Search

Catalysts based on metals of the iron group (Fe, Ni, Co) and their alloys are used in different methods of carbon nanomaterial production. The selection of optimal regimes for these processes calls for fundamental consideration of processes on the catalyst. An investigation of the distribution and thermally activated redistribution of interstitial carbon atoms in the volume and surface of crystalline films has been carried out. These crystals have a b.c.c. lattice and various types of free facets. The kinetic curves of interstitial atom redistribution under changes of temperature have been studied. Correlation has been made between theoretical calculations and experimental data for the graphitization of Fe–C, Ni–C, Co–C alloys and Fe–C alloy with V, Cu, Ti, Co or Ni impurities. The energetic conditions and possibility of macroscopic surface segregation of interstitial atoms have been established.

Calculations are made of the self-energy of an interstitial C 2 molecule placed between the layers of an otherwise perfect graphite crystal. It is essential to allow for deformation of the layers up to two on each side of the interstitial. The maximum displacement of an atom near the interstitial is about 0.7 A. A carbon-carbon repulsion potential due to Crowell is used. The deformation energy of the layers is calculated by a method due to Coulson and Senent . The self-energy of an interstitial C 2 molecule (i.e. energy difference between the interstitial situation and a perfect lattice with the C 2 molecule carefully removed) is about 4.25 eV. The migration energy of the interstitial molecule is estimated to be about 0.24 eV.

Calculations are made of the self-energy of an interstitial carbon atom placed between the layers of an otherwise perfect graphite crystal. The position of minimum energy is confirmed to be directly above an atom in the one plane, and centrally placed below three atoms in the other plane. It is essential to allow for deformation of the layers up to two on each side of the interstitial. The maximum displacement of an atom near to the interstitial is about 0·6 A. The most uncertain feature of the analysis is the form of the carbon-carbon repulsion potential. Various alternatives are compared, and the choice of a function due to Crowell is made. With this function the self-energy of an interstitial (i.e. energy difference between the interstitial situation and a perfect lattice with the extra atoms carefully removed) is about 2·32 eV, of which about 0·12 eV comes from the deformation of second neighbour planes. The migration energy of an interstitial is estimated to be about 0·14 eV, and comparison is made with available experimental measurements.

A simple model of the effect of interstitial atoms on the interlay er properties of a graphite crystal is derived and compared with experimental data. The analysis yields the derivatives of the interaction energy of an interstitial and the adjacent layers. It,is concluded that interstitial carbon atoms due to irradiation above room temperature do not form covalent bonds with the adjacent layers.