Frontiers article
The liquid state of carbon

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Abstract

While carbon provides the basis for all of organic chemistry and biology, elemental carbon compounds themselves play vital roles in our world. Graphite, diamond, fullerenes, graphene, and amorphous carbon solids are used in myriad modern applications, and a number of new, metastable states of carbon with unusual and promising properties have recently been proposed and created. Furthermore, liquid carbon and warm dense carbon are of astrophysical interest, and have been proposed to exist in the cores of large planets and stars (Ross, 1981) [1].

While much is known about the solid forms of carbon, its liquid state remains very poorly understood. Due to difficulties in preparing and studying this material, its structure and properties remain essentially unknown. In this review, we describe recent technological advances which have enabled new experiments on liquid carbon samples, prepared with laser melting and shock compression techniques.

Introduction

The unique properties exhibited by the known allotropes of carbon make this element essential in an increasingly wide variety of industrial and technology contexts, ranging from industrial lubricants [2], [3]) and abrasives [4], to nuclear reactor moderators [5], and to the anodes in lithium ion batteries [6]. The economic importance of carbon technologies is only expected to grow, as newly-emergent carbon materials are applied in modern technology contexts; for example, the market for advanced carbon materials (graphene and carbon nanotubes, foams, fibers) is expected to reach ca. $13 billion by 2025 [7], a growth of ca. 10% in only a few years. This appeal of carbon materials for technology applications derives from the broad array of properties exhibited by carbon allotropes; these, in turn, are engendered by the ability of carbon to form single, double, and triple covalent bonds. This unmatched flexibility in bonding motifs endows carbon materials with a multitude of available atomic structures with widely varying mechanical and chemical properties. By varying the bonding in the solid, these properties can be tuned for a specific application.

As would be expected for such a critical class of materials, a great deal of effort has been devoted to studying the fundamental properties of carbon in its many forms. Beyond the common graphite and diamond allotropes, which have been extensively investigated [8], [9], [10], [11], newer solid carbon materials and nanostructures like nanotubes, graphene, and fullerenes, have increasingly received both experimental and theoretical attention [12], [13], [14], [15]. The properties of the carbon vapor phase have likewise been carefully analyzed via plume studies of ablated carbon plasmas [16], [17] and through spectroscopic studies of various Cn chains [18].

Unlike the solid and gaseous states, liquid carbon has received comparatively little attention, and our understanding of its properties accordingly remains limited. This lack of research on the liquid phase emanates from the difficulty in both preparing and interrogating the liquid state. In fact, as we discuss in detail later, carbon in its liquid form exists only under extreme temperatures and pressures, like those found in planetary and stellar cores. As such, it is very difficult to generate samples of the liquid in the laboratory under equilibrium conditions. Due to that impediment, coupled with the fact that the liquid state is seemingly irrelevant in terrestrial chemistry and physics, the liquid has largely been ignored in modern carbon research. Despite this esoteric nature, it has become increasingly apparent that there are important fundamental and practical reasons to address this knowledge gap, and thus to advance our understanding of liquid carbon properties. This article describes the efforts made thus far to that end.

Section snippets

The carbon phase diagram

As can be seen from its position on the phase diagram shown in Fig. 1, producing carbon in the liquid state presents a formidable challenge. The liquid phase of carbon is favored only under extreme conditions, requiring a temperature of ~5000 K and pressures of ca. tens of megapascal to produce an equilibrium sample. The complexity of achieving these conditions is the chief impediment to studying liquid carbon. Known container materials cannot survive such extreme conditions; hence, maintaining

Why study liquid carbon?

As liquid carbon is a material naturally found in only the most extreme environments, it might seem as though there is little reason to undertake a detailed study of its properties--save for scientific curiosity. However, although rarely observed, understanding the properties of the liquid is potentially important in the modelling of both terrestrial and astrophysical systems, as well as being critical for the manufacture of emerging carbon materials.

Structural simulations

With the high temperatures and pressure necessary to generate equilibrium liquid carbon being so difficult to maintain in a laboratory setting, simulations have become a principal tool for elucidating its properties. One of the earliest simulations of liquid carbon was carried out by Galli and Martin in 1989 [63]. In their molecular dynamics study, an amorphous carbon precursor with an initial density of 2 g/cm3 was heated to a temperature of 5000 K. The material was observed to melt at a

Flash heating experiments

Preparing liquid carbon in a manner compatible with appropriate characterization tools has been and remains a significant challenge. As the conditions necessary to transition to the liquid state are essentially impossible to sustain in the laboratory, due to the extreme temperature and pressure required, liquid samples must be generated transiently and are generally very short-lived. This puts significant constraints on the types of experiment that can be performed on the material, as these

Conclusions

Clearly, many intriguing unanswered questions remain regarding the properties of liquid carbon. While theory has produced a wide array of predictions regarding the structure, phase diagram, and electronic nature of the liquid, as of yet, few of these predictions have been experimentally tested, and several of the predicted properties of the liquid remain controversial. On the experimental side, the results of measurements are often conflicting and provide little concrete insight into the true

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work is supported by the U. S. Army Research Laboratory and the U. S. Army Research Office under grant number W911NF-13-1-0483 and No. W911NF-17-1-0163. S. L. R. received a National Science Foundation Graduate Research Fellowship under Grant No. DGE 1106400. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

Richard Saykally is the Class of 1932 Endowed Professor of Chemistry at the University of California, Berkeley. He received his B.S. in Chemistry from the University of Wisconsin, Eau Claire and his Ph.D. from the University of Wisconsin, Madison. Prof. Saykally has been the recipient of over 75 awards from 15 different countries, including the Irving Langmuir Award in Chemical Physics and Peter DeBye Award in Physical Chemistry from the American Chemical Society and the Faraday Lectureship

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    Christopher Hull received his Ph D. in Physical Chemistry in the Fall of 2019 and his B.S from Northwestern University.

    Sumana Raj received her PhD. in Physical Chemistry in the Fall of 2019 and her B.A. degree in Chemistry from Cornell University.

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