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Journal Science Intensive Technologies №5 for 2011 г.
Article in number:
RESEARCH OF THE SPECIFIC HEAT IN SMALL Ni AND Cu CLUSTERS ON THE BASIS OF TIGHT BINDING POTENTIAL
Keywords: computer simulation molecular dynamics tight-binding
Authors:
S.L. GAFNER, L.V. REDEL, J.J. GAFNER
Abstract:
The method of molecular dynamics with use of the modified potential of tight binding investigates a heat capacity of Cu and Ni clusters with a diameter from 2 to 6 nanometers in a temperature interval 200-800 К. The obtained data for copper nanoparticles with D = 6 nanometers at Т = 200 К exceed experimental values approximately on 1,8 % that testifies to a correct choice of interaction potential. However at more heats (Т = 300 К) experimental values start to exceed our estimations approximately on 3 % and as it will be possible to approximate from the analysis of data with temperature growth such distinction to increase. In our opinion the main reason of distinction is that in direct experiments was actually defined a thermal capacity not for individual clusters, but for nanomaterial consisting of connected particles. The obtained data testify to a number of laws which are in the consent with analytical calculations. The conclusion has been drawn, that in case of individual free clusters the heat capacity can exceed a specific heat of a bulk material, and the given distinction decreases with the nanoparticles growth to proportionally reduction of a number of surface atoms. So relative reduction of ΔС with growth of particles diameter from 2 to 6 nanometers will be about 23 %, and reduction of a share of surface atoms in this case makes approximately 28 %. It has been noticed, that at Т = 200 К heat capacity of copper cluster with D = 6 nanometers exceeds a specific heat of a bulk on 10 %, and nickel on 13 %. Hence, the considerable value of a specific heat noted in experiments for polycrystalline copper and nickel cannot be connected with characteristics of the free clusters, and are defined by other reasons. In our opinion such properties of nanomaterials can be connected with degree of agglomeration of particles. That is strong influence surfaces can render, interphase borders and increase of root-mean-square displacement of atoms on an incorporated surface connected nanoclusters. Hence, on thermal capacity increase in nanostructures the greatest influence renders occurrence of low-frequency atomic fluctuations with the increasing amplitudes, caused by the increased surface agglomerated clusters and interphase borders in nanomaterials.
Pages: 16-22
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