Polycrystalline structure

The kinetics of electrocrystallization conforms to the above description only under precisely defined conditions. The deposition of metals on polycrystalline materials again yields products with polycrystalline structure, consisting of crystallites. These are microscopic formations with the structure of a single crystal. 

Fig. 13.5. As seen, a conventional Prussian blue film is of polycrystalline structure, however, the layer covers the surface completely.

As can be seen from this figure, the heat-resistance was remarkably improved by the drastic changes in the microstructure from amorphous to polycrystalline structure. Another type of SiC-based fiber, SA fiber (2), has a sintered SiC polycrystalline structure and includes very small amounts of aluminum. This fiber exhibits outstanding high temperature strength, coupled with much improved thermal conductivity and thermal stability compared with the Nicalon and Hi-Nicalon fibers. The fabrication cost of the SA fiber is also reduced to near half of that of the Hi-Nicalon Type S [ 17]. The SA fiber makes SiC/SiC composites even more attractive to the many applications [18]. In the next section, the production process, microstructure and physical properties of the SA fiber are explained in detail. 

It is no wonder that the particles are spherical but crystalline, if one considers the formation mechanism. The rather smooth surface of the spherical magnetite may be due to the rapid contact recrystallization of the constituent primary particles (5), forming the rigid polycrystalline structure. Flowever, it must be noted that polycrystalline spheres are also prepared by normal deposition of monomeric solute, as shown in the formation of the uniform spherical polycrystalline particles of metal sulfides in Chapters 3.1-3.3. Thus, while we may be able to predict the final particle shape and structure from the formation mechanism, it is risky to conclude the formation mechanism only from characterization of the product. As a rule, scrupulous analyses are needed for concluding the growth mechanism in a particle system. 

In Ref. 67, the increase in bandgap of the as-deposited film was attributed to a mixed amorphous-polycrystalline structure (apparently no XRD pattern was found for the as-deposited film). The onset of absorption in the transmission spectrum was sharp for the as-deposited film, and any polycrystalUne (nonquantized) material would be expected to give some absorption at lower energies, even if the amorphous phase possessed a higher bandgap. Therefore size quantization seems to be a more reasonable explanation for the high bandgap of the as-deposited film. 

Comparative estimation of hardness of different materials by an indirect method of measurement of the elasticity modulus has shown that the results conform with those expected. This agreement can be estimated especially for ceramic materials whose polycrystalline structure practically precludes correct measurement of hardness using a hardness pin (Vickers, Knoop and other methods). 

Most ceramic shapes do not consist of one single crystal, but are composed of numerous crystals joined together to form polycrystalline structures. The characteristics of the grain boundaries between crystals can influence the strength, chemical stability, and electrical properties as much us do the crystalline structures within Ihe individual grains. 

With a high-pressure hot-pressing method c-BN-TiC/TiN composites are prepared by sintering and subsequently heat treatment between 1000 and 1400 °C. The samples exhibit a dense polycrystalline structure of c-BN-TiN/ TiC, and a thin layer of fine TiB2 is visible at the c-BN-binder interface. Hardness decreases significantly after heat treatment [256]. 

Parker et al. reported (Martin et al., 2007) another atomistic simulation for the ceria NTs. A multilayer structure was proposed for the model and it is believed that the polycrystalline structure would stabilize the oxygen vacancies at surfaces, grain boimdaries, and triple junctions, therefore, the ceria NTs would have enhanced catalytic activities. 

A single phase polycrystalline structure identified by x-ray or electron diffraction, and... 

The major feature of polymers that have been bulk crystallized under quiescent conditions are polycrystalline structures called sphemlites. These are roughly spherical supercrystalline structures which exhibit Maltese cross-extinction patterns when examined under polarized light in an optical microscope. Spheruliies are characteristic of semicrystalline polymers and are also observed in low-molecular-weight materials that have been crystallized from viscous media. Sphemlites are aggregates of lamellar crystallites. They are not single crystals and include some... 

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