Dynamic Properties

Carbon nanocapsules containing nanometer-size particles of molybdenum carbide were irradiated by neutrons of flux 1014 N cm-2 s—1 for 6 days [638]. The y-ray spectrometry showed 1010 Bqg-1 of Mo-99 transformed from Mo-89 and decayed into Tc-99. The TEM investigation indicated that the nanocapsule was robust against the neutron irradiation and was used as a container for radioactive materials and tracer elements. The contents of the capsule in the nanometer scale were activated and transformed into other nuclei.

A reactor chamber was developed for the secure handling of radioactive materials [639]. Using the arc-discharge method for fullerene synthesis, the carbon encapsulation capability of uranium was tested. The uranium was encapsulated as a dicarbide with two types of carbon covers: graphene sheets parallel to the external surface and randomly oriented carbon crystallites. The covers with parallel sheets of graphite were present with two different interlayer spacings. One corresponded to polyhedral particles with an interior gap between the kernel and capsule. The other consisted of rounded-vertex particles with no gap and with interlayer spacing between that of graphite and that corresponding to polyhedral nanocapsules. Extensive studies were described for the resolution of carbon structures and a transformation mechanism was proposed for the interpretation of rounded-vertex onionlike structures. Closed carbon covers chemically protected the actinide kernel and provided safer handling and processing of toxic materials for medical and nuclear purposes.

In-situ and ex-situ TEM observation was performed in copper implanted with carbon ions at temperatures from 570 to 973 K [640]. Carbon onions (concentric graphitic spheres) and nanocapsules (concentric graphitic spheres with cavities) were observed together with amorphous carbon layers. Statistics of cluster size as a function of ion fluence, implantation temperature, and substrate crystallinity gave insights into the nucleation processes of onions and nano-capsules. One was the formation of graphitic layers on grain boundaries to encapsulate copper particles. The other was the nucleation of graphitic cages, probably fullerenes, due to both a high concentration of carbon atoms and a high amount of radiation damage. Simultaneous observation of microstructural evolution under implantation revealed that onions were formed inside the substrate, not the surface, and that they segregate at the surface due to radiation-enhanced evaporation.

A scanning tunneling microscopy study of carbon nano-capsules (onions) was reported [641]. Spherulitic graphite was shown to be purely crystalline graphite based on XRD and TEM studies. Carbon nanocapsules grown on pitch-based carbon fibers were investigated by HRTEM [642]. Two types of nanocapsules were formed during heating up to 2000 °C in 1 atm N2 gas: one was a capsule enclosing a CaS single crystal and the other was a cubic hollow capsule. The growth mechanism of these carbon nanocapsules was discussed by comparing the structures of particles formed at several temperatures.

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