Morphology platelet-like

Most of the HA crystals in young bone are small with a mean crosssectional width of only about 90 A. Occasionally, larger crystals in the 500 to 1000 A range have a thin platelet-like form. Those two crystal populations are morphologically distinct from one another and must have originated under somewhat different conditions3, 48). 

Random copolymers of VF2/F3E when crystallized from the molten state above the Curie temperature show a microstructure in the form of very thin needle-like morphological units which are probably semicrystalline. Figure 5a illustrates the needle-like microstructure of the copolymer 80/20 melt crystallized in the paraelectric phase observed at 140 °C. After codling at room temperature the microstructure of the ferroelectric crystals is such that what appear in the optical microscope as radial fibers are, in fact, stacks of thin platelet-like morphological units (see Fig. 5b). 

The C-S-H phase is an amorphous or nearly amorphous material of the general formula Ca0. Si02.H20, where both x and y may vary over a wide range. On the nanometer scale the C-S-H phase is stracturally related to the crystalline phases 1.4 nm tobermorite and jeimite. In cement pastes hmited amounts of foreign ions may be incorporated into the C-S-H phase. On the micrometer scale the C-S-H phase appears either as a dense amorphous mass or as a microciystalline material with an acicular or platelet-like morphology. The material contains pores with radii between about 1 and 10" nm, and exhibits a specific surface area exceeding 100 m /g. 

Compared to cellulose or chitin, the morphology of constitutive nanocrystals obtained from starch is completely different. Figure 19.12 shows a TEM obtained from a dilute suspension of waxy maize starch nanocrystals. They consist of 5-7 nm thick platelet-like particles with a length ranging 20-40 nm and a width in the range of 15-30 nm. The detailed investigation on the stmcture of these platelet-like nanoparticles was reported [36]. 

The barrier properties of starch nanocrystals/natural rubber nanocomposites were also investigated [39]. For these systems, the water vapour transmission rate, the diffusion coefficient of oxygen, the permeability coefficient of oxygen and its solubility, were measured. It was observed that the permeabiUty to water vapour, as well as to oxygen, decreased when starch nanocrystals wctc added These effects were ascribed to the platelet-like morphology of the nanocrystals. 

It is also believed that the microstructure of a surface film is more complicated than a simple layer. One concept is that the surface film on Mg consists of multi-layers. A schematic diagram illustrating a multi-layer structure of surface film on Mg has been proposed based on transmission electron microscopy (TEM) observations (Nordlien eta/., 1995). For example, a platelet-like morphology has been suggested for the surface film on Mg in water (Vermilyea and Kirk, 1969). It is postulated that a very thin and compact MgO may be present next to the Mg substrate and that the relatively thick and non-compact (porous) outer layer is mainly Mg(OH)2. 

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