Pulsed Laser Annealing of Carbon

Author:

Abrahamson, Joseph Peter

Graduate Program:

Energy and Mineral Engineering

Degree:

Doctor of Philosophy

Document Type:

Dissertation

Date of Defense:

May 05, 2017

Committee Members:

Randy Lee Vander Wal, Dissertation Advisor/Co-Advisor
Randy Lee Vander Wal, Committee Chair/Co-Chair
Semih Eser, Committee Member
Jonathan P Mathews, Committee Member
Adri van Duin, Outside Member

Keywords:

carbon
carbonization
graphitization
graphitizable
graphitizability
odd membered carbon rings
laser annealing
microscopy
electron microscopy
carbon annealing
kinetics of graphitization
carbon black
coke
char
mesophase

Abstract:

This dissertation investigates 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. Annealing trajectories with respect to time at temperature are observed from a commercial carbon black (R250), model graphitizable carbon (anthracene coke) and a model non-graphitizable carbon (sucrose char) 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. Time-temperature-histories with nanosecond temporal resolution and temperature reproducibility within tens of degrees Celsius were determined by spectrally resolving the laser induced incandescence signal and applying multi-wavelength pyrometry. The Nd:YAG laser fluences include: 25, 50, 100, 200, 300, and 550 mJ/cm2. The maximum observed temperature ranged from 2,400 °C to the C2 sublimation temperature of 4,180 °C. The CO2 laser was used to collect a series of isothermal (1,200 and 2,600 °C) heat treatments versus time (100 milliseconds to 30 seconds). Laser heated samples are compared to furnace annealing at 1,200 and 2,600 °C for 1 hour. The material transformation trajectory of Nd:YAG laser heated carbon is different than traditional furnace heating. The traditional furnace annealing pathway is followed for CO2 laser heating as based upon equivalent end structures. The nanostructure of sucrose char after 5 seconds of isothermal annealing at 2,600 °C is comprised almost entirely of quasi-spherical closed shell particles that are free of sp3 and oxygen content. With additional time at temperature the particles unravel and propagative particle opening occurs throughout the material. The irregular pore structure found in the end product is a result of particle unraveling. The structures found in heat treated sucrose char believed to contain odd membered rings are not manufactured during the annealing process due to impinging growth of stacks. Thus, odd membered rings are likely present in the starting non-graphitizable char. 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. The majority of initial oxygen content evolved out during low temperature carbonization. The intermediate species formed after oxygen evolution dictated the resulting carbon skeleton and thus the graphitizability. Carbonization of anthanthrone resulted in a graphitizable coke. It is proposed that carbon monoxide loss from anthanthrone results in the formation of perylene. An obvious resemblance was observed in structure between heat treated sucrose and polyfurfuryl alcohol char as compared to heated treated char embedded with 5 membered rings via carbonization of fluorene. Thus, providing evidence that 5 membered rings are present in the virgin chars and are the cause of non-graphitizability. The heteroatom sulfur effects carbon structure in a different way as compared to oxygen. Sulfur is thermally stable in carbon up to ~ 1,000 °C and thus plays little role in the initial low temperature (500 °C) carbonization. As such it imparts a relatively unobservable impact on nanostructure, but rather acts to cause micro-cracks upon rapid evolution in the form of H2S and CS2, upon subsequent heat treatment. Laboratory generated synthetic soot from benzene and benzene-thiophene were Nd:YAG laser and furnace annealed. Furnace annealing of sulfur doped synthetic soot results in cracks and rupturing due to the high pressures caused by explosive sulfur evolution at elevated temperature. Whereas Nd:YAG laser heating of the sulfur doped sample acted to induce curvature. The observed curvature is owed to annealing occurring simultaneously with sulfur evolution. The unset lamellae are strongly influenced by the defect formed upon sulfur evolution. Coke and char samples were prepared via carbonization in 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 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. The potential technological importance of laser annealing carbon is demonstrated as annealing can be performed continuously and rapidly. Examples of material processing and synthesis not possible via traditional furnace annealing are provided.