Flat sheet precursor

The vertebrate nervous system including neurons, astrocytes, oligodendrocytes, and other cells originates from a flat sheet of neuroepithelial cells, constituent of the inner lining of neural plate along the dorsal surface of embryo (Fujita, 2003). These neuroepithelial cells are the earliest precursors in the developing CNS. 

CMS manhranes are composed of microporous carbon (as libers, on tubes or flat sheets) prepared from carbonization of polymeric precursors under controlled conditions. Precursors that are mostly used are cellulose or Pis. Depending on the manbrane pore size and the process conditions, separation may take place according to (1) molecular sieving < 5 A), (2) selective surface flow (5 A < < 12 A), (3) Knudsen diffusion... 

There are researchers involved in the study of flat sheet homogeneous membrane particularly focused on the development of entropic contributions to diffusiv-ity selectivity as the polymer matrix evolved to a rigid caibon matrix [22]. Polymer precursor membranes pyrolyzed at intermediate steps in the pyrolysis process and finally pyrolyzed membranes were tested for the purpose to study the development... 

Figure 5.46 Schematic representation of helical and twisted ribbons as discussed in Ref. 165. Top Platelet or flat ribbon. Helical ribbons (helix A), precursors of tubules, feature inner and outer faces. Twisted ribbons (helix B), formed by some gemini surfactant tartrate complexes, have equally curved faces and C2 symmetry axis. Bottom Consequences of cylindrical and saddlelike curvatures in multilayered structures. In stack of cylindrical sheets, contact area from one layer to next varies. This is not the case for saddlelike curvature, which is thus favored when the layers are coordinated. Reprinted with permission from Ref. 165. Copyright 1999 by Macmillan Magazines.

The second set of synthesis steps shown in Fig. 9.2 concerns the preparation of the green body for the C3 formation. Steps 5-8 are related to generating a flat or curved sheet from the carbon fiber and the precursor of the binder phase. This process can be complicated because the final part made from C3 cannot be changed in its form nor can it be interconnected to another C3 part by reasonably affordable techniques suitable for mass production. The quality of the final product is here decided by the homogeneity of the material distribution and the exact shaping. 

Flat laminates can be produced using the processes described in other chapters of this handbook. For example, a double belt press or compression moulding can produce the precursor laminate. Figure 5.3a shows thermoplastic composites with unidirectional fibres in each layer consolidated into a laminate with a picture frame mould. A picture frame mould goes into a hot press or autoclave and turns composite lamina into a fused precursor. To some extent, sheet forming adds cost to a final structure because a manufacturer creates two components the flat precursor and the formed structure. 

Special emphasis is paid to a modular approach in which functionalized benzene moieties serve as building blocks for chain-type (ID), disc-type (2D), and dendrimer-type (3D) graphene derivatives (Fig. 1) [104,105]. The critical question, when taking on graphene as a challenge for polymer synthesis, is whether any of these polyphenylenes can serve as precursors for graphenes and how the necessary chemical transformation to flat graphene sheets can be accomplished. 

Phenolic Resin Phenolic resin has also been used to produce CMS membranes. The advantage of phenolic resin over other precursors such as polyimides is low cost. Shusen et al. (1996) have produced asymmetric phenol formaldehyde resin-based CMS membranes. A flat film (0.05-0.10 mm) was formed by thermopressure molding. The membrane was subsequently heated to 850-900°C in nitrogen followed by 0.5-2.0% oxygen. One side was covered by a porous ceramic sheet during this process, and, therefore, the oxidation did not occur symmetrically. The resulting membrane had a reported permeability of 2300 Barrers and an O2/N2 selectivity of 10.65. 

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