Study On Kinetics, Mechanisms And Modeling Of Oxidation Of Carbon Materials

Carbon fiber-reinforced carbon composites and graphite are widely used in aerospace and metallurgy industries owing to the rare combination of physicochemical properties. In an inert atmosphere or vacuum, the mechanical properties can be maintained and even enhanced with increasing of the temperature. Nevertheless, when exposed in air at high temperatures, they will be oxidized at temperatures as low as 400℃, and the mechanical properties are degraded by oxidation. Thus, a great deal studies on oxidation behavior of carbon materials have been carried out. There are some problems existing in research on oxidation behavior of carbon materials for the past 40 years, such as the simplified oxidation kinetic equation and the lack of systematicness. To complete the oxidation kinetic equation and develop the universal methods, a systematic research on the kinetics, mechanisms and modeling of oxidation of carbon materials was conducted by TGA and SEM based on carbon felt/carbon composites, two-dimensionally carbon/carbon composites and graphite in this paper.The isothermal kinetics and mechanisms of the oxidation of carbon/carbon composites were investigated. Compared with former studies on isothermal oxidation behavior, there are two new improvements. Firstly, in non-linear oxidation stage, the oxidation rate was represented by the n-order equation. Secondly, the relation between the post-oxidation microstructure and mechanism was revealed. Then the model-free, model-fitting and reduced-time plot methods were introduced for isothermal oxidation kinetics of carbon/carbon composites and more concrete explanation about oxidation mechanisms were obtained. At the same time, to seek the reason about rapid deterioration of properties at the initial stage of oxidation, in situ SEM observation was implemented.Modeling of the isothermal oxidation curves of carbon materials is presented by the methods of kinetic analysis and of polynomial regression ofα-t/t0.5 curves. By comparing the experimental and simulated TG curves, the two methods can better predict weight loss of the isothermal oxidation of carbon, and the polynomial regression ofα-t/t0.5 curves is a more convenient method.The non-isothermal oxidation of carbon materials, which is more close to the practical situation, was investigated by the model-fitting method, the model-free method, and the method of the shape of the TG-DTG curves, and the applicability of three methods was discussed. Then modeling of non-isothermal oxidation curves of graphite was conducted. Results show that the model-free method is recommended as a trustworthy way for obtaining reliable kinetic information from non-isothermal oxidation of graphite, and the non-isothermal oxidation of graphite exhibits three regimes. The methods of kinetic analysis and of polynomial regression ofα-T/T_0.5 curves do not fit the modeling of the non-isothermal oxidation TG curves of graphite, and the method of linear regression of lgβ-1000/T curves can better model the non-isothermal oxidation TG curves of graphite.