Microstructures of pyrocarbon connect the synthesis of C/C composites with its performance. It is significant to characterize and categorize the pyrocarbon matrices quantitatively, for it is the basis for investigating the rational synthesis condition, as well as the key of understanding the application of C/C composites.
Selected-area electron diffraction (SAED) developed recently, is efficient in classifying pyrocarbons quantitatively, based on its high precision in spatial resolution and that the orientation angle (OA) is proportional to the twist degree of pyrocarbon crystallites. However, OA has only a partial description of the microstructure of pyrocarbon. Moreover, the influence effects of the tilt angle of a sample have, to date, been rarely studied for their reliability of classifying pyrocarbon by SAED. Therefore, in this dissertation, the 3D reciprocal distribution of pyrocarbon has been investigated by an improved tilt method; and the influence of tilt angle on pyrocarbon classification has been discussed. The main results are as follows:
A new kind of pyrocarbon with extraordinarily high graphitization degree (OA＝0°) was discovered. A 3D “dotted-cylinder” reciprocal lattice was reconstructed to describe the highly-graphitized pyrocarbon’s “plane defects”: the turbostratic structure, which has two main defects – rotational and displacive defects – between graphitic crystallites with large crystalline size (La is about 400nm，Lc is about 10nm). Moreover, the “off-plane defects” of the highly-graphitized pyrocarbon are the interfaces between “mature” cones; and the OAs of the pyrocarbons, near fiber surface with lower textural degree, relate to the size of the pyrocarbon cones.
To solve the difficulty of keeping a fixed tilt axis for the continuous reciprocal distribution of polycrystal during a tilt process, we developed the traditional tilt method by quantifying the main and partial tilt angle beforehand. This method allows us to tilt a TEM sample about a set tilt axis in a selective step. So we can also obtain the 3D reciprocal distribution model for polycrystals with complicated structures, so as to fully characterize their microstructural features.
Applying the improved tilt method to C/C composites, we obtained the Polyacrylonitrile (PAN)-based carbon fiber’s 3D “drum-like” reciprocal distribution, which illustrates that the fiber crystallites are stacking isotropically in the direction perpendicular to its growth axis; while, in the parallel direction, the crystallites only have a mild misalign. The 3D “drum-like” reciprocal distribution of pyrocarbon is an “umbrella-like” model with uneven “skirt”, which indicates that pyrocarbon’s graphitic crystallites has a certain preferred orientation in the direction parallel to fiber growth axis.
Further diffraction analysis on the pyrocarbon matrices of a C/C composites showed that the conventional OA determination with a single SAED pattern, based on which pyrocarbons are usually categorized, is largely dependent on the tilt angle of the C/C composites specimen with respect to the incident electron beam. One pyrocarbon can be classified as possessing different types of textural degrees under different tilt angles. To improve the reliability and accuracy of its classification, an optimal OA for pyrocarbon is suggested as the OA in sectional direction, which can be achieved by approaching an even diffraction ring of the PAN-based carbon fiber the pyrocarbon deposits on.
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