Surface Analysis of Nuclear Graphite

Keywords Surface analysis Graphite, XPS, 2 of nuclear graphite Auger d-parameter, sp

Application Note MO426(1) Further application/technical notes are available online at: www.kratos.com

Overview Introduction In this short study we will explore the differences in Graphite is one of the key materials used in the current generation of nuclear reactors in the UK. Since the early days of nuclear fission surface chemistry between two different graphites used graphite has been recognised as an excellent neutron moderator in the nuclear industry. In recent times a wide variety and reflector allowing sustainable controlled fission. of nanostructured carbon forms have been observed in Contaminants in moderator rods are a major problem as they nuclear graphite which vary the graphitic nature of the can act as neutron absorbers. This causes decreased fission and material. The nature of these forms can greatly affect production of unwanted isotopes – a common cause of radioactive waste. The goal of this application is to understand the surface the material’s ability to act as an effective moderator. chemistry of two different graphites: Pile Grade A (PGA) and Here we will discuss the elemental composition of the Gilsocarbon (Gilso). PGA is an early form of nuclear graphite used graphite surface and the extent of graphitic sp2 bonding. in many reactors world-wide, particularly in the UK in the Magnox series. It is derived from a petroleum coke (which is a by-product of the oil refining industry). Gilsocarbon was developed later and is used in the Generation II Advanced Gas-Cooled Reactors. The Gilsonite coke is produced from refining the asphalt. Here we will study the two graphites and try to identify differences in surface chemistry and extent of sp2/sp3 character.

Kratos Analytical Ltd. UK APPLICAION NOES AXIS SERIES Wharfside, Trafford Wharf Road, Manchester M17 1GP Phone: +44 161 888 4400 Fax. +44 161 888 4402 www.kratos.com

400 16.12 eV High resolution C 1s spectra were acquired to identify Experimental Highthe200 resolutionchemical Cstat 1s esspectra of the were carbon acquired in theto identify surface the (figure chemical Figure 3. First derivative C KLL Auger states2). of the carbon in the surface (figure 2). All measurements were acquired using a Kratos Axis Supra XPS. 20.82 eV spectra. The AXIS Supra incorporates several features which make high 0 50 resolution spectroscopic analysis of these types of challenging 230 250 270 290 samples routine: 45 Conclusions -200 40 • A 500 mm Rowland circle Al X-ray monochromator; Two graphite samples were analysed using XPS to 35

dN(E)/ dE • Coaxial charge neutralisation system; determine carbon chemistry at the surface. Small -400 30 concentrations of contaminants were seen on the • Magnetic lens; 25 surface of the Gilsocarbon surface and the high-

• 165 mm hemispherical analyser; 20 resolution C 1s spectrum showed less sp2 character -600 • Delay line detector. 15 than for the PGA sample. Further analysis of the Auger parameter allowed the degree of surface sp2:sp3 The two graphitic samples were cut from larger rods using 10 to be quantified. This study highlights the use of the -800 a mechanical saw. 5 AXIS Supra spectrometer in detecting low kinetic energy / eV Results 0 concentration contaminants and in determining 310 300 290 280 bonding arrangements in carbon materials After introducing the graphite samples into the AXIS Supra, survey Binding energy /eV spectra were acquired to identify the elements present on the surface. Figure 1 shows the spectra post automated elemental peak Acknowledgements identification. Figure 2: C 1s spectra for Gilso (green) and PGA (blue). Figure 2: C 1s spectra for Gilso (green) and Many thanks to Dr Alex Theodosiou of the University C 1s PGA (blue). of Manchester for this collaborative study. For both graphite samples the C 1s peak appears at ~284.4 eV.

