Densely packed crystallites

Fig. 17. Structural diagram (51) for sputtered layers. Zone 1 is a porous structure consisting of tapered crystallites separated by voids. Zone 2 shows columnar grains, and Zone 3 has a recrystallized grain structure. Zone T is a transition structure consisting of densely packed fibrous grains.

Taking, for instance, Al, with a melting point of 660 °C and a web substrate temperature of 50 °C, zone I formations will be created (porous structure, pointed crystallites, large voids) and up to 250 °C, formations in the transitional area (densely packed fibers) will appear. Up to 450 °C zone II (pillar-shaped crystallites), and above this temperature zone III (conglomerate-type crystallites) formations will be seen. Because of the relatively low maximum thermal stress that may be applied to polymer webs, the growth in metallized layers on polymer webs mainly occurs in Zone I or in the transitional zone. The different growth is also evident from comparison of cooling drum and free-span coater methods. 

If no load is applied during carbonization, the fibers shrink also in this phase of the treatment The shrinkage of fully stabilized fiber is of the order of about 10 %. It is not only a consequence of the various condensation and elimination reactions mentioned above, but also of the formation of densely packed graphitic crystallites (ct. Sect. 3.5.2). The shrinkage can, however, be counteracted by a load, and the mechanical properties of the carbon fiber are improved if this is done. 

Although the typical high tensile strength carbon fibers (obtained at heat treatment <1600 °C) are certainly considerably less ordered, and less densely packed, their structure may be assumed to be somewhere in-between the fibrillar structure with some pseudocrystalline lamellar order within the fibrils, as suggested by Warner for stabilized fiber, and the graphitic crystallites suggested by Crawford . The voids determine the relatively low density of the carbon fibers (for PAN based fibers... 

The cluster model assumes availability in the amorphous polymers structure of local order domains (clusters), consisting of several densely packed collinear segments of different macromolecules (amorphous analog of crystallite with stretched chains). These clusters are connected between themselves by tie chains, forming by virtue of this physical entanglements network, and are surrounded by loosely packed matrix, in which all fluctuation free volume is concentrated [107],... 

Lately it was offered to consider polymers amorphous state stmcture as a natural nanocomposite [6]. Within the frameworks of cluster model of polymers amorphous state stmcture it is supposed, that the indicated structure consists of local order domains (clusters), immersed in loosely packed matrix, in which the entire polymer free volume is concentrated [7, 8]. In its turn, clusters consist of several coUinear densely packed statistical segments of different macromolecules, that is, they are an amorphous analog of crystallites with stretched chains. It has been shown [9] that clusters are nanoworld objects (tme nanoparticles-nano clirsters) and in case of polymers representation as natural nanocomposites they play nanofiller role and loosely packed matrix-nanocomposite matrix role. It is significant that the nanoclusters dimensional effect is identical to the indicated effect for particulate filler in polymer nano composites sizes decrease of both nano clusters [10] and disperse particles [11] resrdts to sharp enhancement of nanocomposite reinforcement degree... 

The value is determined as linear defects length per polymer volume unit (see the Eq. (4.1)). Since in the cluster model a segment included in densely packed regions (crystallites or clusters) is assumed as linear defect (dislocation analog) then value p is determined as follows [3] ... 

Isotropic samples of a partially crystalline polymer essentially correspond to an ensemble of densely packed, isotropically distributed stacks of parallel lamellar crystallites. If the extensions of the stacks parallel and normal to the lamellar surfaces are large compared to the interlamellar distance, the scattering behavior can be related to the electron density distribution Pe z) measured along a trajectory normal to the surfaces. This trajectory will pass through amorphous regions with density /c e,a stnd crystallites with a core density pe,c The average density (pe) lies between these two limits. 

Mixed oxides have the ability to form paracrystalline phases that represent an intermediate phase between the crystalline and the amorphous phase. Because of the disordered structure paracrystallinity causes defects that occur due to the inhibition of recrystallization and are initiated by the introduction of additional anions or molecules into the lattice structure. The formation of these deformed crystalline structures depends on the used synthesis method as well as the nature of the cation precursor. Thermal decomposition of the LDHs leads to the formation of the metastable phase that contains divalent and trivalent cations with densely packed configuration [2,32]. After the reduction, tri-valent ions are trapped inside the crystallites of divalent ions causing the formation of active centers. The centers that exhibit higher reactivity are believed to be formed on the surface of the paracrystalline crystals. After deactivation, the grain size of these mixed oxides increases due to the gradual removal of the defects in the crystal lattice [2]. 

The 3D images in Fig. 14 show that there are major qualitative differences in the A" and B" films. The A film contains largely oxidized nickel. Subsequent XPS measurements showed it to be a mixture of Ni(OH)2 and NiO. Whatever oxidized Cu is present is found to be distributed near the oxide/metal interface. By contrast the B film has a significant concentration of oxidized Cu. di.stributed fairly evenly through the film structure. Some measure of the differences in Ni/Cu ratio at individual points in the volume, can be gained from the intensity plots in Fig. 14, where NiO" and CuO intensities are compared. Also, the distribution of oxides on the B alloy surface shows them to be less densely packed and more irregular. In addition, the crystallite size is larger. 

In studying the structure of liquids, the model of dense random packing (DRP-model) suggested by Bernal [6.7] played an important role. In this model determined by the algorithm of the structure formation from spheres interacting according to some law), the correlations in the mutual location of atoms rapidly decrease with distance and the cybotaxis or crystallite orderings are absent. 

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