Pyrolysis processes oxidation stabilization

Carbon fibers can be produced from a wide variety of precursors in the range from natural materials to various thermoplastic and thermosetting precursors Materials, such as Polyacrylonitrile (PAN), mesophase pitch, petroleum, coal pitches, phenolic resins, polyvinylidene chloride (PVDC), rayon (viscose), etc. [42-43], About 90% of world s total carbon fiber productions are polyacrylonitrile (PAN)-based. To make carbon fibers from PAN precursor, PAN-based fibers are generally subjected to four pyrolysis processes, namely oxidation stabilization, carbonization and graphitiza-tion or activation they will be explained in following sections later [43]. 

Schutz et al. studied the thermal properties of a phenolic resin nanocomposite containing silsesquioxanes [80]. The thermal oxidative stability of the nanocomposites was improved, as compared to that of the piu e resin. This effect may be a consequence of the formation of a protective layer of SiG2 during silsesquioxanes pyrolysis at the surface of nanocomposites, which retarded the thermal oxidative degradation. The temperatures characteristic to thermal degradation processes were higher in the case of nanocomposites, compared to the pure resin matrix. 

Carbon fibres are obtained from different organic fibres (precursors) by pyrolysis, which consists of decomposition into smaller molecules at high temperature. The process of fabrication of carbon fibres from special PAN fibres includes two steps oxidative stabilization at low temperature and carbonization at high temperature in an inert atmosphere. Due to the high cost of raw materials (e.g. PAN fibres) and of this production process, carbon fibres are still expensive. Carbon fibres may be also produced from crude oil deposits like pitches or asphalts. Three main groups of carbon fibres are considered as possible composite materials reinforcement ... 

Pretreatment stabilizes the stracture of the precursors, acts to maintain the molecular stracture of the carbon chains, and/or enhance the uniformity of pore formation during the pyrolysis process. Current pretreatment includes oxidation, chemical treatment, physical method such as stretching. Oxidation or thermostabilization is the most popular and commonly used method to pretreat the polymeric precursors. This preteatment stabilizes the stracture of the precursors so that they can withstand the high temperatures in several pyrolysis steps. Thermostabilization can maximize the carbon yields of resultant membranes by preventing excessive volatilization of elemental carbon during pyrolysis. Oxidation has been carried out by Kusuki et al. [74], who thermally treated the precursors in atmospheric air at 400°C for 30 min before pyrolysis. Tanihara and Kusuki [75], Okamoto and co-woikers [76], and David and Ismail [31] have also applied thermostabilization. 

After thermo-stabilization, air in the tube is purged by nitrogen to prevent the oxidation from occurring during the high temperature pyrolysis process. The hollow fiber is then heated to a required pyrolysis temperature and under the required conditions. The resulting carbon membrane is cooled down to ambient temperature in nitrogen atmosphere. Table 5.2 shows the pyrolysis conditions required for the preparation of the PAN caibon hollow fiber membrane. 

It has been observed that solid oxide fuel cell voltage losses are dominated by ohmic polarization and that the most significant contribution to the ohmic polarization is the interfacial resistance between the anode and the electrolyte (23). This interfacial resistance is dependent on nickel distribution in the anode. A process has been developed, PMSS (pyrolysis of metallic soap slurry), where NiO particles are surrounded by thin films or fine precipitates of yttria stabilized zirconia (YSZ) to improve nickel dispersion to strengthen adhesion of the anode to the YSZ electrolyte. This may help relieve the mismatch in thermal expansion between the anode and the electrolyte. 

Pyrolysis can be considered as a three-step process. The first step (stabilization) takes place when poly(acrylonitrile) fibers are heated at 200-300 °C in air. This initiates oxidation and the formation of cross-finks. The second phase (known as carbonization)... 

Bryce and Greenwood studied the kinetics of formation of the major volatile fraction from potato starch, and its components. They limited their interest to the temperature range from 156 to 337 and to the formation of water, as well as of carbon mon- and di-oxide. The results revealed the following facts. Stability toward pyrolysis within the first 20 minutes of the process falls in the order amylose < starch < amylopectin < cellulose. Autocatalysis is absent, as shown by Puddington. Both carbon mon- and di-oxide are evolved as a consequence of each of two first-order reactions. The initial one is fast, and the second is slow. The reasons are not well understood, but they probably involve some secondary physical effects. The amount of both carbon oxides is a direct function of the quantity of water produced from any polysaccharide, which, furthermore, is independent of the temperature. The activation energy for the production of carbon mon-and di-oxide reaches 161.6 kJ/mol, and is practically independent of the polysaccharide formed. At the limiting rates, the approximate ratios of water carbon dioxide carbon monoxide were found to be 16 4 1 for amylopectin, 13 3 1 for starch, 10 3 1 for amylose, and 16 5 1 for cellulose. 

Results presented in Fig. 20.20 show that polysiloxanes, obtained by ordinary sol-gel processing of TEOS, are also effective as nanometric size stabilizers. The role of the SiO covering in the stability of the SnO grains was discussed before. However, since the current interpretation of the phenomenon requires the absorption of hydrophilic species on the solid particles, the temperature treatment reduces the Si-OH population and favors the segregation of SnO this event parallels an important increase in the dimensions of metallic oxide crystallites. In contrast, pyrolysis of glucosidic moieties after treatment produces carboxylic groups able to coordinate metallic ions at the particle surface, and the related size stabilization, prolonged up to 600 °C, appears exclusive of starch. 

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