MECHANISM OF CARBONIZATION UNDER PRESSURE .2. INFLUENCE OF IMPURITIES

Carbonization under pressure (< 2 up to 30 MPa) of natural polymers leads to peculiar carbon microtextures with at least two types of spherical bodies. The formation of such particles depends on the chemical composition of the precursor and of its impurities. Some algae-derived biopolymers have been heat-treated up to 500° and 650°C at 4°C min-1 heating rate in sealed glass tubes (pressure less than 2 MPa) and in gold autoclave tubes (5 MPa and 30 MPa). Morphological and microtextural characterizations were carried out by scanning and transmission electron microscopy, optical microscopy and EDS analysis. In the purest samples (small amount of Si) spheres having a radial microtexture were found. The presence of these spheres is observed from 2 to 30 MPa pressure. As the Si content increases, spherical bodies are replaced by radial shells surrounding small crystals of silica. Traces of phosphor and calcium without silicon promote early carbonization of carbon-black-like particles having a concentric microtexture. The presence of such particles was observed only under 30 MPa pressure. If the P and Ca contents increase and associate with Si, concentric carbonaceous shells surround small crystals of calcium phosphate or silicophosphate. © 1990.