Effects of heat treatment on the structure of LDPE-derived solid carbons

Heat treatment of spherical solid carbons produced by pyrolysis of low-density polyethylene (LDPE) gave faceted particles with spherical geometry. The changes in the structure of the heat-treated (HT) carbons in the temperature range 1300–2600 °C were characterized by elemental analysis, X-ray diffraction (XRD), temperature-programmed oxidation (TPO), Raman spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) which was also used in conjunction with the focused ion beam (FIB) thinning. The HT carbons gave interlayer spacing (d0 0 2) values close to that of graphite, however, the maximum values of La and Lc (around 20 nm) were significantly lower than those of graphite. The TPO profiles showed that the HT carbons contain domains with different structural order and that the contribution of the more ordered domains increases with the increasing treatment temperature. The SEM and TEM examination showed faceting of the initial spherules and FIB analysis showed concentric carbon aromatic layers in a core shell structure. The extent of ordering in the coherent domains increased in the radial direction suggesting that the growth of crystallites was constrained by the size of the initial carbon spherules.

► Incipient structural ordering achieved during pyrolysis of LDPE leads to better aptitude for graphitization of the starting carbons.
► Graphitization takes place through progressive development of multiple domains of different structure that evolve to more ordered domains.
► LDPE derived carbons exhibit an easy development of two-dimensions ordering and incipient three-dimensions ordering at 2600 °C.
► Interlayer spacing values equivalent to that of graphite can be achieved but the spherical shape of starting solids constrains crystal growth.
► The high temperature carbons show concentric layers with a core and shell structure.
► The core of the high temperature carbons contains multiple nuclei and the shell area shows larger crystals with increasing stacking and size of coherent domains in radial direction.