Exposing amorphous 'solid' carbon nanofibers (CNFs) to spark plasma sintering (SPS) converts their tips to diamond at about 1500 °C and atmospheric pressure, but the physics and chemistry of this low-pressure conversion are unexplained. The open problem is to establish the physical mechanism by which the pulsed-DC spark plasma induces sp3 carbon bonding along the observed onion-intermediate pathway and to determine whether the residual iron particles in flame-grown CNFs catalyze the conversion or are merely reduced bystanders.

Background

Diamond is the sp3-bonded (tetrahedral) allotrope of carbon, thermodynamically metastable relative to the sp2-bonded (trigonal, graphite-like) allotrope at ambient pressure. Converting sp2 carbon to diamond conventionally requires high pressure and high temperature (HPHT, the classic industrial route since 1955) or metal-catalyzed HPHT synthesis; chemical vapor deposition (CVD) grows diamond metastably from gas phase at low pressure but only on hot substrates and slowly. A distinct family of experiments converts curved or disordered sp2 nanostructures directly to diamond: Banhart and Ajayan (Nature, 1996) showed that carbon nano-onions — concentric fullerene-like graphitic shells — under intense electron irradiation act as nanoscopic pressure cells whose shells collapse inward to nucleate diamond; carbon nanotubes were later converted by laser irradiation, shock waves, and hydrogen plasma. Against this background, Zhang, Shen, Sun, Zhu, Wang and McCartney (Carbon, 2005) and Shen, Zhang, Sun, Zhu and McCartney (Nanotechnology, 2006) showed that spark plasma sintering converts CVD carbon nanotubes to diamond at very low pressure, and the Rostock/China line of Zhang, Ahmed, Bednarcik/Holzhuter and Burkel extended this to fullerene C60 (J. Crystal Growth, 2012) and argued that diamond forms 'through the generation of plasma' during SPS (Phys. Status Solidi A, 2012), a claim at the level of interpretation rather than an established mechanism. Spark plasma sintering is a densification technique in which a pulsed direct current (typically thousands of amperes, low voltage) passes through a graphite die and the powder compact, producing internal Joule heating and, in the proponents' picture, transient local micro-plasma discharges between particles; separating the thermal, electrical-field, and any genuine plasma effects is intrinsically difficult because no probe directly observes the particle-contact scale during a pulse. Luo, Qi, Pan and Yang (Scientific Reports 5, 13879, 2015) — the source of this problem — converted ethanol-flame-grown 'solid' (amorphous-cored, iron-catalyst-containing) carbon nanofibers to well-crystallized diamond at 1500 °C at atmospheric pressure in 5 minutes of SPS. Using electron microscopy, electron-energy loss spectroscopy (EELS, which distinguishes sp2 from sp3 bonding via the 284 eV pi* and 292 eV sigma* edges), X-ray diffraction and Raman spectroscopy (diamond line at 1332 inverse centimeters), they proposed a structural sequence — disordered CNF to crystallized CNF to bent graphitic sheets to onion-like rings to diamond — and stated two explicit uncertainties: the plasma's precise role is a belief, not a demonstration, and, in their words, 'we do not know whether the iron catalyst particles played a catalysis role on the conversion from the CNFs to diamond during SPS treatment'. X-ray diffraction showed the iron oxide catalyst residues were reduced to body-centered-cubic iron during the run, which is consistent with either a catalytic or a bystander role. They also reported that pure well-crystallized CVD carbon nanotubes failed to convert under identical treatment, so feedstock microstructure matters. The problem stated below is the still-unresolved mechanistic core of that report.

Problem Statement

For the spark-plasma-sintering conversion of amorphous 'solid' carbon nanofibers to diamond at roughly 1500 °C and atmospheric pressure (Luo et al., Scientific Reports 5, 13879, 2015): (1) determine the physical mechanism by which the pulsed-DC spark plasma induces sp3 bond formation along the observed pathway (disordered carbon nanofiber, crystallized nanofiber, bent graphitic sheets, onion-like rings, diamond) — that is, identify which of the candidate drivers (local Joule heating and thermal gradients, transient micro-plasma chemistry, electrostatic field or current effects, local stress at curved onion shells) dominates the sp2-to-sp3 conversion and quantify its contribution relative to the other candidates, sufficiently to predict under what SPS parameters and feedstock microstructures conversion occurs; and (2) determine whether, and to what extent, the residual iron particles in flame-grown carbon nanofibers catalytically assist this conversion or are bystanders that are merely reduced to metallic iron during the run (for example by controlled experiment or by a mechanistic argument). The task is limited to these two sub-questions; large-scale synthesis is out of scope.

References

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