Uptake of gases in bundles of carbon nanotubes

Model calculations are presented which predict whether or not an arbitrary gas experiences significant absorption within carbon nanotubes and/or bundles of nanotubes. The potentials used in these calculations assume a conventional form, based on a sum of two-body interactions with individual carbon atoms; the latter employ energy and distance parameters which are derived from empirical combining rules. The results confirm intuitive expectation that small atoms and molecules are absorbed within both the interstitial channels and the tubes, while large atoms and molecules are absorbed almost exclusively within the tubes.Comment: 9 pages, 12 figures, submitted to PRB Newer version (8MAR2K). There was an error in the old one (23JAN2K). Please download thi

CH 2

Successful growth of diamond by chemical vapor deposition requires that chemisorbed hydrocarbon species, most notably CH2 groups, are able to migrate on the growing surface. Quantum mechanical and hybrid quantum mechanical/molecular mechanical (QM/MM) cluster models are here used to investigate the energetics of CH2 migration on the C{111):H surface and between C{100}:H 2 x 1 terraces separated by a region of C{111}:H surface. Many migration pathways of this type proceeding via structures involving 3-, 4-, and 5-membered rings are found to have relatively low barriers, so that migration should be relatively facile at typical diamond growth temperatures. In contrast, CH2 migration via one particular C{111}:H/C{100}:H 2 x 1 step-edge geometry results in the formation of a very stable 6-membered ring intermediate. The energetics suggest that this process will be irreversible and should thus result in incorporation. This type of step-edge also occurs in the limiting case of two C{100}:H 2 X 1 terraces separated by a monolayer step, and migration of CH2 species along the lower C{100}:H 2 x 1 terrace toward such step edges is predicted to favor incorporation. These findings offer a rationale for the deduced propensity for step-flow growth and the observation of stepped {100} terraces in CVD diamond samples.</p

Democracies, change, sustainability, and transformation: historical perspectives

This article discusses the potential of a historical approach to sustainability transformations. Using environmental issues and governance structures as case studies, it first describes how historical “sustainability transformations” can be conceptualized. It then suggests that 19th-century constitutional reforms can be read as attempts at reaching fiscal sustainability, whereas some social reforms can be interpreted as attempts to render the capitalist economy sustainable. In conclusion, the article highlights that the primary value of historical approaches to sustainability transformations will not lie in models, but in encouraging more creative questions