Paper fiber anisotropy

The technology of mesophase-pitch-based carbon fiber has stimulated the rapid development of the chemistry of mesophase behavior and preparation. The carbonization schemes and mechanisms leading to optical anisotropy via the mesophase, the control of carbonization with emphasis on the preparation of spinnable mesophase, and the mesophase transition and reactivity in relation to the structure of its constituent molecules are summarized in this paper. 

An important consequence of the dynamic conditions used in the papermaking process is the introduction into the paper sheet of a three dimensional anisotropy. During the papermaking process as the fibers are much longer than the paper thickness, they undergo an in-plane orientation. Futhermore, the dynamic conditions cause the fibers to orient themselves, not only in the plane of the sheet, but also in the direction of the moving wire. This direction is called the Machine Direction (MD) and the direction perpendicular to it is the Cross-Direction (CD). Paper anisotropy is one of its more important characteristics, since it influences all its mechanical properties. A schematic illustration of the distribution of the fibers in a paper sheet is shown in Fig. 3- In this article, it will be demonstrated that, the anisotropy also influences the electrical conductivity of the paper. 

Measurements of cardiac tissue tensor conductivities are reported on in only three papers, and these values are not in complete agreement (Clerc, 1976 Roberts et al., 1979 1980). All agree, however, that conductivity along the fiber axis exceeds that across the fiber axis by a substantial factor. Furthermore, this factor (the anisotropy ratio) is substantially different in intracellular vs. interstitial space. 

Another variation of the stochastic reconstraction of the non-woven carbon paper GDL was proposed by Thiedmann et al. Realizations of the 3D structures are generated by stacking thin sections of cyUndrical fiber layers. The thin sections of the GDL are represented by planar random line tessellations, which are built by intersecting lines located at random in the material plane. These lines are dilated in 3D, and each dilated line tessellation represents a thin fiber-layer section. The segmentation of thin section from SEM images mimics the anisotropy in the... 

Zamel et al. presented a study on the estimation of effective thermal conductivity of carbon paper GDL stmctures based on the aforementioned DNS formalism (Eq. 9.12). The 3D carbon paper GDL microstractures were reconstracted using the stochastic method by Schulz et al. They investigated the influence of fiber orientation, anisotropy, compression and binder fraction. Figure 9.27 shows the representative effective thermal conductivity prediction along with experimental data available in the literatirre." " ... 

Paper is an anisotropic material with regard to many physical properties. This anisotropy is due to the anisotropic properties of the individual fibers, which result from the fibrillated microstructure of the fiber rather than the fiber shape [7]. As a result of the fibrillated structure, the fiber can accept, for example, high tensile forces in the direction of the fiber axis with low elongation however, even small tensile forces acting perpendicular to the fiber axis cause high elongations. 

Both fiber aHgnment and shrinkage restraints are responsible for the anisotropy of the moisture expansion of finished paper, which is generally far lower in the machine direction than in the cross machine direction, the latter being nonuniform across the width. Both the above factors also affect the load - deformation... 

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