Defects and vacancies

Fig. 16.2 TEM image of a CNT with indications of topological defects and vacancies. Reproduced with permission from Wiley VCH (2011) [7].

Keywords carbon nanotubes, defects and vacancies, geometry and electronic structures, semi-empirical quantum calculations. 

In general terms, Che role of foreign ions is easy Co explain. Thermal reorganisation involves diffusion via defects and vacancies [10]. If foreign ions occupy these sites, the rate of sintering will be affected. Alternatively, if foreign ions react with the host lattice to form a new chemical compound, then the reorganisation of Che new compound will occur at a different rate. 

By radiation-process of the fabrication, the decomposition gases are evolved from PTFE matrix and the mass decreases with increasing EB dose. The evolved gases will induce the micro defects and vacancies. In the case of RX-PTFE... 

X-ray diffraction studies proved that DMC catalysts, similar to many nonstoichiometric chemical substances, have many defects and vacancies in the crystalline structure [62]. These defects and vacancies are very strong co-ordination points. The oxiranic monomer is strongly co-ordinated by these centres at the oxygen atom and is transformed in a much more reactive structure than the uncoordinated monomer. This activation of the monomer by co-ordination explains the very high catalytic activity of DMC catalysts. 

Point defects can drastically lower the thermal conductivity of the important carbide and nitride high thermal conductivity ceramics. In this respect, oxygen, which is a common impurity, has been found to be very important. For example, silica (Si02), an impurity in silicon nitride (Si3N4), formed by oxidation at high temperatures in air, can react to produce substitutional defects and vacancies in the following way ... 

In Eq. 11.23, Z is the number of distinct orientations of the defect complex it accounts for the configurational entropy. For example, if the vacancies sit on adjacent sites in NaCl, Z would be 12. Figure 11.6 shows calculated concentrations of Schottky defects and vacancy pairs in NaCl and MgO. 

Variations in composition and the presence of defects and vacancies may considerably alter the properties and behavior of these materials. This is reflected by the high spread of values found in the literature. In order to be meaningful, a property should be associated with the composition of the carbide being tested this is often not the case and, although a great deal of information is available, the reported values are sometimes questionable. The data listed in the following chapters must be viewed with this in mind. 

Figure 3.4. Structure of turbostratic graphite. Note lattice defects and vacancies.

Crystallite boundaries, lattice defects, and vacancy sites... 

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