Structural components in needle-cokes as studied by etching with chromic acid

Abstract

Structure within calcined and graphitized cokes from a delayed coker is investigated by optical microscopy, etching with chromic acid solution and scanning electron microscopy. The grains of mosaics of a coal extract delayed coke are clearly identifiable. Etching studies of needle-cokes are informative giving the detail of pore wall structure, of the sub-divisions of structure within the macro-needles (1 mm diameter) of needle-coke, i.e. the acicular flow domains, of disclinations within domains and of a layered “sedimentary” structure within domains. The etching characteristics of graphitized needlecokes are quite distinct from those of calcined needle-cokes. In the former, basal planes of acicular flow domains etch into small bundles, 1 to 2 μm in length, with development of fissures every 1 μm apart and some 0.1 μm across. Features observed in the topography of etching may assist in the understanding of how needle-cokes accommodate thermal and mechanical stresses.