Liquid Carbon

Liquid carbon refers to the liquid phase of carbon resulting from the melting of pure carbon in a solid phase (graphite, diamond, carbon fibers or a variety of other carbons). The phase diagram for carbon shows that liquid carbon is stable at atmospheric pressure only at very high temperatures (the melting point of graphite Tm ~ 4450 K) [4]. Since carbon has the highest melting point of any elemental solid, to avoid contamination of the melt, the crucible in which the carbon is melted must itself be made of carbon, and sufficient heat must be focused on the sample volume to produce the necessary temperature rise to achieve melting [8,51]. Liquid carbon has been produced in the laboratory by the laser melting of graphite, exploiting the poor interplanar thermal conductivity of the graphite [8], and by resistive heating [52], the technique used to establish the metallic nature of liquid carbon.

Although diamond and graphite may have different melting temperatures, it is believed that the same liquid carbon is obtained upon melting either solid phase. It is likely that the melting of carbon nanotubes also forms liquid carbon. Since the vaporization temperature for carbon (~4700K) is only slightly higher than the melting point (~4450K), the vapor pressure over liquid carbon is high. The high vapor pressure and the large carbon-carbon bonding energy make it energetically favorable for carbon clusters rather than independent atoms to be emitted from a molten carbon surface [53]. Energetic considerations suggest that some of the species emitted from a molten carbon surface have masses comparable to those of fullerenes [51]. The emission of fullerenes from liquid carbon is consistent with the graphite laser ablation studies of Kroto, Smalley, and co-workers [10].

Resistivity measurements on a variety of vapor grown carbon fibers provided important information about liquid carbon. In these experiments fibers were heated resistively by applying a single 28 current pulse with currents up to 20 A [52]. The temperature of the fiber as a function of time was determined from the energy supplied in the pulse and the measured heat capacity for bulk graphite [54] over the temperature range up to the melting point, assuming that all the power dissipated in the current pulse was converted into thermal energy in the fiber. The results in Fig. 8 for well-graphitized fibers (heat treatment temperatures Tht = 2300°C, 2800°C) show an approximately linear temperature dependence for the resistivity p(T) up to the melting temperature, where p(T) drops by nearly one order of magnitude, consistent with metallic conduction in liquid carbon (Fig. 8). This figure further shows that both pregraphitic vapor grown carbon fibers (Tht = 1700° C, 2100°C), which are turbostratic and have small structural coherence lengths, and well graphitized fibers, all show the same behavior in the liquid phase, although their measured p(T) functional forms in the solid phase are very different. Measurements of the resistance of a carbon nanotube through the melting transition have not yet been carried out.

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Fig. 8. The electrical resistivity vs temperature for vapor grown carbon fibers with various heat treatment temperatures (Tht = 1700, 2100, 2300, 2800° C). The sharp decrease in p(T) above ~4000K is identified with the melting of the carbon fibers. The measured electrical resistivity for liquid carbon is shown [52]

Fig. 8. The electrical resistivity vs temperature for vapor grown carbon fibers with various heat treatment temperatures (Tht = 1700, 2100, 2300, 2800° C). The sharp decrease in p(T) above ~4000K is identified with the melting of the carbon fibers. The measured electrical resistivity for liquid carbon is shown [52]

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