Studies of Carbon Phase Transitions by the Extended Bond Order Potential

by
Jianwei Che*, Tahir Ç agin, and William A. Goddard, III

Materials and Process Simulation Center,
California Institute of Technology
,
Pasadena, CA, 91125


Abstract

The Generalized extended empirical bond order dependent potential (GEEBOD) is used to study the transitions among different phases of solid state carbon system. This force field builds upon the Brenner FF, but allows the nonbond terms t o build in smoothly as bonds are broken and formed. We are applying this to various phase transitions in carbon.

We find that the diamond to graphite transition (thermodynamically exothermic ) is achieved in Molecular Dynamics (MD) simulations by simply increasing the temperature (to 3000 K). We studied the time scale for this transition and its pressure dependence. As seen from figure 1, these MD simulations show that the transition occurs first at the surface and then gradually penetrates into the diamond bulk phase.

However, the reverse transition, from graphite to diamond, occurs only under more drastic conditions. To study this process we used dynamic shock-wave loading MD. The results show that the transition has an orientational preference. We monitored as a function of time the number of atoms that changes hybridization and find that the transformation depends on shock-wave speed as shown in figure 2.

We also examined the result of heating a one-nanometer cube of diamond crystal rapidly (2000K in one picosecond). We find a rapid transformation in which the surface atoms form chains and sheets, leading to a multiwall fullerene within 500 ps.

The accuracy of the GEEBOD potential for describing fullerene formation is assessed by comparing with DFT calculations.

Acknowledgement: This research was funded by a grant from DOE-ASCI. The facilities of the MSC are also supported by grants from NSF (ASC 92-17368 and CHE 91-12279), ARO (MURI), ARO (DURIP), ONR (DURIP), Chevron Petroleum Technology Co., Asahi Chemical, Owens-Corning, Exxon, Chevron Chemical Co., Asahi Glass, Chevron Research Technology Co., Avery Dennison, BP America, and Beckman Institute.


Figure 1. The process of transition from diamond to graphite at 3000 K.

Figure 2. The number of 4 coordination and 3 coordination C atoms as a function of time during shock-wave simulation at 3 different shocking speed.



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