Kinetics of the graphitization of dispersed diamonds at "low" temperatures

The bulk density of graphitized ultradisperse diamond (UDD) was measured by a gamma-ray attenuation method at 1370-1870 K. These data combined with sm all angle x-ray scattering and true density measurements of the samples hea ted at various fixed temperatures were used to study the graphitization kin etics of the UDD. The reaction rate was modeled as a migration rate of the interface between the developing graphite-like carbon and the remaining dia mond phase. A "reducing sphere" model was used to obtain the rates from the changes in densities. The estimated kinetic parameters in an Arrhenius exp ression, namely the activation energy, E = 45 +/- 4 kcal/mol, and the pre-e xponential factor, A = 74 +/- 5 nm/s, allow quantitative calculations of th e diamond graphitization rates in and around the indicated temperature rang e. The calculated graphitization rates agree well with the graphitization r ates of diamonds with different dispersity estimated from high-resolution t ransmission electron microscopy data. The large difference between the rate s and the kinetic parameters obtained in this study and those estimated by G. Davies and T. Evans [Proc. R. Soc. London 328, 413 (1972)] for the tempe rature range 2150-2300 K indicates that there are different graphitization mechanisms operating in the "low" and "high" temperatures regions. (C) 2000 American Institute of Physics. [S0021-8979(00)07419-3].