Pre-oxidized Fibers Research Articles

CHARACTERIZATION OF OXYGEN DISTRIBUTION IN THE CROSS SECTION OF POLYACRYLONITRILE OXIDIZED FIBERS WITH SCANNING TRANSMISSION X-RAY MICROSCOPY

The distribution of oxygen element in the cross section of polyacrylonitrile (PAN) precursor fibers and oxidized fibers has been investigated by using a scanning transmission X-ray microscope (STXM) with a spatial resolution of 30 nm,combined with dual-energy contrast imaging method. By using the optical density ratio of the corresponding pixels in the two STXM images, the STXM experimental results indicated that the oxygen is homogeneously distributed in the pristine PAN fibers. During stabilization process, two important reactions occur and change the chemistry of PAN structure. In the skin section of PAN fiber,oxidation converts it into a conjugated and hetero-aromatic structure in the presence of oxygen. While in the core section of PAN fiber,cyclization turns linear PAN into a six-member ring structure due to the absence of oxygen. After treatment, an increase in the oxygen concentration in the skin of the PAN fibers is clearly observed while it remains constant in the core of PAN fibers, clearly demonstrating the diffusion process in the treatment. The propagating velocity of oxygen along the radical direction is about 0.033 mu m/min. This research shows that the STXM is a powerful technique to characterize the internal microstructure of the PAN pre-oxidized fibers, and therefore will help to rationally understand the evaluation mechanism of the skin-core structure.

The evolution of crystalline structure in the preparationof PAN-based carbon fibers studiedby whole powder pattern fitting

Based on the Rietveld theory and Maud software, the evolution of crystalline structure of PAN-based carbon fibers was studied by investigating the different stages in the preparation of carbon fibers. The microstructure parameters of PAN fibers, oxidative fibers, and carbon fibers, including cell parameters, apparent crystal grain sizes, textural structure and micro-strain, were obtained from the named whole powder pattern fitting method for XRD result. Based on rigorus theory of crystallography, the whole powder pattern fitting treats the whole XRD powder diffraction pattern rather than individual diffraction peaks, resulting in higher credibility and precision. The results indicated that polymer chains were arranged along the axis of PAN fiber and the apparent crystallite size was about 6.4 nm. After pre-oxidation, the originally ordered structure in PAN fiber was damaged and new less-ordered cyclic ladder structure was formed in the pre-oxidized PAN fibers, while the apparent crystallite size of fiber decreased obviously. With the carbonization process, the cyclic ladder structure of per-oxidized fibers transferred into layer structure similar to that of graphite. The layers were oriented along the axis of carbonized fiber that contributed to the X-ray diffraction, which comprised the apparent crystallite grains. As the carbonization temperature increased, size of the layer structure in the direction perpendicular to fiber axis increased clearly, the preferential orientation was enhanced and the layer structure became much more ordered.

Carbonization of pre-oxidized polyacrylonitrile fibers

The microstructural evolution, elemental composition and gas releasing character of preoxided poly-acrylonitrile (PAN) fibers during continuous carbonization were investigated by Raman spectroscopy, XRD, XPS and TG-MS. Results indicate that the sp2 C—C bond length, crystal size and graphitization degree increase. The increase of orientation degree and the decrease of micropore volume are the fundamental reasons for the improvement of density and tensile strength of carbon fibers. Surface and global carbon contents of pre-oxidized PAN fibers, and both low temperature and high temperature carbonized fibers indicate that the loss of non-carbon atoms during carbonization propagates from the exterior to the interior of the fibers. TG-MS indicates that gases are released mainly at low temperature in carbonization. The cross-linking of trapezoid giant molecular chains to release nitrogen takes place at a high temperature during carbonization.

Development of a New Kind of Sealing Composite Material Reinforced with Aramid and Pre-Oxidized Fibers

A new kind of sealing composite material reinforced mainly with aramid and pre-oxidized fibers were developed by molding preparation method. The effect of different surface treatment methods of the fibers on the heat resistance of the composite material was studied. The optimum prescription of the sealing composite material was obtained by regression design method. The relationship between the transverse tensile strength of the material after ageing at elevated temperatures and its main ingredients was investigated. The performances of the developed sealing composite material were tested, and the results indicate that the main performances of the material coincide with the requirements prescribed in Chinese national standard and some performances are even better than that of the products of some famous corporations.

Kinetics and mechanism of thermal decomposition of pre-oxidized polycarbosilane fibers

The thermal decomposition of pre-oxidized polycarbosilane(PCS)fibers was carried out in a thermogravimetric analyzer under non-isothermal condition.The Coats-Redfern equation was used to estimate the apparent activation energy in thermal decomposition and the mechanism of thermal decomposition was studied by IR,XRD and SEM.It was found that the thermal decomposition of pre-oxidized PCS fibers was a first order reaction and the apparent activation energy was estimated to be 19.826kJ/mol.The thermal decomposition process was a dehydrogenation and dehydrocarbonation condensation with the evaporation of low molecular components at low temperature.With increasing temperature the fibers were converted into an inorganic structure by decomposition of the side chains of the polymer.When the temperature was further increased,the crystallite size of SiC increased and the fibers became completely inorganic;above 1250℃.The mechanical properties of the SiC fibers became lower with the rapidly increasing crystallite size of the β-SiC.