Microstructure evolution of turbostratic carbon studied by different analytical methods

M. Dopita^1*, A. Salomon¹, M. Emmel², C. G. Aneziris² and D. Rafaja¹

¹Institute of Materials Science, Technical University of Freiberg, Gustav-Zeuner-Strasse 5, Freiberg, D-09599, Germany

²Institute of Ceramics, Glass and Construction Materials, Technical University of Freiberg, Agricolastrasse 17, Freiberg, D-09599, Germany

*dopita@gmail.com

A series of high melting synthetic coal-tar pitch Carbores P specimens annealed at different temperatures up to 1400 °C was prepared and the thermally induced microstructural changes were investigated by the combination of different analytical methods. From the structural point of view, the Carbores P is a turbostratic carbon structure, where the individual graphite layers are arranged parallel to each other however with random orientation around the normal to the layers. The temperature evolution of microstructure of investigated samples was studied using the X-ray scattering, neutron scattering, X-ray photo spectroscopy (XPS), Raman scattering, scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDX), transmission electron microscopy particularly with high resolution (TEM/HRTEM) and electron energy loss spectroscopy (TEM/EELS).

The computer program Turbostratic-C, based on the Warren-Bodenstein’s theory of scattering on turbostratic carbon allowing fitting of whole measured scattering powder pattern was written and used for refinement of microstructural parameters of studied samples from measured X-ray and neutron scattering patterns. The program enables the refinement of essential physical parameters of turbostratic carbon materials, the mean lattice parameters a₀, c₀, mean cluster sizes parallel and perpendicular to the graphitic planes La and Lc and its distributions as well as the mean square lattice displacements áu_a²ñ and áu_c²ñ.

a)	b)
Figure 1. The temperature evolution of the mean cluster sizes parallel and perpendicular to the graphitic planes La and Lc (a). The temperature evolution of the mean lattice parameters in-plane of graphitic layers a0, and perpendicular to the graphitic layers c0 (b), refined from the X-ray scattering data.

From X-ray scattering data measured on investigated samples we refined the following, temperature activated, microstructural evolution of the Carbores P. With increasing annealing temperature, the mean cluster size La increased from 1.23 nm in initial powder to 6.05 nm in specimen annealed at 1400 °C. This is connected to an increase of the number of atoms in each individual graphitic layer from 46 in the initial powder to 1107 atoms in specimen annealed at 1400 °C. The mean cluster size parallel to graphitic layers increases from 1.07 nm to 4.82 nm, which corresponds to increase of number of layers from 3 to 14 in initial powder and sample annealed at 1400 °C, respectively. Simultaneously, the width of both distributions increases with growing mean cluster sizes. The evolution of lattice parameter a₀ (the lattice parameter in the graphitic planes, in ab-plane) exhibits pronounced increase in samples annealed at temperatures 800 °C and higher. The a₀ lattice parameter changes from a₀ = 0.2434 nm in the initial powder to a₀ = 0.2449 nm in specimen annealed at 1400 °C. The recalculated in-plane C–C bond length varies from l_C–C = 0.1404 nm to l_C–C = 0.1414 nm in raw powder and specimen annealed at 1400 °C, respectively. However, in all investigated specimens the lattice parameters a₀ and the in-plane bond lengths l_C–C are smaller than its equilibrium values in undisturbed graphite. Contrary to thermal evolution of lattice parameter a₀, the lattice parameter c₀ (lattice parameter perpendicular to the graphitic planes, in c-direction) decreases with increasing annealing temperature, following well the linear dependence on the annealing temperature. The refined c₀ lattice parameters of turbostratic carbon are in all studied samples higher than those of perfect undisturbed graphite. The lattice parameter c₀ of initial powder is c₀ = 0.7102 nm, which is about 6 % more than in unperturbed graphite. With increasing annealing temperatures, the difference between the unperturbed graphitic and turbostratic lattice parameters decreases. In sample annealed at 1400 °C the refined lattice parameter is c₀ = 0.6883 nm, which is about 2.6 % higher value than in perfect graphite. The temperature evolution of mean cluster sizes La and Lc and lattice parameters a₀ and c₀ are shown in Fig. 1. Using the refined lattice parameters, the X-ray structural density of studied samples was calculated. Calculated densities are smaller than the density of undisturbed graphite, and smallest density r = 2.19 g/cm³ shows the initial powder. With increasing annealing temperature, the density increases up to roughly 800 °C where it reaches the value of approximately r = 2.23 g/cm³, which is about 1.5 % less than the density of perfect graphite. The mean square atomic displacements áu_a²ñ and áu_c²ñ decreases with increasing annealing temperature, whereas the refined values are in all specimens under study higher than the values of undisturbed graphite. The changes in the refined lattice parameters together with the decay of the mean square atomic displacements, all values tending towards the values of graphite, are attributed to decreasing degree of disorder and concentration of defects with increasing annealing temperature.

The microstructural changes of Carbores P estimated from measured X-ray scattering data were correlated with results obtained from other analytical methods. The evolution of the mean cluster sizes La are in good correspondence with data obtained from the Raman spectroscopy measurements. The information about the bound types and its changes were determined from the XPS and TEM/EELS measurements. The TEM/EELS additionally confirmed the density changes in the annealed samples. The morphological changes of studied samples were investigated using the SEM and TEM. Finally, the changes on the atomic scale, growth of the cluster sizes and decay of the disorder with increasing annealing temperature was confirmed by TEM/HRTEM.

1. B. E. Warren & P. Bodenstein, Acta. Cryst., 18, (1965), 282.

2. M. Dopita, M. Rudolph, A. Salomon, M. Emmel, C. G. Aneziris & D. Rafaja, Adv. Eng. Maters., 15, (2013) 1280.

The authors would like to thank the German Research Foundation (DFG) for supporting the subproject A05, which is a part of the Collaborative Research Centre 920 (CRC 920) “Multi-Functional Filters for Metal Melt Filtration - A Contribution towards Zero Defect Materials”.