Crystalline solids polycrystalline materials

Takeuchi etal. (1985) have examined the HDS catalyst deposits in detail with XPS, ESR, x-ray diffraction, and electron microscopy. X-ray diffraction revealed the presence of the crystalline V3S4 phase, a nonstoichiomet-ric, polycrystalline solid with sulfur-to-vanadium ratios of 1.2 to 1.5. This polycrystalline material was observed by microscopy as 10-/um-long, rod-shaped crystals on the outer surface of the catalyst and about 0.1 /urn in length within the catalyst pores. The x-ray diffraction technique will not reveal any amorphous phases present. Electron spin resonance spectra revealed the presence of a vanadyl on the surface that was coordinated with 4S and distinctly different from the 4N coordination of the crude oil... 

Most solids are not crystalline on their surface. This is certainly true for amorphous solids. It is also true for most crystalline or polycrystalline solids because for many materials the molecular structure at the surface is different from the bulk structure. Many surfaces are for example oxidized under ambient conditions. A prominent example is aluminum which forms a hard oxide layer as soon as it is exposed to air. Even in an inert atmosphere or in ultrahigh vacuum (UHV) the surface molecules might form an amorphous layer on the crystalline bulk solid. 

Glass-matrix materials can be considered as a non-crystalline solid with the frozen-in structure of a liquid. Characteristics of some important varieties of glass are given in Table 3.7. Glass-matrix materials are polycrystalline materials having fine ceramic crystallites in a glass matrix. Important glass-ceramic matrix materials are as follows. 

Trapped radicals are produced in both glasses and crystalline solids and have been studied mainly by ESR. In glasses the yield of radicals is typically 3-4 (100eV) and in polycrystalline material 4-6 (100 eV)" (In perdeuterated compounds this is about a factor of 1.3 lower" ". )... 

In recent years, more complex types of transport processes have been investigated and, from the point of view of solid state science, considerable interest is attached to the study of transport in disordered materials. In glasses, for example, a distribution of jump distances and activation energies are expected for ionic transport. In crystalline materials, the best ionic conductors are those that exhibit considerable disorder of the mobile ion sublattice. At interfaces, minority carrier diffusion and discharge (for example electrons and holes) will take place in a random environment of mobile ions. In polycrystalline materials the lattice structure and transport processes are expected to be strongly perturbed near a grain boundary. 

Amorphous solid Crystalline solid Materials science Polycrystalline sohd Solid state Section 15.8 Covalent network sohd Ionic crystal Metallic crystal Molecular crystal... 

Solid core fibers guide the laser radiation through total internal reflection. These fibers are made of different kinds of glasses, single crystalline materials and polycrystalline materials. 

While the studies discussed so far were concerned with the sintering of metals, the question of material transport in sintering is as pertinent for ionic solids as it is for metals. At the temperatures where sintering is important extensive plastic deformation by dislocation motion is possible in both single crystals and polycrystalline aggregates of ionic solids. It was therefore decided to adapt the Brett and Seigle technique of sintering with fiducial markers to an ionic crystalline solid, calcium fluoride. 

Chapter 2 was concerned primarily with the various types of atomic bonding, which are determined by the electron structures of the individual atoms. The present discussion is devoted to the next level of the structure of materials, specifically, to some of the arrangements that may be assumed by atoms in the solid state. Within this framework, concepts of crystallinity and noncrystallinity are introduced. For crystalline solids, the notion of crystal structure is presented, specified in terms of a unit cell. The three common crystal structures found in metals are then detailed, along with the scheme by which crystallographic points, directions, and planes are expressed. Single crystals, polycrystalline materials, and noncrystalline materials are considered. Another section of this chapter briefly describes how crystal structures are determined experimentally using x-ray diffraction techniques. 

If we were to measure the birefringence, AN, of a polycrystalline material in the solid state, it would not represent the overall orientation. AVconsists of contributions from the amorphous and crystalline regions ... 

Polymers in the solid state can be completely amorphous, partially crystalline, or almost completely crystaUine. Polymer crystals have the requiranent that they must accommodate the covalent axis within an ordered structure. For some time the interpretation of X-ray diffraction patterns was that individual polymer molecules were partly crystalline and partly amorphous. The longest dimension of the crystallites in polycrystalline materials is usually about 5-50 nm, which is a small fraction of the length of a fully extended polymer molecule. A graphical representation of this once popular model is shown in Figure 3.12. Here a long polymer chain wanders... 

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