Novel graphite crystals show exceptional strength, chemical stability for potential use in laboratory equipment and medical devices
The new forms of carbon, known as graphite polyhedral crystals (GPCs), were reported in the Oct. 13 issue of Science by a team of materials scientists from the University of Chicago and the Tokyo Institute of Technology in Japan. The group was led by Yury Gogotsi of Chicago, now at Drexel University in Philadelphia.
According to the account, GPCs are microscopic polyhedrons with shapes that range from needles, rods, rings and barrels to double-tipped pyramids. Their cores consist of carbon nanotubes and their faces are made of graphite. They are also described as exhibiting unusual sevenfold, ninefold, or more complex axial symmetry. The crystals are up to 1 micrometer in cross section and 5 micrometers in length. The authors write that "they can probably be grown in much larger sizes."
The glassy carbon in which the GPCs are embedded is typically made by pyrolysis of three-dimensionally crosslinked polymers. Glassy carbon is known for chemical resistance and is therefore sometimes used in laboratory equipment exposed to corrosive environments. It is also frequently used as an electrode material in polarographic studies. Discovery of the new crystals was achieved by fracturing a glassy carbon sample and examining its pores with a transmission electron microscope (TEM).
What makes the GPCs of more than academic interest is their reported physical properties. The authors of the Science paper write that the crystals appear to have electrical conductivity and optical properties similar to graphite. They appear to be even more chemically stable than the glassy carbon in which they're embedded. In fact, the crystals survive autoclave conditions that cause dissolution of glassy carbon. Nor did the crystals shatter in pieces when the glassy carbon was crushed to study its pore structure, or when hydrothermally treated samples were ground for TEM or Raman spectroscopy studies.
The researchers write they expect GPCs to possess at least the mechanical properties of graphite whiskers along their axis, and probably that of nanotubes. Indentation tests carried out on a microscopic scale show the crystals have hardness values up to 5.8 GPa (compared with 3.2 GPa for glassy carbon and about 1 GPa for graphite). The Young's modulus of portions of the crystals are reported to be more than twice that of glassy carbon. And the polyhedral structure of the crystals makes them more rigid than conventional cylindrical nanotubes. Nanotubes in the cores of the crystals are resistant to bending. Because of this, the researchers note that GPCs could provide "a better reinforcement to composites compared with cylindrical nanotubes, vapor-grown fibers, and whiskers because of their faceted shape." The carbon shells of the crystals cannot rotate relative to each author, the researchers add, which enhances the torsional stability of the crystals.
Because the sizes of the GPCs are controlled by the pore sizes in the glassy carbon hosts, it should be possible to grow the crystals larger by making glassy carbon with larger pores. In fact, it may be possible to grow nanotubes to the size of a pencil, the researchers write, while still maintaining their structural perfection. If this can be done, they conclude, "tailored micro- and macroscopic carbon shapes can be produced with a degree of perfection never seen before."
For more information, contact Yury Gogotsi, Dept. of Materials Engineering, Drexel University, at 215-895-6446, or gogotski@drexel.edu.
By Gordon Graff
Managing Editor, Laboratory Network.com
ggraff@vertical.net