On phase changes of carbon under dynamic conditions as observed by STEM

Abstract

A mechanism of diamond formation under dynamic conditions was proposed recently [1, 2]. In its essence, four stages were discussed of nucleation, growth and survival of diamond particles formed by application of ultrashort pressure pulses at very high temperatures to mixtures of graphite and copper or iron. Formation of diamond nuclei via a solid-gas-liquid-solid sequence of phase transitions was suggested. The results of numerous experiments were found to be in satisfactory agreement with the proposed model and are described. elsewhere [1-3] . Many analyses of the recovered samples were carried out with a Philips 300 transmission electron microscope. The resolution, however, of a conventional TEM is not always sufficient to detect very small particles. Therefore, additional work was carried out using a STEM (VG Microscopes Ltd, HB5). The very unique structures observed will be briefly described in this note. Experiments were performed on mixtures of natural graphite with copper and/or iron powder, encapsulated in special sample holders. These mixtures were subjected to very high temperature and pressure pulses [1,3]. The metals were separated from the sample by different techniques [1]. A variety of allotropic carbon forms was found in the remaining part of the sample [1, 3-51. Fig. 1 shows an agglomerate of carbon particles recovered from a typical experiment, and analysed with the aid of a STEM. The coexistence of different carbon forms suggests a complicated transition mechanism with extremely steep pressure and temperature gradients leading to metastable "frozenin" states. Figs. la and b show a spherical structure of turbostratic (carbon black) graphite surrounded by a rim of well-ordered hexagonal grapite, t This common feature can be explained by a mechanism involving a high temperature pulse vaporizing carbon at the surface of the graphite grains and building micropockets of gaseous carbon instantaneously. Upon reaching saturation the gaseous carbon forms liquid droplets at surface sites where favourable nucleation conditions (weak spots [1], etc.) exist. When the temperature pulse decays crystallization of the droplets starts, propagating from the periphery towards the centre. This model is in part corroborated by the findings of Whittaker and Kintner [6]. The existence of grains with p-diamond ([1], Fig. lc), and cubic diamond (Fig. le) structures in close proximity to turbostratic (Fig. lb) and amorphous (Fig. 1 d) carbon indicate very steep pressure gradients (pressure "pockets") over regions of hundreds of nm 2. Moreover, the presence of particles (Fig. l f) with diffraction patterns similar to those of some carbynes [4, 7, 8] renders it possible the carbynes are either formed by reconversion of diamond [1] or are solely high temperature forms of carbon without any pressure contribution [9]. If pressures are low as in rarefied areas, phase transition to diamond is not achieved. Instead, formation of disordered graphite structures is observed. Fig. 2 shows particles in dark field (left, centre) and bright field (right, centre) illumination. The diffraction patterns of Figs. 2b, c and d exhibit (0 0 1) planes of hexagonal graphite, those of Figs. 2a and e show (h k 0) orientation. In conclusion, diamond synthesis by application of flash-heating and shock pressures requires a delicate timing and balancing of the two pulses to adjust the conditions most favourable for nucleation of diamond, and its growth and survival upon