Experimental measurements of defects

The yield strengths of defect-free SWNTs may be higher than that measured for Bacon s scroll structures, and measurements on defect-free carbon nanotubes may allow the prediction of the yield strength of a single, defect-free graphene sheet. Also, the yield strengths of MWNTs are subject to the same limitations discussed above with respect to tube slippage. All the discussion here relates to ideal nanotubes real carbon nanotubes may contain faults of various types that will influence their properties and require experimental measurements of their mechanical constants. 

Some of the defects of the hard-sphere model for the reaction cross section are apparent. It assumes that all the line-of-centers kinetic energy can be used for overcoming the threshold. It does not account for the dependence of Q( , /, j) on the internal states of the reactants. It does not consider any effect due to molecular structure or to the details of the collision process. Improvement requires either direct experimental measurement of g( , Uj) or equivalently, a calculation which takes into account the interaction potential between colliding molecules in specific internal states. 

As part of the same application, the uses of electrochemistry are discussed. Electrochemical investigations can range from generalized studies aimed at understanding the mechanisms of corrosion of a metal in a particular medium to measurements of defect areas of failed specimens from the field. For the latter, defect areas can be masked off from noncorroded areas on the metal surface and numerous electrochemical procedures can then be applied in order to dc-tennine the film porosity and the chemical activity of the region. The studies on Monel 400 alloy corrosion described below represent a comprehensive approach to a corrosion problem. While, by itself, the work cannot claim to have solved" the problem at hand, it has helped to develop confidence in the mechanisms proposed earlier, which were based on much less solid evidence. The experimental approach followed is based on some of the most practical choices facing a scientist or engineer who wishes to employ the techniques of modern surface science and electrochemistry to solve a practical problem. 

Furthermore, the relationships between the transport coefficients have become evident, enabling an interpretation in the atomistic picture. Rate constants of the hopping process, mobility and the diffusion coefficient of the hopping particle (point defect) are closely related parameters. Equation (6.35) emphasizes the importance of the specific conductivity as a transport parameter, which extends beyond its role as a valuable measurement parameter and an electrical material property. Equation (6.32) demonstrates that (close to equilibrium) it is proportional to the equilibrium concentration of the defect under consideration and its mobility. The proportionality to S was exploited extensively in Chapter 5 for experimental verification of defect chemistry. 

While it is possible that surface defects may be preferentially involved in initial product formation, this has not been experimentally verified for most systems of interest. Such zones of preferred reactivity would, however, be of limited significance as they would soon be covered with the coherent product layer developed by reaction proceeding at all reactant surfaces. The higher temperatures usually employed in kinetic studies of diffusion-controlled reactions do not usually permit the measurements of rates of the initial adsorption and nucleation steps. 

From measurements of conductivities, transfer numbers (electro-migration of charged species), lattice constants and experimental densities, it has been shown that Frenkel defects predominate (Lidlard -1957). This means that ... 

It is always easy to calculate idealized scattering curves for perfect networks. The experimental systems vary from the ideal to a greater or lesser degree. Accordingly, any estimate of the correctness of a theoretical analysis which is based on an interpretation of experiment must be put forth with caution since defects in the network may play a role in the physical properties being measured. This caveat applies to the SANS measurement of chain dimensions as well as to the more common determinations of stress-strain and swelling behavior. 

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