Experimental study of the microtextural and structural transformations of carhonaceous materials under pressure and temperature

A coal and two reference synthetic-cokes, a lamellar graphitizing anthracene-based coke (AC) and a microporous nongraphitizing saccharose-based coke (SC), were held at pressures up to 8 GPa and temperatures up to 1473 K under dry and hydrous conditions. Their subsequent structural and microtextural modifications were characterized by high-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy (microspectroscopy and area-mode spectroscopy). No significant transformation was observed in products held up to 95 hours at temperatures below 1273 K. At 1273 K and above, for constant run duration, the higher the pressure, the better is the carbonaceous material organization. The effect of pressure on the graphitization mechanisms is most obvious in the case of the non-graphitizing SC for which the main role of pressure is to transform the microporous microtexture towards a lamellar one. Graphitization is initiated in pore walls as a result of pore growth and thus appears as a local phenomenon. Triperiodic graphite was detected by HRTEM in all SC samples synthesized at 2 GPa and above (1273 K, 95 hours). In the case of the AC material, microtexture remains lamellar and the graphitization results in the reorientation and the in-plane growth of the aromatic layers and rather appears as a bulk phenomenon. The evolution of the coal is intermediate between the two synthetic cokes, as this precursor was microtexturally heterogeneous, composed of an intermediate microtexture between lamellar and microporous ones. Graphitization under pressure appears to be a progressive and continuous process that proceeds heterogeneously through the carbonaceous matrix. The products recovered from high-pressure experiments are structurally and microtexturally heterogeneous, and this heterogeneity raises important problems with respect to the characterization scale and transformation rate in the experimental products. Indeed, in comparison to graphitization temperature in natural samples, it is expected that longer run duration would have led to triperiodic graphite under most experimental conditions achieved here. However, our experimental results show that pressure mainly speeds up the graphitization process, or even makes it possible in the case of the SC, by inducing microtextural and subsequent structural transformations. In all cases, whatever the pressure, temperature defines the highest structural state.