Carbonization Model Of Pitch-Based Carbon Materials

With the development of science and engineering, the demands of high materials properties become more imperious. It requires strict control from the design and manufacture of the materials. Computer aid technology has been a powerful tool for design and preparation of various materials. On the basis of experimental and experienced data, by calculating preparation processes, capabilities and structure and so on theoretically, it is possible to accurately predict the materials properties. In this way, we can optimize the process of preparation and improve the materials properties furthest, furthermore, it helps us avoiding the cost of "try and error" for experiments.Because of the complicacy of the components of coal tar pitch, the simulation of preparation process of pitch-based composites has not been developed widely. In this paper, attempts were made to research the carbonization process of pitch and of pitch-based composites by numerical method, so as to we can get deep into understanding the manufacture process of pitch-based composites.The themogravimetry of typical coal-tar pitch is investigated. On the basic of pyrolysis mechanism of coal-tar pitch, the carbonization process of pitch is divided into three partly overlapping stages. Series reactions of γ,β and α resins of coal-tar pitch are regarded in each stage. Thermogravimetric models of different stages are proposed by Coats-Redfern's method through one TG curve. It is found that the each stage of the pyrolysis of coal-tar pitch is first order reaction. The obtained kinetic parameters are close to the results reported in some literatures. The influences of different heating reat and different contents to carbonization process of coal-tar pitch, modulated with γ,β and α resins, are forecasted by the proposed mathematical model. It is shown that the predictions of the model agree well with the experimental tendency. The established model is verified in some degree.The growth of bubbles caused by the volatiles during pyrolysis of pitch is investigated. Based on the TG curve and the balance equation of mechanics, dynamicmodel of growth of bubbles are proposed. Dynamic of bubbles growth is theoretically calculated by the proposed mathematical model. It is found some general conclusions as following. With the increase of heating rate, growth rate of bubble decreases at low temperature range, but it rapidly increases at higher temperature range. Three marked sequential stages are found as clear decrease, approximate invariability and complicated change with rate of mass loss when the interface velocity of bubble is discussed. The influence of heating rate to the transformation tendency of interface velocity is negligible. The radical velocity of liquid around the bubble rapidly increases with heating rate at the same temperature.Carbonization mechanism of C/C composites is analyzed. One-dimensionaldynamic mathematical model of the carbonization process of C/C composites wasdeveloped combining heat transfer, mass transfer with the degradation of the matrixequations. The deduced equations are solved by the partial differential toolbox ofMatlab software. The model was used to provide the distributions of temperature,pressure, degree of degradation and porosity depending on the temporal and spatiallimitations in the specimen. The effects of heating rate, sample thickness or externalpressure on the results mentioned above are theoretically calculated and discussed. Theinfluences of these factors on the properties of C/C composites are expatiated also. Thegenerally results are obtained as that the gradients in the distribution of temperature,pressure, degree of degradation and porosity all increase with the heating rate increase.It is shown that, with the increasing of heating rate, the maximum temperature of thetemperature difference between the surface and the center of the sample obviously moveto the higher temperature. The gradients of the distribution of both temperature andpressure increase with the thickness of the sample increase, and the distribution of thedegree of degradation and porosity becomes uneven inside the matrix. However, thelarger the environment pressure is, the smaller pressure difference between the surfaceand the center of the preform is. It is beneficial to get the uniform and high capabilitiesC/C composites.