The Existence of Defects

The properties of the defect solid state are fundamental to our understanding of all reacting systems involving a solid in fact, there is little of the metallurgical and chemical industries which is not based on the chemical properties of defect solids. A little reflection will make this so obvious that there is no necessity to enumerate specific examples even the electrical industry depends on the ability to produce materials having controllable defect properties, e.g. luminescent materials for fluorescent lamps and cathode-ray tube screens, oxide materials for cathode coatings, a variety of semi-conductors for resistors, rectifiers, detectors and photoelectric devices. 

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Multi-walled CNTs (MWCNTs) are produced by arc discharge between graphite electrodes but other carbonaceous materials are always formed simultaneously. The main by-product, nanoparticles, can be removed utilizing the difference in oxidation reaction rates between CNTs and nanoparticles [9]. Then, it was reported that CNTs can be aligned by dispersion in a polymer resin matrix [10]. However, the parameters of CNTs are uncontrollable, such as the diameter, length, chirality and so on, at present. Furthermore, although the CNTs are observed like cylinders by transmission electron microscopy (TEM), some reports have pointed out the possibility of non-cylindrical structures and the existence of defects [11-14]. 

The existence of defects with metastable geometrical configuration (structures) has been studied extensively in recent years (Watkins, 1989). The majority of these defects appear to involve interstitials, but they are often complexes formed by the association of two defects, one of which is... 

Whereas in good-conducting doped or polymeric dyes ft-or -type conductivity can be explained without difficulty by analogy with inorganic semiconductors, the p- and -type photoconductivity in insulating (intrinsic) dye films cannot be explained in this manner. It is necessary to take into consideration the existence of defect states (lattice defects, dislocations, impurities etc.) distributed at different depths in the forbidden zone between valence and conduction band these defect states are able to trap electrons and holes, respectively, with different probability 10,11,88),... 

Therefore, we are able to discuss the existence of defect states (recombination centers and traps) in the forbidden gap of solid organic dyes characterized by different trapping probabilities (ranging from 10-12 cm2 to 10-20 cm2) for electrons and holes. Hence, asymmetric trapping of electrons and holes leading to n- and -photoconductivity is very probable. 

At all levels of supermolecular structure, the transformations caused by mechanical stress stem from the anisotropy of the mechanical properties—by the existence of defects at the macroscopic level by aniso-... 

It is a well-known fact that it is not feasible to manufacture crystals that are perfect in all aspects, because of the presence of crystal defects. This effect occurs because of the fact that over absolute zero, the existence of defects in crystals is thermodynamically necessary. 

If the semiconductor is an ionic solid, then electrical conduction can be electronic and ionic, the latter being due to the existence of defects within the crystal that can undergo movement, especially Frenkel defects (an ion vacancy balanced by an interstitial ion of the same type) and Schottky defects (cation and anion vacancies with ion migration to the surface). This will be discussed further in Chapter 13, as ionic crystals are the sensing components of an important class of ion selective electrodes. 

In electron microscopy a sample is bombarded with a finely focused beam of monochromatic electrons. Products of the interaction of the incident electron beam with the sample are detected. If the sample is sufficiently thin—up to 200 nm thickness—the beam is transmitted after interacting with the sample, leading to the technique of transmission electron microscopy (TEM). TEM is used to probe the existence of defects in crystals and phase distributions. Scanning TEM instruments have been recently developed to obtain images over a wider area and to minimize sample degradation from the high-intensity beams. 

IR (22), excessive and Al-independent ion-exchange capacity (22)] of the existence of defect sites in Si—rich ZSM-5 are mentioned. Since the occurrence and distribution of defect sites may give a clue about the operating synthesis mechanism (cf. the introductory comments and Figure 1) they have been studied in closer detail. 

The spectroscopic technique using 129xe-NMR together with OMCTS as a blocking agent allows the study of individual crystallite microporous volume of differents zeolites. The xenon as an internal probe can discriminate if a molecule penetrates or not in a given zeolite microporosity. Furthermore the xenon probe can follow the diffusion of other molecules in the internal zeolite microporosity and finally is very sensitive to the existence of defects that cannot be characterized by other techniques like X-rays. 

To exemplify the widespread significance of defects in the chemistry of the solid state we may list the types of reaction which have, on the basis of experiment, been shown to require the existence of defect structures for them to proceed (34). These reactions are ... 

