Good to Know Series – 08 – Graphitising and Non-Graphitising Carbons
Before diving into synthetic graphite, it’s important to understand where graphitising and non-graphitising carbons come from—and why they behave so differently. In previous episodes, we explored natural graphitisation and the transformations that occur in organic matter. This episode shifts the focus to artificial graphitisation: the transformation of various carbon precursors in high-temperature laboratory furnaces.
This discussion would be incomplete without acknowledging the pioneering work of Rosalind E. Franklin—the scientist I most admire. While best known for her work on DNA, Franklin’s groundbreaking studies on coal and carbon precursors helped shape the field of carbon science (see Harris and Suarez-Martinez, 2021). Her insights will guide us through much of this episode.
Early Works on Coal
Many people don’t realise that Franklin began her career studying coal. During her PhD in the 1940s, she worked at the British Coal Utilisation Research Association, where she investigated coal porosity using helium—an approach still widely used today to determine density and adsorption capacity (Franklin, 1949a).
She found that as coal rank increases, so does its density, and that porosity changes not only in size but in connectivity—how easily gases or liquids move between pores. She later applied these observations to a range of carbonised coals (Franklin, 1949b), confirming that porosity depends on both the starting material and the temperature of carbonisation. Although her work was originally focused on gas mask applications during WWII, its implications for gas transport, permeability, and diffusion remain deeply relevant—especially for understanding coal seam gas and underground gas storage.
Graphitising and Non-Graphitising Carbons
While working at the Laboratoire Central des Services Chimiques de l’État in Paris, Franklin turned her attention to carbon materials and the behaviour of different precursors at temperatures up to 3000°C. Using X-ray diffraction (XRD), she conducted one of the earliest studies of carbon structural changes at such high temperatures.
Her 1951 paper, “Crystallite growth in graphitizing and non-graphitizing carbons”, became foundational. It explained why some carbon precursors undergo graphitisation—and why others don’t, even at extreme temperatures.
Franklin identified two key conditions required for graphitisation:
Weak cross-links between crystallites in the precursor matrix, and
Pre-orientation, where neighbouring crystallites are already nearly parallel.
As temperature rises, these weak cross-links—often formed by heteroatoms—break, allowing crystallites to align and coalesce. Crystallite growth proceeds by eliminating micropores as near-parallel clusters gradually shift into truly parallel arrangements.
Materials like petroleum coke, pitch coke, and even polyvinylchloride fulfil these criteria, making them graphitisable and useful in synthetic graphite production.
In contrast, when the cross-links are too strong or the structure too disordered, even extreme heat can’t align the crystallites. These are the non-graphitising carbons (or hard carbons), which include charcoal, activated carbon, carbons derived from polyvinylidene chloride, and low-rank coals (e.g., lignite to high-volatile bituminous coal).
What About Anthracites?
Franklin discovered that anthracites behave differently. Unlike lower-rank coals, they begin to graphitise above 2500°C. Why? Because, while they retain significant microporosity up to ~1600°C, it disappears as temperatures rise beyond 2500°C—yielding a compact, highly graphitic structure.
Additionally, anthracites are typically optically anisotropic, a sign that their internal structure is already somewhat ordered. This anisotropy suggests a pre-orientation of basic structural units—again, a crucial prerequisite for graphitisation.
Beyond Franklin: Other Approaches to Induce Graphitisation
Franklin’s work was later expanded by researchers like Agnes Oberlin (Oberlin, 1984), who further explored the transformation pathways of carbon. These efforts laid the groundwork for various strategies to graphitise otherwise non-graphitising materials:
Pressure-assisted graphitisation, combining high pressure with high temperature to aid crystallite rearrangement (e.g., De Fonton et al., 1980).
Catalytic graphitisation, using metals to promote graphitic structure formation (e.g., Oya and Otani, 1979).
Pre-treatment by partial oxidation, which removes defect sites and edges, easing later structural alignment (e.g., Gharpure and Wal, 2023).
High-temperature pulse heating, where rapid heating to ~3000°C occurs in seconds, compared to the hours needed in conventional furnaces (Fogg et al., 2020).
These techniques aim to achieve graphitic carbon with greater crystallinity and purity—but even after decades of research, the process remains complex and not always economically viable.
Final Thoughts
Rosalind Franklin’s early insights—rooted in coal science—still resonate today. She showed that structure, not just temperature, governs graphitisation. Her work reminds us that understanding carbon isn’t just about reaching high temperatures—it's about knowing how carbon is built and how it can evolve.
As we continue to push the limits of what carbon materials can do, Franklin’s legacy guides us—not just in science, but in curiosity, rigour, and the pursuit of clarity.
References:
De Fonton, S., Oberlin, A., Inagaki, M., 1980. Characterization by electron microscopy of carbon phases (intermediate turbostractic phase and graphite) in hard carbons when heat-treated under pressure. Journal of Material Science 15, 909-917.
Fogg, J.L., Putman, K. J., Zhang, T., et al., 2020. Catalysis-free transformation of non-graphitising carbons into highly crystalline graphite. Communications Materials 1, 47. (open access)
Franklin, R.E., 1949a. A study of the fine structure of carbonaceous solids by measurements of true and apparent densities. Part I. Coals. Trans. Faraday Soc., 45, 274-286.
Franklin, R.E., 1949b. A study of the fine structure of carbonaceous solids by measurements of true and apparent densities. Part II. Carbonized coals. Trans. Faraday Soc., 45, 668-682.
Franklin, R.E., 1951. Crystallite growth in graphitizing and non-graphitizing carbons. Proc. Roy. Soc. Lond., 209, 196-218.
Gharpure, A., Wal, R. V., 2023. Improving graphemic quality by oxidative liberation of crosslinks in non-graphitizable carbon. Carbon 209, 118010.
Harris, P.J.F., Suarez-Martinez, I., 2021. Rosalind Franklin, carbon scientist.
Oberlin, A., 1984. Carbonization and graphitization. Carbon 22, 521-541.
Oya, A., Otani, S., 1979. Catalytic graphitization of carbons by various metals. Carbon 17, 131-137.
