Synthesis and Crystal Structure of n-Diamond From Catalyzed Carbon Black

摘要

After pyrogenation of carbon black and nanometer-sized iron catalyst at atmospheric pressure and a temperature of 1100℃ , n-diamond nanometer-particles and diamond-like carbon (DLC) nanocrystals were synthesized. The products were analyzed by x-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), and electron-probe microanalysis (EPMA). The average size of the DLC nanometer powders was found to be about 20 nm. The lattice constant of n-diamond was determined as 0.3594 nm. In addition, the final powder samples were analyzed by XRD after various aging-treatment times. Time-evolutional XRD results indicate that n-diamond is a metastable phase and it decomposes slowly at room temperature. A "defective diamond" model with a fractional occupation site was proposed to explain the time-dependent variations of XRD reflection intensities for the characteristic peaks of n-diamond. In this model, the zero occupancy at the defective vacancy sites corresponds to a face-center-cubic (fcc) crystal, while full occupation leads to a perfect diamond structure. All-electron density functional theory computations further confirm the trend of increasing stability during the evolution from fee to diamond structure within our model. Therefore, we suggest that n-diamond is indeed an intermediate state between the low-density fcc-like structure and the high-density diamond structure.