The binding energy, surface energy, surface atom coordination numbers, and bond lengths and angles of small clusters at low temperatures are calculated with parameters fitted to Ni using both the Lennard-Jones (LJ) potential and the embedded atom (EA) potential to assess the ~ensitivity of. interatomi~ potential on cluster structure. Simulations are performed by ImplementatlOn of the Simulated annealing method in a canonical ensemble Monte Carlo tec~nique. We e~amine clusters with n<;;;34 atoms and we find that they are noncrystalline (with the exceptlOn of n = 6). The most stable structure of clusters consisting of n <;;; 15 atoms and n = 19 atoms is the same for both potentials (with the exception of n = 8). However, the most s~able structure of clusters with n;:;. 16 atoms is different for the two potentials (with the exception of n = 19). Smeared angular distribution and pair distribution functions are found for many EA clusters whereas sharp, well defined peaks exist for LJ clusters. A discontinuous transition from polyicosahedral to quasicrystalline structure is found from n = 30 to n = 31 atoms for t~e l.! potential. This transition occurs at smaller n for the EA potential. Surface atom coordmatlOn numbers are found to be nonmonotonic functions of cluster size. The existen~e 0: multiple structures of small clusters and the effect of quenching rate during crystalhzatIon on the final shape of clusters are also examined.
The binding energy, atom coordination numbers, bond lengths, surface restructuring, and bulk melting behavior of small clusters versus temperature are compared for the Lennard-Jones (LJ) potential and embedded atom (EA) potential using the Monte Carlo method with parameters fitted to Ni. We find that EA clusters are more thermally stable than LJ clusters with regard to evaporation. For small clusters whose minimum energy structure is polyicosahedral, a smooth change of physical properties with temperature is observed for both potentials. However, for clusters whose minimum energy structure is quasicrystalline, a structural phase transition analogous to a first order transition of bulk materials can be found for both potentials. This structural phase transition is manifested by discontinuous changes of atom coordination numbers and bond length, and in some cases, of energy. Implications of this transition in catalysis are discussed. Isomerization between minima of each one of the two potential hypersurfaces along with its dependence on temperature are examined. The many body nature of the EA potential results in lower isomerization probabilities between EA isomers as contrasted to LJ isomers.
Approved for public release; distribution unlimited. ii REPORT DOCUMENTATION PAGEForm Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing the burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. REPORT DATE (DD-MM-YYYY)November 2008 ARL-TR-4642 SPONSOR/MONITOR'S ACRONYM(S) 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) SPONSOR/MONITOR'S REPORT NUMBER(S) DISTRIBUTION/AVAILABILITY STATEMENTApproved for public release; distribution unlimited. SUPPLEMENTARY NOTES ABSTRACTBased on Density Functional Theory -Generalized Gradient Approximation (DFT-GGA) calculations, we provide a theoretical model for the effect of the catalytic support (alpha alumina oxide (αAl 2 O 3 )) on the dissociation of molecular hydrogen (H 2 ), molecular oxygen (O 2 ), hydroxyl (OH), water (H 2 O), and the surface diffusion of oxygen and hydrogen species along the αAl 2 O 3 (0001) surface. These processes are key to understanding the "inverse spillover effect" that occurs during hydrogen combustion on alumina surfaces. Our results indicate the dissociation of O 2 is not thermodynamically favored on the αAl 2 O 3 surface. However, both H 2 and H 2 O can dissociate, forming hydroxyls with oxygen atoms in the second atomic layer. Once dissociated, oxygen species can diffuse locally but encounter a large barrier to long-range surface diffusion in the absence of defects or other species. In contrast, the barrier to the long-range surface diffusion of hydrogen is modest under ideal conditions.
Approved for public release; distribution unlimited. Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing the burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. ii REPORT DOCUMENTATION PAGE
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