Assessing Crystallite Dimensions of Graphite in Calcined Petroleum Coke

Calcined Petroleum Coke (CPC) is a graphite-based material commonly used in metallurgy, aluminum production, and fuel production. It is a raw petroleum coke derivative created during high-temperature processes. Containing high amounts of carbon, CPC has superior electrical conductivity.

The potential quality of CPC as an electrode is determined by the crystallite size (CS, L_c ), making evaluating the crystallite size of the CPC vital.

An ideal method for determining this is X-Ray diffraction (XRD), which is in accordance with the ASTM D5187 standard.

 L_c is arrived at using the following formula:

 Δpo = 2(sinθ₂  – sinθ₁)/λ

where: θ₁ = lower angle at half peak intensity width

θ₂ = higher angle at half peak intensity width

λ = incident wavelength

The Scherrer equation establishes this formula from scattering domains and the basic relationship between the size and the peak width.¹

L_c = (Kλ)/((β_obs – β_inst)cosθ)

K = arbitrary constant 0.89–1.39

λ. = incident wavelength

β_obs = measured peak line breadth

β_inst = instrument peak line breadth contribution

θ = angular location of the diffraction peak in degrees

An expression for L_c valid for calcined petroleum coke can be derived in latter formulas.

L_c = 0.89/Δpo

The Williamson-Hall plot is a sophisticated approach based on the deconvoluting contributions of strain and size on the peak broadening, allowing for obtaining more precise L_c values. ²

ε_str = strain component 

Instrument and Software 

Pictured in Figure 1 is the Thermo Scientific™ ARL™ X’TRA Companion X-Ray Diffractometer. It is user-friendly and simple as a benchtop XRD system for process control and advanced applications. The ARL X’TRA Companion uses a 160 mm radius θ/θ goniometer in Bragg-Brentano geometry matched with a 600 W X-Ray source (Cu or Co).

The beam axial and radial adjustment is operated by Soller slits and divergence. Air scattering is limited through the use of a variable beam knife. An integrated water chiller is an optional feature.

The ARL X’TRA Companion offers impressively quick data collection with a state-of-the-art solid state pixel detector (55 x 55 μm pitch). It comes equipped with automated result transmission to a LIMS and one-click Rietveld quantification capabilities.

Figure 1. ARL X’TRA Companion diffraction system. Image Credit: Thermo Fisher Scientific – Production Process & Analytics

Experimental 

Derived from R&D carbon, graphite-certified reference material RDC-1104 was measured in reflection for two minutes using Cu Kα (1.541874 Å) radiation. To minimize penetration depth error, sample spinning was used during acquisition, and the sample was prepared in a zero-background sample holder.

Figure 2. Measurement of RDC-1104 Graphite sample. Data were obtained at room temperature. Image Credit: Thermo Fisher Scientific – Production Process & Analytics

Results

Use of the Scherrer formula with the FWHM of the [002] reflection of graphite results in a L_c of 28.0 Å (see Figure 2). This is comparatively ideal to the certified value of 28.5(15) Å. The FWHM was obtained through profile fitting the graphite [002] reflection with a Pseudo-Voigt function. Since this is a computational approach to determining L_c, this method is compliant with ASTM D5187.

There is no identifiable influence of the Williamson-Hall method on the results of the analysis. The graphite sample displays no strain.

Conclusion

The ARL X’TRA Companion is an appropriate option for ascertaining the crystallite size in CPC samples, according to ASTM D5187.

References and Further Reading

1. P. Scherrer, Göttinger Nachrichten Gesell. 1918 2, 98-100.
2. G. K. Williamson, W. H. Hall, Acta Metall. 1953, 1, 22-31.

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Production Process & Analytics.

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