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PP8.02 - Thermodynamics of Diamond Nanocrystal 
Date/Time:
April 24, 2014   3:00pm - 3:30pm
 
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The thermodynamic stability of diamond nanocrystals considered in a number of papers, largely theoretically. Because in nanodiamond (ND) crystal of 4-5 nm size, the fraction of the surface atoms is ~15 %, the main influence to the enthalpy provide the chemical state of the surface atoms . Experimental data show enhanced ND enthalpy to 30.72 kJ / mol measured through combustion, far exceeding the difference for bulk diamond and graphite ( 1.8 kJ / mol) [1]. In this paper, we attempt to estimate the diamond nanooctahedra standard enthalpy in the approximation to the C - C, C = C and C -H bond energies. Obviously, the highest value of the enthalpy should have a diamond nanocrystals with unreconstructed (111 ) faces covered with broken ( ‘dangling’ ) bonds, ≡ C. After the Pandey surface reconstruction the C = C bond formation take place with markedly enthalpy decrease. Finally, it is possible to consider unreconstructed monohydride (111 ) faces with a monolayer of C-H bonds formation with next dramatic enthalpy decreasing. In all cases, the energy of the atoms at the vertices and edges of the crystals was not considered. It is easy to show that the deviation of the considered nanocrystals from bulk diamond energy will increase with a decrease in the size a - the length of the edge of the octahedron. Assuming that the excess energy of the surface atoms on the unreconstructed (111 ) is 177.82 kJ/ mol, we can show that for an octahedron with a = 5 nm have excess relative the bulk diamond crystal enthalpy amounted 28.1 kJ/mol. Chemisorption of hydrogen provides significant reduction in surface energy and further reduction of the diamond nanocrystals enthalpy. Abovementioned looks as interesting and helpful approach to the size ranges consideration between higher diamantoids and diamond nanoparticles. Moreever, according to many CVD diamond studies the hydrogen plays an important role in the diamond nucleation, because a significant decrease in the sub-nm crystals enthalpy and their surface energy takes place.REFERENCES[1]. G.V.Sakovich, et al. Detonation nanodiamond. Synthesis. Properties. Application. Science and Technology in Industry. 2011. # 4. PP. 53-61.
 


 
 
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