic Si:C:N ratio and various boron contents. The major part of the experimental investigations is dedicated to XRD measurements of the heat treated samples in order to analyze the volume fraction of the crystallized phases growing within the amorphous PDCs during the crystallization course as a function of annealing time and temperature. In addition to the XRD measurements, further experimental investigations were carried out using high temperature thermal gravimetric analysis (HT-TGA), transmission electron microscopy (TEM), energy-filtering [...]
doi:10.18419/opus-1275 fatcat:eivy3d4dlfg6tkmugrn5dukdva