Determining Crystallite Size in Calcined Petroleum Coke

Calcined Petroleum Coke (CPC) is manufactured from “green” petroleum coke, which is a byproduct of oil refining. The electrodes used in the metal smelting industry are typically made of CPC due to the sponge-like structure which enhances the contact between electrode and metal. The crystallite size greatly influences the electrical conductivity, and as a result also the quality of electrodes, which limits the effectivity of the entire smelting process. For this reason, it is vital to evaluate the crystallite size of the raw CPC material.

Instrument

Thermo Scientific™ ARL™ EQUINOX 100 uses a custom-designed Cu (50 W) or Co (15 W) micro-focus tube with mirror optics. Thanks to the unit’s low power consumption, it is completely transportable and does not require an external water chiller. The same unit can be transported between laboratories with no special infrastructure required.

The ARL EQUINOX 100 provides more rapid data collection times than other diffractometers because of its unique curved position sensitive detector (CPS) that measures all diffraction peaks at once and in real time, making it suitable for reflection as well as transmission measurements (Figure 1).

Figure 1. ARL EQUINOX 100 X-ray diffractometer.

Experimental

Measurement of an A508 graphite anode sample from RD-Carbon was performed in reflection mode. The sample was analyzed under Cu Kα (1.541874 Å) radiation for 2 minutes while the sample rotates throughout the analysis. L_c was determined from the FWHM of the graphite [002] reflection using MAUD, JADE and following the method explained in the ASTM D5187 standard.

Results

The mean crystallite size of the graphite particles is obtained by fitting the [002] reflection of graphite using Rietveld’s method implemented in MAUD and JADE. Furthermore, both the programs account for the influence of micro-strain. In the methods described in the ASTM D5187 standard, a simplified version of the Scherrer equation is used. While all three methods give similar results (MAUD: 48.1 Å; JADE: 42.0 Å; ASTM D5187: 18.5 Å), the differences are generally due to slightly different algorithm used by the programs and the susceptibility to errors of the FWHM determined according to the ASTM D5187 standard.

Figure 2. Rietveld fit (MAUD) of graphite [002] reflection.

Conclusion

From the results, it is evident that the ARL EQUINOX 100 is well suited to determine the crystallite size in CPC samples using Rietveld’s method with MAUD and JADE as well as in accordance with the ASTM D5187.

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific - Elemental Analyzers.

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