Cell orientation

In the presence of external forces, plastic foams in which the cells are elongated or flattened in a particular direction may be formed. This cell orientation can have a marked influence on many properties. The results of a number of studies have been reviewed (59,60). 

The scoring of foci was carried out according to the recommended guidelines. Only foci considered as positive (type III), showing deeply basophilic, dense multilayering of cells, random cell orientation at all parts of the focus edge, invasion into the surrounding contact-inhibited monolayer, and domination of spindle-shaped cells, were counted. 

Figure 3.9. The relation between a primitive rhombohedral cell and an equivalent tripleprimitive hexagonal cell is shown. The cell orientations have been selected according to the criteria adopted by the International Tables (Hahn 2002).

In the frame of electrocatalysis and fuel cell-oriented research, a vast amount of data was collected during the last decades. 

Below the yield point, however, stress/strain behaviour was found to be independent of initial cell orientation, due to the threefold symmetry of the hexagonal cellular array [54], This allows a correlation between shearing and extensional deformations to be made [55], namely that shear can be considered as elongation followed by rotation. Thus, information on one type of deformation can be obtained by solving expressions for the other. 

Using planar tetrakaidecahedra as the model, on the other hand, causes the square faces to shrink to zero area at the yield point. The unit cell therefore resembles a true rhombic dodecahedron. The elastic response was found to be anisotropic (i.e. dependent on initial cell orientation) for the planar model, up to the elastic limit. This is in contrast to the monodisperse 2D case, which is... 

The anatomy of softwoods will be described first because it is less complex than hardwoods. The two main cell types which constitute softwoods are tracheids, which conduct and support, and parenchyma which store food. These two cell types can be further classified as to their orientation, that is longitudinal or transverse. Cells oriented in the longitudinal direction have the long axis of the cell oriented parallel to the vertical axis of the tree trunk whereas transversely oriented cells have their long axis at right angles to the vertical axis of the tree stem. 

Obviously, softwood anatomy is relatively simple as only two types of cells, longitudinal tracheids and ray parenchyma, constitute the bulk of the wood. Hardwoods have a more complex anatomy as more kinds of cells are present. The roles of conduction and support are carried out by different cells and in addition to the transverse ray parenchyma, food-storage cells oriented in the longitudinal direction are present. Parenchyma oriented longitudinally are called longitudinal or axial parenchyma. Vessel segments perform the conduction role, and fibers the support role. 

A modern, non-stoichiometric, fuel-cell-oriented, perovskite material, Cco Gdo iOj 95 (COG), has been developed by Professor Steele at Imperial College London, as recorded in Steele etal. (2000a, b). Professor Steele has found the way, via COG, to a porous ferritic stainless steel, tubular or flat square IT/SOFC operating at 500 °C, superb achievement. 

In the presence of external forces, the cells may be elongated or flattened. Cell orientation can influence many properties [22,23]. An important characteristic of the cell structure is the extent of communication with other cells. This is... 

Figure 8.10 (a) The MgO(IIO) Miller plane and (b) a cell orientated to make the d-vector perpendicular to this surface. 

The microstructure of ceramic honeycombs not only affects physical properties like CTE, strength, and structural modulus, but has a strong bearing on substrate/washcoat interaction, which, in turn, affects the performance and durability of the catalytic converter [28-30]. The coefficient of thermal expansion, strength, fatigue, and structural modulus of the honeycomb substrate (which also depend on cell orientation and temperature) have a direct impact on its mechanical and thermal durability [22). Finally, since all of the physical properties are affected by washcoat formulation, washcoat loading, and washcoat processing, they must be evaluated before and after the application of washcoat to assess converter durability [28-30]. 

Fig. 31. Thomoconductivity X of CFCh-foamed polyurethane as a function of cell orientation a = D /D- relative to heat flux D" and are the mean cell diameters paralld and perpendicular to the heat flux respectively ...

FIGURE 2.15. Some faces of sodium chloride crystals, labeled for a unit cell oriented as in Figure 2.5. Sodium cations are denoted Na, and chloride anions are denoted Cl. (a) When a crystal grows as a cube, its faces have equal numbers of sodium and chloride ions on their surfaces, (b) When the crystals grow as octahedra, the faces contain only one type of ion, either sodium or chloride. 

Dysplasia is defined as abnormal growth of a tissue with respect to shape, size, and the organization of component cells. Normal cell-to-cell orientations are disorganized or disrupted, and the cells themselves vary in size and shape (Figure 4). When present, dysplasia may be associated with chronic irritation, occur with metaplasia, and be seen in neoplastic transformation. It is a change that is a hallmark of increased risk for development of neoplasia. Like metaplasia, dysplasia is a potentially reversible tissue alteration. It is also considered in some circumstances as a preneoplastic change. 

Figure 10. Relationships between unit cell orientations for lAlmcm, Pnma and Cmcm structures and the reference axes, X, Y, Z. Cell edges of the Pm3m unit cell are parallel to the reference axes. The P lmbm structure has the same unit cell as the IMmcm strac-ture, except that the c repeat of the former is half that of the latter (from Carpenter et al. 2000b).

---