Catalytic graphitization of three-dimensional wood-derived porous scaffolds
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A catalytic technique to enhance graphite formation in nongraphitizing carbons was adapted to work with three-dimensional wood-derived scaffolds. Unlike many synthetic graphite precursors, wood and other cellulosic carbons remain largely disordered after high temperature pyrolysis. Using a nickel nitrate liquid catalyst and controlled pyrolysis conditions, wood-derived scaffolds were produced showing similar graphitic content to traditional pitch-based graphite while retaining the high-aspect ratio pores of the precursor wood microstructure. Graphite formation was studied as a function of processing time and pyrolysis temperature, and the resulting carbons were analyzed using x-ray diffraction, Raman spectroscopy, x-ray photoelectron spectroscopy, and electron microscopy techniques.
I. INTRODUCTION
Wood-derived scaffold structures have received recent attention due to the advantageous thermal and structural properties that result from the retention of the naturally optimized open porous microstructure of the precursor wood.1–3 While previous efforts have focused primarily on the development and characterization of biomorphic silicon carbide from wood,4–10 biomorphic graphitic scaffolds are of particular interest due to the attractiveness of porous graphite for use in thermal management devices.11,12 Graphitic scaffolds combine low density, low thermal expansion coefficient, and high thermal conductivity, making them ideal candidates for many thermal management applications.13 A common graphite precursor used for the fabrication of commercial porous graphitic scaffolds is petroleum pitch, which, like several other petroleum-based carbons, is graphitizable upon heat treatment to temperatures ranging from 1700 to 3000 °C.11,14 When wood carbon is exposed to a similar graphitization heat treatment, turbostratic carbon is produced.15,16 Unlike graphite, where carbon layers have well defined stacking and a regular planar spacing (3.354 Å), turbostratic carbon is composed of misoriented carbon layers with a distribution of planar spacings greater than that of ideal graphite.17 Due to the formation of turbostratic carbon under graphitization heat treatment conditions, wood and other cellulose-based materials are considered to be nongraphitizable or hard carbons.15–18 To enhance ordering in nongraphitizable carbons, catalytic treatments have been developed that utilize Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2010.88 J. Mater. Res., Vol. 26, No. 1, Jan 14, 2011
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II. EXPERIMENTAL METHODS A. Processing of graphitic scaffolds
Wood-derived graphitic scaffolds were produced from two species: red oak (Quercus rubra) and beech (Fagus sylvatica). Samples were first cut into cubes measuring 2 cm per side, then allowed to dry thoroughly in an oven
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transition metal elements such as iron,19,20 cobalt,19,21 nickel,19–21 and copper22 to promote the formation of graphitic carbon at pyrolysis temperatures as low as 500 °C.19 Using the excess free en
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