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Molecular Dynamics Simulation of Nano-Diamond Synthesis by Shock Wave

Mahnama, Maryam | 2013

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 45010 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Naghdabadi, Reza; Movahhedy, Mohammad Reza
  7. Abstract:
  8. In the field of high-pressure material science, diverse carbon systems under pressure have been intensively studied with interest in synthesizing new phases. A variety of these synthetic phases which have met various applications in today technology are called the amorphous diamond. The pressure-induced structural transition of carbonaceous material to amorphous diamond is realized by shock compression and rapid quenching. The shock compression and rapid quenching generate the high pressure (several GPa) and the temperature (several hundred K) in a fraction of a microsecond.Since the mechanical and electrical properties of the synthetic diamond are severely sensitive to the atomic structure, the synthesis process should be well controlled to acquire any desired property from the diamond according to its specific application. Therefore, the simulation of the phase transformation from fullerene into diamond has been considered in the last decade. These studies focus on the simulation of fullerenes (C60 or C70) melting and its quenching with the aid of quantum mechanics schemes. In the present work, the atomic interactions of carbon atoms in fullerene C60 under shock compression is considered as a mean to detect the phase transition and material structure. In this study, the molecular dynamics (MD) simulation with AIREBO potential function is employed to discover the phenomena during shock propagation in the fullerene C60.One of the important issues in this MD simulation is the modeling of the quenching process during the shock compression, which affects the structural stability of the resultant phase. In the present work a new NVΔT ensemble is developed to model the heat transfer during shock compression. Moreover, the constant-strain and constant-stress Hugoniostats with some changes have been employed. Utilizing above approaches, the MD simulation results show close conformity with the experimental data in the literature.Modifying the constant-stress Hugoniostat, the Hugoniot curve is obtained in a single simulation run for the first time. Having this curve along low-index crystal orientations of fullerene, different material behavior along these directions is obvious. The Hugoniot curve in each orientation shows the Hugoniot elastic limit and phase transition. The amorphous structure of the resultant diamond phase is concluded from the overlapping of the Hugoniot curves in low-index directions after the phase transition.Also, the radial distribution function, angel distribution function, cohesive energy and nearest neighbor distribution are employed to detect the new phase characteristics. The obtained results show a tetrahedral structure for the carbon atoms in amorphous diamond, which its high content is the measure of structure hardness. The obtained results show a high fraction of carbon atoms in sp3 hybridization (up to 90%). Moreover, the effect of cooling rate parameter and crystal orientation on the sp3 fraction in amorphous diamond is investigated. The results show that the <100> direction and the cooling rate of about 1013 K/s is the best for the structure hardness.

  9. Keywords:
  10. Molecular Dynamic Simulation ; Shock Compression ; Fullerene C60 ; Amorphous Diamond ; Hugoniot Curve

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