Browsing by Author "Tavakoli, Amir Hossein"

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    Thermodynamic and kinetic studies on the thermal stability of amorphous Si-(B-)C-N ceramics
    (2010) Tavakoli, Amir Hossein; Bill, Joachim (Prof. Dr.)
    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 structural 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 TEM (EFTEM) and field-emission scanning electron microscopy (SEM). For the chemical analysis of the samples, inductively coupled plasma-atomic emission spectroscopy together with a combination of various analysis equipment based on the combustion techniques were applied. Thermodynamic modeling of the amorphous Si–C–N domains proves that the addition of boron increases the driving energy of the crystallization. The experimental results of this study demonstrate that the increase of the boron content promotes the formation of the nanocrystallites (SiC) in accordance with the thermodynamic computations. A further result of this study corresponds to the role of boron on the fraction of alpha and beta modifications of Si3N4 in the boron-containing ceramics. The ratio alpha / beta is reduced with increasing the boron content. According to the experimental investigations, the crystallization of SiC is initiated at temperatures above 1300°C. This stage of crystallization is followed by the second stage of crystallization including the formation of Si3N4 nanocrystallites in addition to the evolution of further SiC nanocrystallites. It is demonstrated that the crystallization process according to the experimental analysis is to a large extent in agreement with the formation sequence of the nanocrystalline phases within the amorphous state anticipated by the model of metastable phase equilibria. As a result of a comprehensive kinetic analysis of the Si3N4 crystallization in Si–B–C–N PDCs, the continuous nucleation of the crystallites was identified as the dominant mechanism of the nucleus formation followed by a three-dimensional growth process. Moreover, the obtained results imply that the determined crystallization mechanisms are independent on the boron content. However, the activation energy of the Si3N4 crystallization significantly increases with increasing the boron content. A further analysis points to the crucial role of the nucleation kinetics in the progress of the crystallization course and also demonstrates the significant increase of the energy barrier for the nucleation process with increasing the boron content. The chemical instability of these materials corresponds to the carbothermal reduction of Si3N4 (Si3N4 degradation) according to the reaction Si3N4 + 3C = 3SiC + 2N2. The study of the kinetic effect of boron on the chemical stability of the Si–(B–)C–N PDCs indicates that the effective activation energy of the Si3N4 degradation noticeably increases by the presence of boron. Based on the obtained kinetic parameters, the chemical reaction at the interface of the reactants (Si3N4 and C) and the formation kinetics of SiC as the reaction product are two potential processes which individually or together conduct the progress of the Si3N4 degradation for the boron-free ceramic. In the case of the boron-containing ceramic, the local diffusion of C out of BNCx turbostratic layers, surrounding the Si3N4 nanocrystals, and the gas (N2) outflow from the reaction zone have been concluded to be the most plausible processes limiting the progress of the Si3N4 degradation.