Direct observation of the mesophase in coal

Thin sections of vitrinite from Pittsburgh seam, Hazard No. 4, Illinois No. 6, and Lower Kittanning coking coals were ion-thinned to electron transparency and examined in a Philips EM300 transmission electron microscope. A heating stage was used in the microscope to observe their transformation to semicoke. Before heating, the vitrinite samples showed little structure, except for the Hazard No. 4, which contained pre-existing dark spots. Upon heating, small dark spots began to appear, growing in size and number with increasing temperature. These spots were identified as liquid crystals (mesophase). As heating continued, the mesophase in the Pittsburgh seam vitrinite coalesced into irregular units about 300 nm in diameter. In the Hazard No. 4 vitrinite, coalescence also occurred but at a slower rate. In the lllinois No. 6 vitrinite, only scattered spots of mesophase appeared. They grew to about 100 nm but did not coalesce. In the Lower Kittanning vitrinite, the mesophase formed at a higher temperature and grew at a slower rate than in the other samples. After one hour at the maximum stage temperature, small rods appeared, and their growth was easily observed with the electron microscope. Electron diffraction patterns obtained while the specimens of the four coals were hot consisted of diffuse rings. The observations in the electron microscope at elevated temperatures confirm the existence of the mesophase in the coal-to-coke transformation. The relation between the observed mesophase formation and the coking properties of the coals is discussed.