Microstructure of turbostratic carbon studied by X-ray scattering

M. Dopita¹, A. Salomon¹, M. Emmel², Ch. G. Aneziris² and D. Rafaja¹

¹Institute of Materials Science, Technical University of Freiberg, Gustav-Zeuner-Straße 5, Freiberg, D09599, Germany

²Institute of Ceramics, Glass and Construction Materials, Technical University of Freiberg, Agricolastraße 17, Freiberg, D09599, Germany

dopita@gmail.com

The carbon phases, high melting coal-tar resins or pitches and carbon blacks are an important components of the carbon bonded refractories. Since other constituents of these refractories, i.e. refractory oxides and graphite, exhibit exceptional thermal stability, the microstructure of the carbon binder phases changes at elevated temperatures. This is one of the main reasons for the degradation of the materials properties, because they depend strongly on the microstructure of carbon binder phases.

From the structural point of view, the high melting coal-tar resins or pitches and carbon blacks are turbostratic structures, where the graphite layers are arranged parallel to each other however with random orientations around the normal to the layers. Such structural disorder leads to the formation of pronounced 00l peaks comming from the scattering on mutually disoriented parallel layer groups and asymmetrical hk bands originating from the scattering on individual layers, in the diffraction pattern. Mutually random orientations of individual graphitic layers, as well as other defects creating turbostratic structure as are random fluctuations in the parallel layer spacings, random lateral translations of graphitic layers and curvatures of individual graphite layers, prevent the formation of distinct diffraction lines with the general indices hkl. From measured diffraction 00l and hk profiles one can estimate the mean cluster dimensions Lc and La (average number of disoriented graphite layers and its mean distance and average lateral size of individual layers) as well as the disorientation degree of individual parallel layers.

Simulations of scattered intensity distributions from two and three dimensional carbon structures of different shapes and sizes were done using the general Debye scattering equation [1]. The influence of the lattice defects typical for the turbostratic structure, i.e. random fluctuations in the parallel layer spacings, random lateral translations of graphitic layers and mutual disorientations of individual parallel layers around the layers normal direction, on the resulting simulated scattered intensities were studied and discussed. The microstructure-induced changes in the line broadening, in the shape parameter in the Scherrer formula [2] and in the lattice parameters determined from the positions of the X-ray diffraction lines are discussed in particular. The set of presented Scherrer parameters allows the calculation of the cluster sizes along and normal to the basal planes from the measured X-ray scattering. The reliability of the Warren-Bodenstein approach [3] for scattering on turbostratic carbon structures was proven. Intensity distributions simulated using the Warren-Bodenstein approach were compared to those obtained using the general Debye scattering equation. It was confirmed that both approaches yield, for particular cluster size, similar results.

A series of high melting coal-tar resin specimens annealed at different temperatures up to 1400°C was prepared. Measured X-ray scattering patterns were fitted using the Warren-Bodenstein approach to describe the thermal evolution of main microstructural parameters. Necessary corrections, influencing strongly the X-ray scattering intensity distributions from low absorbing carbon material, i.e. absorption and polarization corrections, incoherent part of scattered intensity corrected for the influence of radiation pressure (Breit-Dirac correction) and to the finite width of spectral window of monochromator, cutting out some part of incoherently scattered intensity (Ruland correction), as well as the multiple scattering were token into consideration. The thermal evolution of mean lateral cluster size La, number of parallel layers and consequently the cluster size in c-direction Lc, its distributions, mean lattice parameters a₀ and c₀, and graphitization degree of the parallel layers group, were determined.

Figure 1. Single carbon layer of radius 10 Å (a). Calculated X-ray scattering patterns from the 2-dim carbon layers of different radius (b); thick curve corresponds to the scattering from layer shown in Fig 1a. Parallel layers cluster of dimensions La and Lc (c). Calculated X-ray scattering patterns from parallel layers groups with La = 10 Å; number of parallel layers varied between 1 and 15 (d). Thick curve corresponds to the scattering from parallel layers group shown in Fig 1c.

1. P. Debye, Ann. Phys., 351, 6, (1915), 809.

2. P. Scherrer, Göttinger Nachrichten Gesell., 2, (1918), 98.

3. B. E. Warren and P. Bodenstein, Acta. Cryst., 18, (1965), 282.

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”.