The C 1s spectrum for PGA has an asymmetric line shape towards highFor binding both graphite energy. This samples peak shape the isC typical 1s peak of graphitic appears carbon. at References There~284.4 is also eV. a pronounced The C 1s peak spectrum at >290 eVfor – a shake-upPGA has satellite an 2 [1] T.L. Barr, S. Seal, J. Vac. Sci. Technol. A 13(3) (1995) previouslyasymmetric attributed line shapeto sp bonding. towards In higharomatic binding systems energy. this * 1239. structureThis peak has shapebeen shown is typical to be of due graphitic to π→π carbon.transitions There involving is thealso two a highest pronounced filled orbital peak and at the >290 lowest eV unfilled – a shake orbital.- upIn [2] Scaglione et al. Appl. Surf. Sci., 47 (1991) 17-21. general, the more pronounced the loss peak the2 greater degree Intensity satellite previously attributed to sp bonding. In of sp2 bonding. The Gilso C 1s spectrum differs greatly from PGA. Therearomatic is no longersystems a clearly this structuredefined shake-up has been feature shown and toa shoulder be due to π→π* transitions involving the two highest filled Na 1s O 1s is clear on the high binding energy side of the primary peak ~285- C KLL orbital and the lowest unfilled orbital. In general,3 the O KLL 286 eV. This shoulder is most probably increased sp character and somemore C-O pronounced chemistry. the loss peak the greater degree of sp2 bonding. The Gilso C 1s spectrum differs greatly To gain further information regarding the bonding states of the 1200 1000 800 600 400 200 0 from PGA. There is no longer a clearly defined shake- Binding energy /eV two different graphite samples, carbon KLL Auger electron lines wereup featurealso acquired. and a Following shoulder ionization is clear byon photoelectron the high binding emission Figure 1: Survey spectra for Gilso (green) and PGA (blue). anenergy outer shell side electron of the can primary fill the createdpeak ~285vacancy-286 and eV the. Thisenergy releasedshoulder can is result most in probablythe emission increased of an Auger sp3 electron. character Analysis and Quantification of the survey spectra for the two samples (table 1) ofsome the X-ray C-O inducedchemistry. C KLL Auger peak can help distinguish the showed as expected mainly carbon on the surface with a small bondingTo gain states further in a semi-quantitative information regarding manner for thenon-functionalized bonding amount of surface oxygen. The oxygen can be attributed to a thin samples.states ofScaglione the two et al.different proposed graphite a simple methodsamples, of quantifyingcarbon film of adventitious carbon which is present on all air-exposed the relative amount of sp2/sp3 character in a carbon material KLL Auger electron lines were also acquired. Following samples. Adventitious carbon is generally comprised of short chain, using this first derivitave2. The energy difference between the perhaps polymeric non-graphitic hydrocarbons species with small spectrumionization maximum by photoelectron and minimum isemission known as anthe outerd-parameter shell amounts of both singly and doubly bound oxygen functionality1. andelectron is linearly can related fill the to the created concentration vacancy of spand2 character. the energy Figure No other elements were detected in the PGA surface however 3 releasedshows the firstcan derivativeresult in spectra the foremission the two ofgraphite an Augersamples. several contaminants of low concentration were identified in the Hereelectr thereon. is Analysisa clear distinction of the inX- energyray induced difference C KLLbetween Auger the Gilso sample. peakpeak maximum can help and disti minimumnguish forthe the bonding two graphites. states inPGA a semishows- ~74%quantitative sp2 character manner compared for nonto ~20%-functionalized sp2 character samples.of Gilso. The Element C 0 Si S Na N CI correlationScaglione of resultset al .obtained proposed from thea highsimple resolution method C 1s andof C KLLquantifying spectra indicate the relative that the amountPGA sample’s of sp surface2/sp3 character contains ain higher a PGA 98.70 1.30 - - - - - concentrationcarbon material of graphitic using carbon.this first derivitave2. The energy Gilso 93.92 5.16 0.11 0.07 0.19 0.52 0.03 difference between the spectrum maximum and minimum is known as the d-parameter and is linearly related to the concentration of sp2 character. Figure 3 Table 1: Surface quantification for Gilso and PGA graphite surfaces. shows the first derivative spectra for the two graphite samples. Here there is a clear distinction in energy difference between the peak maximum and minimum 2 for the two graphites. PGA shows ~74% sp character compared to ~20% sp2 character of Gilso. The correlation of results obtained from the high resolution C 1s and C KLL spectra indicate that the PGA sample’s surface contains a higher concentration of graphitic carbon.

Kratos Analytical Ltd. UK APPLICAION NOES AXIS SERIES Wharfside, Trafford Wharf Road, Manchester M17 1GP Phone: +44 161 888 4400 Fax. +44 161 888 4402 www.kratos.com

ECNICAL NOES AXIS SERIES Conclusions 400 16.12 eV Two graphite samples were analysed using XPS to determine carbon chemistry at the surface. Small concentrations of contaminants were seen on the surface of the Gilsocarbon surface and the high- 200 resolution C 1s spectrum showed less sp2 character than for the PGA sample. Further analysis of the Auger parameter allowed the 20.82 eV degree of surface sp2:sp3 to be quantified. This study highlights the 0 use of the AXIS Supra spectrometer in detecting low concentration 230 250 270 290 contaminants and in determining bonding arrangements in carbon materials -200 Acknowledgements Many thanks to Dr Alex Theodosiou of the University of Manchester dN(E)/dE for this collaborative study. -400 References [1] T.L. Barr, S. Seal, J. Vac. Sci. Technol. A 13(3) (1995) 1239. -600 [2] Scaglione et al. Appl. Surf. Sci., 47 (1991) 17-21.

-800 kinetic energy /eV

Figure 3. First derivative C KLL Auger spectra.