Abstract
In order to accommodate an increasing demand for glassy carbon products with tailored characteristics, one has to understand the origin of their structure-related properties. In this work, through the use of high-resolution transmission electron microscopy, Raman spectroscopy, and electron energy loss spectroscopy it has been demonstrated that the structure of glassy carbon at different stages of the carbonization process resembles the curvature observed in fragments of nanotubes, fullerenes, or nanoonions. The measured nanoindentation hardness and reduced Young’s modulus change as a function of the pyrolysis temperature from the range of 600–2500 °C and reach maximum values for carbon pyrolyzed at around 1000 °C. Essentially, the highest values of the mechanical parameters for glassy carbon manufactured at that temperature can be related to the greatest amount of non-planar sp2-hybridized carbon atoms involved in the formation of curved graphene-like layers. Such complex labyrinth-like structure with sp2-type bonding would be rigid and hard to break that explains the glassy carbon high strength and hardness.
Highlights
The disordered, non-graphitizing glassy carbons, called glass-like carbons, are typically synthesized by pyrolysis of polymeric precursor such as phenolic resins or polyfurfuryl alcohol [1,2,3,4]
The presence of curvature has been attributed to the topological defects in the form of non-hexagonal carbon rings such as pentagons and heptagons that were directly observed by the high-resolution transmission electron microscope (HRTEM) [9]
Bearing in mind the results from HRTEM and Raman spectroscopy which indicate that the structure of glassy carbon at different stages of pyrolysis has features of fullerene-like units, we suggest that the energy loss spectroscopy (EELS) spectra of glassy carbons can resemble the spectra of fullerenes
Summary
The disordered, non-graphitizing glassy carbons, called glass-like carbons, are typically synthesized by pyrolysis of polymeric precursor such as phenolic resins or polyfurfuryl alcohol [1,2,3,4]. Due to their relative ease of production and a diverse range of physical properties, such as high thermal resistance, extreme chemical stability, low density and great hardness compared with other carbons, gases impermeability, and high electrical conductivity, these carbons have been extensively industrially applied since decades. The most recent studies have suggested that glassy carbons have a fullerene-related structure. Even after heat treatment at temperatures of 3000 °C and above the glassy carbons cannot be transformed into crystalline graphite [7, 10] and preserve the general type of atomic disorder remaining features of the paracrystalline structure [12]
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