R. Vander Wal, J. Abrahamson
Penn State University,
Keywords: carbon, laser, TEM, nanostructure, graphitization, additive
Summary:This presentation describes laser heating of carbon materials. The carbon industry has been annealing carbon via traditional furnace heating since at least 1800, when Sir Humphry Davy produced an electric arc with carbon electrodes made from carbonized wood. Much knowledge has been accumulated about carbon since then and carbon materials have become instrumental both scientifically and technologically. However, to this day the kinetics of annealing are not known due to the slow heating and cooling rates of furnaces. Additionally, consensus has yet to be reached on the cause of non-graphitizability. The annealing trajectories with respect to time above temperature from a model graphitizable carbon (anthracene coke) and a model non-graphitizable carbon (sucrose char) are observed at the earliest stages of annealing via rapid laser heating. Materials were heated with 1064 nm and 10.6 µm laser radiation from a Q-switched Nd:YAG laser and a continuous wave CO2 laser, respectively. A pulse generator was used reduce the CO2 laser pulse width and provide high temporal control. Temperatures between 2,427 and the C2 sublimation temperature of 4,184 °C were achieved by varying the Nd:YAG laser energy. The total time above graphitization heat treatment temperatures is 1.5 µs. A maximum temperature of 2,600 °C was achieved via CO2 laser annealing and hold times ranged between 100 ms and five minutes. Furnace annealing of cokes and chars produced from: oxygen containing compounds (polyfurfuryl alcohol and anthanthrone), from a five membered ring containing poly-aromatic hydrocarbon (fluorene), and from sulfur containing decant oil and a blend of anthracene-dibenzothiophene were compared to furnace annealed anthracene coke and sucrose char. A commercial carbon black (R250) and laboratory generated carbon black from benzene and benzene-thiophene were laser and furnace annealed. Cokes and char samples were prepared via carbonization is sealed tubing reactors. The extent of mesophase development was assessed by measuring the materials optical anisotropy with a polarized light microscope. Physical and chemical transformations from annealing were measured with high resolution electron microscopy, energy dispersive X-ray spectroscopy, selected area electron diffraction, and electron energy loss spectroscopy. Virgin samples and traditional furnace annealed samples available in bulk were analyzed with X-ray diffraction. Laser annealed materials were compared to furnace annealed samples held at matching temperatures for one hour. To resolve the detailed morphological and nanostructural changes, high resolution transmission electron microscopy was used to examine the carbons before and after annealing. The potentially enormous technological importance of laser annealing carbon is demonstrated as annealing can be performed continuously and rapidly. Additionally, examples of material processing and synthesis not possible via traditional annealing are provided.