The existence of defects has a simple thermodynamic basis. The creation of a defect in a perfect structure has an unfavorable effect on the enthalpy some coulombic or bonding energy must be sacrificed to create it. However, the introduction of some irregularities into an initially perfect array markedly increases the entropy aTAS term large enough to cancel the unfavorable AH term will thus arise up to some limiting concentration of defects. It is possible to write an expression for the concentration of defects in equilibrium with the remainder of the structure just as though a normal chemical equilibrium were involved. 

A more obvious but perhaps underappreciated problem with surface roughness is the existence of defect sites on a surface, i.e., sites that would not be exposed on a perfectly smooth surface. This type of defect is separate from classical defects like stacking faults, subgrain boundaries and dislocations, and is due just to non-uniform expression of the substrate structure in an uneven surface (Fig. 9) such as could occur with the local development of vicinal faces. As surface characterization methods are generally poor except in the case of a small suite of oxides and silicates, this effect has probably not been fully considered to date. For example, it is possible to imagine a low roughness (hkl) surface that is entirely terminated by small faces with other (hkl) orientations, so that the exposed surface functional groups differ both in density and orientation from what is expected. 

Silicon is the most widely used material in the electronics industry. To develop silicon-based devices for optoelectronic applications, one would like to make silicon a photon-emitting material. Unfortunately, silicon is an indirect gap semiconductor and, thus, the efficiency of photon emission is extremely low since the radiative recombination of the electron-hole pair is not allowed without the assistance of a momentum-conserving phonon. Moreover, the existence of defects leads to an almost total quenching of this rather unlikely process. 

It is shunless to have some kinds of defects in solids, especially in transition metal oxides and rare earth oxides, because molecules of oxygen, which is a counter component of the metal oxide, are highly volatile even at low temperatures and are able to escape easily from the solid. More basically, the entropy term of the state requires the existence of defects. Most of the transport phenomena in solids, for example, diffusion of ions, are controlled by defects [12, 13]. 

Different types of crosslinks play a key role in the higher structure of intermediate filament networks. As shown in previous studies, the intermediate filament network is involved not only in the mechanical role but also in the cellular role. It is intriguing that the intermediate filament network can withstand extreme deformation >100% without rupmre, even with the existence of defects [14,... 

Such variation in the density of surface states can be explained by the existence of defect centers associated with dangling bonds arising from the discontinuity of Si—O or Si—H bonds and oxygen danghng bonds (—O) [20]. For strucmres with d = 5.0 nm, the increase in the density of surface states may be due to the simultaneous formation of two types of defects, trivalent Si atoms (=Si.) and oxygen dangling bonds (—O) [21]. The defects arise when the Si—O bonds cleave under UV irradiation, as proven by the decrease in the vlo absorption band of Si-0 modes (see Fig. 6.2). 

It appears, therefore, that while some of the details of this picture of the sickling process are as yet conjectural, the proposed mechanism is consistent with experimental observations at hand and offers a chemical and physical basis for many of them. Furthermore, if it is correct, it supplies a direct link between the existence of defective hemoglobin moleeules and ibe pathological consequences of sickle cell disease. 

Quality of various types of materials, particularly of fibers, for their use in various applications is checked by their mechanical properties, which may also throw light on the defects in the fibers. It is well known now that many factors such as morphological structure, fiber dimensions, and its fracture mechanism contribute to the tensile behavior of the fiber. Whether the fiber is of synthetic or natural origin, the defects are supposed to be responsible for the large scatter in their properties. In both these type of fibers, there will be inherent defects related to internal structure-morphological factor. Moreover, one should also expect the existence of defects caused by the processing/extraction process, mainly in the case of namral fibers, which may be distributed nonuniformly and difficult to quantify. These are... 

It becomes apparent that as the translational symmetry of the chain is removed by the existence of defects, any periodicity is lost and Eq. (3-43) cannot be used. Ratter, Eq. (3-17) must be used which corresponds to the dynamical case of a finite molecule. Since in this case our molecular model is huge and has no symmeti7, the size of the secular equation to be solved becomes extremely large. Moreover, such types of study require the freedom to try many models with different kinds, concentrations, and distributions of defects. Last, but not least, in order to play a game as close as possible to reality in these studies, molecular models must be composed of as many monomer units as possible. 

The orientation of the LC polymer films is usually carried out between treated glass slides. Consequently the orientation process, the existence of defect structures and variations in orientation, as well as boundary effects can be directly visualized by means of polarizing optical microscopy. 

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